PARTNERSHIP OF NORFOLK DISTRICT COUNCILS
STRATEGIC FLOOD RISK ASSESSMENT
SUBSIDIARY REPORT A
NORTH NORFOLK DISTRICT COUNCIL AREA
DECEMBER 2007
REPORT REF: 7293A/21/CW/06-07/1778
PARTNERSHIP OF NORFOLK DISTRICT COUNCILS
STRATEGIC FLOOD RISK ASSESSMENT
SUBSIDIARY REPORT A
NORTH NORFOLK DISTRICT COUNCIL AREA
DECEMBER 2007
REPORT REF: 7293A/21/CW/06-07/1778
CLIENT:
North Norfolk District Council
ENGINEER: Millard Consulting
The Atrium
Merchants Court
St Georges Street
Norwich
Norfolk
NR3 1AB
Tel:
Fax:
01603 610916
01603 620631
Report Prepared By:
.....................................................
Christopher Ward, BSc (Hons), CEng, MICE, MIHT, MCIWEM
Report Checked By:
Brian Coghlan, BSc (Hons), CEng, PhD, MCIWEM
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
REGISTRATION OF AMENDMENTS
Revision
and Date
Amendment Details
Draft
Revision
Prepared
By
CDW
Revision
Approved
By
JPC
Draft
31/07/07
Second
Draft
14/09/07
Final
21/12/07
Amendments to text and plans
CDW
BC
Final amendments to text and plans
CDW
BC
Ref: 7293A/21/CW/06-07/1778
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SUBSIDIARY REPORT A: NORTH NORFOLK DISTRICT COUNCIL AREA
CONTENTS
1.
INTRODUCTION
1.1
Terms of Reference
1.2
North Norfolk District Council Area
1.3
Local Development Framework and Scope of Stage 2 SFRA
1.4
Shoreline Management Plan
1.5
Coastal Habitat Management Plan
1.6
Catchment Flood Management Plan
1.7
Internal Drainage Boards
1.8
North Norfolk Joint Flood Plan
2.
SOURCES OF FLOODING IN NORTH NORFOLK
2.1
Coastal
2.2
Fluvial and Tidal
3.
ENVIRONMENT AGENCY FLOOD ZONES
4.
HYDRAULIC MODELLING AND FLOOD PROBABILITY MAPPING
5.
COASTAL DEFENCE OVERTOPPING AND BREACH SCENARIOS
6.
SUSTAINABLE DRAINAGE
7.
RECOMMENDATIONS
8.
SITE SPECIFIC FLOOD RISK ASSESSMENTS
9.
CONCLUSIONS
10.
REFERENCES
11.
GLOSSARY
APPENDICES
A.
CORRESPONDENCE
B.
INTERNAL DRAINAGE BOARDS
C.
PPS25 PRACTICE GUIDE: SITE SPECIFIC FRA PRO-FORMA
Ref: 7293A/21/CW/06-07/1778
Page 1
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
PLANS (BOUND SEPARATELY)
General
7293A/21/INDEX 1 NORTH NORFOLK DISTRICT COUNCIL: SFRA DRAWINGS INDEX
SHEET 1
7293A/21/INDEX 2 NORTH NORFOLK DISTRICT COUNCIL: SFRA DRAWINGS INDEX
SHEET 2
7293A/21/200
NORTH NORFOLK DISTRICT COUNCIL: THE STUDY AREA
7293A/21/201
RIVERS AND CATCHMENTS
7293A/21/202
ENVIRONMENT AGENCY FLOOD ZONES AND EXISTING FLOOD
DEFENCES – WEST
7293A/21/203
ENVIRONMENT AGENCY FLOOD ZONES AND EXISTING FLOOD
DEFENCES – EAST
Flood Probability Maps (showing probability of flooding now and for the future climate
change scenarios)
7293A/21/210
ALDBOROUGH ZONE 3b FUNCTIONAL
WITHOUT CLIMATE CHANGE
7293A/21/211
ALDBOROUGH ZONE 3a HIGH PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/212
ALDBOROUGH ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/213
ALDBOROUGH FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/214
CATFIELD ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/215
CATFIELD ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/216
CATFIELD ZONE 2
CLIMATE CHANGE
7293A/21/217
CATFIELD FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/218
CORPUSTY ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
Ref: 7293A/21/CW/06-07/1778
MEDIUM
FLOODPLAIN
PROBABILITY
WITH/
WITH/WITHOUT
Page 2
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7293A/21/219
CORPUSTY ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/220
CORPUSTY ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/221
CORPUSTY FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/222
ERPINGHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/223
ERPINGHAM ZONE
CLIMATE CHANGE
7293A/21/224
ERPINGHAM ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/225
ERPINGHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/226
FAKENHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/227
FAKENHAM ZONE
CLIMATE CHANGE
7293A/21/228
FAKENHAM ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/229
FAKENHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/230
GIMINGHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/231
GIMINGHAM ZONE
CLIMATE CHANGE
7293A/21/232
GIMINGHAM ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/233
GIMINGHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/234
HICKLING ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/235
HICKLING ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
Ref: 7293A/21/CW/06-07/1778
3a
3a
3a
HIGH
HIGH
HIGH
PROBABILITY
PROBABILITY
PROBABILITY
WITH/WITHOUT
WITH/WITHOUT
WITH/WITHOUT
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7293A/21/236
HICKLING ZONE 2
CLIMATE CHANGE
7293A/21/237
HICKLING FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/238
HORNING ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/239
HORNING ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/240
HORNING ZONE 2
CLIMATE CHANGE
7293A/21/241
HORNING FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/242
HORSEY ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/243
HORSEY ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/244
HORSEY ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/245
HORSEY FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/WITHOUT
CLIMATE CHANGE
7293A/21/246
HOVETON ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/247
HOVETON ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/248
HOVETON ZONE 2
CLIMATE CHANGE
7293A/21/249
HOVETON FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/250
INGWORTH ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/251
INGWORTH ZONE
CLIMATE CHANGE
Ref: 7293A/21/CW/06-07/1778
MEDIUM
MEDIUM
MEDIUM
3a
HIGH
PROBABILITY
PROBABILITY
PROBABILITY
PROBABILITY
WITH/WITHOUT
WITH/WITHOUT
WITH/WITHOUT
WITH/WITHOUT
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7293A/21/252
INGWORTH ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/253
INGWORTH FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/254
LANGHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/255
LANGHAM ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/256
LANGHAM ZONE 2
CLIMATE CHANGE
7293A/21/257
LANGHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/258
LUDHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/259
LUDHAM ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/260
LUDHAM ZONE 2
CLIMATE CHANGE
7293A/21/261
LUDHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/262
MUNDESLEY ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/263
MUNDESLEY ZONE
CLIMATE CHANGE
7293A/21/264
MUNDESLEY ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/265
MUNDESLEY FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/266
POTTER HEIGHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/
WITHOUT CLIMATE CHANGE
7293A/21/267
POTTER HEIGHAM ZONE 3a HIGH PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/268
POTTER HEIGHAM ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
Ref: 7293A/21/CW/06-07/1778
MEDIUM
MEDIUM
3a
HIGH
PROBABILITY WITH/WITHOUT
PROBABILITY
PROBABILITY
WITH/WITHOUT
WITH/WITHOUT
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7293A/21/269
POTTER HEIGHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b
WITH/WITHOUT CLIMATE CHANGE
7293A/21/270
ROUGHTON ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/271
ROUGHTON ZONE
CLIMATE CHANGE
7293A/21/272
ROUGHTON ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/273
ROUGHTON FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/274
SCULTHORPE ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/275
SCULTHORPE ZONE 3a HIGH PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/276
SCULTHORPE ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/277
SCULTHORPE FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/278
STALHAM ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/279
STALHAM ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/280
STALHAM ZONE 2
CLIMATE CHANGE
7293A/21/281
STALHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/282
STIFFKEY ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/283
STIFFKEY ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/284
STIFFKEY ZONE 2
CLIMATE CHANGE
Ref: 7293A/21/CW/06-07/1778
3a
HIGH
MEDIUM
MEDIUM
PROBABILITY
PROBABILITY
PROBABILITY
WITH/WITHOUT
WITH/WITHOUT
WITH/WITHOUT
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7293A/21/285
STIFFKEY FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/286
SUTTON ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/287
SUTTON ZONE 3a HIGH PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/288
SUTTON ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT CLIMATE
CHANGE
7293A/21/289
SUTTON FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/WITHOUT
CLIMATE CHANGE
7293A/21/290
THORNAGE ZONE 3b FUNCTIONAL FLOODPLAIN WITH/WITHOUT
CLIMATE CHANGE
7293A/21/291
THORNAGE ZONE
CLIMATE CHANGE
7293A/21/292
THORNAGE ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/293
THORNAGE FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
7293A/21/294
WALSINGHAM ZONE 3b FUNCTIONAL
WITHOUT CLIMATE CHANGE
7293A/21/295
WALSINGHAM ZONE 3a HIGH PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/296
WALSINGHAM ZONE 2 MEDIUM PROBABILITY WITH/WITHOUT
CLIMATE CHANGE
7293A/21/297
WALSINGHAM FLOOD PROBABILITY ZONES 2, 3a AND 3b WITH/
WITHOUT CLIMATE CHANGE
3a
HIGH
PROBABILITY
WITH/WITHOUT
FLOODPLAIN
WITH/
Coastal Overtopping and Breaching
7293A/21/2900
SEA PALLING BREACH 1 IN 200 YR EVENT PLUS CLIMATE CHANGE
MAXIMUM FLOOD DEPTH AND EXTENT
7293A/21/2901
SEA PALLING BREACH 1 IN 200 YR EVENT PLUS CLIMATE CHANGE
HAZARD TO PEOPLE
7293A/21/2902
MORSTON BREACH AND OVERTOPPING 1 IN 200 YR EVENT PLUS
CLIMATE CHANGE MAXIMUM FLOOD DEPTH AND EXTENT
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7293A/21/2903
MORSTON BREACH AND OVERTOPPING 1 IN 200 YR EVENT PLUS
CLIMATE CHANGE HAZARD TO PEOPLE
7293A/21/2904
WELLS-NEXT-THE-SEA BREACH AND OVERTOPPING 1 IN 200 YR
EVENT PLUS CLIMATE CHANGE MAXIMUM FLOOD DEPTH AND
EXTENT
7293A/21/2905
WELLS-NEXT-THE-SEA BREACH AND OVERTOPPING 1 IN 200 YR
EVENT PLUS CLIMATE CHANGE HAZARD TO PEOPLE
7293A/21/2906
WELLS-NEXT-THE-SEA OVERTOPPING ONLY 1 IN 200 YR EVENT
PLUS CLIMATE CHANGE MAXIMUM FLOOD DEPTH AND EXTENT
7293A/21/2907
WELLS-NEXT-THE-SEA OVERTOPPING ONLY 1 IN 200 YR EVENT
PLUS CLIMATE CHANGE HAZARD TO PEOPLE
Sustainable Drainage
7293A/21/298
SUITABILITY OF SUSTAINABLE DRAINAGE ACROSS THE NNDC
AREA
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
1.0
INTRODUCTION
1.1
Terms of Reference
1.1.1
Millard Consulting were appointed to carry out the Stage 2 Strategic Flood Risk Assessment
(SFRA) for the administrative areas covered by a consortium of District Councils consisting
of Broadland District Council, North Norfolk District Council, the Broads Authority,
Norwich City Council and South Norfolk Council. The particular Terms of Reference for this
Stage 2 study are set out in the JBA Consulting document “Stage 1 Inception Report and
Terms of Reference for Stage 2” dated October 2006. The Stage I Study consisted essentially
of the compilation of a data inventory together with a review of the various planning and
development related flood risk matters considered important to the five authorities. The Stage
1 Study also set out a Scope for the Stage 2 work.
1.1.2
This Report covers the North Norfolk District Council area.
1.1.3
The SFRA is intended to be utilised as a planning tool to enable local planning authorities
and others to meet the strategic objectives set out in the Department for Communities and
Local Government Planning Policy Statement 25 (PPS25) Development and Flood Risk
published in 2006.
1.1.4
PPS25 requires each local planning authority to carry out an SFRA to inform the preparation
of local development documents (LDDs), and to enable the LPA to apply the sequential
approach in the site allocation process.
1.1.5
PPS25 covers the general scope of an SFRA in paragraphs E5, E6 and E7. Key paragraphs
are:
“Decision-makers should use the SFRA to inform their knowledge of flooding, refine the
information on the Flood Map and determine the variations in flood risk from all sources of
flooding across and from their area. These should form the basis for preparing appropriate
policies for flood risk management for these areas. The SFRA should be used to inform the
Sustainability Appraisal (incorporating the SEA Directive) of the Local Development
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Documents (LDDs), and will provide the basis from which to apply the Sequential Test and
Exception Test in the development allocation and development control process.
Where decision-makers have been unable to allocate all proposed development and
infrastructure in accordance with the Sequential Test, taking account of the flood
vulnerability category of the intended use, it will be necessary to increase the scope of the
SFRA to provide the information necessary for application of the Exception Test. This should
additionally, consider the beneficial effects of flood risk management infrastructure in
generally reducing the extent and severity of flooding when compared to the Flood Zones on
the Flood Map. The increased scope of the SFRA will enable the production of mapping
showing flood outlines for different probabilities, impact, speed of onset, depth and velocity
variance of flooding taking account of the presence and likely performance of flood risk
management infrastructure.
There may be considerable benefits in several LPAs, within a catchment area of high
development pressure or a designated development area, joining together to undertake a subregional Strategic Flood Risk Assessment. This will help LPAs to consider the issues raised
by flooding on the wider scale (of the river catchment and/or coastal cell). This will enable
them to contribute to, and take account of, the River Basin Management Plans required to be
published by 2009 by the Environment Agency as part of the implementation of the EC Water
Framework Directive.”
1.1.6
Other guidance relating to the scope of the SFRA is contained in the February 2007
Communities and Local Government Web-based document “Development and Flood Risk, A
Practice Guide Companion to PPS25 – Living Draft, A Consultation Paper”. This document
defines two “Levels” of SFRA, the Level 2 SFRA being more exhaustive than the Level 1
SFRA, each to be adopted in the following circumstances:
“In local authority areas where flooding is not a major issue and where development
pressures are low, a less detailed approach will be required relative to that necessary in
areas where there is high development pressure and flooding is a significant issue. A staged
approach is therefore recommended in PPS25, designed to allow flexibility in the level of
assessment required from one local authority area to another.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Where the need to apply the Exception Test is identified, due to there being an insufficient
number of suitably available sites for development within zones of lower flood risk, the scope
of the SFRA should be widened. This increased scope SFRA, (see paragraph E6 of PPS25) is
referred to a Level 2 SFRA…”.
The “PPS25 Practice Guide” includes a list of the key outputs required from a “Level 1
SFRA”. Table 1.1 below shows the “Practice Guide” requirements and identifies where in
this NNDC SFRA the required information can be found.
The “Practice Guide” also
includes at paragraph 2.41 a list of key outputs required for a “Level 2 SFRA”. Some of
these outputs are relevant to this NNDC SFRA and Table 1.2 below identifies where in this
NNDC SFRA these outputs can be found.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Table 1.1: Required “Level 1” SFRA Outputs
“Practice Guide Companion to PPS 25”
Section in this NNDC
Required “Level 1” SFRA output
SFRA
Plans showing the LPA area, Main Rivers, ordinary Flood Probability Maps
watercourses and flood zones, including the functional and other Plans. Report
floodplain where appropriate, across the local authority area as Section 4. GIS layers.
defined in Table D1 of PPS25, as well as all allocated
development sites
An assessment of the implications of climate change for flood Flood
Probability
risk at allocated development sites over an appropriate time Maps. Report Section 4.
period, if this has not been factored into the plans above
GIS layers.
The location of any flood risk management measures, Report Section 2
including both infrastructure and the coverage of flood warning
systems
Locations where additional development may significantly Report Section 7
increase flood risk elsewhere
Guidance on the preparation of FRA’s for allocated Report Section 8
development sites
Guidance on the likely applicability of different sustainable Report Section 6
drainage systems (SuDS) techniques for managing surface
water run-off at key development sites
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Table 1.2: Required “Level 2” SFRA Outputs
“Practice Guide Companion to PPS 25”
Section in this NNDC SFRA
Required “Level 2” SFRA output
An appraisal of the current condition of flood Not included
defence infrastructure and of likely future
policy with regard to its maintenance and
upgrade
An
appraisal
of
the
probability
and Report Section 5 - Coastal Defence Breach
consequences of overtopping or failure of and Overtopping Scenarios. Coastal Breach,
flood
risk
management
infrastructure, Overtopping
and
Hazard
Maps,
and
including an appropriate allowance for animations, which all include a climate
climate change
change allowance
Maps showing the distribution of flood risk Not included
across flood zones
Guidance on the appropriate policies for sites Not included
which satisfy parts a) and b) of the Exception
Test, and requirements to consider at the
planning application stage to pass part c) of
the Exception Test
Guidance on the preparation of FRA’s for Report Section 8
varying risk across the flood zone
1.1.7
The “PPS25 Practice Guide” also states:
“PPS25 requires that LPAs prepare Strategic Flood Risk Assessments (SFRAs) to an
appropriate level of detail to allow the Sequential Test to be applied in the site allocation
process. This is an essential part of the pre-production/evidence gathering stage of the plan
preparation process. Consideration should be given to working jointly with other local
authorities and stakeholders to prepare a sub-regional SFRA. The SFRA should take into
consideration any regional guidance prepared by the Regional Planning Body.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
The SFRA will provide useful baseline information to the Sustainability Appraisal scoping
and assessment stages and will also provide the evidence base for application of the
Sequential Test and the Exception Test in the land use allocation process. The LPA should
demonstrate that it has considered a range of options in conjunction with the flood zone
information from the SFRA and applied the Sequential Test, and where necessary the
Exception Test, in the site allocation process. This can be undertaken directly or, ideally, as
part of the Sustainability Appraisal. Where other sustainability criteria outweigh flood risk
issues, the decision making process should be transparent in the Sustainability Appraisal
report. The process should take account of any locational criteria included in guidance
prepared by the Regional Planning Body.”
1.1.8
The Stage 1 Study says that the NNDC SFRA is required to:
•
provide a reference and policy document to inform Local Development Frameworks
and any subsequent plans;
•
ensure that the Planning Authorities meet their obligations under the latest planning
guidance;
•
provide a reference and policy document to advise and inform private and commercial
developers of their obligations under the latest planning guidance.
1.1.9
The particular Scope relevant to the NNDC area, as set out in the SFRA Stage 1 study, is
itemised in paragraphs 1.3.5 and 1.3.6 below.
1.2
North Norfolk District Council Area
1.2.1
North Norfolk District covers an area of 994.4 sq km with a coastline of 82.6 km. The area is
very diverse; the predominant land use is arable farming with some livestock. Other major
land uses include forestry, gravel and sand working, nature reserves on the coastal plain and
tourism. The western and central areas are predominantly arable and characterised by
heathland, valley mires, woodland, and coastal cliffs. The topography has a greater relief
than other areas of Norfolk; the Holt-Cromer ridge, formed of sands and gravels, reaches an
elevation of 100 m near Sheringham. The major part of the coastal strip is designated as an
Area of Outstanding Natural Beauty, containing several SSSIs, and is recognised as one of
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
the finest natural coastlines in the British Isles. It includes large areas of saltmarshes
developing behind sand dunes and shingle with extensive areas of intertidal sand and
mudflats. The eastern area is rather bleaker, with less woodland, and the coast fringed with
cliffs of varying height and composition. Coastal erosion is a feature of the North Norfolk
coastline. The dynamic processes involved and the policies associated with the management
of the retreating coastline are covered in the Kelling to Lowestoft Ness Shoreline
Management Plan (SMP), which is currently under review.
1.2.2
To the south and east of the North Norfolk area lie the Norfolk Broads, the premier wetland
within the UK, associated with the Rivers Ant, Thurne, Bure. Where the Broads Authority
area overlaps the North Norfolk District Council area, the Broads Authority is the local
planning authority.
1.2.3
There is a thriving tourism industry in North Norfolk, principally centred around the wildlife
habitats of the coastal areas, the seaside towns, and the towns and villages associated with the
Broads.
1.2.4
North Norfolk is bounded by Kings Lynn and West Norfolk Borough Council to the west,
Breckland District Council and Broadland District Council to the south and Great Yarmouth
Borough Council to the south east. Kings Lynn and West Norfolk Borough Council adopted
an SFRA in 2005. SFRAs also exist for Great Yarmouth Borough Council (Great Yarmouth
and Gorleston Strategic Flood Risk Assessment – Capita Symonds – June 2006) and for
Breckland (Breckland Strategic Flood Risk Assessment – Mott Macdonald – June 2005).
Both the Kings Lynn and West Norfolk Borough Council SFRA and the Great Yarmouth and
Gorleston SFRA are being updated to be PPS25 compliant. There are no plans to update the
Breckland SFRA.
1.3
Local Development Framework and Scope of Stage 2 SFRA
1.3.1
The NNDC LDF Core Strategy submission document is now complete. The Examination in
Public is scheduled for December 2007. The NNDC LDF Submission document (June 2007)
proposes that most of the new development in North Norfolk should be concentrated in four
of the larger towns of the District, ie Cromer, Fakenham, Holt and North Walsham, which are
identified as “principal settlements”.
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A lesser level of development is proposed in
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“secondary settlements” identified as Hoveton, Sheringham, Stalham, and Wells-Next-TheSea. In addition, the Core Strategy proposes that some small scale development is necessary
to meet local needs in some of the larger villages and that the service role of these villages
should be protected and enhanced. These villages are nominated as “service villages”,
identified as Aldborough, Briston and Melton Constable, Catfield, Corpusty and Saxthorpe,
Horning, Little Snoring, Little Walsingham, Ludham, Roughton, and Southrepps. Some of
these villages have proposed housing allocations and others will only be permitted infill
development in designated areas.
1.3.2
Five Coastal Service Villages, Bacton, Blakeney, Happisburgh, Mundesley, Overstrand and
Weybourne are also identified.
1.3.3
The LDF Site Specific Proposals Preferred Options Consultation Document (for consultation
25 September 2006 to 6 November 2006) identifies the preferred options (residential, retail,
employment, etc) on specific sites associated with these settlements, selected on the basis of a
range of criteria, including sustainability.
1.3.4
The Stage 1 SFRA report inspected each of these preferred option sites against the
Environment Agency Flood Zone Maps, which do not take account of climate change, or
defences, or hydraulic structures, and determined that on the basis of these maps all but three
appeared to be totally within PPS25 Flood Zone 1, ie for which there is no significant risk of
flooding from main rivers or sea and which could therefore be considered sequentially
preferable to all others. The proposed allocation in Ludham that is partly in Flood Zone 2 is
intended for housing. The site in Wells similarly affected is intended as a car park, but is at
risk only in the event of breaching or overtopping of the defences.
1.3.5
This analysis, made in the Stage 1 SFRA on the basis of the Environment Agency Flood
Maps, is of limited usefulness, because PPS25 requires that the spatial planning process
should take account of climate change. It is conceivable that some sites currently within
Flood Zone 1 may be in Flood Zone 3 in the future. This Stage 2 SFRA, which includes an
assessment of climate change, is intended to determine whether sites might be at a higher risk
than previously identified.
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1.3.6
The Stage 1 SFRA study included a review of the apparent flood risk at each of the
settlements identified in the LDF consultation. The following scope for the Stage 2 SFRA
work was set out:
•
determine and map the PPS25 Zone 3b “functional floodplain” (the 1 in 20 year return
period flood event), the Zone 3a floodplain (the 1 in 100 year return period event),
and the Zone 2 floodplain (the 1 in 1,000 year event), with and without the PPS25
allowance for climate change (appropriate to residential development), at the
following settlements: Fakenham, Hoveton, Stalham, Sculthorpe, Roughton,
Mundesley, Corpusty, Horning, Ludham, Thornage, Erpingham, Gimingham,
Stiffkey, Langham, Potter Heigham, Hickling, Horsey, and Ingworth. The modelling
and mapping was intended to utilise the existing Environment Agency hydraulic
models (prepared by other consulting engineers) and where models had not been
constructed, the Stage 1 document noted that: “an alternative approach will be needed
including possibly some model development”. The modelling and mapping was
required to represent the “with defences” scenario, and to include hydraulic structures
such as bridges and defences. This is opposed to the Environment Agency Flood
Zone Maps which do not account for all of these structures;
•
carry out breach and overtopping modelling to produce maximum extent, depth and
hazard maps at three locations along the North Norfolk coast, at Sea Palling,
Blakeney/Morston and Wells-Next-The-Sea, the exact location to be agreed with
NNDC and the EA;
•
carry out a general assessment of areas being considered for the LDF in the context
of the Sequential Test, covering surface water drainage and sustainable drainage.
1.3.7
The exact scope of this Stage 2 SFRA was refined during the early period of the work to take
into account the more detailed requirements of NNDC (see Appendix A).
1.4
Shoreline Management Plan
1.4.1
NNDC, together with Great Yarmouth Borough Council, Waveney District Council, the
Environment Agency and Natural England are joint sponsors of the Kelling to Lowestoft
Shoreline Management Plan (SMP).
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This “second generation” SMP is one of several
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compiled for various stretches of the UK coastline.
The SMP is a coastal defence
management document which aims “to promote sustainable management policies, for a
coastline for the 22nd century, which achieve objectives without committing to unsustainable
defences…”. It covers policies and scenarios for three epochs (time periods); 0 to 20 years,
20 to 50 years and 50 to 100 years. There are four main objective areas: the framework
objective (policies to comply with current defence management framework associated with
public funding), the technical objective (policies to have no adverse effect on any physical
processes which bring benefits), the environmental objective (policies to take due
consideration of biodiversity) and the socio-economic objective (policies to consider current
regional development policies and statutory planning policies). The coastline is divided into
particular cells to which are assigned coastal defence policies appropriate to the three epochs,
which might be “advance the (defence) line”, “hold the line” “managed retreat” or “no active
intervention”. The SMP Review was published in 2006 but its consequences have been
considered to be so severe by many stakeholders that authorities are reticent to adopt and
implement it until suitable mitigation measures are in place.
1.5
Coastal Habitat Management Plan
1.5.1
NNDC, together with Defra, the EA, English Nature, the Natural Environment Research
Council are joint sponsors of the “North Norfolk Coast Coastal Habitat Management Plan –
Royal Haskoning Ltd – 2003” otherwise known as “ChaMP”. It is prepared as one of the
inputs to the SMP and is intended to direct the long term strategic view of likely losses and
gains to habitats and species as a result of coastal processes, climate change and coastal
defence policies. It seeks to optimise policies towards offsetting losses. It is linked to
national obligations associated with the EU Habitats Directive, the EU Birds Directive and
the Ramsar International Convention on Wetlands.
1.6
Catchment Flood Management Plan
1.6.1
NNDC area is the subject of two ongoing Catchment Flood Management Plans (CFMP’s).
The work is being carried out for the EA by its framework consultants. CFMP’s are broad
scale strategic plans intended to assess how flood risks across a catchment might change and
be sustainably managed over the next 50 to 100 years. The work usually considers fluvial
catchment systems, across administrative boundaries, and is linked to the UK’s obligations in
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respect of River Basin Management Plans under the EU Water Framework Directive. The
Plans are intended to set out strategic guidance for future flood risk management but not to
specify actual flood risk reduction measures or management approaches to be put in place.
The date for final publication is uncertain. The two studies covering North Norfolk are:
•
The North Norfolk Catchment Flood Management Plan, which covers the river
systems draining northwards (including part of the Borough of Kings Lynn and West
Norfolk), ie the Rivers Hun, Burn, Glaven, Stiffkey, Mun and Spring Beck. The
settlements of Wells, Stiffkey, Little Walsingham, Blakeney, Langham, Thornage,
Holt, Weybourne, Sheringham, Cromer, Mundesly, Bacton and Happisburgh are
therefore covered by this Plan.
•
The Broadland Rivers Catchment Flood Management plan, which covers all the river
systems draining southward and eastwards towards the River Yare. This includes the
Rivers Bure, Wensum, Ant and Thurne in North Norfolk.
The settlements of
Fakenham, Sculthorpe, Briston/Melton Constable, Corpusty/Saxthorpe, Roughton,
Aldborough, Erpingham, Ingworth, North Walsham, Lessingham, Ingham, Horsey,
Hickling, Stalham, Sutton, Catfield, Ludham, Horning, Hoveton and Potter Heigham
are covered by this Plan.
1.7
Internal Drainage Boards
1.7.1
An Internal Drainage Board (IDB) is a statutory public body operating under primary
legislation to provide a water level management service within its prescribed drainage
district.
It raises income through the direct rating of agricultural land and buildings in its
drainage district and recovers through a special levy placed on constituent district councils or
unitary authorities an income in recognition of the benefit arising from its work to all nonagricultural land and property. The IDB owns, operates and maintains pumping stations,
water control structures, sluices and other assets and has the power to designate watercourses
(other than EA Main River) as Board Main Drains which it then maintains. It is the relevant
operating and drainage authority within its district and can control third party works through
its Byelaws made under the Land Drainage Act 1991.
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1.7.2
The IDB consortium operating in North Norfolk is the Water Management Alliance, based in
King’s Lynn and information relating to its areas and policies is included in Appendix B.
The relevant Alliance members are:
•
The Norfolk Rivers IDB – formed in 2005 by the amalgamation of seven Boards
(North Norfolk IDB, Upper Nar IDB, River Wensum IDB, Upper Yare & Tas IDB,
Upper Bure IDB, and the gravitational areas of the Smallborough IDB and Middle
Bure IDB.
•
The Broads IDB – first formed in 2005 by the amalgamation of eight Boards
(Happisburgh to Winterton IDB, Repps, Martham & Thurne IDB, Muckfleet & South
Flegg IDB, Lower Bure, Halvergate Fleet & Acle Marshes IDB and the pumped areas
of the Smallburgh IDB and the Middle Bure IDB. The Lower Yare First IDB and the
Lower Yare Fourth IDB also joined the Broads IDB; the new amalgamation is the
Broads (2006) IDB.
1.7.3
The IDB’s are not statutory consultees in the planning process, but the Environment
Agency informs applicants and the Local Planning Authority that a particular
development may lie within an IDB district and that its Byelaws will apply. The
Water Management Alliance has a planning officer and planning applications are
scrutinised regularly. An IDB is also entitled to receive payment in the form of a
development contribution from any development either within its district, or from
outside the district but within the catchment, for managing any additional flows. This
charge is index-linked and is in the order of £40,000 per hectare of additional
impermeable area.
1.7.4
In common with many authorities, IDB’s support the principles of sustainable
drainage systems as a method of reducing the impact of flooding. IDB’s regard it as
essential that agreement is reached between the developer, the Local Authority, IDB
and Environment Agency, etc., in order to determine and secure the future
maintenance of these systems.
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1.8
North Norfolk Joint Flood Plan
1.8.1
The NNDC Emergency Planning Section, together with other interested parties (the EA, the
Police, Fire and Rescue Service, etc) has compiled the “Seawitch” Joint Flood Plan. It sets
out warning and emergency evacuation procedures for all the settlements which are thought
to be at risk of coastal flooding.
It prescribes and nominates in specific detail all the
individual responsibilities, the evacuation routes, the location of rest centres, and the
operational procedures which will be triggered in the event of flooding along the coast. The
document was issued in 2006 and is updated annually.
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2.0
SOURCES OF FLOODING IN NORTH NORFOLK
2.1
Coastal
2.1.1
The North Norfolk coastline includes approximately equal lengths of eroding cliffs and low
lying land which is susceptible to flooding. The low lying areas are found at each end of the
central length (from Kelling Hard to Cart Gap). At the western end of this central stretch
there is saltmarsh and the shingle ridge leading to Blakeney Point. In the east there are sand
dune areas which protect the Norfolk Broads. All these low lying areas are vulnerable to
coastal flooding, and encroachment into the Broads area remains possible. The worst coastal
flood recorded is the well documented event of 31 January 2007.
2.1.2
Where cliffs exist, erosion is the major problem. The cliffs are comprised of silts, sands, clays
which are eroded by wave action. This, together with instability of the cliffs, results in the
loss of material and retreat of the cliff line. Defences exist at the main settlements in order to
limit coastal erosion and protect these settlements.
2.1.3
Coastal flooding exhibits the highest hazard in North Norfolk. Hazard is especially high
where defence failure might occur as flood velocities and depths are likely to be extreme
following any defence breach.
2.2
Fluvial and Tidal
2.2.1
Fluvial flooding, or a combination of fluvial and tidal flooding, is a threat to settlements in
North Norfolk particularly towards the east and south of the District where the Broads river
system exists and where the tidal influence dominates. The onset of flooding is slow and is
generally associated with tidal effects which are predictable and the subject of Environment
Agency flood warnings. Flow velocities during flood conditions are relatively slow and
depths relatively shallow.
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3.0
ENVIRONMENT AGENCY FLOOD ZONES
3.1
Flood Zones are defined in Table D.1 of PPS25 “Development and Flood Risk”. The zones
delineate areas at risk of fluvial, tidal or coastal flooding. Table D.1 notes that these flood
zones refer to the probability of river and sea flooding, ignoring the presence of defences.
This is an important qualification, and it should be noted that the Environment Agency Flood
Maps available on the EA website, and elsewhere, are drawn on this basis. The EA flood
maps therefore give a worst case scenario.
o
Flood Zone 1
Flood Zone 1 (Low Probability) is defined in PPS25 as land assessed as having a less
than 1 in 1,000 annual probability of river or sea flooding in any year (<0.1%). These
areas are on higher ground than the areas defined by Zones 2, 3a and 3b.
o
Flood Zone 2
Flood Zone 2 (Medium Probability) is defined in PPS25 as land assessed as having
between a 1 in 100 and 1 in 1,000 annual probability of river flooding (1% – 0.1%) or
between a 1 in 200 and 1 in 1,000 annual probability of sea flooding (0.5% – 0.1%) in
any year.
o
Flood Zone 3a
Flood Zone 3a (High Probability) is defined in PPS25 as land assessed as having a 1
in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater
annual probability of flooding from the sea (>0.5%) in any year.
o
Flood Zone 3b
Flood Zone 3b (Functional Floodplain) is defined in PPS25 as land where water has to
flow or be stored in times of flood. SFRAs should identify this Flood Zone (land
which would flood with an annual probability of 1 in 20 (5%) or greater in any year or
is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed
between the LPA and the Environment Agency, including water conveyance routes).
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3.2
PPS25 requires that SFRA’s should adopt a more detailed and rigorous approach and take
account of the presence of flood defences in flood zone mapping. The Flood Probability
Maps included with this SFRA are therefore drawn on this basis, and the flood zones
indicated, although using the same category designation (Zones 3b, 3a, and 2), do take
account of the existing flood defences. This is noted on the drawings themselves.
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4.0
HYDRAULIC MODELLING AND FLOOD PROBABILITY MAPPING
4.1
In order to carry out the hydraulic modelling and flood mapping for this project, well
established hydraulic modelling and flood mapping techniques were applied. This involved
one- and two-dimensional (1-D/2-D) unsteady hydrodynamic modelling and the application
of GIS techniques for further processing of the modelling results.
4.2
The flood mapping analyses the flood risk associated with North Norfolk river systems. T he
maps therefore indicate the probability of flooding associated with the river systems,
including where the rivers are tidally influenced. Certain coastal areas are also at risk of
direct coastal flooding associated with failure or overtopping of coastal defences.
Information relating to the risk of coastal flooding is available from the Environment Agency.
These SFRA flood probability maps should therefore be read in conjunction with EA
information relating to the probability of coastal flooding.
4.3
Most of the flood mapping was based on existing ISIS models prepared by others. These
existing models were identified in the Stage 1 SFRA. For areas not already covered by one
of the existing ISIS models, new hydrodynamic models were created. The level of
sophistication of these 1-D models is appropriate to the strategic nature of this mapping
exercise.
4.4
Some areas were modelled using ESTRY, the 1-D component of the TUFLOW modelling
software.
4.5
The following settlements were identified in the Stage 1 Inception Report as requiring flood
risk mapping and the delineation of the Functional Floodplain: Fakenham, Hoveton, Stalham,
Sculthorpe, Roughton, Mundesley, Corpusty, Horning, Ludham, Thornage, Erpingham,
Gimingham, Stiffkey, Langham, Potter Heigham, Hickling, Horsey and Ingworth. Table 4.1
below indicates the hydraulic model used for each.
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Table 4.1: Flood Mapping Locations and Hydraulic Models
North Norfolk District Council
Settlement
Watercourse
Fakenham
Wensum
Hoveton
Thornage
Erpingham
Gimingham
Bure Tributary
1-D ESTRY Model/Millard
Mun
Bure
Mun
Bure
BESL Broads
Model
BESL Broads
Model
Glaven
Tributary
Bure Tributary
North Norfolk
Rivers
North Norfolk
Rivers
Stiffkey
Tributary
BESL Broads
Model
BESL Broads
Model
BESL Broads
Model
North Norfolk
Rivers
1-D ISIS Model/EA
1-D ISIS Model/EA
Bure
Thurne/
Womack Water
Glaven
Tributary
Bure Tributary
Mun
Stiffkey
Langham
Stiffkey
Tributary
Potter Heigham
Horsey
Thurne
Hickling Broad
Horsey
Mere/New Cut
Ingworth
4.6
1-D ISIS Model/EA
Bure Tributary
Stiffkey
Hickling
Upper Wensum
Wensum
Ant
Mundesley
Corpusty
Horning
Ludham
Model Type and Source
BESL Broads
Model
BESL Broads
Model
Upper Wensum
Bure
Stalham
Sculthorpe
Roughton
Model
Bure
1-D ISIS Model/Halcrow
1-D ISIS Model/Halcrow
1-D ISIS Model/EA
1-D ISIS Model/Halcrow
1-D ISIS Model/Halcrow
1-D ISIS Model/Millard
1-D ISIS Mode/Millard
1-D ISIS Model/EA
1-D ISIS Model/EA
1-D ESTRY Model/Millard
1-D ISIS Model/Halcrow
1-D ISIS Model/Halcrow
1-D ISIS Model/Halcrow
1-D ISIS Model/EA
For each settlement affected by fluvial flooding there are individual maps showing the 1 in 20
year flood outline (Functional Floodplain – Zone 3b), with and without climate change; the 1
in 100 year outline (Zone 3a – High Probability), with and without climate change; and the 1
in 1,000 year outline (Zone 2 – Medium Probability), with and without climate change. All
the SFRA modelling and mapping takes account of the existence of flood defences, whereas,
as noted above, the EA flood maps do not.
4.7
Where a particular watercourse reach is tidally influenced, the mapping shows the 1 in 20
year flood outline (Functional Floodplain – Zone 3b), with and without climate change; the
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outline for a combination of the fluvial 1 in 100 year event combined with the 1 in 200 year
tidal event (Zone 3a – High Probability), with and without climate change; and the 1 in 1,000
year outline (Zone 2 – Medium Probability) with and without climate change.
4.8
The climate change outline is that appropriate to the year 2115, appropriate to the design life
of residential development. The climate change increment calculated for use in the hydraulic
modelling is that recommended by PPS25, ie in fluvially dominated reaches an increase in
peak fluvial flow of 20%. In tidally dominated reaches the tidal climate change increment
appropriate to the year 2115 is calculated as set out in PPS25, Annex B, Table B.1. Where
appropriate the fluvial and tidal climate change increments are added together in order to
obtain the most conservative result.
4.9
There is also a map for each settlement showing all six flood outlines.
4.10
Map scales vary depending upon the size of each settlement and the intention has been to
maximise the clarity of the mapping for the purpose of informing the strategic planning
process and sequential test decisions.
4.11
The modelling and mapping take account of existing defences and structures whereas the
Environment Agency Flood Zone Maps do not.
4.12
The maps are supplied as prints and as GIS layers compatible with MapInfo and ESRI GIS.
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5.0
COASTAL DEFENCE OVERTOPPING AND BREACH SCENARIOS
Breach in Coastal Defences (200 year event with Climate Change) – Residual Risk
Mapping of Maximum Flood Depth and Extent
5.1
The Stage 1 Inception Report identified three locations where it was considered important to
examine Residual Risk associated with the failure of coastal defences. Specific breach
modelling was carried out at Sea Palling, Morston and Wells-Next–the-Sea. The locations
were agreed with NNDC and the EA. The breach locations are chosen based on a qualitative
judgement of the magnitude of consequences, rather than on a quantative assessment of the
probability of failure of the defence.
•
The Sea Palling breach location is chosen near to communities at risk. The land
locally is slightly lower than the surrounding area and it is near to the end of the reef
defences.
•
At Morston the village is protected by a flood bank and the breach scenario assesses
the effect on the village if the bank breaches.
•
At Wells–Next-The-Sea the breach location is chosen at the point in the West Bank
which is known to have breached during the 1978 event.
5.2
The breach locations were chosen in consultation with the EA and the Coastal Planning Team
at NNDC. A breach could occur at any location, and the probability of a breach occurring at
any particular position is not possible to determine in any quantitative way, unless there are
known defects or weaknesses or particular characteristics relating to a certain location. The
probability of breach will be influenced by the design standard of defence, its condition and
maintenance regime. Coastal erosion and loss of beach material are also factors.
5.3
The purpose of the breach analysis is to predict what would happen in this very worst
scenario, for the purposes of emergency planning, and in order that planning decisions for
areas that benefit from the defences, and decisions relating to the provision of funding for
new defence infrastructure and for the maintenance and improvement of existing defences,
can be made.
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5.4
The breach analysis provides detailed information indicating where the floodwater will go, at
what speed, how deep it is, and how long it will take to get there, across the hinterland behind
the defences. There will be a “Rapid Inundation Zone” immediately behind the defence
where there will be an extreme degree of hazard to people and property (usually considered
to be the general area which floods to a depth of 500mm within 30 minutes, unless otherwise
analysed). The computer modelling is “dynamic”, ie it operates against time steps. The
analysis produces maps of water depth and extent at any given time after the breach, together
with water velocities, and an animation of the progress of the whole flood is produced.
5.5
If the whole coastline were to be analysed with breaches modelled at many points, the
resulting flood line would be similar to the EA Flood Maps, which ignore the presence of
flood defences. (Note that the breach analyses carried out in this SFRA include an allowance
for future climate change, and therefore the extent of flooding from a series of breaches close
together along the coastline would produce a worse flood outline than the EA Flood Maps, as
these do not include climate change). Many areas of the North Norfolk Coastline are not at
risk of flooding by breach of defences; many stretches of the coastline are high enough to be
unaffected by breach or overtopping scenarios, even with the predicted sea rise associated
with climate change. However the threat of coastal erosion remains in these areas of the
coastline: this is not a topic covered by this SFRA and reference should be made to the SMP
for guidance on policies and planning there.
5.6
The breach analysis also allows an assessment of “Hazard to People” on the basis that the
combination of flood speed, and flood depth, will affect people more or less, depending upon
physical strength and size. The analysis will produce “Hazard Maps” showing areas in which
during the period of the inundation the flood will produce “danger to some”, “danger to
most” and danger to all”, as defined by EA guidance – see below.
5.7
In areas where land values are high behind defences or where development pressures mean
that existing development needs to be maintained, or new development behind defences is
necessary, the analysis of breach scenarios can guide the location of new development, or
essential infrastructure, to areas at least risk even if the defences fail. The breach analysis
also allows meaningful evacuation strategies to be formulated.
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5.8
In North Norfolk although breach scenarios might show the extent of the spread of inundation
behind a defence to be less than the EA maps, it does not mean that the risk is less. All
defended areas are special in that they can be inundated very quickly, to a significant depth,
although the probability of occurrence is small. In North Norfolk it is unlikely that
development pressure by itself will be sufficiently severe to necessitate planning decisions to
be made on the basis of breach scenarios. The breach scenarios should be used to inform the
preparation of Evacuation Plans and the planning of the emergency services. In these areas
land use planning decisions can still be meaningfully made on the basis of the EA “without
defences” flood maps.
5.9
Site specific FRAs prepared by developers should include a breach analysis, if a site behind
existing defences is being considered.
5.10
The overtopping and breach scenarios at Sea Palling, Morston and Wells-Next-The-Sea were
created by applying two-dimensional unsteady hydrodynamic modelling using the TUFLOW
software.
5.11
The TUFLOW software was specifically developed to simulate flow patterns of twodimensional nature as can be observed on floodplains, coastal waters and estuaries. In
TUFLOW models the study area is represented as a 2-D domain, whose properties, such as
elevation or surface roughness, is assigned by a series of GIS layers.
5.12
The topography of the TUFLOW 2-D domains was based on LiDAR data surveyed between
1999 and 2001 and supplemented by ground survey data of existing flood defence structures.
5.13
Manning’s roughness over the study area was assigned using land-use information supplied
by the Environment Agency (see Table 5.1 below).
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Table 5.1: Manning’s Roughness based on Land-Use
Manning’s n
0.02
0.025
0.025
0.025
0.025
0.025
0.03
0.03
0.03
0.035
0.035
0.035
0.045
0.07
0.45
5.14
Land-Use Category
Commercial
Industrial
Playing Fields
Theme Parks
Town
City
Intensive Pasture
Village
Beach (sand and shingle)
Other
Extensive Pasture
Arable
Horticulture
Forest, Scrub
Formal Parks
The boundary conditions for the TUFLOW models are based on a 1 in 200 year tidal event,
with an allowance for future climate change appropriate to the next 100 years, with the
addition of a three day storm surge to the design event, as indicated in Table 5.2.
Table 5.2: Tide Hydrographs for SFRA Breach Models
Sea Palling
Peak Flood Level (200 year)
3.790m AOD
Allowance for the effects of climate change
1.017 m AOD
(According to table B1 of PPS25)
Total
4.807 m AOD
Wells-Next-the-Sea
Peak Flood Level (200 year)
5.240 m AOD
Allowance for the effects of climate change
1.017 m AOD
(According to table B1 of PPS25)
Total
6.257 m AOD
Morston (Blakeney)
Peak Flood Level (200 year)
4.990 m AOD
Allowance for the effects of climate change
1.017 m AOD
(According to table B1 of PPS25)
Total
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6.007 m AOD
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5.15
For defences with crest levels less than the flood level of the design event, the breach is
assumed to occur at the point in the surge event where the defence first reaches maximum
loading (ie at point of overtopping). Where the height of defences is greater than the design
event the breach is assumed to occur such that the highest surge peaks are included within the
breach period. The breach width (in “hard” defences) is assumed to be 50m wide and to be
repaired 36 hours after the initial breach.
5.16
All boundary conditions are in accordance with EA national guidance and flood defence
levels were agreed beforehand with the Environment Agency.
5.17
Local drains and watercourses have not been considered in these broad scale breach
scenarios.
5.18
The mapping shows the maximum extent of inundation together with the maximum depth of
the floodwater at any point experienced during the whole period of the breach event.
Breach Model Setups
5.19
The Model setup for Sea Palling was as follows:
•
The study area was modelled with a 2-D domain grid size of 25 m x 25 m
•
Sea defences were modelled with a height of 5.65 m AOD at the breach location
(50 m) and 200 m either side ie dunes removed for a length of 450 m
•
The depth of the breach extended down to the landward toe of the sea defence at
3.3 m AOD
•
The breach opening was assumed to occur at the third peak at 28 h (3.5 m AOD) until
the sixth peak at 64 h (3.5 m AOD). See Figure 5.1 below:
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7.00
6.00
Water Level (m AOD)
5.00
4.00
3.00
2.00
1.00
0.00
-1.00 0
50
100
150
200
-2.00
-3.00
Time (hours)
W ith Surge
Defence Level
W ithout Surge
Timing of Breach
Figure 5.1: Model Setup for Sea Palling Breach
5.20
The Model setup for Morston was as follows:
•
The study area was modelled with a grid cell size of 25 m
•
Sea defences were modelled with a height of 4.8 m AOD (Morston West Bank), 5.4 m
AOD (East Bank) and 4.2 m AOD (Blakeney West and East Bank)
•
The depth of the breach was at the landward toe of the sea defence at 2.3 m AOD
•
The breach opening was assumed to occur from the time of overtopping of the sea
defence at 38 h for 36 hours. See Figure 5.2 below:
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7.00
Water Level (m AOD)
6.00
5.00
4.00
3.00
2.00
1.00
0.00
-1.00 0
50
100
150
200
-2.00
Time (hours)
W ith Surge
W ithout Surge
W est Bank
East Bank
Blakeney
Timing of Breach
Figure 5.2: Model Setup for Morston Breach
5.21
The Model setup for Wells-Next-The-Sea was as follows:
•
Study area modelled with a grid of 15x15 m resolution
•
Sea defences were modelled with a height of: 5.3 – 6.3 m AOD (Wells West Bank to
Holkham Gap), 5.0 m AOD (Freeman Street, The Quay), and 6.5 m AOD (Wells East
Bank)
•
The depth of the breach was set to the level at the landward toe of the sea defence
(0.5 m AOD).
•
The breach opening was assumed to occur from the fifth peak at 52 h (6.26 m AOD)
until 88 h (2.39 m AOD). See Figure 5.3 below:
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7.00
Water Level (m AOD)
6.00
5.00
4.00
3.00
2.00
1.00
0.00
-1.00
0
50
100
150
200
-2.00
with surge
Freeman Street
Time (hours)
without surge
East Bank
West Bank
Timing of Breach
Figure 5.3: Model Setup for Wells-Next-the-Sea Breach
5.22
It should be noted that in the case of Sea Palling there is no overtopping associated with the
breach scenario. In the case of Morston however there is overtopping associated with the
breach. Similarly in the case of Wells-Next-The-Sea there is overtopping associated with the
breach, and an additional “overtopping only” scenario has been modelled for this location.
5.23
Table 5.3 summarises the breach modelling parameters for the three locations.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Table 5.3: Summary of Breach and Overtopping Model Setup Parameters
Model Parameter
Sea Palling
Morston
Wells
Grid Size
25m x 25m
25m x 25m
15m x 15m
Defence Crest Levels
5.65m AOD
4.8m
AOD 5.3m – 6.3m AOD (Wells
(Morston
West Bank to Holkham Gap),
West Bank), 5.0m AOD (Freeman Street,
5.4m
AOD The Quay), and 6.5m AOD
(East
Bank) (Wells East Bank)
and
4.2m
AOD
(Blakeney
West
and
East Bank)
Peak Surge Tide Level 4.81m AOD
6.01m AOD
6.26m AOD
(1 in 200 year event
plus
climate
change
allowance plus storm
surge)
Width of Breach
50m (and dunes 50m
removed
50m
for
200m each side of
Breach)
Base of Breach Level
3.3m
AOD
at 2.3m AOD
0.5m AOD
breach and dunes
removed down to
5.65m
AOD
200m either side
Period of Breach
Repaired
36hrs
5.24
after Repaired
Repaired after 36hrs
after 36hrs
In addition to maps and GIS outputs this study has also produced animations of the breach
modelling.
These show the flood direction and speed, together with depth, across the
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hinterland behind the breach, in 15-minute steps.
These are useful for the analysis of
evacuation routes, time of escape and the location of evacuation centres.
Breach in Coastal Defences (200 year event with Climate Change) – Residual Risk
Hazard to People Mapping
5.25
The “Hazard to People” rating was modelled in accordance with the guidelines set out in the
DEFRA/EA Document “Flood Risks to People, Phase 2, R&D Technical Report FD2321
TR1”. The hazard rating is a function of flood depth and velocity and is defined as:
HR = ((v + 0.5) × D ) + DF
where HR = hazard rating (dimensionless),
v = velocity (metres per sec),
D = depth (metres),
DF = debris factor (set to 1).
5.26
Other guidance relating to Flood Risks to People is given in “Flood Risks to People, Phase 2,
R&D Technical Report FD2321 TR2”, and “Flood Risk Assessment Guidance for New
Development, Phase 2, R&D Technical Report FD2320/TR1 and TR2”.
5.27
The groups of people considered within the “Hazard to People” Rating are categorised as
follows:
Danger for Some – includes children, the elderly and the infirm
Danger for Most – includes the general public
Danger for All – includes emergency services
5.28
The hazard modelling results were categorised into four degrees of flood hazard i.e. low,
moderate, significant and extreme, based on the assessment set out in the R&D paper, in
accordance with the Table 5.4 below. The hazard mapping was carried out using the
TUFLOW Software.
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Table 5.4: Hazard to People as a Function of Flood Velocity and Depth (source: table3.2,
p.8 DEFRA/EA R&D “Flood Risk to People Phase 2 FD2321 TR1)
5.29
Hazard Rating
<0.75
Degree of Flood Hazard
Low
0.75 – 1.25
Moderate
1.25 – 2.5
Significant
>2.5
Extreme
Description
Caution
“Flood zone with shallow flowing water
or deep standing water”
Dangerous for some (i.e. children)
“Danger: Flood zone with deep or fast
flowing water”
Dangerous for most people
“Danger: flood zone with deep fast
flowing water”
Dangerous for all
“Extreme danger: flood zone with deep
fast flowing water”
The Hazard Maps show the maximum degree of flood hazard experienced at the particular
location during the entire period of the breach.
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6.0
SUSTAINABLE DRAINAGE SYSTEMS (SUDS)
Introduction
6.1
Built development is likely to reduce the permeability of at least part of the site resulting in a
change to natural drainage patterns and an increase in peak rates and total quantities of
surface water runoff. Traditionally, developed areas have disposed of excess surface water
via below ground piped networks, discharging at uncontrolled rates into local watercourses.
In areas where both foul water and surface water was transported through the same sewer (ie
combined sewers), additional strain was added to sewage treatment works due to the
additional volumes of water and during heavy rainfall events these sewers often overflowed
causing pollution incidents across neighbouring areas. However, over the past 50 years these
systems have been constructed separately thus markedly reducing the number of pollution
incidents to the local environment. This has not necessarily solved the issues associated with
pollutant runoff from urban areas, which in turn drains into the surface water sewer network
and subsequently into local watercourses. With this arrangement and no controls at the
outfall locations, the volume and peak runoff rates of the surface water from development
areas will increase, resulting in a potential change in the receiving watercourse regime and
leading to a potential increase in flood risk to others upstream and downstream.
6.2
Sustainable drainage systems (SuDS) have therefore been developed to tackle these issues by
minimising the impacts from the development on the quantity and quality of the runoff and to
maximise biodiversity. The main objectives of sustainable drainage include the reduction of
runoff rates and volumes, increased natural recharge of the underlying soils and aquifers,
reduction of pollution concentrations in storm water and the enhancement of biodiversity.
Figure 6.1 illustrates some of the matters that require consideration during the SuDS design
process.
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Figure 6.1: SuDS design and planning: issues for consideration (taken from Figure 2.2
of CIRIA C697)
6.3
In order to mimic natural catchment processes as closely as possible, a “management train”
can be adopted, using drainage techniques in series to incrementally reduce pollution, flow
rates and volumes. A typical hierarchy for the management train is as follows:
•
Prevention – good site design and site housekeeping measures to prevent runoff and
pollution (eg sweeping to remove surface dust and detritus from car parks), and
rainwater reuse/harvesting. Prevention policies should generally be included within
the site management plan.
•
Source control – control of runoff at or very near its source (eg soakaways, other
infiltration methods, green roofs, pervious pavements).
•
Site control – management of water in a local area or site (eg routing water from
building roofs and car parks to a large soakaway, infiltration or detention basin).
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•
Regional control – management of runoff from a site or several sites, typically in a
balancing pond or wetland.
6.4
The management train is summarised in Figure 6.2. Wherever possible, stormwater should be
managed in small, cost-effective landscape features located within small subcatchments
rather than being conveyed to and managed in large systems at the bottom of drainage areas
(end of pipe solutions). The techniques that are higher in the hierarchy are preferred to those
further down so that prevention and control of water at source should always be considered
before site or regional controls. However, where upstream control opportunities are restricted,
a number of lower hierarchy options should be used in series. Water should be conveyed
elsewhere only if it cannot be dealt with on site.
Figure 6.2: SuDS Management Train (taken from Figure 1.5 of CIRIA C697)
6.4
Annex F of PPS25 recommends the maximum practical use of Sustainable Drainage Systems
(SuDS) within proposals for new sites.
PPS25 requires that SuDS be installed where
appropriate, in order to limit the amount of surface water runoff entering drainage systems
and to return surface water into the ground to follow its natural drainage path.
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Typical SuDS Components
6.5
The following descriptions of the various types of SuDS techniques demonstrate how
particular SuDS components are suited for providing a particular drainage function. For
example, ponds, lagoons and basins are generally used for attenuation of flows and volumes
prior to discharge into the watercourse whereas soakaways and infiltration trenches are
generally used to dispose of surface water directly below ground thus recharging aquifers.
The development layout and site characteristics together with the sub-surface soil types will
influence which types of SuDS component are suitable.
Filter Strips
6.6
These are wide vegetated strips of gently sloping land, which accept and treat runoff by
filtering particulate material from adjacent paved areas. They require high land take, which
may restrict their use in high density areas, however they provide a positive amenity value.
Swales
6.7
Swales are broad, shallow depressions covered by grass or other vegetation. They receive
storm water runoff from adjacent paved areas and convey this water to a pipe or other
structure. Infiltration can occur across the length of the swale if the soils permit and some
level of treatment can be facilitated with appropriate floral species. They are useful for
conveying surface water during small events, however are less effective for higher return
period events. They also require suitably sized verge areas, which may not be available in
high density developments.
Infiltration Basins
6.8
These comprise of dry depressions with a stripped or vegetated base. They are located across
highly permeable soils thus deposing surface water through infiltration. The sensitivity of the
underlying groundwater needs to be taken into consideration when proposing such measures.
Although land take can be significant, they can be located within open space areas and as a
single entity rather than several systems across the whole development.
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Pervious Surfaces
6.9
Pervious surfaces are frequently used for car parking areas or pedestrianised areas. They
allow surface water to infiltrate through gaps between blocks (ie permeable) or through the
block surfaces themselves (ie porous). The sub-base is suitably constructed in order to
promote infiltration or to attenuate surface water depending on the soil types. Pervious
surfaces act not only as an effective way to store or dispose of surface water but have also
been shown to act as a filter and retainer for pollutants, in particular oil.
Infiltration Devices
6.10
These devices allow rainwater to be temporarily stored and to percolate into the ground.
Soakaways are infiltration devices commonly used to drain surface water from roofs. They
comprise of below ground concrete perforated rings with a granular surround although plastic
geocellular units are available. Soakaways are very useful for reducing the volume of runoff
from a development, however in order to be effective they require suitably permeable soils.
The soakaways should be a minimum distance of 5m from any structure with foundations and
located within the back garden. Runoff rates from a roof into a soakaway is generally higher
than the natural runoff across a greenfield surface and infiltration into the ground surface.
Therefore, soakaways have a propensity to reduce the volume of surface water which is lost
to runoff and evaporation (especially in summer) on the ground surface by returning it
directly into the sub-soils of the site. This is also useful when natural recharge is limited due
to periods of frost and snow cover in winter or when evaporation rates in the summer are
limiting the recharge of the underlying water bearing strata.
6.11
Infiltration trenches are based on similar principles but are linear and tend to be shallower.
Green Roofs
6.12
These are multi-layer systems that cover the roof of a building with vegetation. Below this
system is an appropriately designed drainage layer, together with other layers which provide
protection and waterproofing.
Green roofs are effective in providing attenuation and a
reduction in runoff: however there is a disadvantage in that surface water drainage from the
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roof area can carry an increased sediment load to soakaways or piped networks. There are
also maintenance and aesthetic implications associated with this SuDS component.
Rainwater Harvesting
6.13
Rainwater from roofs and hardstanding can be stored and used across the development area.
This re-use of rainwater (eg for flushing toilets or washing machines) can reduce the demand
and dependence from mains supply and can also reduce the volumes and rates of runoff from
the site. The water is typically stored within an external storage tank and piped to appropriate
areas of the building.
6.14
Water butts are connected to rainwater down pipes and provide temporary attenuation during
rainfall events. They typically hold approximately 0.5 cu m of rainwater and can be used as
part of the rainwater harvesting regime of the building or used to provide attenuation prior to
discharge into a soakaway. As long as they are regularly emptied, they have the potential to
reduce runoff volumes from small events although they have no ecological benefits.
Wet Ponds
6.15
Wet Ponds are permanently wet basins providing both attenuation of runoff rates and
treatment of the received storm water. Side slopes of the ponds should be shallow for health
and safety reasons and the base and sides of the ponds are normally lined and vegetated.
Some volume reduction is achieved through evaporation and transpiration and runoff rates
into neighbouring watercourses can be controlled through suitably sized outlet structures.
Ponds provide good ecological benefits if appropriately designed and maintained, however
they require significant land take if other SuDS measures such as soakaways are not
incorporated across the remainder of the site.
6.16
Wetlands are very similar to wet ponds; however they are more complex, requiring shallower
zones to enhance the biodiversity of the area. The focus of these areas is storm water
treatment and therefore wetland areas are dependant on controlled water levels and other
design criteria, which in turn can result in significant land take.
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Dry Ponds (Detention Basins)
6.17
These normally consist of dry depressions, which provide attenuation of runoff rates and
volumes from the development and to provide some treatment to the storm water (although
this is not as effective as ponds). Detention basins require less land take than ponds as
greater attenuation depths and runoff losses can be achieved.
6.18
Table 6.1 shows a multi-criteria selection matrix for these SuDS components, which can be
used to assist with the decision-making process.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Table 6.1: Selection matrix for SuDS components (adapted from Table 5.2, 5.4 and 5.9 of CIRIA C697)
SuDS Technique
Soils
Available
Low
Space?
Density
Residential
Local
Commercial
Brownfield
Roads
Contaminated
Maintenance
Land
Community
Cost
acceptability
Habitat
creation
Low
High
Permeable
Impermeable
potential
Filter Strips
Y
Y
N
Y
Y
Y
Y
Y
Y
Y
H
H
M
H
Swales
Y
Y
N
Y
Y
Y
Y
Y
Y
Y
L
M
L
M
Infiltration Basins
N
Y
N
Y
Y
Y
Y
Y
Y
Y
L
M
L
M
Pervious Surfaces
Y
Y
Y
Y
Y
Y
N
Y
Y
Y
M
M
M
L
Infiltration devices
N
Y
Y
Y
Y
Y
Y
Y
Y
Y
L
M
M
L
Green Roofs
Y
Y
Y
Y
Y
Y
N
Y
Y
Y
H
H
H
H
Y
Y
Y
Y
Y
Y
N
Y
Y
Y
H
M
H
L
Ponds/Wetlands
Y
Y
N
Y
Y
Y
Y
Y
Y
Y
H
H
H
H
Detention Basins
Y
Y
N
Y
Y
Y
Y
Y
Y
Y
L
H
L
M
Rainwater
Harvesting
Y = Yes
Ref: 7293A/21/CW/06-07/1778
N = No
H = High
M = Medium
L = Low
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SuDS Suitability Across the Study Area
6.19
The suitability of the various SuDS techniques across the study area can be related to the soil
types and geology. The permeability of the underlying soils across development areas is a
key factor when determining the practical use of the various SuDS components. Other
factors such as water table and contaminated land also determine SuDS suitability and will
vary on a site by site basis. These factors have not been taken into account in this study.
6.20
The superficial deposits (or Quaternary drift deposits) which comprise the upper layers of the
soil types are likely to be the most relevant when considering SuDS applications. These
deposits include; Alluvium; Glacial Sands and Gravels; Boulder Clay and Contoured Drift,
Norwich Brickearth and Loams. The infiltration capacity and percolation ability of the soils
can vary markedly from one soil type to another across the study area. For example, Boulder
Clay is likely to be relatively impermeable and therefore not particularly suited to infiltration
devices; however Glacial Sands and Gravels are likely to posses a sufficient volume of voids
to effectively allow surface water to percolate through the soil horizon. Table 6.2 sets out the
soil types which exist across the study area.
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Table 6.2: Expected soil types and characteristics across the study area
Geological Succession (Drift)
Soil Characteristics
Alluvium
These deposits consist of the latest material
transported by streams and form areas of flat
land. They comprise silty deposits of sandy
clay or peaty marl with occasional gravels.
River Gravels
Gravel deposits associated with material
transported by streams.
Glacial Sands and Gravels
Sand and gravel deposits associated with
meltwater close to an ice front.
Boulder Clay
A chalky glacial till deposit with occasional
fine interbedded sands.
Contoured Drift, Norwich Brickearth and Mainly unbedded clays and loamy sands.
Loams
Geological Succession (Solid)
London Clay
Consists of blue-grey impermeable clays.
Norwich Crag
Consists mainly of interbedded sands and
gravels with shelly sands and lenticular clays.
Upper Cretaceous Chalk
A soft, white fine-grained limestone.
SuDS Suitability Across the North Norfolk District Council Area
6.21
There are four main soil types expected to be encountered across the North Norfolk area:
Alluvium; Glacial Sands and Gravels, Boulder Clay and Contoured Drift, Norwich
Brickearth and Loam.
Some London Clay deposits are also likely to be encountered
especially around the main river valleys. Table 6.3 describes where these deposits exist and
indicates their suitability to the SuDS techniques as detailed earlier.
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Table 6.3: General applications of SuDS in relation to soil types across the North Norfolk District area
Soil Type
Locations
Infiltration
Appropriate SuDS components
Capacity
Alluvium
These soils are restricted mainly to the River networks within
Average – dependant
Filter Strips, Swales, Infiltration Basins (depending on
this area (ie Thurne, Ant and Bure) and do not generally
on the proportions of
depth to water table), Pervious Surfaces, Infiltration devices
occupy higher ground where settlements are typically located
each
(dependant on depth to water table and proportion of soil
(apart from at Hoveton).
encountered.
soil
type
types), Green Roofs, Water butts, Ponds/Wetlands and
Detention Basins.
Glacial Sands and Gravels
These deposits are concentrated towards the north of the
Good
Filter Strips, Swales, Infiltration Basins (depending on
district area (ie at Holt, Fakenham, Felbrigg, North Walsham
depth to water table), Pervious Surfaces, Infiltration devices
and Knapton).
(dependant on depth to water table), Green Roofs, Water
butts, Ponds/Wetlands and Detention Basins.
Boulder Clay
These deposits are located mainly to the west of the district
Poor
area covering settlements including Great Snoring, Briston and
Filter Strips, Swales, Pervious Surfaces, Green Roofs,
Water butts, Ponds/Wetlands and Detention Basins.
Langham.
Contoured Drift, Norwich
These soil types cover much of the central and north east of the
Brickearth and Loams.
district encompassing settlements which include Aldborough,
Swanton
Abbott,
Lessingham,
Hickling,
Poor
Filter Strips, Swales, Pervious Surfaces, Green Roofs,
Water butts, Ponds/Wetlands and Detention Basins.
Happisburgh,
Sheringham, Cromer and Sea Palling.
London Clay
These are located generally around the main River networks
and adjacent to the alluvial deposits.
Settlements such as
Poor
Filter Strips, Swales, Pervious Surfaces, Green Roofs,
Water butts, Ponds/Wetlands and Detention Basins.
Homing, Sutton, Potter Heigham and Ludham are all located
within these areas.
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SuDS Suitability Across the LDF Preferred Sites
6.22
Table 6.4 below sets out the results of a site specific assessment of the suitability of particular
SuDS elements, for each of the preferred development sites identified in the NNDC
September 2006 “Site Specific Proposals Preferred Options Consultation Paper”. The
analysis is made from an assessment of soils data, taken largely from the current BGS
Geological Maps, together with some borehole data where available.
Anglian Water
6.23
The analysis below also includes where possible relevant information from Anglian Water
relating to the available use and capacity of sewer assets if SuDS components cannot be
utilised. The AWS data is not exhaustive and is in many cases inconclusive. The AWS
company view has been stated to us as follows:
“In general we would try to push for surface water from new development to go to soakaway
or direct connection to existing watercourse, where possible. If these are not possible and a
surface water sewer exists we would seek to limit the discharge rate as much as possible to
protect sewer and watercourse capacity. For brownfield sites we would look to limit flows to
existing or preferably less and for greenfield sites the restriction would be as much as is
reasonably practicable. If we have made specific reference to surface water sewer capacity
being an issue then we would look for a delay in development to enable time to plan, design
and construct improvements.”
Nevertheless AWS has certain statutory obligations under the 1989 Water Act relating to the
provision, adoption and maintenance of surface water drainage infrastructure for new
developments and the company view stated above does not affect these obligations.
Typical Attenuation Storage Areas
6.24
Table 6.4 also includes an assessment of the approximate size for a typical attenuation
storage area appropriate to the particular settlement. The typical attenuation storage area has
been calculated using Figure A3.2 of the HR Wallingford document “Use of SUDS in High
Density Developments – Guidance Manual” (2005) - reproduced at Fig 6.3 below. Appendix
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
3 of that document describes the methodology. The calculations are based upon a 1 in 100
year return period storm event. The various parameters contributing to Fig 6.3 below are:
•
the assumed percentage impermeable area (PIMP) which is generally assumed to be
70%;
•
a climate change factor of 10%;
•
specific hydrological zones for the region;
•
an assumed 100% runoff from paved areas;
•
Standard Annual Average Rainfall (SAAR) for the location;
•
the Winter Rainfall Acceptance Potential (WRAP) for the soil type.
For example, to find the attenuation volume and storage area for an allocation in Cromer, the
SAAR value is extracted from the data within the Flood Estimation Handbook (FEH) CDROM. The settlement is located on the WRAP map contained within the Flood Studies
Report (FSR) 1977, and this determines the WRAP category appropriate to the settlement
(categories range from 1-5 depending on the soil composition). The SAAR value and WRAP
category are then used to define the corresponding attenuation volume in cubic metres per
site hectare. For Cromer, the SAAR value of 649mm and the WRAP category of 1 results in
an attenuation volume of approximately 510 m3/ha. Assuming that the attenuation feature
(e.g. a dry pond) would be 1m in depth for health and safety purposes, the area of the feature
would equate to approximately 510 m2/ha.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Fig 6.3: Attenuation Storage Volume for South and East England - for Developments
with Percentage Impermeable Area (PIMP) = 70%. Taken from Fig A3.2 in Appendix
3 of “The Use of SuDS in High Density Developments – Guidance Manual: HR
Wallingford, 2005”.
6.25
In Table 6.4 the preferred development sites are considered by settlement, in the following
order: Cromer, Fakenham, Holt, Hoveton, North Walsham, Sheringham, Stalham, Wells,
Aldborough, Blakeney, Briston and Melton Constable, Catfield, Corpusty and Saxthorpe,
Horning, Little Snoring, Little Walsingham, Ludham, Roughton, Sculthorpe, Southrepps,
Tattersett, Mundesley, Weybourne, Overstrand, Happisburgh and Bacton.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Table 6.4: APPLICABILITY OF SUDS AT NORTH NORFOLK SETTLEMENTS
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Cromer
Gravelly Head is beneath
Average
2
Filter Strips, Swales, Infiltration Basins
510m /ha
with
sites C11, ROSS2 and
(depending upon depth to water table),
discharge into local
CP1.
Pervious Surfaces, Infiltration devices
watercourses.
No spare capacity
(depending upon depth to water table and
Head, which is mainly
Average
proportion of soil types) Green Roofs,
stony, sandy clay and
Water
clayey sand is beneath sites
Detention Basins
Butts,
Ponds/Wetlands
and
CO2, CO3.
Head is also predominant
Average
beneath sites CO5 and
CO6 although
undifferentiated contorted
clay, silts, sands and gravel
are to be found in eastern
areas.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Undifferentiated contorted
Average
clay, silts sands and
gravels exist beneath sites
CO1, ROSS 3, ROSS 4,
C12 and ROSS1.
The greatest extent of site
Average
CO4 is over
undifferentiated contorted
clay, silts sands and
gravels although the
eastern area covers
Briton’s Lane Sand and
Gravel Member.
Briton’s Lane Sand and
Good
Filter Strips, Swales, Infiltration Basins,
Gravel Member is to be
Pervious Surfaces, Infiltration devices,
found beneath sites C17,
Green
CO8, C15, C16, C22, HC1,
Ponds/Wetlands and Detention Basins.
Roofs,
Water
Butts,
CO9, C18, C19 and HC2.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Sites C10 and C13 overlay
Average
Filter Strips, Swales, Infiltration Basins
a mix of gravelly Head and
(depending upon depth to water table),
undifferentiated contorted
Pervious Surfaces, Infiltration devices
clay, silts, sands and
(depending upon depth to water table and
gravels.
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
CO7 is over a mix of
Good for Briton’s
Across Sand and Gravel where infiltration
Briton’s Lane Sand and
Lane
capacity is good, Filter Strips, Swales,
Gravel Member and
Gravel Deposits
Sand
and
Infiltration Basins, Pervious Surfaces,
Infiltration devices, Green Roofs, Water
gravelly Head.
Butts, Ponds/Wetlands and Detention
Basins.
Average
gravelly Head
for
Filter Strips, Swales, Infiltration Basins
(depending upon depth to water table),
Pervious Surfaces, Infiltration devices
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Ref: 7293A/21/CW/06-07/1778
Page 55
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
The eastern area of C14 is
Good in the east
Across Sand and Gravel where infiltration
over Briton’s Lane Sand
capacity is good, Filter Strips, Swales,
and Gravel Member. The
Infiltration Basins, Pervious Surfaces,
southern site is over Head
Infiltration devices, Green Roofs, Water
and the western site is a
Butts, Ponds/Wetlands and Detention
mix of Head and
Basins.
undifferentiated contorted
clay, silts, sand and gravel.
Average
Filter Strips, Swales, Infiltration Basins
elsewhere
(depending upon depth to water table),
Pervious Surfaces, Infiltration devices
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Fakenham
for
Filter Strips, Swales, Infiltration Basins
410m2/ha
Pebbly
(depending upon depth to water table),
discharge into IDB
Pervious Surfaces, Infiltration devices
drainage ditches or
covers the vast majority of
(depending upon depth to water table and
the River Wensum.
F01 except for a small
proportion of soil types) Green Roofs,
pocket
Water
Till
comprising
chalky,
Average
pebbly clay is beneath all
Chalky
of F05, F06 and F09. Also
Clay
at
the
eastern
boundary, and F07 except
Butts,
Ponds/Wetlands
with
No spare capacity
and
Detention Basins
for an area adjacent to the
southern boundary. Areas
of
site
not
over
Till,
overlays sand and gravel.
Good
for
and Gravel
Sand
Across Sand and Gravel where infiltration
capacity is good, Filter Strips, Swales,
Infiltration Basins, Pervious Surfaces,
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
F02, F03, and ROSS 7, are
Good
over sand and gravel.
Across Sand and Gravel where infiltration
capacity is good, Filter Strips, Swales,
Infiltration Basins, Pervious Surfaces,
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
F04 and F08 are above
Poor
Filter Strips, Swales, Pervious Surfaces,
Alluvium comprising silt
Green
Roofs,
Water
Butts,
and clay, with subordinate
Wetlands and Detention Basins
Ponds/
peat.
The north of ROSS 6 is
Good in north
Across Sand and Gravel where infiltration
over sand and gravel with
capacity is good Filter Strips, Swales,
the southern part of the site
Infiltration Basins, Pervious Surfaces,
over Alluvium of silt and
Infiltration devices, Green Roofs, Water
clay with subordinate peat.
Butts, Ponds/Wetlands and Detention
Basins.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Filter Strips, Swales, Pervious Surfaces,
Poor in south
Green
Roofs,
Water
Butts,
Ponds/
Wetlands and Detention Basins
Holt
Filter Strips, Swales, Infiltration Basins,
510m2/ha
for Holt, H01, H02, H03,
Pervious Surfaces, Infiltration devices,
discharge into local
H04, H05, H06, H07, H08,
Green
watercourses.
H09, H10, H11, H12 and
Wetlands and Detention Basins.
All site specific proposals
Good
Roofs,
Water
Butts,
Ponds/
with
No spare capacity
E2 are above Briton’s Lane
Sand and Gravel Member.
Hoveton
Sites HV01, HV02 and the
majority
of
HV03
above sand and gravel.
are
Good
Filter Strips, Swales, Infiltration Basins,
470m2/ha
Pervious Surfaces, Infiltration devices,
discharge into the
Green
River Bure.
Roofs,
Water
Butts,
Ponds/
with
Limited
capacity
at
HV01
otherwise no spare capacity
Wetlands and Detention Basins.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
E3 and HV04 are over
Average
Filter Strips, Swales, Infiltration Basins
Crag which is also present
(depending upon depth to water table),
beneath
Pervious Surfaces, Infiltration devices
the
north
east
corner of HV03.
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
North Walsham
Filter Strips, Swales, Infiltration Basins,
510m2/ha
east of North Walsham
Pervious Surfaces, Infiltration devices,
discharge into the
NW01,
NW02,
NW04,
Green
North
NW16,
NW17,
NW18,
Ponds/Wetlands and Detention Basins.
NW20,
NW21,
NW22,
or
NW23,
NW24,
NW26,
watercourses.
NW27,
NW33,
NW34,
The majority of sites to the
Good
Roofs,
Water
Butts,
with
No spare capacity
Walsham
and Dilham Canal
local
NW35, NW36, E5, ROS8
overlay sands and gravels.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
NW19
is
above
Head
Average
Filter Strips, Swales, Infiltration Basins
comprised mainly of stony,
(depending upon depth to water table),
sandy clays and clayey
Pervious Surfaces, Infiltration devices
sands.
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
The
British
Geological
Good
Filter Strips, Swales, Infiltration Basins,
Survey Map immediately
Pervious Surfaces, Infiltration devices,
to
Green
the
west
of
North
Walsham is not published
Roofs,
Water
Butts,
Ponds/
Wetlands and Detention Basins.
at a scale of 1: 50,000.
Reviewing the information
shown it is highly likely
that sands and gravels are
to be found in this area as
they
are
prevalent
elsewhere to the east. This
has
been
confirmed
verbally with the British
Geological
Survey
who
consulted the 1:10 000 map
of the area and spoke with
the District Geologist. Sites
to which this applies are
listed as follows:
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
NW03,
NW05,
NW06,
NW07,
NW08,
NW09,
NW10,
NW11,
NW14,
NW15,
NW25,
NW28,
NW29,
NW30,
NW31,
NW32, ED1
Sheringham
Filter Strips, Swales, Infiltration Basins
510m2/ha
over
(depending upon depth to water table),
discharge into local
undifferentiated contorted
Pervious Surfaces, Infiltration devices
watercourses.
clay, silts, sand and gravel.
(depending upon depth to water table and
Sites SH01, SH08 and
SH07
are
Average
with
No spare capacity
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Gravelly Head is to be
Average
found beneath sites ROS5
and SH02.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Gravelly
Head
and
Average
undifferentiated contorted
clay, silts, sand and gravel
are to be found beneath
sites SH04, SH05, SH09
and SH10.
Sites SH11 and SH06 have
areas
Average
above
undifferentiated contorted
clay, silts, sand and gravel
as well as Britons Land
Sand and Gravel Member.
POS 3 is above Briton’s
Good
Across Sand and Gravel where infiltration
Lane Sand and Gravel
capacity is good, Filter Strips, Swales,
Member.
Infiltration Basins, Pervious Surfaces,
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
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Page 64
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Stalham
Areas of allocation sites over Diamicton
470m2/ha
with
Limited capacity at ST01 and
ST08, ST09, ST14 are
and glacio-lacustrine silts with poor
discharge
into
ST02
above
infiltration capacity: Filter Strips, Swales,
Stalham Staithe.
Sites ST04, ST05, ST07,
Diamicton
Poor
and
glacio-lacustrine silts.
otherwise
no
spare
capacity
Pervious Surfaces, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins
Sites ST02, ST10, ST11,
Good
Over Sand and Gravel Areas with good
ST12, ST13 and a great
infiltration capacity: Filter Strips, Swales,
proportion of E4 and ST15
Infiltration Basins, Pervious Surfaces,
overlay sand and gravel.
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
The northern half of ST06
is over
Diamicton and
glacio-lacustrine silts.
Ref: 7293A/21/CW/06-07/1778
Poor (North)
Filter Strips, Swales, Pervious Surfaces,
Green
Roofs,
Water
Butts,
Ponds/Wetlands and Detention Basins
Page 65
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Over Sand and Gravel Areas with good
The south is above sand
Good (South)
and gravel.
infiltration capacity: Filter Strips, Swales,
Infiltration Basins, Pervious Surfaces,
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
The west of site ST01 is
Good (West)
Over Sand and Gravel Areas with good
above sand and gravel with
infiltration capacity: Filter Strips, Swales,
the remainder of the site
Infiltration Basins, Pervious Surfaces,
above
Infiltration devices, Green Roofs, Water
Diamicton
and
glacio-lacustrine silts.
Butts, Ponds/Wetlands and Detention
Basins.
Poor elsewhere
Filter Strips, Swales, Pervious Surfaces,
Green
Roofs,
Water
Butts,
Ponds/
Wetlands and Detention Basins
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
The north of site ST03 is
Good (North)
Over Sand and Gravel Areas with good
above sand and gravel with
infiltration capacity: Filter Strips, Swales,
the south above Diamicton
Infiltration Basins, Pervious Surfaces,
and glacio-lacustrine silts.
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
Poor (South)
Filter Strips, Swales, Pervious Surfaces,
Green
Roofs,
Water
Butts,
Ponds/Wetlands and Detention Basins
Wells
Filter Strips, Swales, Pervious Surfaces,
510m2/ha
with
Limited capacity at W01 and
Survey 1: 50,000 scale
Green
discharge into the
W02 otherwise no spare capacity
maps
Wetlands and Detention Basins
The
British
are
Geological
yet
to
be
Poor
Roofs,
Water
Butts,
Ponds/
IDB
drainage
published for the area.
ditches or Wells
Liaison with the British
Salt Marshes.
Geological
Survey
has
established
that
the
superficial geology in the
vicinity
of
W01
is
comprised of glacio-fluvial
sand and gravel with some
Diamicton to the south-east
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
and head to the north and
south of the site.
Borehole logs 700m from
the allocation site indicate
sandy/silty clay overlain by
made ground.
It is considered prudent to
base the assessment on the
least favourable geology
for all sites, W01, W02,
W03, W04 and CP2 due to
the lack of information and
the presence of silty clay
and Diamicton.
Aldborough
Filter Strips, Swales, Infiltration Basins
510m2/ha
beneath site ALD02 is
(depending upon depth to water table),
discharge into local
identified as Hanworth Till
Pervious Surfaces, Infiltration devices
watercourses
Member: sandy, pebbly,
(depending upon depth to water table and
IDB
clayey, silt.
proportion of soil types) Green Roofs,
ditches.
The
surface
geology
Average
Water
Butts,
Ponds/Wetlands
with
No Data
or
drainage
and
Detention Basins
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
ALD01 is above Head:
Average
mainly stony, sandy clay
and clayey sand.
Alluvium: mainly clay, silt
Average
and sand are beneath ALD
03.
ALD04 is likely to overly
Average
Head, Till and Alluvium
referred to above.
Blakeney
Over Sand and Gravel Areas with good
510m2/ha
over Briton’s Lane Sand
infiltration capacity: Filter Strips, Swales,
discharge into Agar
and Gravel Member and
Infiltration Basins, Pervious Surfaces,
Creek.
Esker is also shown in
Infiltration devices, Green Roofs, Water
close proximity west of the
Butts, Ponds/Wetlands and Detention
site
Basins.
BLA01 is predominantly
on
the
Good
British
with
No Data
Geological Survey map.
Briston
and
Melton Constable
Following correspondence
with the British Geological
Ref: 7293A/21/CW/06-07/1778
Poor
Filter Strips, Swales, Pervious Surfaces,
510m2/ha
Green
discharge into the
Roofs,
Water
Butts,
Ponds/
with
No Data
Page 69
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Survey the area in the
Wetlands and Detention Basins.
River
Bure
at
vicinity of BRI01 is shown
Briston and local
as having surface geology
watercourses
comprised of Diamicton.
IDB
A borehole within 190m
ditches at Melton
indicates the presence of
Constable.
and
drainage
sand and clay.
Catfield
Site CAT01 lies above
Diamicton
and
Poor
glacio-
Saxthorpe
and
470m2/ha with no
Green
obvious outfall.
Roofs,
Water
Butts,
No Data
Ponds/Wetlands and Detention Basins
lacustrine silts.
Corpusty
Filter Strips, Swales, Pervious Surfaces,
Filter Strips, Swales, Pervious Surfaces,
510m2/ha
with the British Geological
Green
discharge into the
Survey the area in the
Ponds/Wetlands and Detention Basins.
Following correspondence
Poor
Roofs,
Water
Butts,
River
vicinity of COR1 is shown
IDB
as having surface geology
ditches.
with
Bure
No Data
and
drainage
comprised of Diamicton
and Head.
A borehole approximately
half a kilometre east of
COR1 does record the
presence
of
Ref: 7293A/21/CW/06-07/1778
sand
and
Page 70
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
gravel which could indicate
that the site may have
better
infiltration
characteristics. Until such
time as more detailed, site
specific
information
is
made available then the
recommendation is to base
the
infiltration
capacity
upon the worst case surface
geology scenario.
Horning
The north east of site
HOR01
Diamicton
Poor
overlays
and
Filter Strips, Swales, Pervious Surfaces,
470m2/ha
Green
discharge into the
Roofs,
Water
Butts,
Ponds/Wetlands and Detention Basins
glacio-
with
No Data
River Bure.
lacustrine silts.
The
remainder
of
the
allocation is above Crag.
Average
Filter Strips, Swales, Infiltration Basins
(depending upon depth to water table),
Pervious Surfaces, Infiltration devices
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Ref: 7293A/21/CW/06-07/1778
Butts,
Ponds/Wetlands
and
Page 71
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Detention Basins
Little Snoring
SNO1 is within an area
Average
across
Filter Strips, Swales, Infiltration Basins
460m2/ha
shown to comprise clay,
Sheringham Cliff
(depending upon depth to water table),
discharge into IDB
silt,
Formation areas
Pervious Surfaces, Infiltration devices
drainage ditches.
sand
and
(Sheringham
Formation).
gravel
and
Sand
gravel
Cliff
(depending upon depth to water table and
and
proportion of soil types) Green Roofs,
Diamicton
Water
deposits are also present.
Butts,
Ponds/Wetlands
with
No Data
and
Detention Basins
Good across Sand
Over Sand and Gravel Areas with good
and Gravel areas
infiltration capacity: Filter Strips, Swales,
Infiltration Basins, Pervious Surfaces,
Infiltration devices, Green Roofs, Water
Butts, Ponds/Wetlands and Detention
Basins.
Poor
Ref: 7293A/21/CW/06-07/1778
across
Filter Strips, Swales, Pervious Surfaces,
Diamicton
Green
Roofs,
Water
Butts,
deposits areas
Ponds/Wetlands and Detention Basins.
Page 72
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Walsingham
Filter Strips, Swales, Infiltration Basins
510m2/ha
shown to comprise clay,
(depending upon depth to water table),
discharge into the
silt,
gravel
Pervious Surfaces, Infiltration devices
River Stiffkey.
Cliff
(depending upon depth to water table and
as
proportion of soil types) Green Roofs,
WAL01 is in an area
sand
and
(Sheringham
Formation)
as
well
Average
Diamicton, according to
Water
the
Detention Basins
correspondence
Butts,
Ponds/Wetlands
with
No Data
and
received from the British
Geological Survey.
Poor
Ludham
across
Filter Strips, Swales, Pervious Surfaces,
Diamicton
Green
deposits areas
Wetlands and Detention Basins
Average
Filter Strips, Swales, Infiltration Basins
510m2/ha
with
The
(depending upon depth to water table),
discharge
into
majority of LUD02 is also
Pervious Surfaces, Infiltration devices
Womack Water.
above Crag although Head
(depending upon depth to water table and
deposits are also shown
proportion of soil types) Green Roofs,
entering the site from the
Water
west.
Detention Basins
Sites LUD01 and LUD03
are
above
Ref: 7293A/21/CW/06-07/1778
Crag.
Roofs,
Butts,
Water
Butts,
Ponds/Wetlands
Ponds/
No Data
and
Page 73
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
The majority of LUD04 is
Average
above
Crag areas
across
Filter Strips, Swales, Infiltration Basins
Crag
with
and
glacio-
Pervious Surfaces, Infiltration devices
lacustrine silts in evidence
(depending upon depth to water table and
to the east.
proportion of soil types) Green Roofs,
Diamicton
(depending upon depth to water table),
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Poor across Silt
Filter Strips, Swales, Pervious Surfaces,
areas
Green
Roofs,
Water
Butts,
Ponds/
Wetlands and Detention Basins
Roughton
Sites ROU5, ROU6 and
ROU7 lie above Crag.
Average
Filter Strips, Swales, Infiltration Basins
510m2/ha
with
(depending upon depth to water table),
discharge
into
Pervious Surfaces, Infiltration devices
Hagon Beck.
No Data
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Ref: 7293A/21/CW/06-07/1778
Page 74
Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Sites
ROU3
is
Average
predominantly above Head
comprised mainly of stony,
sandy clays and clayey
sands
ROU2 and ROU4 have
Average
areas above Crag with
Head in other locations.
Site ROU1 is above both
Head
and
Average
Alluvium:
mainly clay, silt and sand.
Sculthorpe
Sand and gravel is beneath
site WG02.
Good
Over Sand and Gravel Areas with good
510m2/ha
infiltration
Strips,
discharge into IDB
Pervious
drainage ditches or
Swales,
Surfaces,
capacity:
Infiltration
Infiltration
Filter
Basins,
devices,
Green
with
No Data
River Wensum.
Roofs, Water Butts, Ponds/Wetlands and
Detention Basins.
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
The major part of site
Predominantly
Over Sand and Gravel Areas with good
WG01 is above sand and
Good
infiltration
gravel
although
Till
Swales,
capacity:
Infiltration
Basins,
Strips,
Pervious
comprising chalky, pebbly
Surfaces,
clay may be encountered at
Roofs, Water Butts, Ponds/Wetlands and
the very north of the site.
Detention Basins
Average
very north
at
the
Infiltration
Filter
devices,
Green
Filter Strips, Swales, Infiltration Basins
(depending upon depth to water table),
Pervious Surfaces, Infiltration devices
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Southrepps
Filter Strips, Swales, Infiltration Basins
510m2/ha with no
all the Southrepps sites,
(depending upon depth to water table),
obvious outfall.
SOU1,
SOU3,
Pervious Surfaces, Infiltration devices
SOU4, SOU5 and SOU6.
(depending upon depth to water table and
Head will also be found
proportion of soil types) Green Roofs,
beneath SOU5.
Water
Crag is prevalent beneath
SOU2,
Average
Butts,
Ponds/Wetlands
No Data
and
Detention Basins
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Tattersett
Over Sand and Gravel Areas with good
510m2/ha
Chalk are to be found
infiltration
discharge into the
beneath site E7.
Swales,
Sands and gravel as well as
Good
capacity:
Infiltration
Surfaces,
Infiltration
Filter
Basins,
Strips,
Pervious
devices,
with
No data
River Tat.
Green
Roofs, Water Butts, Ponds/Wetlands and
Detention
Mundesley
Filter Strips, Swales, Infiltration Basins
510m2/ha
with
geological stratum in the
(depending upon depth to water table),
discharge
into
area.
Pervious Surfaces, Infiltration devices
Mundesley Beck.
Crag
is
the
dominant
Average
No Data
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Weybourne
Till is prevalent with seams
Poor for Till
of Head
Filter Strips, Swales, Pervious Surfaces,
510m2/ha
with
Green
discharge
into
Roofs,
Water
Butts,
Ponds/Wetlands and Detention Basins
Average for Head
No Data
Spring Beck.
Filter Strips, Swales, Infiltration Basins
(depending upon depth to water table),
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Pervious Surfaces, Infiltration devices
(depending upon depth to water table and
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
Overstrand
The major part of town
Predominantly
Filter Strips, Swales, Infiltration Basins
510 m2/ha with no
overlays Gravelly Head.
Average
(depending upon depth to water table),
obvious outfall.
Undifferentiated contorted
Pervious Surfaces, Infiltration devices
clay, silts sand and gravel
(depending upon depth to water table and
is found in the south,
proportion of soil types) Green Roofs,
north-west, south-west and
Water
east of the town.
Detention Basins
Briton’s lane Sand and
Good
Gravel
outskirts of town.
Member
is
at
the
Butts,
Ponds/Wetlands
No Data
and
Over Sand and Gravel Areas with good
infiltration
capacity:
Filter
Strips,
prevalent at the outskirts of
Swales,
the town with seams of
Surfaces,
Gravelly Head.
Roofs, Water Butts, Ponds/Wetlands and
Infiltration
Infiltration
Basins,
devices,
Pervious
Green
Detention
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Happisburgh
Most of Happisburgh is
above
Diamicton
Poor for Silts
and
Glacio Lacustrine Silts.
510 m2/ha with no
Green
obvious outfall.
Roofs,
Water
Butts,
No Data
Ponds/Wetlands and Detention Basins
Sand and Gravel is found
Good
beneath
and Gravel
the
Filter Strips, Swales, Pervious Surfaces,
western,
for
Sand
Over Sand and Gravel Areas with good
infiltration
eastern and southern parts
Swales,
of the town.
Surfaces,
capacity:
Infiltration
Infiltration
Filter
Basins,
devices,
Strips,
Pervious
Green
Roofs, Water Butts, Ponds/Wetlands and
Detention
Happisburgh
Most of Happisburgh is
above
Diamicton
Poor for Silts
and
Glacio Lacustrine Silts.
Good
beneath the north-western,
and Gravel
south-eastern
eastern
Happisburgh.
and
parts
510
Green
discharge
Roofs,
Water
Butts,
Ponds/Wetlands and Detention Basins
Sand and Gravel is found
farof
for
Sand
m2/ha
Filter Strips, Swales, Pervious Surfaces,
with
into
No data
a
local watercourse.
Over Sand and Gravel Areas with good
infiltration
Swales,
Surfaces,
capacity:
Infiltration
Infiltration
Filter
Basins,
devices,
Strips,
Pervious
Green
Roofs, Water Butts, Ponds/Wetlands and
Detention
Ref: 7293A/21/CW/06-07/1778
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
SETTLEMENT
SOIL TYPES
AS DEFINED IN
INFILTRATION
APPROPRIATE SUDS
TYPICAL
ANGLIAN WATER
CAPACITY
COMPONENTS
ATTENUATION
CAPACITY AND
STORAGE
SCHEDULE FOR
AREA*
IMPROVEMENT
LDF
Bacton
Sand
and
Gravel
is
prevalent beneath the entire
Predominantly
Over Sand and Gravel Areas with good
510 m2/ha with no
Good
infiltration
obvious outfall.
area.
Swales,
capacity:
Infiltration
Surfaces,
Infiltration
Filter
Basins,
Strips,
No data
Pervious
devices,
Green
Roofs, Water Butts, Ponds/Wetlands and
Detention
Gravelly
Alluvium
and
Head consisting of mainly
Average in the
Filter Strips, Swales, Infiltration Basins
south
(depending upon depth to water table),
stony, sandy clays and
Pervious Surfaces, Infiltration devices
clayey
(depending upon depth to water table and
sands
are
present in the south.
also
proportion of soil types) Green Roofs,
Water
Butts,
Ponds/Wetlands
and
Detention Basins
* Based on Figure A3.2 of HR Wallingford document entitled “Use of SuDS in High Density Developments”. Assumes 70% impermeable area and a 1 in 100 year return
period (plus 10% climate change) storm event. Area is based on a 1m deep attenuation feature.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
7.0
RECOMMENDATIONS
7.1
Development proposals in North Norfolk should be tested against the flood probability maps
in this SFRA, which indicate flood probability associated with river systems, together with
the sustainable drainage suitability maps and tables, and the Environment Agency maps
indicating risk of coastal flooding.
Coastal flooding and flooding associated with defence
failure are likely to produce the most significant consequences and greatest hazard because of
the speed of onset of the flood, the high water velocities and the deep water. Decisions
associated with development proposals for coastal areas should be influenced by and linked
to the funding status and the implementation programme for associated flood defence
maintenance and improvement. Flooding from tidally influenced systems, typically in the
eastern and central areas of North Norfolk, is likely to be less hazardous, because of the
slower onset. Fluvial flooding associated with upstream areas of individual catchments can
arise rapidly, but in the North Norfolk terrain the catchments are not normally “flashy” and
the hazard from these floods, excepting unusual meteorological conditions, is least onerous.
7.2
Consideration of the flood risk at a particular location should take account of the climate
change enhanced flood outlines in this SFRA in accordance with PPS 25.
7.3
Any development encroaching within any of the plotted flood zones will increase flood risk
to adjacent areas, and the effect on flood risk of a number of small encroachments is
cumulative. If the requirements of PPS25 are met in full then additional development will not
increase flood risk elsewhere.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
8.0
SITE SPECIFIC FLOOD RISK ASSESSMENTS
8.1
PPS25 Appendix E paragraphs E8, E9 and E10 set out the general circumstances in which a
site specific flood risk assessment is required to be submitted by the proposer of new
development:
“E8. At the planning application stage, an appropriate FRA will be required to demonstrate
how flood risk from all sources of flooding to the development itself and flood risk to others
will be managed now and taking climate change into account. Policies in LDDs should
require FRAs to be submitted with planning applications in areas of flood risk identified in
the plan.
E9. Planning applications for development proposals of 1 hectare or greater in Flood Zone 1
and all proposals for new development located in Flood Zones 2 and 3 (see Table D.1, Annex
D) should be accompanied by a FRA. This should identify and assess the risks of all forms of
flooding to and from the development and demonstrate how these flood risks will be
managed, taking climate change into account. For major developments in Flood Zone 1, the
FRA should identify opportunities to reduce the probability and consequences of flooding. A
FRA will also be required where the proposed development or change of use to a more
vulnerable class may be subject to other sources of flooding (see Annex C) or where the
Environment Agency, Internal Drainage Board and/or other bodies have indicated that there
may be drainage problems.
E10. The FRA should be prepared by the developer in consultation with the LPA. The FRA
should form part of an Environmental Statement when one is required by the Town and
Country Planning (Environmental Impact Assessment) (England and Wales) Regulations
1999 as amended.”
8.2
PPS25 Appendix E paragraphs E1 to E3 set out the general principles involved and the
requirements for the scope of a site specific FRA:
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
“The Assessment of Flood Risk
General Principles
E1. Properly prepared assessments of flood risk will inform the decision-making process at
all stages of development planning. There should be iteration between the different levels of
flood risk assessment.
E2. Any organisation or person proposing a development must consider whether that
development will not add to and should where practicable reduce flood risk. The future users
of the development must not be placed in danger from flood hazards and should remain safe
throughout the lifetime of the plan or proposed development and land use.
E3. At all stages of the planning process, the minimum requirements for flood risk
assessments are that they should:
•
be proportionate to the risk and appropriate to the scale, nature and location of the
development;
•
consider the risk of flooding arising from the development in addition to the risk of
flooding to the development;
•
take the impacts of climate change into account (see Annex B);
•
be undertaken by competent people, as early as possible in the particular planning
process, to avoid misplaced effort and raising landowner expectations where land is
unsuitable for development;
•
consider both the potential adverse and beneficial effects of flood risk management
infrastructure including raised defences, flow channels, flood storage areas and other
artificial features together with the consequences of their failure;
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
•
consider the vulnerability of those that could occupy and use the development, taking
account of the Sequential and Exception Tests and the vulnerability classification (see
Annex D), including arrangements for safe access;
•
consider and quantify the different types of flooding (whether from natural and human
sources and including joint and cumulative effects) and identify flood risk reduction
measures, so that assessments are fit for the purpose of the decisions being made;
•
consider the effects of a range of flooding events including extreme events on people,
property, the natural and historic environment and river and coastal processes;
•
include the assessment of the remaining (known as ‘residual’) risk (see Annex G)
after risk reduction measures have been taken into account and demonstrate that this
is acceptable for the particular development or land use;
•
consider how the ability of water to soak into the ground may change with
development, along with how the proposed layout of development may affect drainage
systems; and
•
be supported by appropriate data and information, including historical information
on previous events.”
8.3
The “PPS25 Practice Guide” considers that FRA’s can be classified into three levels of
detail, as shown in Table 8.1 below:
Table 8.1: Levels of Site Specific Flood Risk Assessment (taken from the “PPS 25 Practice
Guide” Table 2.3)
FRA Level
Description
Level 1
Screening study to identify whether there are any flooding or surface water
management issues related to a development site that may warrant further
consideration. This should be based on readily available existing information,
including the SFRA, Environment Agency Flood Map and Standing Advice. The
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
screening study will ascertain whether a FRA is required.
Level 2
Scoping study to be undertaken if the Level 1 FRA indicates that the site may lie
within an area that is at risk of flooding or that the site may increase flood risk due to
increased run-off. This study should confirm the sources of flooding which may affect
the site. The study should include the following:
•
an appraisal of the availability and adequacy of existing information
•
a qualitative appraisal of the flood risk posed to the site, and potential impact
of the development on flood risk elsewhere
•
an appraisal of the scope of possible measures to reduce the flood risk to
acceptable levels.
The scoping study may identify that sufficient quantitative information is already
available to complete a FRA appropriate to the scale and nature of the development.
Level 3
Detailed study to be undertaken if the Level 2 FRA concludes that further quantitative
analysis is required to assess flood risk issues related to the development site.
The study should include:
•
quantitative appraisal of the potential flood risk to the development
•
quantitative appraisal of the potential impact of development site on flood risk
elsewhere
•
quantitative demonstration of the effectiveness of any proposed mitigation
measures.
8.4
The “Practice Guide” also identifies sources of information appropriate to each level of
FRA, as shown in Table 8.2 below:
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
Table 8.2: Typical Sources of Information for FRA’s (taken from the “PPS25 Practice
Guide” Table 2.4)
FRA
Typical Sources of Information
Level
1
Environment Agency Flood Map
Environment Agency Standing Advice
PPS25 table D.1
SFRA
2
Regional or local policy statements or guidance (eg Regional Spatial Strategies, Local
Development Documents)
Regional Flood Risk Appraisals (RFRAs)
Strategic Flood Risk Assessments (SFRAs)
Catchment Flood Management Plans (CFMPs)/Shoreline Management Plans SMPs)
Surface Water Management Plans
Consultation with the LPA/Environment Agency or other flood risk consultees to
identify, in broad terms, what issues, related to flood risk, need to be considered
including other sources of flooding
Historic maps
Local libraries and newspaper reports
Interviews with local people
Walkover survey to assess:
•
Potential sources of flooding
•
Likely routes for flood waters
•
The site’s key features, including flood defences, and their condition
Site survey to determine:
•
General ground levels across the site
•
Levels of any formal or informal flood defences relevant to the site
All other documents listed in Appendix B of this Guide.
3
As above, plus
Detailed topographical survey
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Detailed hydrographic survey
Site-specific hydrological and hydraulic modelling studies
Monitoring to assist with model calibration/verification
Continued consultation with the LPA, Environment Agency and other flood risk
consultees.
Content of a Site Specific FRA
8.5
The “PPS25 Practice Guide” gives the following guidance on the contents of a site specific
FRA:
“The content of a FRA should always be appropriate to the scale and nature of the
development. A FRA may include some, but seldom all, of the following outputs. The detailed
scope of the FRA should be agreed in advance with the LPA in consultation with the
Environment Agency and any other relevant flood risk consultees. A typical Level 2 or Level
3 FRA might cover the following:
1.
Development description and location
•
What type of development is proposed and where it will be located?
•
What is its Vulnerability Classification?
•
Is the proposed development consistent with the Local Development Documents?
•
Evidence that the Sequential Test or Exception Test has been applied in the selection
of this site for the development type proposed.
2.
Definition of the flood hazard
•
What sources of flooding could affect the site?
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•
For each identified source, describe how flooding would occur, with reference to any
historic records wherever these are available.
•
What are the existing surface water drainage arrangements for the site?
3.
Probability
•
Which flood zone is the site within?
•
If there is a Strategic Flood Risk Assessment (SFRA) covering this site, what does it
show?
•
What is the probability of the site flooding taking account of the contents of the SFRA
and of any further site-specific assessment?
•
What are the existing rates and volumes of run-off generated by the site?
4.
Climate change
•
How is flood risk at the site likely to be affected by climate change?
5.
Detailed development proposals
•
Details of the development layout, referring to the relevant drawings.
•
Where appropriate, demonstrate how land-uses most sensitive to flood damage have
been placed in areas within the site that are at least risk of flooding.
6.
Flood risk management measures
•
How will the site be protected from flooding, including the potential impacts of
climate change, over the development’s lifetime?
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7.
Off site impacts
•
How will you ensure that the measures to protect your site from flooding will not
increase flood risk elsewhere?
•
How will you prevent run-off from the completed development causing an impact
elsewhere?
8.
Residual risks
•
What flood-related risks will remain after you have implemented the measures to
protect the site from flooding?
•
How, and by whom, will these risks be managed over the lifetime of the
development?”
8.6
The “PPS25 Practice Guide” includes a draft Site Specific FRA pro-forma, with Guidance
Notes, which can be issued to a developer by the LPA, for submission with the developer’s
planning application. This pro-forma is included in Appendix C.
8.7
The PPS25 Practice Guide” also includes some other guidance relating to site specific
FRA’s:
“Use of modelling software
The modelling software chosen for detailed assessments should be capable of producing the
required output. It will generally be appropriate to choose commercial hydraulic/river
modelling software that is in widespread use. In certain circumstances, for example where
the applicability of a model to a specific situation has not been previously demonstrated, it
will be necessary for those conducting the FRA to have independent benchmarking tests
carried out to demonstrate model performance using standard data. Examples of how this
may be achieved under a range of scenarios are provided in the Defra/Environment Agency
R&D Report “Benchmarking of hydraulic river modelling software packages” (WS-105).
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This is available on the Joint Defra/Environment Agency Flood and Coastal Erosion Risk
Management R & D Programme web site.
In reporting on any hydraulic modelling carried out as part of the FRA, a technical
description of the model should be provided. This should include the name and version of the
software used. Where non-standard software has been used, evidence should be provided to
demonstrate the applicability of the model(s) to the situation in question.
Allowing for uncertainty
Flood risk assessments may require complex analyses and the use of specialist techniques
and software, particularly in the design of measures to protect vulnerable properties from
flooding. Hydrologists and hydraulic modellers seldom have all the data they require in
order to accurately determine the flows and flood levels associated with events with annual
probabilities as low as 1 per cent. LPAs should liaise with the Environment Agency to ensure
that, where such studies are undertaken, the approach adopted has taken adequate account
of the need to:
•
calibrate and verify numerical models using all relevant information reasonably
available
•
allow for uncertainties in the input parameters
•
consider the sensitivity of modelling results to errors in the input parameters and
adopt a precautionary approach, particularly where errors could have serious
consequences.
Compensatory Flood Storage/Conveyance
Undefended areas
Where development is proposed in undefended areas of floodplain, which lie outside of the
functional floodplain, the implications of ground raising operations for flood risk elsewhere
needs to be carefully considered. There are few circumstances where provision of
compensatory flood storage or conveyance will not be required for undefended fluvial
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floodplain areas. This is because, whilst single developments may have a minimal impact, the
cumulative impact of many such developments can be significant.
In undefended tidal areas, raising the ground is unlikely to impact on maximum tidal levels
and provision of compensatory storage should not be necessary.
Defended areas
When proposing new development behind flood defences, the impact on residual flood risk to
other properties should be considered. New development behind flood defences can increase
the residual flood risk, should these defences breach or overtop, by disrupting conveyance
routes (flow paths) and/or by displacing flood water. If conveyance routes that allow flood
water to pass back into a river or the sea following failure of a flood defence are blocked,
this will potentially increase flood risk to existing properties. If there is a finite volume of
water able to pass into a defended area following a failure of the defences, then a new
development, by displacing some of the flood water, will increase the risk to existing
properties.
It is recommended that, should any land allocation be proposed in a defended flood area, the
potential cumulative impact of loss of storage at the allocation sites on flood risk elsewhere
within the flood cell should be considered. Such assessment should be appropriate to the
scale and nature of the proposed development and flood risk. If the potential impact is
unacceptable, mitigation should be provided.
Run-off Rates and Volumes from New Development
The policies in Annex F of PPS25 state that both the rates and volumes of run-off from new
developments should be ‘no greater than the rates prior to the proposed development, unless
specific off-site arrangements are made which result in the same net effect’. This may have
significant implications for new developments, which developers will need to factor into the
earliest stages of their site assessments.”
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9.0
CONCLUSIONS
9.1
This SFRA is compiled in accordance with the recommendations of PPS25. It will allow
appropriate strategic land use planning decisions to be made in areas which may be at risk of
flooding and will provide the basis from which to apply the Sequential Test and Exception
Test in the development allocation and development control process.
9.2
The information in this SFRA, particularly the flood probability maps and the sustainable
drainage schedules and maps, will allow development to be allocated to areas which are not
at risk of flooding, and to areas which are most able to accommodate sustainable drainage.
9.3
As well as being relevant to planning decisions, the hazard maps and animations associated
with the coastal defence breach scenarios are intended as guidance for evacuation strategies
in the specific settlements covered.
9.4
Climate change predictions are likely to be revised from time to time with ongoing research,
and planning policy will change, so it would be prudent to review the basis of this SFRA
periodically, possibly every four or five years.
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10.0
REFERENCES
i.
Communities and Local Government 2006.
Planning Policy Statement 25:
Development and Flood Risk. HMSO.
ii.
Communities and Local Government February 2007. Development and Flood Risk:
A Practice Guide Companion to PPS25 “Living Draft” – A Consultation Paper
(Web-only publication).
iii.
DEFRA/EA 2005. Flood Risk Assessment Guidance for New Development, Phase 2,
Flood and Coastal Defence R&D Programme, R&D Technical Report FD2320/TR2..
Water Research Council.
iv.
DEFRA/EA 2005a.
Flood Risks to People, Phase 2, R&D Technical Report
FD2321/TR2, Flood and Coastal Defence R&D Programme.
Water Research
Council.
v.
DEFRA/EA 2005b.
Flood Warning for Vulnerable Groups:
A review of the
literature, Flood and Coastal Defence R&D Programme. Environment Agency.
vi.
DEFRA/EA 2005c.
Preliminary rainfall runoff management for developments,
Flood and Coastal Defence R&D Programme, R&D Technical Report W5074/A/TR/1, Revision C. Water Research Council.
vii.
DEFRA/MAFF 2000. Flood and Coastal Defence Project Appraisal Guidance.
viii.
Environment Agency Guidance Note 3, Planning Policy Statement 25: Development
and Flood Risk Assessments – Development in Flood Zones 3 and 2.
ix.
Halcrow/HR Wallingford 1999. ISIS [CD-ROM]. Version 2.4.4.
x.
Hulme, M., Jenkins, G.J., Lu, X., Turnpenny, J.R., Mitchell, T.D., Jones, R.G.,
Lowe, J., Murphy, J.M., Hassell, D., Boorman, P., Mcdonald, R. and Hill, S. 2002
Climate Change Scenarios for the United Kingdom:
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The UKCIP02 Scientific
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Report, Tyndall Centre for Climate Change Research, School of Environmental
Sciences, University of East Anglia, Norwich, UK. 120pp
xi.
Marshall, D.C.W. and Bayliss, A.C. 1994. Flood Estimation for Small Catchments,
IH Report 124. Institute of Hydrology.
xii.
NERC 2006. Flood Estimation Handbook [CD-ROM]. Institute of Hydrology.
xiii.
NERC 1975. Flood Studies Report (FSR). Institute of Hydrology.
xiv.
Newman, A.P.
2004.
Protecting groundwater with oil-retaining pervious
pavements: historical perspectives, limitations and recent developments. Quarterly
Journal of Engineering Geology and Hydrogeology, 37, pp 283-291
xv.
ODPM 2003. Preparing for Floods. London: ODPM.
xvi.
Reed, R., Faulkner, D. and Bayliss, A. 1999. Flood Estimation Handbook (FEH), 5
Volumes. Institute of Hydrology.
xvii.
WinDes 2006. Micro Drainage. Version 10.4.
xviii. Woods-Ballard., et al. 2007. The SUDS Manual, Report C697. London: CIRIA.
xix.
Bettess, R. 1996. Infiltration drainage – Manual of good practice, Report C156.
London: CIRIA.
xx.
ADAS 1980. MAFF Report 5, Pipe size design for field drainage.
xxi.
BRE 1991. Digest 365. Soakaway Design.
xxii.
British Geological Survey 2001. Solid Geology Map, UK South Sheet, 1:625,000.
Coventry: Clifford Press Ltd.
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xxiii. Chatwin, C. P. 1961. British Regional Geology: East Anglia and Adjoining Areas.
Fourth Edition, London: HMSO.
xxiv. DOE 1981. The Wallingford Procedure: Design and Analysis of Urban Storm
Drainage. HR Wallingford.
xxv.
DOE 1981.
Modified Rational Method:
The Wallingford Procedure.
HR
Wallingford.
xxvi. Institute of Geological Sciences 1976.
Hydrogeological map of Northern East
Anglia, 1:125,000. London: Cook, Hammond and Kell Ltd.
xxvii. Institute of Geological Sciences 1977. Quaternary Map of the United Kingdom,
1:625,000. Ordnance Survey.
xxviii. Martin, P. et al. 2001. Sustainable urban drainage systems – best practice guide,
Report C523. London: CIRIA.
xxix. Martin, P. et al. 2000. Sustainable urban drainage systems - Design manual for
England and Wales, Report C522. London: CIRIA.
xxx.
National SuDS Working Group. 2004. Interim Code of Practice for Sustainable
Drainage Systems.
xxxi. Pratt, C., Wilson, S., and Cooper, P.
2002.
Source control using constructed
pervious surfaces; hydraulic, structural and water quality performance issues,
Report C582. London: CIRIA.
xxxii. Water UK 2006. Sewers for Adoption 6th Edition, A design and construction guide
for developers. Water Research Council.
xxxiii. Wavin 2002. Stormwater Management, Design and Installation Manual.
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xxxiv. Wilson, S., Bray, R. and Cooper, P.
2004.
Sustainable Drainage Systems;
hydraulic, structural and water quality advice, Report C609. London: CIRIA.
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11.0
GLOSSARY
Actual Risk
The qualitative assessment of risk taking into account the performance
of any flood defences.
Breach
A failure of flood defences.
Catchment
The area of land contributing to the flow in a watercourse.
Coastal
Pertaining to the coast, or open sea. Coastal flooding can occur when
beaches, cliffs or defences are overtopped, or where coastal erosion
occurs.
cu m/sec
Unit of volume rate of fluid flow; cubic metres per second, sometimes
written cumecs, or cu m/s
DCLG
Department for Communities and Local Government.
DEFRA
Department for Environment, Food and Rural Affairs.
DEM
Digital Elevation Model; a digital representation, or map, of
topography.
Design Flood Event
Flood event of given probability of occurrence which is used for design
purposes or for flood probability mapping.
DPD
Development Plan Document (see under LDF below).
DTM
Digital Terrain Model; a digital representation, or map, of topography,
usually more accurate than a DEM, which may have certain features
filtered out.
EA
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Environment Agency
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Exception Test
In exceptional circumstances, and following the PPS25 Sequential Test,
there may be a valid reason for a development type on a site which is
not compatible with the level of flood risk. The Exception Test must be
applied. There are three stringent requirements, roughly that a) the
development provides wider sustainability benefits that outweigh flood
risk, b) must be on previously developed land (or shown that there are
no reasonable alternative sites on previously developed land, and c), a
site specific FRA must demonstrate that the site will be safe, not
increase flood risk elsewhere, and will reduce overall flood risk if
possible. The exact requirements for the Exception Test are given in
PPS25 Annex D paragraph D9.
FEH
Flood Estimation Handbook (1999); contains the current national
guidance and methodology for rainfall and river flood flow estimation.
The Handbook is accompanied by a CD-ROM of records and other data
relating to all UK catchments and river and rainfall gauging stations.
Flood Resilience
Measures involved in preventing or limiting damage to property which
otherwise might be damaged as a result of flooding. Can be associated
with preventing floodwater entering the property, or the use of designs
and constructions materials which will not be damaged if wet.
Flood Zone
Flood probability zone as defined by PPS25. Delineates areas at risk
of fluvial, tidal or coastal flooding by mapped zones categorised as
“high”, “medium” and “low” risk. The mapped zones are available on
the EA website; the EA mapping takes no account of existing defences.
Risk is defined as probability of occurrence of flooding. PPS25 defines
a further category of “functional floodplain” for areas of land where
water has to flow or be stored in times of flood. Certain restrictions and
requirements covering built development are associated with each of
the four flood zones.
It is important to note that the Environment Agency Flood Maps
(available on the EA website) ignore the presence of defences, whereas
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the Flood Probability Maps within this Strategic Flood Risk
Assessment do take account of existing defences.
Flood Zone 1
Flood Zone 1 (Low Probability) is defined in PPS25 as land assessed as
having a less than 1 in 1,000 annual probability of river or sea flooding
in any year (<0.1%). These areas are on higher ground than the areas
defined by Zones 2, 3a and 3b.
Flood Zone 2
Flood Zone 2 (Medium Probability) is defined in PPS25 as land
assessed as having between a 1 in 100 and 1 in 1,000 annual probability
of river flooding (1% – 0.1%) or between a 1 in 200 and 1 in 1,000
annual probability of sea flooding (0.5% – 0.1%) in any year.
Flood Zone 3a
Flood Zone 3a (High Probability) is defined in PPS25 as land assessed
as having a 1 in 100 or greater annual probability of river flooding
(>1%) or a 1 in 200 or greater annual probability of flooding from the
sea (>0.5%) in any year.
Flood Zone 3b
Flood Zone 3b (Functional Floodplain) is defined in PPS25 as land
where water has to flow or be stored in times of flood. SFRAs are
required to identify this Flood Zone (land which would flood with an
annual probability of 1 in 20 (5%) or greater in any year or is designed
to flood in an extreme (0.1%) flood, or at another probability to be
agreed between the LPA and the Environment Agency, including water
conveyance routes).
Fluvial
Pertaining to “fresh” water, ie not seawater. Fluvial flooding occurs in
inland catchments, where there is no tidal influence.
FRA
Flood Risk Assessment.
Freeboard
The difference between water level and the top of riverbank, sea wall,
defence structure, etc. Often used to refer to the extra height provided
on a flood defence or other structure to allow for uncertainties in water
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level prediction, the effects of future climate change, wave run-up,
future settlement of foundations of the structure, etc.
FSR
Flood Studies Report (1975), used prior to the FEH for the purpose of
estimating flood peaks.
Functional Floodplain
Functional Floodplain is defined in PPS25 as land where water has to
flow or be stored in times of flood. Equivalent to Flood Zone 3b.
SFRAs are required to identify this Flood Zone (land which would
flood with an annual probability of 1 in 20 (5%) or greater in any year
or is designed to flood in an extreme (0.1%) flood, or at another
probability to be agreed between the LPA and the Environment
Agency, including water conveyance routes).
Hydrograph
Diagram showing fluid flow rate varying with time.
IDB
Internal Drainage Board. A statutory public body operating under the
1976 Land Drainage Act to provide drainage service within its
prescribed drainage district. It raises income through the direct rating of
agricultural land and buildings in its drainage district and levies
constituent district councils or unitary authorities an income in
recognition of the benefit arising from its work to all non-agricultural
land and property. An IDB owns and maintains drains, pumping plant
and other assets and is the relevant operating authority for ordinary
watercourses within its district.
INFOWORKS RS
Computer software, which allows the investigation, routing and
mapping of river flows.
ISIS
Computer software used for the one-dimensional analysis of flow in
channels. Used to create a “river model” for the predictive analysis of
water levels etc. in river systems.
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LDF
Local Development Framework. The planning system introduced by
the Planning and Compulsory Purchase Act 2004 requires local
planning authorities to compile an LDF comprising of local
development documents (LDD’s - statutory planning documents), and
supplementary planning documents (providing greater detail). Usually
includes a Core Strategy and Site Specific Allocations. LDF’s are
intended to be guided by Regional Spatial Strategies (RSS’s) compiled
by the appropriate regional body. For Norfolk the RSS is the East of
England Plan produced by the East of England Regional Authority
(EERA).
LiDAR
Light Detection and Ranging; a ground survey which is carried out
from aircraft and used for the creation of a ground model or DTM.
LPA
Local Planning Authority.
m
metre.
Main River
Statutory category assigned to major watercourses (usually large rivers
and streams but can include smaller watercourses of local significance).
Marked on official Main River maps. The EA has permissive powers
covering flood defence allowing works to be carried out by the EA.
m AOD
Metres Above Ordnance Datum; a measure of height, expressed
relative to the particular zero datum used in the UK. All topographical
ground levels, water levels and river bed levels are expressed in this
form. As a very general guide, mean sea level is zero, ie Ordnance
Datum is approximately mean sea level.
Mitigation
The management or reduction of flood risk by special measures or
planned actions.
mm
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millimetre.
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m/sec
metres per second, a unit of velocity, sometimes written m/s.
ODPM
Office of the Deputy Prime Minister.
Ordinary Watercourse
A watercourse not classified as Main River or part of Main River.
Usually the smaller watercourses where the local authority or IDB is
the operating authority. Permissive powers for defence or other works
on ordinary watercourses are usually retained by the local authority or
IDB.
Overtopping
Flood water rises higher than the river bank, flood defence or sea
defence crest level and flows over the defence, flooding the area behind
the defence.
PPG25
Planning Policy Note 25: Development and Flood Risk. Issued in 2001
and now superseded by PPS25.
PPS25
Planning Policy Statement 25: Development and Flood Risk. Issued in
December 2006. Requires flood risk to be taken into account at all
stages of the planning process in order to avoid inappropriate
development in areas at risk and to direct new development away from
areas at highest risk. Requires that flood risk is appraised, managed and
reduced as part of the development planning process. Classifies flood
risk in terms of zones, defines functional floodplain, specifies which
type of development is appropriate to each zone, and classifies the
vulnerability of each type of development. Requires flood resilience
and SuDS in new development. Requires the formulation of regional
flood risk assessments, district wide strategic flood risk assessments
and site specific flood risk assessments with development proposals.
Probability
A statistical concept expressing the likelihood of occurrence of a
particular event within a certain time period or within a sample group
of events. The probability of occurrence of a flood of a certain
magnitude can be expressed either as a) a percentage likelihood of
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occurrence within (typically) one year, or as b) the average time period
until the next flood of the same magnitude. For example, the flood of
certain magnitude, which has a 1% probability of occurrence within
any one year, is likely to occur on average once in a period of 100
years. This flood event has a “1% annual probability of occurrence” or
a “return period of 100 years”. Because this is a statistical concept used
to define a flood of a particular magnitude, it is important to recognise
that this flood can in fact occur tomorrow, or tomorrow and again next
week.
Rapid Inundation Zone
Typically areas behind defences where, due to a defence failure or
overtopping, inundation is likely to occur at a rapid pace. The area
would then be subject to high water velocities and significant water
depths, with significant risk to persons and property. Evacuation plans
are essential in these areas. Computer modelling of overtopping and
breach scenarios is necessary to quantify the risk, the degree of hazard
to groups of persons, and in order to formulate evacuation strategies. A
convenient definition of the rapid inundation zone is the area that
would flood to a depth of 500mm within 30 minutes of a defence
breach or overtopping.
However a rigid definition is not always
appropriate.
Reach (of river)
A certain length of river.
Return Period
The average time before the next occurrence of a flood of the specified
(or greater) magnitude. Usually expressed in years.
RFRA
Regional Flood Risk Assessment.
Run-off
The flow of water from an area caused by rainfall.
Sequential Test
The method of treating development sites in order of preference such
that those sites which are not at risk of flooding are considered most
preferable and those most at risk are considered least preferable or not
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preferable at all.
PPS25 requires this approach from planning
authorities. Defined in PPS25 Annex D.
SFRA
Strategic Flood Risk Assessment.
SMP
Shoreline Management Plan; a coastal defence management document.
The coastline is divided into particular cells to which are assigned
defence policies based upon an assessment of technical, environmental,
and socio-economic objectives.
Defence policies for a particular
stretch of coastline might be “advance the (defence) line, “hold the
line”, “managed retreat” or “no active intervention”, based on an
assessment of all objectives.
sq km
A unit of area equivalent to a square of 1km side length. Sometimes
written km2.
SuDS
Sustainable Drainage Systems. These are surface water drainage
techniques which mimic natural drainage patterns and avoid or
minimise the use of piped systems. Piped systems can, because of
fixed or limited capacity, cause flooding or overflowing during high
rainfall.
Piped systems also speed up and accentuate the flow of
drainage water to rivers, concentrating the flows into higher peaks,
exacerbating flooding in the watercourse. SuDS techniques return the
water to the ground rather than directing it to pipes, and slow down,
attenuate or store any water flows which do have to be directed to
watercourses. SuDS techniques are very useful in the reduction of
flood risk especially in urban areas. PPS25 requires the use of SuDS.
Tidal
Affected by tides. Estuaries and the lower downstream lengths of rivers
are affected by tidal influence from the open sea. Flooding in tidally
influenced lengths of rivers can be caused either by fluvial flood flows
originating in the inland catchment, or from tidal influence causing
water to flow back up the estuary, or from a combination of both.
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TUFLOW
Computer software which allows the two dimensional analysis of flood
flow across floodplains.
Vulnerability Classification PPS25 classifies the different types of development into classes
according to how vulnerable the development type is to flooding, and
to assess whether the development type will be appropriate in a
particular flood risk zone.
WINFAP-FEH
A computer software package associated with the Flood Estimation
Handbook.
1-D
One dimensional.
2-D
Two dimensional.
1 in 20 year return period flood event
A measure of flood magnitude. The flood event that is
of such magnitude that it is likely to occur (return) once
within any 20-year period on average. It is equivalent to
the flood event that has a 5% annual probability, ie it
has a 5% probability of occurring in any one year.
1 in 100 year return period flood event
A measure of flood magnitude. The flood event that is
of such magnitude that it is likely to occur (return) once
within any 100-year period on average. It is equivalent
to the flood event that has a 1% annual probability, ie it
has a 1% probability of occurring in any one year.
1 in 200 year return period flood event
A measure of flood magnitude. The flood event that is
of such magnitude that it is likely to occur (return) once
within any 200-year period on average. It is equivalent
to the flood event that has a 0.5% annual probability, ie
it has a 0.5% probability of occurring in any one year.
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1 in 1,000 year return period flood event
A measure of flood magnitude. The flood event that is
of such magnitude that it is likely to occur (return) once
within any 1,000-year period on average. It is
equivalent to the flood event that has a 0.1% annual
probability, ie it has a 0.1% probability of occurring in
any one year.
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Partnership of Norfolk District Councils SFRA: Subsidiary Report A - North Norfolk District Council
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