MOLD FLOOD ALLEVIATION SCHEME
FLOOD CONSEQUENCE ASSESSMENT
FINAL
MAY 2014
Civil Engineering Solutions Ltd
Victoria Buildings
Flintshire County Council
County Hall
Unicorn Lane
Dolgellau
Gwynedd
LL40 1LN
Mold
Flintshire
CH7 6NF
Tel: 01341 421 911
fax: 01341 423 311
Email: info@ces.eu.com
Web: www.ces.eu.com
Flintshire County Council
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Mold Flood Alleviation Scheme Flood Consequence Assessment
REVISION HISTORY
Revision Reference
Date Issued
Amendments
Issued to
Draft
2/4/14
Flintshire County Council
Final
2/5/14
Flintshire County Council
TERMS OF REFERENCE
This report has been commissioned by Flintshire County Council. Mark Roberts of Civil Engineering
Solutions Ltd has undertaken the work.
Prepared By: …………………………………….
Mark A Roberts B.Eng C.Eng MICE
Director
Approved By: …………………………………….
Mark A Roberts B.Eng C.Eng MICE
Director
CONTACT DETAILS
Prepared by:
Client:
Civil Engineering Solutions Ltd
Victoria Buildings
Flintshire County Council
County Hall
Unicorn Lane
Dolgellau
Gwynedd
LL40 1LN
Mold
Flintshire
CH7 6NF
Tel: 01341 421 911
fax: 01341 423 311
Email: info@ces.eu.com
Web: www.ces.eu.com
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DISCLAIMER
This document has been prepared solely as a Flood Consequence Assessment Report for Flintshire
County Council. Civil Engineering Solutions Ltd accepts no responsibility or liability for any use, which is
made of this document other than by the client for the purposes for which it was originally commissioned
and prepared.
COPYRIGHT
The copyright for this document (including its electronic form) shall remain vested in Civil Engineering
Solutions Ltd, but Flintshire County Council shall have a license to copy/reproduce and use the document
for the purpose for which it was provided. Civil Engineering Solutions Ltd shall not be liable for the use by
any person of this document for any purpose other than for which the same was provided by Civil
Engineering Solutions Ltd. This document shall not be reproduced in whole or in part or relied upon by
third parties for any use whatsoever without the express written authority of Civil Engineering Solutions Ltd
2014.
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EXECUTIVE SUMMARY
The Mold Flood Alleviation Scheme has been designed to reduce flood risk and consequences to 167
mixed residential and commercial properties within Mold.
The scheme has been designed for Flintshire County Council and comprises of 2.7km of new high level
cut off culvert (with some open channel sections) on farmland to the west of Mold. Existing edge of field
ditches will be used where possible to deliver overland flow into the new culvert system and new drainage
ditches will be provided to capture additional flow as required. The cut off channel will discharge into the
River Alyn at Pool House, close to the Rhyd Y Galau (Denbigh Road) bridge.
Attenuation within the town will also be provided which have been designed to remove flood risk to Cae
Bracty and Brook Street. In total 9000m3 of underground storage to be used in times of heavy rainfall will
be sited under Maes Bodlonfa and also Ysgol Glanrafon playing fields. These areas will be reinstated
following construction of the tanks.
A great deal of study and assessment has been undertaken in the design of the Mold FAS. This involves
detailed catchment hydrological and flood modelling for the River Alyn, Mold western Catchments and the
existing drainage infrastructure to the town itself. The FAS design has evolved following the review of the
information contained in the various studies which are available as separate volumes with some extracts
being presented within the Flood Consequence Assessment report.
The design proposed is considered to be the most cost effective, technically sound and environmentally
sustainable scheme achievable. The scheme is not without disruption to landowners, residents and the
wider community but offers substantial benefits in the reduction of flood risk.
As with any flood alleviation scheme design, it is not possible to completely remove flood risk. Some
subjectivity is required when assessing residual flood risk against the overall benefits to the larger
community.
This report therefore recommends that the Mold FAS is considered to satisfy the requirements of TAN15.
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CONTENTS
REVISION HISTORY ................................................................................................................... III
TERMS OF REFERENCE ............................................................................................................ III
CONTACT DETAILS .................................................................................................................... III
DISCLAIMER ............................................................................................................................... IV
COPYRIGHT ................................................................................................................................ IV
EXECUTIVE SUMMARY............................................................................................................... V
CONTENTS .................................................................................................................................. VI
1 INTRODUCTION ...................................................................................................................... 9
2 REQUIREMENTS OF TECHNICAL ADVICE NOTE 15 ........................................................ 11
3 FLOOD ALLEVIATION SCHEME PROPOSALS .................................................................. 13
4 CONSEQUENCES OF THE SCHEME DESIGN ON FLOOD RISK ..................................... 15
5 CONCLUSION ....................................................................................................................... 29
6 RECOMMENDATIONS .......................................................................................................... 30
7 APPENDICES ........................................................................................................................ 31
LIST OF TABLES
Table 1: Property Identification ................................................................................................ 21
Table 2: Existing and Proposed Property Risk Comparison Alyn Q100cc (m) ................... 23
Table 3: Do Nothing-Attenuate Scenario Depth Band Property Count ................................ 27
LIST OF FIGURES
Figure 1: Welsh Government Development Advice Map for Mold as of 31st March 2014 . 12
Figure 2: High level cut off culvert route ................................................................................ 13
Figure 3: Mold main surface water infrastructure (existing). ................................................ 14
Figure 4: River Alyn model extents ......................................................................................... 16
Figure 5: River Alyn 100yCC hydrographs (19 hour storm duration) .................................. 17
Figure 6: MFAS 100yCC hydrographs (6 hour storm duration) ............................................ 18
Figure 7: Alyn_Q100cc Existing scenario maximum depths ................................................ 19
Figure 8: Alyn_Q100cc Proposed scenario maximum depths ............................................. 20
Figure 9: Alyn_Q100cc maximum depth comparison (Proposed - Existing) ...................... 21
Figure 10: Property Locations ................................................................................................. 22
Figure 11: Culvert flows at Cae Bracty 100y CC event .......................................................... 24
Figure 12: Brook Street & Gas Lane pipe capacities ............................................................. 26
Figure 13: xx52 Hydrographs ................................................................................................... 26
Figure 14 Do Nothing Q100cc Maximum depth contours ..................................................... 27
Figure 15: Q100cc existing maximum flood depths .............................................................. 28
Figure 16: Q100cc Attenuate scenario maximum flood depths ........................................... 29
APPENDICES
APPENDIX A:
APPENDIX B:
APPENDIX C:
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MOLD FAS CATCHMENT HYDROLOGY
MOLD FAS GENERAL ARRANGEMENTS
RIVER ALYN FLOOD DEPTH & EXTENTS
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1 INTRODUCTION
Flintshire County Council commissioned Ymgynghoriaeth Gwynedd Consultancy (YGC) I March 2011 to
design a flood alleviation scheme for the county town of Mold in Flintshire. The flood scheme was deemed
necessary following frequent flooding to large areas of the town including significant flooding to property in
November 2000 and also the receipt of a Project Appraisal Report dated 2005. Civil Engineering Solutions
Ltd (CES Ltd) were appointed alongside YGC as part of the design team to develop the technical design
solutions.
This Flood Consequence Assessment seeks to support the planning process as required by Technical
Advice Note 15 (TAN15). The report identifies the main mechanisms for flooding from catchment
generated flows to the west of Mold and direct rainfall on the town itself.
1.1
HISTORICAL FLOODING
A Project Appraisal Report (PAR) was prepared by consultants in 2005 following Pre-Feasibility studies
undertaken in-house by Flintshire County Council in 2002 for Queens Park and Pwll Glas areas of Mold.
The following is an extract from the PAR which sumarises the causes and extent of experienced flooding.
“In general terms watercourses from catchments to the southwest of Mold pass through the urban areas
of the town in both open channel and pipes or culverts many of which, for a variety of reasons, cannot
cope with serious flooding events
Flooding has been experienced to both residential and commercial property from water overflowing
from two separate watercourses within the Pwll Glas and Queens Park areas of Mold.
Flooding, most recently occurred during the October and November 2000 storms, more specifically on
31 October and 06 November 2000. In addition to the damage caused to a significant number of
residential and commercial property and interests, widespread disruption to areas of Mold was
experienced. Emergency services were called upon to assist Flintshire County Council staff during the
event and clean-up operations. This is not an isolated incident as similar flooding apparently occurred
in 1971 and several properties have also flooded in the years in between [1991 and 1998 have been
mentioned by residents].
Generally the problems manifest themselves when water, shed off the four steep catchments to the west
of Mold arrives at the flatter westernmost limits of the town. Water flows through the town via a
combination of culverts and open watercourses/ditches before discharging into the River Alyn east of
Mold.
From an analysis of available LIDAR survey data and from site inspections, it would appear that
floodwater bifurcates from the main catchment, under storm conditions, approximately 850 metres
west of the Queens Park area of the town as it passes under a highway below Maesgarmon Farm. From
here a proportion of the water continues into the watercourse leading to Queens Park with the
remainder veering north and eastwards across the fields leading to the rear of Fir Grove.
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Water flowing down the established watercourse towards Queens Park is joined by ditches carrying
flows from the Gwernymynydd catchment to the southwest. The ditches combine approximately 250
metres west of Queens Park where it enters a canalised watercourse crossing Flintshire County Council
land towards the town. Under dry weather conditions the watercourse flows approximately 100 –
150mm deep but breaks its banks in more severe events.
As this watercourse enters Queen Park it changes from open ditch to culverts as it passes under the
access roads to various residential areas before being culverted adjacent to the junction of Queens Park
Road and Clayton Road near to Bryn Glas School.
The diverted water flowing north and westwards from the crossroads below Maesgarmon Farm travels
along the access track before overflowing across the fields to the Pwll Glas area where it is collected in a
disused access track before decanting into Fir Grove and Gwernaffield road. Under dry weather
conditions these fields do not have a watercourse crossing them.
Areas in and adjacent to New Street, including Cae Bracty flood as a result of floodwater from the above
areas moving further down the catchment as it has not been able to re-enter any of the watercourses.
Other low-lying areas such as Maes y March, also flood for similar reasons.”1
The flooding event of October/November 2000 resulted in internal flooding to some forty properties to
varying depth from the direct rainfall and catchment derived flows. The PAR considered the magnitude of
this storm and suggested that it was of a once in one hundred year return period. The report also went on
to discuss the likely consequences of a “do nothing” option as set out by the then DEFRA/WAG guidance
indicating that this level of flooding would likely be considered as a one in fifty year rarity. More extensive
flooding was assessed to result for the then 200 year design standard of service to an anticipated 91
commercial and residential properties along with associated disruption to local transport and infrastructure.
1.2
OPTIONS FOR FLOOD ALLEVIATION
The PAR identified a number of strategic options for reducing flood risk to Pwll Glas and Queens Park
areas specifically from flows generated by the western catchments. These options included, inter alia:
1)
2)
3)
a western cut off channel on farmland to intercept flows and deliver them into the River Alyn
above the town;
a new watercourse to the east of the town to discharge into the River Alyn downstream of Mold;
and
upgrades and improvements to the existing surface water culverts and watercourses through
Mold.
Strategic options 2 and 3 above were discounted in the original PAR as being expensive and technically
difficult. As a result the strategic option to intercept catchment flows to the west of Mold was taken
forward. Specific options were then considered for the western cut off channel with some six variations on
route and composition being considered within the PAR. The preferred option was a high level gravity
route which was originally considered in open lined channel of varying capacity depending on the
combined contributing catchment flows. This preferred option discharged the intercepted flows into the
River Alyn upstream of the town and the areas subjected to flooding from the river in 2000.
Extensive flooding experienced in 2000 from the River Alyn which flooded through the Bro Alyn and Lead
Mills areas of Mold. Following this event, the then Environment Agency Wales (EAW), undertook
1
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Mold – Project Appraisal Report June 2005 by Martin Wright Associates pg 6
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improvements to the River Alyn (circa 2005) in order to alleviate extensive flooding to the lower areas of
Mold such as Bro Alyn and Lead Mills. At this time, EAW were informed of the likely form of the Flintshire
Flood Alleviation scheme to be completed later and to make allowances within their flood alleviation works
to receive anticipated flows from the FCC scheme.
“Discussions have been held with Halcrow Consultants in Wilmslow [who are retained by the
Environment Agency] in order to assess the effect on the River Alyn. Halcrow have been provided with
all the relevant information from the catchments involved to the southwest of Mold and have concluded
that there will be little affect on the River Alyn from the input of some 4.5 cumecs as the maximum flows
are some 23 hours apart. There is a small residual effect which marginally adds to the River Alyn flood
level in Mold. The Environment Agency have verbally confirmed that, in principle, there is unlikely to be
a problem gaining consent to carry out work to divert this quantity of water into the River Alyn above
Mold.”2
At commencement of detailed design in March 2011 for the high level western cut off channel, it was
considered appropriate to review the original PAR document dated 2005 and to establish if the
assumptions and hence conclusions for the choice of the preferred option were still valid.
CES undertook this review in May/June 2011 and concluded that the original PAR preferred option “3”
remained the most cost effective solution. Although costs had increased significantly, the scheme would
still pass economic criteria for benefits set against construction cost. This review also identified that there
were deficiencies within the existing drainage infrastructure through the town which would result in flooding
to property post cut off channel construction, without improvements to the existing culvert systems.
Subsequent modelling of drainage infrastructure and design work has identified the use of sub surface
attenuation areas within public open spaces within Mold to store floodwater during times of heavy rainfall
and to release it in a controlled manner to the existing culvert systems. This option requires the cut off
channel to divert floodwater away from the Pwll Glas, Queens Park and Ruthin Road areas of Mold before
being effective.
A strategic scheme is therefore proposed to address flooding for Mold following the EAW (now Natural
Resources Wales) improvements to the River Alyn.
2 REQUIREMENTS OF TECHNICAL ADVICE NOTE 15
Planning Policy Wales Technical Advice Note 15 (TAN15) relates to Development and Flood Risk. The
guidance was issued by Welsh Assembly Government (now Welsh Government) in July 2004 and
provides a framework for planning authorities within Wales to draw upon when assessing development
and sustainability with respect to flooding.
TAN15 is not specifically well suited to applications for the development of Flood Alleviation Schemes, as
its intended purpose is to prevent inappropriate development within flood risk areas and to reduce legacy
issues for inappropriate development. The Mold Flood Alleviation Scheme has been designed to reduce
flood risk and consequence resulting from historical development within Mold and as such may be
considered fall into a flood legacy remediation category.
2
Mold – Project Appraisal Report June 2005 by Martin Wright Associates pg 16
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The proposed scheme also introduces flow to the River Alyn above the town and there are concerns within
the community that the River Alyn has flooded prior to implementation of the EAW scheme in 2005.
Naturally these are valid concerns which relate to flood risk/consequence of implementing such a scheme.
It is entirely proper therefore that these issues are identified and taken account of within the scheme
design and this Flood Consequence Assessment Report.
Furthermore Flintshire County Council needs to demonstrate that the consequences or reduction of
flooding resulting from the development of such a scheme are acceptable and provide such advice to the
relevant planning committee for consideration.
2.1
DEVELOPMENT ADVICE MAPPING
Development Advice Maps are maintained by Welsh Government and identify areas of flood risk within
Wales for use when considering Flood Consequence Assessments.
Figure 1: Welsh Government Development Advice Map for Mold as of 31st March 2014
Figure 1 above identifies the flood zone classification areas for Mold. Flood zones are categorised in
Wales as follows:
Zone C1 (Green): Served by significant infrastructure, including flood defences;
Zone C2 (Blue): without significant flood defence infrastructure;
Zone B (Yellow): Areas known to have been flooded in the past; and
Cone A (White): Considered to be at little or no risk of fluvial or coastal flooding.
Flood zone areas for Mold are identified and being mainly C1 and B for the river valley upstream of above
A494 at Pont Pentre and predominantly C2 downstream.
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There is a band of varying flood zone classification running through the central urban/commercial areas of
Mold, which could be largely categorised as C2 (blue) running from the agricultural area between Hendy
Road and Gwernaffield Road in a west north west to east south easterly direction. This area has pockets
of zone C1 areas, notably at Queens Park/Clayton Road; New Street and Cae Bracty. There are fringe
areas of zone B flanking the northern and southern extremes of this band. The main reason for this flood
zone classification is considered to be water shedding off the western catchments leading towards
Gwernaffield.
3 FLOOD ALLEVIATION SCHEME PROPOSALS
3.1
DESIGN CONCEPT
The Mold Food Alleviation Scheme seeks to reduce flows from the western catchments by intercepting
them into a new high level cut off culvert, with associated feeder ditches, delivering them to the River Alyn
via the “Pool House” culvert under the A541 (Denbigh Road). The consequence of adding this water to
the river Alyn at this point has been considered as part of the River Alyn Modelling Study (March 2014)
which has been passed to the Natural Resources Wales for review.
The high level cut off culvert has been designed to operate under gravity without pumping or mechanical
sluices. Figure 2 below identifies the general culvert route along with catchments and existing culvert
infrastructure through Mold.
Figure 2: High level cut off culvert route
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The culvert route is identified by the green line extending from Ruthin Road to the River Alyn. The main
contributing catchments to flooding in Mold are referenced from south to north as being Ruthin Road,
Queens Park and Gwernaffield. The “Factory Pond” catchment is considered to fall towards the River Alyn
and is unlikely to contribute to flows within the proposed culvert. Similarly there is a small catchment area
unaccounted for on the above figure which is located to the north of the Gwernaffield road. This land
generally falls from south to north and as such contributes to flows entering the River Alyn. A analysis of
catchment hydrology has been prepared and is presented in Appendix A
The 2005 PAR document and subsequent PAR review in 2011 identified that an open channel design
would best suit the requirements of the scheme. This would in effect provide a physical barrier to overland
flow and ensure that all floodwater is intercepted from the upper catchment areas. Landowners have
objected to this approach claiming that it will result in large areas of land being taken out of production and
in some cases may make farming businesses unviable. For this reason, FCC has elected to culvert
almost the entire route. To facilitate capture of channelled and overland flow, a series of interceptor
structures and edge of field ditches has been designed. The edge of field ditches are not shown on the
above figure for clarity reasons but are identified on a copy of the General Arrangement plan reference
CES205_GEN/17 which is provided in Appendix B. Responsibility for inspection and future maintenance of
these ditches and structures will be undertaken by FCC.
The proposed culvert route has been designed to intercept as much of the catchments as possible whilst
maintaining a gravity solution. This will ensure that the interceptor culvert will be operational without
reliance on power supply or manual intervention. As such the route chosen will be incapable of capturing
all of the flow generated by the western catchments and rain falling on those parts of the catchment to the
east of the culvert route will continue to discharge through the existing surface water infrastructure through
the town. The existing culverts and open ditch network through Mold are generally identified on the Figure
3 as blue lines.
Figure 3: Mold main surface water infrastructure (existing).
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As is shown in the above figure, surface water is generally conveyed through Mold flowing from west to
east and discharging to the River Alyn downstream of the town after skirting around the sewerage works.
The Mold Flood Alleviation Scheme design has considered the capacity of the existing ditches and culverts
as they pass through the town and a model has been prepared to assess the current flood risk along with
the flood risk following construction of the proposed high level cut off culvert. It was concluded that even
with the reduced catchment flows, flooding would result to specific areas of the town, notably Cae Bracty,
Brook Street and Gas Lane. Indeed Cae Bracty has flooded twice in the past 18 months and has an
estimated existing standard of service of four years against flooding.
In order to assess the effectiveness of the works required within the town a surface water network model
has been prepared. The “Mold Surface Water Modelling Study” (February 2014) details the modelling
approach and results and is available as a separate volume.
The diversion of the bulk of the catchment flows to the River Alyn has been found to improve flood risk for
large areas of Mold and make attenuation schemes viable to prevent flooding to Cae Bracty, Brook Street
and Gas Lane. In total some 9000m 3 of mixed online and offline underground storage is proposed. The
sites are located under the playing fields at Maes Bodlonfa and Ysgol Glanrafon. General Arrangement
plan reference CES205_GEN/14 (B) and CES205_GEN15 (B) is provided in Appendix B detailing the
attenuation proposals.
Water will be diverted into each of the underground storage areas by constricting flow in strategic culverts.
The storage areas will therefore fill under gravity without need for mechanical sluices or pumping.
The Maes Bodlanfa Storage area will then be pumped out at a maximum rate of 100 l/s back into the
surface water culvert using a small pumping station, designed to meet “Sewers for Adoption 7” standards,
downstream of the constriction to regulate flows in the existing pipe network.
The Ysgol Glanrafon paying field storage is divided into two areas; the upper area receives flow from the
Ruthin Road surface water culvert network and discharges via a Hydrobrake with a maximum design flow
rate of 200l/s. The second storage area receives flow from Cae Bracty via a new culvert and gulley
arrangement and is located in the lower area of the playing field to achieve a gravity inflow. This second
storage area will be drawn down by a small surface water pumping station to discharge a maximum 100l/s
back into the surface water culvert in Brook Street.
The attenuation options only become viable if the majority of the catchment flow is diverted to the River
Alyn.
4 CONSEQUENCES OF THE SCHEME DESIGN ON FLOOD RISK
4.1
HIGH LEVEL CUT OFF CULVERT
The high level cut off channel has been designed to reduce surface water flows entering the existing
surface water channels and culverts running through Mold. The benefit of this design is to significantly
reduce flood risk to large areas of the town although it will not be sufficient on its own to remove flood risk
from Cae Bracty, Brook Street and Gas Lane.
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In order to asses consequences of flood risk to properties sited within the floodplain for the receiving River
Alyn, it was considered necessary to construct a numerical computer model of river processes and flood
risk. At the commencement of the design process, the then Environment Agency Wales had made a
number of their own models of the River Alyn available to assist in this assessment. A review of these
models highlighted that the models were compiled from a number of datasets of varying quality and age.
The models themselves were considered to be acceptable for their original purpose but could not be
adequately relied upon to provide an objective assessment for such a significant scheme.
As a result, FCC instructed that a new model be prepared for the River Alyn to adequately cover the river
processes and to assess implications on flood risk resulting from the proposed addition of storm flows to
the river post construction of the high level cut off culvert.
The new model comprised of 4.5km of the river Alyn and 2.7km of the Black Brook which were resurveyed to inform the model. The model was prepared as an ISIS/TUFLOW model with the river
channels being represented in ISIS and the floodplain areas modelled in TUFLOW. This is a standard
industry approach for flood modelling in the UK and has been used in support of flood schemes for a
number of years.
Figure 4: River Alyn model extents
A review of the river hydrology was undertaken working in partnership with EAW hydrology staff. EAW
(now NRW) provided river flow and rain gauge information in order to calibrate the model to improve
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reliability and confidence in model outputs. Full details of the model build process, assumptions,
calculations, calibration and outputs are provided within the separate modelling study report 3.
The study report considered the main catchment response times and flow rates for the River Alyn
alongside those of the Mold western Catchments. The catchment response time was of particular interest
as a 100 year plus climate change event for the River Alyn main catchment was significantly longer than
the same rarity event for the Mold western catchments being 19 hours and 6 hours respectively. This
effectively means that for a 19 hour storm would provide the highest flow rate for the River Alyn whilst a 6
hour storm would generate the highest flows for the high level cut off culvert.
The following table and figures compare peak flow rates in cubic metres per second abstracted from the
Alyn modelling study with respect to the 19 and 6 hour storms.
River Alyn
Black Brook (BB)
MFAS
Alyn_100 (19 hour)
48.21
1.51
4.54
Alyn_100cc (19 hour)
57.85
1.83
5.38
MFAS_100 (6 hour)
40.53
1.87
5.54
MFAS_100cc (6 Hour)
48.64
2.25
6.16
Apr_2012 (actual)
18.12
0.35
3.38
Nov_2000 (actual)
43.30
0.30
3.25
60
Alyn
50
BB
MFAS
Flow (m3/s)
40
30
20
10
0
0
5
10
15
20
25
30
35
40
Time (hrs)
Figure 5: River Alyn 100yCC hydrographs (19 hour storm duration)
The 100yCC storm for the 19 hour Alyn catchment is estimated to deliver a peak flow rate of 57.85 cubic
meters per second. For the same rainfall event, the Mold Flood Alleviation Scheme (MFAS) high level cut
3
River Alyn Modelling Study (Draft Report) dated March 2014 by Civil Engineering Solutions Ltd.
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off culvert would contribute an estimated maximum 5.38 cubic metres per second. The timing of the peaks
however, are such that the MFAS flow is at its maximum at approximately 13 hours, whereas the river
flows peak at around 18 hours. The resultant effect is therefore that flow in the MFAS culvert will be
declining when the River Alyn flow peaks although flows of circa 3 cubic metres per second are still likely.
50
Alyn
BB
40
Flow (m3/s)
MFAS
30
20
10
0
0
5
10
15
20
25
Time (hrs)
Figure 6: MFAS 100yCC hydrographs (6 hour storm duration)
The critical storm for the Mold western catchments of 6 hours duration will result in higher flows being
discharged into the river as can be seen in Figure 6 above. The river flows however are not expected to
reach the same magnitude for the shorter duration event and the combined flows when the Alyn reaches
its peak flow will be less than that of the critical 19 hour storm event for the River Alyn alone as detailed in
Figure 5.
The flood modelling study has considered a number of combinations of flood events and timings. Outputs
of the 100 year plus climate change flood depths and extents for the 19 hour Alyn critical storm are
presented below and also in smaller scale in Appendix C. These are provided for the “existing” and
“proposed” cases.
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Figure 7: Alyn_Q100cc Existing scenario maximum depths
The above figure identifies the flood extent and general depth reported by the modelling for the anticipated
1 in 100 year river flows with allowances of 20% increase in flow rates to take account of climate change.
It is evident that three bridge structures attenuate flow by restricting conveyance in the river channel.
These are located at Rhyd y Golau, Leadmills and the A541/A494 roundabout and are evidenced by the
increase in predicted flood depths immediately upstream of each structure.
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Figure 8: Alyn_Q100cc Proposed scenario maximum depths
The following Figure 9 shows the difference in modelled maximum flood depths for the proposed and
existing scenarios during the 100yCC design event. This is useful in assessing the consequences of
introducing flow from the proposed MFAS culvert into the River Alyn.
The comparison depth grid suggests that there is a general increase in flood depth across the whole area.
This is to be expected given that flow rates in the river will increase during the more significant storm
events. The increase in depth is most notable upstream of the A541/A494 suggesting that there is some
spare capacity for the “existing” case at Rhyd y Golau and Leadmills bridges although it is noted that the
depth of flooding to the floodplain areas increases in these areas generally by between 10mm and
125mm.
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Figure 9: Alyn_Q100cc maximum depth comparison (Proposed - Existing)
Results from the modelling work undertaken identified that predicted flood levels in the vicinity of a small
number of properties will increase marginally for the 100 year plus climate change event critical River Alyn
flows. These properties are located in and around the Leadmills and the end of Wood Green. The
following table and figure identify these properties.
Table 1: Property Identification
# ID
Ground Level
(m AOD)
Easting
Northing
Description
1
100.17
324808
363631
2
101.09
324794
363691
3
100.99
324680
363857
At the end of Wood Green
44
101.85
324736
363781
Bridge Cottages on Chester
Rd
Between A541 & A494
4
The Mastermap shows 2 semi-detached at this location, whereas Google Streetview shows only 1 semidetached
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Ground Level
# ID
(m AOD)
Easting
Civil Engineering Solutions Ltd
Northing
5
104.80
324188
364589
6
105.02
324164
364595
7
104.55
324094
364485
8
104.93
324098
364477
9
104.68
324074
364497
10
105.39
324064
364501
11
105.14
324028
364557
12
105.15
324026
364559
13
105.16
324006
364571
11, 12 & 13
Description
Bryn Awelon
Downstream of Leadmills
Alyn right bank
Leadmills Bridge Inn
Bro Alyn
5&6
7&8
9 & 10
3
4
1&2
Figure 10: Property Locations
Table 2 below summarises the anticipated increase in flood depth reported by the modelling. Generally
the predicted impacts, where they exist at all, are expected to be minor and only a couple of centimetres.
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The two properties most impacted by the scheme are numbers 3 and 4 (Bridge Cottages and the property
at the end of Wood Green). This is consistent with the predicted flood depth comparison map shown
earlier in this section (Figure 9) which predicted the largest changes to occur at Old Pentre Bridge, where
these properties are located. The biggest predicted impacts are to these two properties, for the
Alyn_Q100cc scenario where an additional 20 cm of water is predicted on an already substantial depth of
flood water. It is also relevant to note that both of these properties are located adjacent to an abandoned
mill race. This mill race has been included within the model DTM and may also influence the LiDAR. For
instance, Bridge Cottages appear to be at the same height as the road which remains flood free and is
approximately a metre higher than the ground level at the reported location as, in the LiDAR the ground
slopes steeply towards the millrace. Similarly for the property at the end of Wood Green the difference in
ground level between the reported point and the adjacent road is over 2m, which may or may not be
representative of the floor levels of this property.
Table 2: Existing and Proposed Property Risk Comparison Alyn Q100cc (m)
4.2
Property
ID #
Alyn Q100cc
Existing
Proposed
Difference
1
0.14
0.24
0.10
2
0.10
0.18
0.07
3
0.82
1.08
0.26
4
0.26
0.50
0.25
5
0.00
0.01
0.01
6
0.01
0.02
0.02
7
0.11
0.14
0.02
8
0.00
0.00
0.00
9
0.22
0.27
0.05
10
0.01
0.02
0.00
11
0.00
0.01
0.01
12
0.00
0.01
0.01
13
0.00
0.00
0.00
MOLD UNDERGROUND ATTENUATION
The anticipated reduction in flows passing through Mold as a result of the proposed high level cut off
culvert provides significant benefit to flood risk within the town. The cut off channel on its own however will
not provide protection to Cae Bracty, Brook Street and Gas Lane areas. This is primarily due to the lack of
capacity in the existing culverts serving those areas to adequately convey the storm water.
In order to assess the scale of residual flooding and to consider viable design options for dealing with
floodwater, a flood model was developed using a combination of LiDAR information, survey data, CCTV
information along with foul and surface water sewer information provided by Dwr Cymru Welsh Water
(DCWW). The associated modelling report5 is available as a separate volume and details the modelling
approach and assessment of storage options to alleviate flooding.
5
Mold Surface Water Modelling Study (Draft Report) February 2014 by Civil Engineering Solutions Ltd
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Figure 11: Culvert flows at Cae Bracty 100y CC event
The above graph shows the predicted flow rates and volume of water arriving at Cae Bracty during the 100
year plus climate change design event at various stages of the high level cut off culvert construction. The
red line shows that estimated maximum flow rates for the existing situation are of the order of 2.7m3/s.
The purple line estimates flow rates is when the cut off culvert is constructed to a point whereby the
watercourse passing through Queens Park is diverted to the River Alyn. The green line shows the residual
flows post completion of the cut off culvert. It is interesting that the Ruthin Road catchment delivers water
to the site up to two hours before the more significant Queens Park catchment.
Cae Bracty is a development consisting of first and second floor housing association flats constructed circa
1987 and is located within a hollow or depression with higher ground on all sides. The development is
particularly vulnerable to flooding as it is also sited alongside the confluence of culverted watercourses
serving Ruthin Road, Queens Park and Fir Grove catchments. Cae Bracty also receives surface water
shed off the town Centre via New Street and New Street Car Park. The current assessed standard of
protection against flooding for this development is once in four years based on rainfall data provided by
NRW following the April 2012 flooding event, which resulted in flooding to a number of the ground floor
residences.
Flood water arrives at Cae Bracty and overwhelms the existing surface water infrastructure. A dedicated
public surface water pumping station at Cae Bracty is unable to cope with the volume of water arriving at
the site and the site fills up to a maximum depth of approximately 800mm. At this level the water decants
down Brook Street causing flooding to adjacent properties and on towards Gas Lane.
In order to alleviate this flooding, an attenuation scheme has been designed. The proposed attenuation
consists of three underground storage systems in two sites which will provide a total 9000m3 of total
storage. The attenuation sites are located on parkland and playing fields at Maes Bodlonfa and Ysgol
Glanrafon.
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Modular storage tanks are proposed which comprise of a single tank at Maes Bodlonfa of 3072m3; this
tank is proposed to be constructed under the playing fields currently used as football amenity areas. The
Maes Bodlonfa tank has been designed as offline storage and is generally described on drawing reference
CES205_GEN/14 (B) provided in Appendix B. The proposal for this attenuation site is to place
constrictions within the existing surface water culverts, which serve the Fir Grove and Queens Park areas
of Mold, passing along the perimeter of the amenity area. Dry weather flow will be permitted to be
conveyed in the existing surface water culvert with any additional water decanting into the storage. A
small pumping station has been specified to evacuate the attenuation back into the existing surface water
culvert downstream of the constrictions. A maximum pump rate has been specified as 100l/s which will
enable the storage volume to be emptied within 24 hours of the storm back into the surface water culvert
system. This pump rate has been assessed alongside the proposed Ysgol Glanrafon attenuation
discharge and capacity of the existing surface water culverts running in Brook Street.
The proposed storage and attenuation at Ysgol Glanrafon playing field has been designed as two separate
tanks.
The general arrangement of both storage areas is provided on drawing reference
CES205_GEN/15 (B) in Appendix B.
The first tank has been designed as offline storage and attenuation to the culvert conveying surface water
from Ruthin Road. The existing 800mm diameter surface water culvert is located within the northern
boundary of the school field and flows in a north easterly direction towards Cae Bracty. The proposal is to
place a constriction on this culvert to permit dry weather flows to flow in the existing culvert unimpeded.
Surplus surface water will decant into the adjacent storage tank which has been designed as providing
3200m3 of offline storage. This tank will be evacuated using a Hydrobrake with a discharge capacity of
200l/s back into the existing surface water culvert located within Brook Street via a new section of culvert
under the school field. A high level overflow will also be provided to link the Ruthin Road storage area with
the Cae Bracty storage area which is proposed to be sited lower in the school field.
The second tank provides online storage for surface water arriving at Cae Bracty and has a design
capacity of 2856m3. The tank is sited in the lower areas of the school field adjacent to the junction of
Brook Street and Bryn Coch Lane. A new culvert and highway gully network will be constructed in Cae
Bracty which will provide a positive drainage pathway into the storage. A small surface water pumping
station will be constructed alongside Bryn Coch Lane which will be used to draw down the storage and lift
the water back into the surface water culvert in Brook Street. The pumping station maximum discharge
rate of 100l/s has been considered alongside the assessed capacity of the Brook Street culvert and the
discharge received from Maes Bodlonfa and Ruthin Road storage areas.
The proposed design of the three storage and attenuation areas will significantly reduce flood risk to
property in Cae Bracty, Brook Street and Gas Lane. The attenuation areas and infrastructure will be
maintained by FCC.
The following information has been taken from The Mold Surface Water Modelling Study report (February
2014) which details the modelling undertaken in assessing the design of the attenuation aspects of the
scheme. Of particular concern is the rate at which attenuated floodwater can be reintroduced to the
existing surface water system. To this end, an assessment of the capacities of the surface water culvert
capacity in the receiving system in Brook Street was undertaken.
Figure 12 below identifies the assessed capacities of each of the surface water culverts in Brook Street
leading to Gas Lane. Pipe ID xx52 is located immediately downstream of our proposed discharge point for
the Ysgol Glanrafon attenuation areas.
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1.6
1.4
Flow (m3/s)
1.2
1
0.8
0.6
0.4
0.2
xx70
xx69
xx68
xx67
xx66
xx65
xx64
xx63
xx62
xx61
xx60
xx59
xx58
xx57
xx56
xx55
xx54
xx53
xx52
xx51
xx50
xx25
0
Pipe ID
Figure 12: Brook Street & Gas Lane pipe capacities
Figure 13 below shows modelling results for this culvert for the existing and attenuate scenarios. The line
referenced as “residual“ has been included to assess the maximum base flow to be permitted to pass the
Maes Bodlonfa and Ruthin Road storage areas and for sizing of the acceptable maximum discharge rates
for each attenuation tank.
0.7
0.6
Flow (m3/s)
0.5
0.4
0.3
Exist
0.2
Attenuate
Residual
0.1
0
0
2
4
6
8
10
12
14
Time (hrs)
Figure 13: xx52 Hydrographs
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The modelling undertaken to assess existing infrastructure and proposed scheme design has also been
used to make an assessment of the numbers of properties considered to be at risk of flooding in Mold
under “do nothing” conditions and contrast them with the post flood alleviation scheme (including
attenuation) situation.
The original PAR suggested that 91 commercial and residential properties were at risk of flooding. Our
new assessment suggests that some 360 properties have some degree of flood risk. Of these 167
properties will be completely taken out of flood risk following the implementation of the scheme and many
others will have flood risk reduced.
Figure 14 Do Nothing Q100cc Maximum depth contours
The above figure identifies the extent and depth of anticipated flooding for the “Do Nothing” scenario. The
plan has been contoured to distinguish between the relevant depth bandings contained within TAN15.

Red – Considered as extreme flooding with depth >0.6m;

Orange – Considered as significant flooding with depth 0.15m to 0.6m; and

Green – Considered as minor flooding (presuming a 150mm threshold height) with flood depth
<0.15m.
The following table provides results in terms of property numbers using this analysis. This information was
requested by FCC and WG to consider the scale of benefits now that better tools exist to make a more
objective assessment.
Table 3: Do Nothing-Attenuate Scenario Depth Band Property Count
Green (0.05 > d < 0.15)
Orange (0.15 > d < 0.6)
Red (0.6 > d)
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Attenuate
200
152
8
127
66
0
Final
Difference
73
86
8
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Many of the properties considered to remain at flood risk following construction of the FAS are located in
areas of the town remote form the focus of the scheme. The modelling does not include for local drainage
infrastructure which is likely to reduce flood levels further.
Figure 15: Q100cc existing maximum flood depths
Figures 15 and 16 show modelled flood depths for the 100 year plus climate change event for the urban
areas of Mold under existing and post FAS construction respectively.
Some flood risk can be seen to remain following construction of the flood alleviation scheme however the
extent and depth of residual flooding is significantly reduced.
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Figure 16: Q100cc Attenuate scenario maximum flood depths
5 CONCLUSION
The Mold FAS as proposed removes flood risk to 167 residential and commercial properties. This will be
achieved by reducing the influence of catchment generated flow on the existing drainage infrastructure
through Mold. Attenuation provision of 9000m3 will provide benefits to Cae Bracty, Brook Street and Gas
Lane which is in keeping with the best principals of SuDS.
Diversion of catchment flow into the River Alyn above the town will marginally increase flood risk to a small
number of properties in and around the Lead Mills Bridge area. These areas have previously been
subjected to significant flooding, notably during the October/November 2000 flood events, and have since
been provided with protection in the form of the Environment Agency Wales (now NRW) flood alleviation
scheme for the River Alyn which was constructed in 2005.
Flood depths at these sites have been generated using LiDAR data and as such FFL’s have not been
specifically accounted for within the modelling process. Of the property locations identified, three were
considered to be flood free for the existing 100 year plus climate change event. Maximum predicted flood
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depth for the 100 year plus climate change with the addition of flows from the proposed high level cut off
culvert were 0.01m. The scale of these predicted flood depths may be considered to be beyond modelling
accuracy and tolerance. In any event such depths are unlikely to result in internal flooding to property.
6 RECOMMENDATIONS
A great deal of study and assessment has been undertaken in the design of the Mold FAS. This involves
detailed catchment hydrological and flood modelling for the River Alyn, Mold western Catchments and the
existing drainage infrastructure to the town itself. The FAS design has evolved following the review of the
information contained in the various studies which are available as separate volumes with some extracts
being presented within the Flood Consequence Assessment report.
The design proposed is considered to be the most cost effective, technically sound and environmentally
sustainable scheme achievable. The scheme is not without disruption to landowners, residents and the
wider community but offers substantial benefits in the reduction of flood risk.
As with any flood alleviation scheme design, it is not possible to completely remove flood risk. Some
subjectivity is required when assessing residual flood risk against the overall benefits to the larger
community.
This report therefore recommends that the Mold FAS is considered to satisfy the requirements of TAN15.
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7 APPENDICES
APPENDIX A:
APPENDIX B:
APPENDIX C:
MOLD FAS CATCHMENT HYDROLOGY
MOLD FAS GENERAL ARRANGEMENTS
RIVER ALYN FLOOD DEPTH & EXTENTS
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Appendix A:
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MOLD FAS CATCHMENT HYDROLOGY
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Appendix B:
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MOLD FAS GENERAL ARRANGEMENTS
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Appendix C:
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RIVER ALYN FLOOD DEPTH & EXTENTS
Final