Environment Agency
Boston Waterways Link - Retaining Wall and
Scour Protection
Flood Risk Assessment
February 2008
Halcrow Group Limited
Environment Agency
Boston Waterways Link - Retaining Wall and
Scour Protection
Flood Risk Assessment
February 2008
Halcrow Group Limited
Halcrow Group Limited
Vineyard House 44 Brook Green London W6 7BY
Tel +44 (0)20 7602 7282 Fax +44 (0)20 7603 0095
www.halcrow.com
Halcrow Group Limited has prepared this report in accordance with
the instructions of their client, Environment Agency, for their sole
and specific use. Any other persons who use any information
contained herein do so at their own risk.
© Halcrow Group Limited 2016
Halcrow Group Limited
Vineyard House 44 Brook Green London W6 7BY
Tel +44 (0)20 7602 7282 Fax +44 (0)20 7603 0095
www.halcrow.com
Environment Agency
Boston Waterways Link - Retaining Wall and
Scour Protection
Flood Risk Assessment
Contents Amendment Record
This report has been issued and amended as follows:
Issue
Revision
Description
Date
Signed
Final
01
For planning
15-02-08
AJN
Contents
1
Introduction
1
2
Development Description and Location
2.1 Development Type and Location
1
1
3
Definition of the Flood Hazard
2
4
Probability
2
5
Climate Change
3
6
Detailed Development Proposals
3
7
Flood Risk Management Measures
4
8
Off Site Impacts
4
9
Management of Residual Risks
5
10
Appendices
6
1
Introduction
1.1.1
This Flood Risk Assessment accompanies a planning application submitted by
Halcrow on behalf of the Environment Agency in order to seek planning
permission to construct civil engineering works associated with the Boston
Waterways Link.
1.1.2
The site is located at Black Sluice in Boston, Lincs. Outline planning permission
has been approved (B/07/0140) for converting the existing no 1 sluiceway to a
navigation lock to allow vessels to pass from the Haven to the South Forty Foot
Drain (SFFD). A number of conditions are in the process of being discharged.
1.1.3
A new scour protection structure and retaining wall is the subject of this Flood
Risk Assessment.
2
Development Description and Location
2.1
2.1.1
Development Type and Location
2.1.2
The vulnerability classification for the site is ‘Water-compatible’ since it can be
termed ‘flood control infrastructure’.
2.1.3
The Boston Borough Council online Flood Risk Matrix tool was consulted in
determining whether this type of development is acceptable in principle on flood
risk grounds. Under the sequential test, the site is within Flood Zone 3b
(functional floodplain), however the exception test is not required, since it is a
water compatible development.
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The proposed development is located at Black Sluice in the town of Boston, within
south east Lincolnshire. The location of the site is shown on Figure 1 in Appendix
A. Drawings for the structures are enclosed in Appendix B.
1
3
Definition of the Flood Hazard
3.1.1
In 2002, Boston Borough Council commissioned a Strategic Flood Risk
Assessment for the whole of Boston and the final report was published in
November 2002. The assessment gives a full account of the types of flooding
which may be expected. These are consistent with the types of flooding expected
to affect the site of the scour protection. In summary, these are:



Fluvial flooding from the South Forty Foot Drain
Fluvial flooding from the River Witham
Tidal flooding from The Haven
Source of flooding
Fluvial flooding from the South
Forty Foot Drain
Fluvial flooding from the River
Witham
Tidal flooding from The Haven
How flooding occurs
Flows are discharged by gravity through Black
Sluice. When the sluice is tide locked, the
pumping station is triggered.
Flows are controlled by Grand Sluice which is
located some miles to the north of the
development site.
Tidal water levels present a risk of flooding, in
particular, when coupled with high fluvial
flows.
4
Probability
4.1.1
The site is within Flood Risk Zone 3b: High Risk (Functional Floodplain). The site
will flood on a daily basis being within the tidal zone.
4.1.2
The structure will be on the riverward side of existing flood structures which
currently protect the houses in the area. There will be no net loss of flood plain as
a result of this structure. The flooding regime of the adjacent area will not be
affected.
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2
5
Climate Change
5.1.1
The scour protection structure will be unaffected by climate change because the
structure is under water.
5.1.2
The existing steel sheet pile wall retains the earth embankment which provides
flood protection to the town of Boston. The new sheet pile retaining wall will
replace the existing sheet pile retaining wall. The height of the flood defences will
remain unchanged and so climate change is not being addressed within the scope
of this project. These issues are being addressed in the Boston Haven Flood
Management Strategy Study.
6
Detailed Development Proposals
6.1.1
The Boston Waterways Link is the first project to be progressed as part of the
Fens Waterways Link (FWL), which proposes an exciting new vision for the east
of England with plans to connect navigable waterways between Lincoln, Boston,
Peterborough and Ely.
6.1.2
A key component of this is the construction of a lock to provide a navigable link
between the Haven and South Forty Foot Drain. The new lock must also sustain
the existing Flood Risk Management function of Black Sluice. The structure will
therefore be required to discharge flood water through the downstream sector
gates.
6.1.3
Under this mode of operation it is probable that erosion will occur downstream of
the structure when the sector gates are opened. An erosion control structure is
required.
6.1.4
The proposed erosion control structure will extend 35m from the end of the lock
and 15m out from the existing steel sheet piled retaining wall. The first 25m will be
an underwater reinforced concrete slab along the line of adjacent steel sheet piles.
Surrounding this slab will be a 5m width of 1.8t boulder rock armour. The
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3
finished level of -3.18m AOD has been agreed with the Port of Boston to ensure
that it will not pose a hazard to navigation. Localised dredging will be carried out
by the Port of Boston before placing the erosion control material.
6.1.5
The new steel sheet pile retaining wall will continue for 30m along the line of the
existing embankment as shown in drawing IMAN/00969/030. The existing sheet
piled wall will be re-used as part of the new design.
7
Flood Risk Management Measures
7.1.1
Both the scour protection structure and the retaining wall will have flood defence
functions: the scour protection structure provides continuing stability to the
retaining wall, which in turn adds to the retained height of the flood defences.
7.1.2
The design life of both structures is for 100 years.
8
Off Site Impacts
8.1.1
An 200m2 area of silt adjacent to the new retaining wall will be dredged to locally
reduce the level of the channel bed. It has been determined that this will not
increase flood risk within The Haven channel. The flood risk management
operation of the lock will be carried out by the relevant Environment Agency
function. This will be managed to ensure that flood flows discharged through
Black Sluice do not have detrimental effects downstream.
8.1.2
Since the structure is within the watercourse, there are no additional run-off issues
to consider.
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9
Management of Residual Risks
9.1.1
The Environment Agency will be responsible for maintaining the works
throughout the lifetime of the flood defences. This will include periodic
inspections to ensure the integrity of the retaining wall and flood defence
embankment. These will be conducted either by boat or from land using safe
systems of work. An inspections will be co-ordinated with staff at Black Sluice
Pumping Station.
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5
10
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Appendices
6