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Alkborough scheme reduces extreme water levels in the Humber Estuary and creates new habitat

Authors:
  • Jacobs Engineering Group

Abstract and Figures

There are a number of large scale managed realignment schemes that have been undertaken across Europe under the umbrella of the FRaME project. Alkborough is one such scheme, situated on the south bank of the Humber on the eastern side of the confluence between the River Humber and River Trent. The site is bounded to the south east by the escarpment of the Lincolnshire Heights with the remaining boundaries of the site being formed by the shoreline of the estuary, approximately 6km in length. The site is principally in the ownership of Natural England and the Environment Agency. The promotion of the Alkborough scheme was driven by strategic estuary wide requirements and has significant benefits with regard to sustainable estuary management, namely by: • Providing flood storage to reduce peak tide levels in the inner estuary and tidal rivers, thereby deferring the need for work elsewhere in the estuary to address sea level rise; and • Contributing to habitat creation responsibilities under the European Union Wild Birds and Habitats Directives by creating 170ha of new inter-tidal habitat and 200ha of other natural habitats. This paper describes some of the work that was undertaken prior to construction starting on site, in particular: • Hydrodynamic modelling and geomorphological analysis to inform the assessment of the external impacts of the development and ensure that these were alleviated by appropriate design; • Economic assessment; and • Environmental impact assessment. The key environmental issue with respect to the design of the scheme was the potential tide level reductions in the estuary and the effects of this on navigational interests. As a result normal tidal inundation of the site is by way of a fixed weir breach structure. This structure restricts the inundation into the site to volumes that have negligible effect on tide levels in the estuary. The construction of an overspill weir by lowering part of the existing tidal defence permits floodwaters to enter the site on surge tides within the estuary. The scheme demonstrates how strategic reductions in water level can be made within an estuarine environment whilst simultaneously creating new areas of inter-tidal and other habitats. It is anticipated that the habitat and species supported by the scheme will, in time, allow the site to be designated as a National Nature Reserve (subject to designation by Natural England’s Council) and may ultimately allow it to be included within the European Site boundary. Keywords Flood risk management, habitat creation.
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1
1 INTRODUCTION
There are a number of large scale managed re-
alignment schemes that have been undertaken
across Europe under the umbrella of the FRaME
Flood Risk and Management in Estuaries project.
Alkborough is one such scheme, situated on the
south bank of the Humber Estuary on the eastern
side of the confluence between the River Humber
and River Trent. The scheme demonstrates how
strategic reductions in water level can be made
within an estuarine environment whilst simultane-
ously creating new areas of inter-tidal and other
habitats.
This paper explains the drivers for the project,
the site background and the main elements of the
scheme. The paper then describes some of the
studies and activities that were undertaken prior to
construction starting on site, in particular:
Cali, M., Parsons, A.,Batty, L., Duggan, S., Miller, P, Pontee, N., Miller, P., (2008). Managing coastal change: Walberswick to Dunwich.
FLOODrisk 2008 - The European conference on flood risk management research in to practice 30 September - 2 October 2008 Keble College,
Oxford, UK.
Alkborough scheme reduces extreme water levels in the
Humber Estuary and creates new habitat
D. Wheeler
Halcrow Group Ltd, Endeavour House, Forder Way, Hampton, Peterborough PE7 8GX
S. Tan, N. Pontee
Halcrow Group, Burderop Park, Swindon, SN4 0QD
J. Pygott
Enviroment Agency, Phoenix House, Global Avenue, Leeds LS11 8PG
ABSTRACT:
The Alkborough Scheme
is a fundamental part in
Humber Estuary Flood Risk Management Strategy providing both flood storage to reduce extreme water lev-
els in the estuary and creating new inter-tidal habitat to contribute to the future integrity of the estuary’s envi-
ronmental status. This paper describes the development of the scheme and the studies which were undertaken
to minimize the effects of the scheme on other uses of the estuary. The paper outlines the partnership ap-
proach which was adopted during the scheme’s preparation and concludes with a brief description of the con-
tinuing development of the Alkborough site since the major construction works were completed in autumn
2006.
ABSTRACT: The Alkborough Scheme is a fundamental part in the delivery of the Environment Agency’s
Humber Estuary Flood Risk Management Strategy providing both flood storage to reduce extreme water lev-
els in the estuary and creating new inter-tidal habitat to contribute to the future integrity of the estuary’s envi-
ronmental status. This paper describes the development of the scheme and the studies which were undertaken
to minimize the effects of the scheme on other uses of the estuary. The paper outlines the partnership ap-
proach which was adopted during the scheme’s preparation and concludes with a brief description of the con-
tinuing development of the Alkborough site since the major construction works were completed in autumn
2006.
Figure 1. Aerial view of inundated site, September 2006.
2
Hydrodynamic modelling and geomorphologi-
cal analysis to inform the assessment of the ex-
ternal impacts of the development and ensure
that these were alleviated by appropriate de-
sign;
Environmental impact assessment;
Economic assessment;
Consultation; and,
Partnerships and contributions.
The paper also gives an update of how the site
has developed since its opening in late 2006.
2 PROJECT DRIVERS
2.1 Flood risk reduction
The plains around the Humber Estuary and its
tributaries cover approximately 1,500km
2
and con-
tain areas of great value for industry, commerce,
agriculture and wildlife, as well as population cen-
tres such as Goole, Kingston-upon-Hull, Grimsby
and Cleethorpes. Much of this land is at risk from
flooding under the highest tides. The estuarial tidal
defences are increasingly threatened by rising sea
levels (from global warming and natural sinking of
the land) and by complex sediment movement
processes which erode the foreshore in places. The
terrestrial assets within the estuarial plain of the
Humber are protected from flooding by some
235km of tidal defences which are generally in rea-
sonable condition. However, the standard of pro-
tection is low in places and will reduce progres-
sively in the future as sea level rises.
An outline strategy for managing the tidal de-
fences, that is aimed at providing an appropriate
and sustainable standard of protection to people
and property while safeguarding the natural and
human environment, was published as the Humber
Estuary Shoreline Management Plan (HESMP) in
2000. Assessments have been made of the required
future flood defence expenditure in the estuary and
it is estimated that the present value cost of im-
proving the existing defences over the next 100
years is approximately £240 million.
The Alkborough Flats scheme supports the key
findings of the HESMP that flood risk manage-
ment proposals in the estuary should:
consider realigning the defences where this
will reduce the cost or improve the sustain-
ability of managing the system; and,
support the creation of new inter-tidal habi-
tat as necessary to maintain the estuary’s
conservation status.
A key finding of the supporting work for the
strategy was that the Humber Bridge marks a sig-
nificant change in the estuary’s behaviour. Re-
aligning defences downstream of the bridge will
have little effect on conditions elsewhere whilst
upstream realignments, such as at Alkborough, will
reduce tidal levels during extreme events and,
therefore, have a flood defence benefit.
2.2 Habitat creation
As part of the development of the HESMP, a
Coastal Habitat Management Plan (CHaMP) was
prepared. This assessed the amount of inter-tidal
habitat predicted to be lost due to coastal squeeze
and as a result of any unavoidable encroachment of
new or improved tidal defences. It also sets out the
rate at which replacement habitat is likely to be
necessary.
The CHaMP identified that approximately
420ha of new inter-tidal area will need to be pro-
vided over the 50 years in the estuary upstream of
Grimsby to mitigate coastal squeeze losses. In ad-
dition, a further 45ha will need to be provided to
compensate for any unavoidable encroachment re-
sulting from the improvement and maintenance of
the tidal defences within this part of the estuary. It
is intended that a staged approach is taken to the
provision of replacement/compensatory habitat in
the estuary upstream of Grimsby. The initial devel-
opment of Alkborough Flats will provide signifi-
cant habitat in the area for at least 15 years or
longer should sea levels rise at a rate less than
6mm per year.
3 SCHEME OBJECTIVES
The principal objectives of the project were to
provide flood storage on Alkborough Flats so as to
facilitate flood defence benefits and to create new
inter-tidal habitat that contributes to ensuring that
the integrity of the ‘European Sites’, such as the
Special Protection Area (SPA) and the possible
Special Area of Conservation (pSAC), is main-
tained in the long-term and is compliant with the
Habitats Regulations, in light of predicted habitat
losses identified in the CHaMP arising from
coastal squeeze/sea level rise and to compensate
for losses arising from tidal defence works within
the estuary.
3
Secondary objectives include:
The provision of enhanced habitats, over and
above those required to meet the second prin-
cipal objective, which are able to meet UK
Biodiversity Action Plan (BAP) or other high
level targets; and,
The enhancement of the amenity value of the
area.
4 SITE DESCRIPTION
Alkborough Flats is situated on the south bank
of the Humber Estuary on the eastern side of the
confluence between the River Humber and the
River Trent (Figure 2). It is bounded to the south
east by the escarpment of the Lincolnshire Heights,
a line of limestone hills rising to over 50m OD in
level. The remaining boundaries of the site are
formed by a tidal defence approximately 6km in
length which tapers out at either end into the lime-
stone escarpment. The village of Alkborough is lo-
cated at the top of the limestone escarpment. There
are four access tracks down the escarpment to the
Flats and two public rights of way cross part of the
Flats.
Figure 2. Location of Alkborough Flats.
The existing tidal defence comprised a grassed,
earth embankment set back to varying degrees be-
hind saltmarsh and tidal grazing marsh. The side of
the estuarial channel forms the limit of the marsh
area, seaward of which is inter-tidal mudflat.
The site has a total area of 450ha, of which
370ha lie between the tidal defence and the 5m OD
contour at the base of the escarpment. The remain-
ing 80ha lie between the tidal defence and the low
water mark. Land levels within the site vary be-
tween 2m OD and 4m OD.
Most of the site was formerly used for arable
agriculture (Grade 2 land). The defence embank-
ment and the tidal marsh were used for grazing
livestock. The only buildings on the site were a
derelict farmstead (Flatts Farm) and a barn/crop
drier. A sewage treatment works and a riding stable
are located at the base of the escarpment. The site
was formerly drained by a network of ditches and
watercourses which discharged to the estuary via a
tidal sluice.
The Humber Estuary is an important site for
wildlife with significant areas falling within sites
designated under the European Union (EU) Wild
Birds and Habitats Directives. These are, respec-
tively, the Humber Estuary pSAC and the Humber
Estuary SPA, potential SPA (pSPA), Ramsar and
proposed Ramsar site, which recognise the interna-
tional nature conservation interest of the inter-tidal
habitats and the wetland birds that they support.
Part of Alkborough Flats is located within the
Humber Estuary Site of Special Scientific Interest
(SSSI).
5 SCHEME DESIGN
The managed realignment at Alkborough in-
volved the creation of a 20m tidal exhange struc-
ture in the existing defence to permit tidal inunda-
tion of the Flats. A distribution channel was also
constructed to aid the inundation of the newly cre-
ated site. This resulted in the tidal defence align-
ment being set back to a natural defence line pro-
vided by the toe of the escarpment of the
Lincolnshire Heights, apart from a 1.1km length of
new embankment adjacent to Alkborough Village,
which protect the sewage treatment works and the
riding stable. The scheme also included the lower-
ing of the crest level of a 1.5km length of the exist-
ing tidal defence, thereby allowing over-spilling of
tidal waters into Alkborough Flats during extreme
tidal events.
The other main construction elements of the
scheme were:
the construction of a habitat separation bund to
maintain a proportion of the southern end of
the site as freshwater habitat;
the refurbishment of the rock armour protection
of the existing tidal defence at the north eastern
and south western ends of the site;
the construction of a pumping station to convey
flows from land drainage and from the sewage
treatment works over the realigned defence
embankment; and,
4
the demolition of the barn/crop drier and Flats
Farm.
The general arrangement of the elements is
shown on Figure 3.
Figure 3. General arrangement of the works.
6 DETAILED DESIGN STUDIES
6.1 Hydrodynamic studies
Hydrodynamic modeling was undertaken to as-
sess the inundation characteristics of the site, and
hence its suitability for habitat development, as
well as the potential impacts of the site’s develop-
ment on the local and wider estuary. The modelling
showed that the initial large breaches of the exist-
ing tidal defence which were considered would
lead to significant reductions in water levels within
the main estuary and substantial inundation of the
site by estuarine waters. Whilst reductions in wa-
ter levels within the wider estuary under high tides
were acknowledged to be beneficial in terms of re-
ducing flood risk, reductions under mean spring
tides were seen as disadvantageous to navigation
and other environmental interests.
The chosen option of a tidal exchange structure
together with an overspill weir minimises the po-
tential for negative effects on navigation depth, the
frequency of tidal inundation of the Royal Society
for the Protection of Birds’ (RSPB’s) reserve at
Blacktoft Sands, the Trent Aegir (tidal bore) and
the geomorphology of the estuary, whilst maintain-
ing the flood risk benefits of reducing peak water
levels. As a further safeguard against adverse ef-
fects, the Environment Agency has entered into
agreements with Associated British Ports (ABP)
and the RSPB to provide protection measures
should the need for these arise in the future.
The predicted annual tidal inundation of the site
including frequency of inundation and area covered
is shown on Figure 4.
In order to optimise the flood storage facility at
Alkborough Flats and thereby the flood defence
benefits, varying lengths and levels of the overspill
weir were investigated. The chosen arrangement
was for a weir 1,500m long, of which half is set at
a level of 5.1m OD with the remainder set at 5.45m
OD. The reductions in peak tide levels are a maxi-
mum close to the site and in the Rivers Trent and
Ouse, reducing in the estuary towards the Humber
Bridge (Table 1).
Table 1. Predicted reduction in peak tide levels for a 1500m
long two stage overspill weir.
Reduction in Peak Tide Levels
(mm)
Return Period
(1 in ….. years)
Location
10 50 100 200
Hull 10 11 10 8
Humber Bridge 10 16 18 28
Brough 18 45 55 87
Trent Falls 32 72 92 145
Blacktoft 39 84 103 155
Goole 27 61 72 86
Burton Stather 49 85 104 156
Keadby 48 82 101 157
6.2 Environmental assessment
The Environmental Impact Assessment (EIA)
for the proposed scheme identified a number of
temporary, locally adverse impacts. These mainly
related to general construction disturbance, such as
increased noise, disruption to local traffic, tempo-
rary closure of footpaths, but also potential eco-
logical impacts (for example, disturbance to pro-
tected species), impacts on the European Sites and
the potential for disturbance to birds within the
SPA. However, none of the impacts were consid-
ered to be significant following the implementation
of appropriate mitigation measures such as good
working practices, limited working hours, close li-
aison with residents, adequate safety procedures,
development and agreement of appropriate work-
ing method with key stakeholders and ongoing
consultation. In addition, works in or within 100m
of the European Sites were only carried out be-
tween April and September. The short-term nega-
tive impacts were considered acceptable given the
significant long-term positive benefits provided by
the completed scheme.
5
Figure 4 Normal annual tidal inundation.
The EIA process established that the preferred
scheme would have the following potential perma-
nent impacts:
Beneficial Impacts
Development of approximately 170ha of new
inter-tidal habitat, which meets one of the two
primary objectives of the project by contribut-
ing to the replacement of the coastal
squeeze/sea level rise losses that are predicted
in the CHaMP and thus assist in maintaining
the integrity of the European Site in the long
term. It is anticipated that the habitat and spe-
cies supported on the site will, in time, allow
its designation as a National Nature Reserve
(subject to Natural England’s Council) and the
site’s ultimate inclusion within the European
Site boundary;
Creation of other habitats within the remainder
of the site through appropriate management
(such as grazing marsh, saline pools, wet and
dry reed bed habitats, a freshwater area, hedge-
rows and areas of grassland/scrub);
Contribution to Department for Environment,
Food and Rural Affairs’ (Defra’s) high level
targets and BAP targets;
Substantial contribution to the future manage-
ment of the tidal defences in the Humber Estu-
ary;
Provision of long-term recreational opportuni-
ties and economic, environmental and social
benefits for the local community;
Beneficial effect on the setting of the Alkbor-
ough village Conservation Area; and,
Change in land use from agricultural land to
the creation of diverse habitats.
Adverse Impacts
Permanent closure of a small section of one of
the existing rights of way due to the construc-
tion of the overspill weir;
Loss of hedgerows, trees, shrubs and bird nest-
ing habitat;
Impact on farmland birds due to the loss of ag-
ricultural land.
Potential scour and erosion close to the breach,
affecting archaeological remains in front of the
tidal defences on the foreshore;
Impacts within the European Sites associated
with the placement of rock armour and con-
struction of the tidal exchange structure and
overspill weir;
Potential impact at Blacktoft Sands nature re-
serve due to changes in water level;
6
Potential impacts on navigation due to water
level changes; and,
Potential impacts on inter-tidal areas elsewhere
in the estuary as a result of water level changes.
The mitigation measures identified to manage
these impacts were as follows:
The loss of a section of existing right of way
was compensated for by the provision of new
Public Rights of Way and permissive paths
within the site;
Tree and hedgerow losses were mitigated by
enhanced existing hedgerows and replanting
others. Trees were replaced at a ratio of four to
one of those lost;
Suitable provisions to reduce the impact on
farmland birds were incorporated within the
Site Management Plan (for example, rotational
set-aside; game crops for cover; uncropped
field margins; regenerating surrounding hedge-
rows and hedge trees; dead timber piles);
Regular monitoring by the Environment
Agency, in consultation with North Lincoln-
shire Council Sites and Monuments Records
Officer, of the effects of scouring and erosion
and recording any exposure of archaeological
deposits where necessary;
Losses/changes in habitat within the European
Sites were offset by the habitat created within
the site;
The hydrodynamic modeling work outlined
previously was undertaken to estimate the
magnitude of the potential impacts on water
levels associated with alternative scheme de-
signs and to inform the choice of scheme that
had the least detrimental impact on the habitats
outside the Alkborough site, especially at
Blacktoft Sands and on navigation. The pre-
ferred scheme minimises the reduction in tidal
levels and the small reductions predicted are
not considered significant, particularly when
set against the consequences of sea level rise.
The Environment Agency agreed appropriate
mitigation measures with the RSPB regarding
their nature reserve at Blacktoft and Associated
British Ports (ABP) with regard to navigation
issues; and,
The work also estimated that in the long term
the reduction in the inter-tidal area of the estu-
ary outside of the site as a result of the scheme
will be about 5ha. This is unlikely to be detect-
able within the natural variation in inter-tidal
area in the estuary.
As part of the EIA process an Environmental
Action Plan was prepared to mitigate and manage
any potential impacts on the environment prior to,
during and after construction.
7 COSTS AND BENEFITS
7.1 Costs
The estimated capital cost of the scheme was
£11.14 million (Table 2). This included construc-
tion costs, salaries, fees, an inflation allowance and
a project contingency, together with costs which al-
ready had been incurred. The latter costs, which to-
talled £3.5 million, were for the purchase of the
majority the site and for the implementation of ad-
vance construction works to facilitate the main
scheme works. The present value cost of the
scheme was estimated to be £8.7 million.
Table 2. Estimated scheme costs.
Description Estimated Capital
Cost (£000)
Land purchase 4,670
Advance works 300
Construction works 4,670
Salaries, fees and other costs 450
Inflation allowance 70
Construction contingency 850
Project contingency 130
Overall Scheme Cost 11,140
The cost of future management and mainte-
nance of Alkborough Flats following construction
of the scheme is estimated at £25,000 per annum.
7.2 Benefits
The development of Alkborough Flats produces
both flood defence and environmental benefits.
These benefits were quantified as described below
and assessed over a 100 year period to a January
2005 price base date.
The flood defence benefits arise from the reduc-
tion in peak tide levels during extreme events de-
ferring the need for raising the level of the tidal de-
fences in the estuary upstream of the Humber
Bridge to offset the increased flood risk as a con-
sequence of sea level rise.
Based on the reduction in peak tide levels iden-
tified by the hydrodynamic modeling, the defer-
ment of flood defence expenditure at various loca-
tions in the estuary resulting from the development
of Alkborough Flats was estimated. Assessments
were made of the required future flood defence ex-
penditure in the estuary without the development
of Alkborough Flats (the baseline case) and with
the implementation of flood storage facilities on
the site.
7
The present value cost of the works in the base-
line expenditure plan was £238.8 million. With de-
ferment of expenditure on defences throughout the
estuary, the present value cost of the works re-
duced to £226.5 million. The flood defence benefit
of the Alkborough Flats development was there-
fore calculated to be £12.3 million.
The environmental benefits of converting Alk-
borough Flats into an inter-tidal area were assessed
based on the value of habitat creation of £944 per
hectare per year. This value was taken from a
meta-analysis of a number of valuation studies by
Woodward and Wui (2001). It relates to a move
from “no wetland or one providing little habitat
value” to “wetland providing single service func-
tion as habitat”. The present value of the environ-
mental benefits was estimated to be (370ha x
£944/ha x 29.8 discount factor x 1.089 inflation al-
lowance) = £11.3 million.
The overall present value of the benefits of de-
veloping Alkborough Flats, accounting for both
flood defence and environmental benefits was as-
sessed to be £23.6 million.
7.3 Benefit Cost Assessment
The present value of the cost of developing
Alkborough Flats was £8.7 million. The present
value cost of the benefits arising was £23.6 million
which gave the scheme an average benefit to cost
ratio of 2.7.
Figure 5. Construction of the overspill weir, Summer 2006.
8 CONSULTATION
A wide range of interested parties were con-
sulted throughout the development of the proposed
scheme. This process included consultations with
various relevant Environment Agency functions,
the statutory consultees Natural England, English
Heritage, the Countryside Agency and North Lin-
colnshire Council and non-statutory consultees, in-
cluding landowners, local organisations and indi-
viduals with an interest in the area. Overall, there
was a good level of support shown towards the
scheme by the various stakeholder groups.
The consultation process included:
Dissemination of an Initial Consultation
Document;
Consultation on a Scoping/Consultation Paper;
Meetings with key stakeholders and statutory
consultees;
Local stakeholder meetings and presentation
and discussion of outline designs for the pro-
ject to consultees, including the parish council
and local community; and,
Meetings of the Alkborough Flats Management
Group.
Several meetings were also held to provide an
update on the project’s progress in terms of the
modelling studies, structural requirements and de-
sign issues and how these had been incorporated
into an outline Site Management Plan. These meet-
ings were technical in nature and so attendance
was limited to project team members, Management
Group representatives and other key consultees.
8
Public consultation was primarily led a by the
Alkborough Flats Management Group, through the
production and issue of newsletters to the residents
of Alkborough. These continued to be produced
during construction of the scheme. In addition, a
public exhibition was held in Alkborough village.
The majority of people provided positive feedback
in relation to the proposed scheme. A few ex-
pressed concerns about specific aspects of the pro-
posals. These were mainly in relation to the pro-
posed site access route for construction vehicles,
the temporary increase in traffic during the con-
struction phase, together with general concerns
about the potential future traffic generated by visi-
tors to the site.
In addition to the consultation activities de-
scribed above, the Environment Agency undertook
detailed discussions with a number of stakeholders,
including ABP, landowners, the RSPB, and Scun-
thorpe Internal Drainage Board. These one-to-one
discussions related to very specific aspects of the
project, which needed to be addressed through di-
rect liaison rather than general discussion at any of
the meetings.
9 PARTNERSHIPS AND CONTRIBUTIONS
A partnership approach was taken to the devel-
opment and design of the project to reflect the keen
interest of the various parties and organizations.
The management group which was set up com-
prised the following partners:
Environment Agency;
Natural England;
North Lincolnshire Council;
The Countryside Agency;
English Heritage;
Lincolnshire Wildlife Trust; and,
Associated British Ports.
The management group oversaw the develop-
ment of the scheme and was the focus for the asso-
ciated benefits and impacts. It commissioned work
to consider the long-term management of the site
and the production of a site management plan. The
following long-term management objectives were
set by the management group:
To contribute to the practical implementation
of the HESMP and CHaMP;
To demonstrate sustainable flood defence
planning and implementation in the face of sea
level rise;
To create new inter-tidal habitats, which con-
tribute to BAP targets and help to maintain the
favourable conservation status of the European
Site;
To create a new National Nature Reserve;
To provide a catalyst for improvements to the
local economy through tourism, diversified ag-
ricultural uses and new business development;
To link to and add value to wider regeneration
initiatives in the North Lincolnshire area;
To provide access and interpretation for the
wider community;
To provide a demonstration project for good
practice across all aspects of planning and im-
plementation of similar projects; and,
To establish Europe-wide links in relation to
flood risk management in estuaries.
North Lincolnshire Council is taking the leading
role in the long term management of the site based
on the objectives of the site management plan.
The scheme was funded from a number of
sources with approximately £2 million from the
European Union via the Interreg programme and
from the Regional Development Agency (York-
shire Forward). The remaining project cost was
met by Defra.
10 SCHEME CONSTRUCTION
Construction of the scheme commenced in July
2005 and was completed in September 2006. Al-
though parts of the construction works were de-
layed following the discovery of relic ordnance, the
target date for construction completion of autumn
2006 was achieved.
Following completion the site was inundated by
the highest predicted astronomical tide of 2006.
The area inundated was very similar to that pre-
dicted by the hydrodynamic studies (Figure 3).
During construction a 20m wide “starter chan-
nel” had been excavated across the existing
saltmarsh from the tidal exchange structure to the
estuarial channel. The invert of the channel was
set at the same level as the weir of the exchange
structure.
It was always envisaged that the saltmarsh
channel would substantially widen and deepen
over time as a result of hydrodynamic action.
However, the speed at which this did actually oc-
cur was not foreseen. Due to a perceived risk of
9
undermining of the exchange structure weir, addi-
tional rock armour was installed to the sides and
along the base of the channel to the riverward side
of the weir in October 2005. Since then the
saltmarsh channel appears to have stabilized and
no further remedial works have been necessary.
11 SITE DEVELOPMENT SINCE
CONSTRUCTION
The works which were completed in October
2005 provided the facilities for the tidal inundation
of and flood storage within the site. Since then
over 5 km of new footpaths within the site have
been created, of which half are for disabled access.
Three bird hides have been installed and prepara-
tory work has been undertaken for the provision of
a freshwater reed bed at the southern end of the
site.
Since the initial inundation of the site saltmarsh
and mudflat have developed slowly with up to
0.6m of sedimentation recorded in a several places.
Over 10,000 golden plovers and 6,500 lapwings
were estimated to be on the site during the
2007/2008 winter. The overspill weir has not yet
operated.
Figure 6. The tidal exchange structure, Autumn 2006.
12 CONCLUSIONS
The Alkborough Flats scheme has two primary
benefits:
Provides flood storage to reduce peak tide lev-
els in the estuary during extreme events,
thereby providing a saving of approximately
£12 million by deferring works to improve the
existing tidal defences elsewhere in the estu-
ary;
Contributes to habitat creation responsibilities
under the EU Wild Birds and Habitats Direc-
tives as identified in the Humber Estuary
CHaMP, by creating 170ha of new inter-tidal
habitat and 200ha of assorted other natural
habitats.
The scheme provides flood defence and envi-
ronmental benefits of present value £23.6 million
and has a benefit to cost ratio of 2.7.
The scheme will also deliver a number of addi-
tional enhancement opportunities:
Creation of other habitats within part of the
site through appropriate management (such as
grazing marsh, saline pools, wet and dry reed
bed habitats, a freshwater area and hedgerow
areas of grassland/scrub);
Provision of long term recreational opportuni-
ties and economic, environmental and social
benefits for the local community;
10
Beneficial effect on the setting of the Alkbor-
ough Village Conservation Area; and,
Change in land use from agricultural land to
the creation of diverse habitats.
The key environmental issue with respect to the
design of the scheme was the potential tide level
reductions in the estuary and the effects of this on
navigational interests. As a result normal tidal in-
undation of the site is by way of a fixed weir tidal
structure. This structure restricts the inundation
into the site to volumes that have negligible effect
on tide levels within the estuary. The construction
of an overspill weir by lowering part of the existing
tidal defence permits floodwaters to enter the site
during surge tides within the estuary.
The scheme demonstrates how strategic reduc-
tions in water level can be made within an estua-
rine environment whilst simultaneously creating
new areas of inter-tidal and other habitats. It is an-
ticipated that the habitat and species supported by
the scheme will, in time, allow the site to be desig-
nated as a National Nature Reserve (subject to
Natural England’s Council) and may ultimately al-
low it to be included within the European Site
boundary.
The scheme also demonstrates how a partner-
ship approach, stakeholder participation and exten-
sive consultation are able to facilitate the develop-
ment of such a project as Alkborough. Apart from
land purchase, it took three years from com-
mencement of the appraisal work to completion of
construction.
13 REFERENCES
Environment Agency. 2000. Planning for the Rising Tides-
The Humber Estuary Shoreline Management Plan.
Black&Veatch for Environment Agency. 2005. Humber Es-
tuary Coastal Habitat Management Plan.
Polhill, R.M. 1982. Crotalaria in Africa and Madagascar.
Rotterdam: Balkema.
Woordward & Wui, 2001. The economic value of wetland
services: A meta-analysis
Environment Agency. 2005. Humber Estuary Tidal Defences,
Alkborough Tidal Defence Scheme. Project Appraisal
Report.
... Symonds and Collins, 2007;Kadiri et al., 2011), increased intertidal habitat (e.g. Wheeler et al., 2008;Vuik et al., 2016), controlled tidal restoration (e.g. Cox et al., 2006;Oosterlee et al., 2020) and managed retreat (e.g. ...
... The inlet width and geometry of the transitional polder mainly affected the net import of sand and mud (Fig. 7e,h,i,l). This is understandable, as a smaller inlet width focuses and increases flow velocities at the inlet up to a point where the friction of the inlet width becomes a limiting factor; an example is the managed realignment at Alkborough, where a 20 m narrow inlet greatly limits tidal exchange and the input of sediments (Wheeler et al., 2008). Consequently, transitional polders with complete dike removal experienced weaker tidal exchange flows and significantly less sand import (Fig. 7e,h). ...
Article
Full-text available
Dikes are the conventional means of flood defence along rivers and estuaries. However, dikes gradually lead to the superelevation of waterbodies compared to the subsiding embanked areas, resulting in a rapidly increasing unstable situation under sea-level rise. Therefore, future flood management requires new, sustainable strategies that not only minimise flood risk, but also steer land-level rise. An example is a transitional polder, where a dike-protected area is temporarily reopened to the tide to capture sediment until it has risen well above mean sea-level, after which it could be returned to its original function. This study explores how the sequence of opening transitional polders affects sediment capture and large-scale estuary dynamics through 2D modelling in Delft3D. To this end, different opening sequences were tested along a large estuary, using the Western Scheldt (NL) as an example. Findings show land-level rise in all permutations. However, polders opened later in an opening sequence temporarily experience a lag in muddy sediment capture, most likely due to a deficit in fines. Opening more upstream located polders alone or at the start of an opening sequence generally causes a stronger reduction in mean tidal range than opening more downstream located polders. This is explained by increased friction due to (1) locally added intertidal width and (2) shallowing of the main channels because of increased flood dominance. An upstream-to-downstream opening sequence caused the greatest reduction in mean tidal range, but this is negligible compared to the increase in tidal range due to historic dredging within the estuary for navigational purposes. Further work is needed to determine how dredging, closure of transitional polders and storm surges may negate this benefit to flood safety.
... The literature has covered design guides (CIRIA, 2004) and lessons from existing schemes (e.g. Cooper, 2003;Wolters et al., 2005;Pontee, 2003;2007;Rupp-Armstrong and Nicholls, 2007;Wheeler et al., 2008;Dixon et al., 2008;Morris, 2013). There are also online resources drawing together key facts and figures for different managed realignments (e.g. ...
... Conversely large areas of realignment near the mouth of the estuary were shown to cause increases in water levels. Subsequently the Alkborough flood storage scheme was developed as part of the programme of works arising from HFRMS (Wheeler et al., 2008). This scheme is located at the confluence of the River Ouse and River Trent and was predicted to reduce water levels on extreme events by around 10cm at Blacktoft in the River Trent and Burton Stather in the River Ouse (Environment Agency, 2005). ...
... Whilst some schemes, such as Welwick 1 in the Humber, estuary have been undertaken purely to provide recreate habitat lost to port development in the estuary, many other schemes have involved both of drivers in some way. The following examples illustrate this: @BULLET The Alkborough scheme in the Humber Estuary (Figure 29) was driven by the need to provide compensatory habitats to replace those due to the lost due to the direct and indirect losses caused by flood defences throughout the estuary, but also by the opportunity to reduce extreme water levels and thus reduce flood defence costs elsewhere in the estuary (see Wheeler et al., 2008). @BULLET Hesketh Out Marsh West in the Ribble Estuary (Figure 30) was developed to create additional intertidal habitat to compensate for habitat losses in Morecambe Bay, while maintaining flood defence standards in the local area of the Ribble (see Tovey et al., 2009). ...
... Cost savings can also arise if schemes act as flood storage areas, thereby leading to lower water levels during storms 3 and reduced maintenance costs elsewhere (e.g. Wheeler, et al., 2008). managed realignment may not, however, be the cheapest option in all situations. ...
Chapter
Full-text available
Managed realignment can offer a sustainable long-term management option for coasts and estuaries by reducing the pressures on flood defences and compensating for habitat lost due to developments or coastal squeeze. This chapter discusses some of the factors influencing the long-term sustainability of managed realignment taking into consideration the UK context and experience. The sustainability of managed realignment at the local level depends on the availability and suitability of land to deliver set objectives and greatly on stakeholder support. The implementation of more than 50 schemes in the last two decades in the UK indicates that these factors have not hindered the implementation of managed realignment at the national scale. However, managed realignment is not the best option for all coastal areas. It is certain that managed realignment schemes will continue to be built in the UK. However, managed realignment is not a universal panacea, it is not possible to implement this policy everywhere, and schemes are unlikely to be viewed as a success from all perspectives. External funding sources and extensive stakeholder consultation are required to support the scale of managed realignment implementation planned in the future.
... The literature has covered design guides (CIRIA, 2004) and lessons from existing schemes (e.g. Cooper, 2003;Wolters et al., 2005;Pontee, 2003;2007;Rupp-Armstrong and Nicholls, 2007;Wheeler et al., 2008;Dixon et al., 2008;Morris, 2013). There are also online resources drawing together key facts and figures for different managed realignments (e.g. ...
... Conversely large areas of realignment near the mouth of the estuary were shown to cause increases in water levels. Subsequently the Alkborough flood storage scheme was developed as part of the programme of works arising from HFRMS (Wheeler et al., 2008). This scheme is located at the confluence of the River Ouse and River Trent and was predicted to reduce water levels on extreme events by around 10cm at Blacktoft in the River Trent and Burton Stather in the River Ouse (Environment Agency, 2005). ...
... The literature has covered design guides (CIRIA, 2004) and lessons from existing schemes (e.g. Cooper, 2003;Wolters et al., 2005;Pontee, 2003;2007;Rupp-Armstrong and Nicholls, 2007;Wheeler et al., 2008;Dixon et al., 2008;Morris, 2013). There are also online resources drawing together key facts and figures for different managed realignments (e.g. ...
... Conversely large areas of realignment near the mouth of the estuary were shown to cause increases in water levels. Subsequently the Alkborough flood storage scheme was developed as part of the programme of works arising from HFRMS (Wheeler et al., 2008). This scheme is located at the confluence of the River Ouse and River Trent and was predicted to reduce water levels on extreme events by around 10cm at Blacktoft in the River Trent and Burton Stather in the River Ouse (Environment Agency, 2005). ...
Article
Full-text available
Large realignment sites near the mouth of an estuary have the potential to raise water levels throughout the estuary. This increase is produced by the newly created intertidal area drawing additional water into the estuary on the flood tide. Although some of this additional water enters the realignment site, a proportion also bypasses the realignment site and continues upstream, thus increasing water levels in these areas of the estuary. The modelling results described in this paper suggest that an important determinant on estuary water levels is the rate at which the realignment site fills. Schemes that fill more slowly are predicted to have less impact than those that fill more rapidly, even if high water levels within the schemes are similar. For the schemes tested in this paper, the rate at which the site flooded appeared to depend less on breach dimensions and more on the width of the site that the flood waters had to cross in order to cause full inundation. It is proposed that those options with the shortest distances result in the flood tidal waters experiencing less frictional losses. These options showed a more rapid rise in water levels within the site, and produced a correspondingly larger increase in water levels within the estuary.
... It occurs after the creation of any new set back defences and other features within the intertidal areas of the site (e.g. artificial creeks, lagoons, fresh water control structures ;Wheeler, et al., 2008: Tovey et al., 2009Scott et al., 2015;Latham et al., 2016). It might be assumed that breaching is a rapid and binary process, however, it typically occurs over several days or weeks and often involves increasing degrees of hydrodynamic connectivity and inundation. ...
Article
The ingress and egress of water from managed realignment sites in macrotidal estuaries commonly causes erosion at the breaches and the formation of channels across the fronting intertidal areas. However, the rate of development of these features and their final dimensions have not been well documented. This case study paper details the development of the breach area and the erosional processes in the main and exit channels associated with the Steart Marshes managed realignment site in south west England. The paper shows type of data, along with its accuracy, which is currently available for this type of study. It uses a combination of UAV and LiDAR data that was collected as part of a monitoring campaign to record the development of scheme over a period of 3 years and 7 months following site opening. This data would normally reside within consultancy reports and thus be unavailable to the wider scientific community. The paper demonstrates that further work is needed to improve the prediction of erosion rates associated with managed realignment schemes. The monitoring data at Steart Marshes show the development of a large exit channel along the line of the former channel that led across the existing intertidal mudflats. The highest rates of erosion occurred in the first 3 months following opening. After this time rates declined markedly and, approximately 7 months after opening, the breach and exit channel region began to show periods of accretion as well as erosion. The non-linearity in erosion rates is similar to previously reported sites, but the rates of development were more rapid due to higher flow speeds generated by large tidal prism and the single breach design. Although the volume of the exit channel region appears to have stabilised, a secondary channel is starting to form, which suggests that further changes are likely. Within the site, further recession of the step feature is expected before a stable condition is reached. The paper documents these changes in more detail and draws out those behaviours that are likely to apply to other schemes.
Article
Full-text available
Managed realignment is the landward relocation of flood infrastructure to re-establish tidal exchange on formerly reclaimed land. Managed realignment can be seen as a nature-based flood defence system that combines flood protection by the realigned dike (artificial) and restored saltmarshes (nature-based). So far, research on coastal managed realignment is primarily directed to saltmarsh restoration on formerly reclaimed land. This study focuses on the realigned dikes. The aim of this research is to characterize realigned dikes and to indicate the characteristics that offer opportunities for nature-based flood protection. We categorized 90 European coastal managed realignment projects into two realigned dike groups: (1) Newly built landward dikes and (2) Existing landward dikes of former multiple dike systems. The second group has two subcategories: (2a) Former hinterland dikes and (2b) Realignments within summer polders. For each group we present the realigned dike characteristics of a representative case study. We consider that the use of existing landward dikes or local construction material make realignment more sustainable. From a nature-based flood protection perspective, the presence of an artificial dike is ambiguous. Our results show that targeted and expected saltmarsh restoration at managed realignment does not necessarily result in a greener realigned dike design that suits for combined flood protection with restored saltmarshes. We recommend coastal managers to explicitly take combined flood protection into account in the realigned dike design and steer the topography of the realignment site to facilitate nature-based flood protection and promote surface elevation increase seaward of the realigned dike in response to sea level rise. This makes managed realignment a nature-based flood defence zone for now and for the future.
Planning for the Rising Tides-The Humber Estuary Shoreline Management Plan. Black&Veatch for Environment Agency
  • Environment Agency
Environment Agency. 2000. Planning for the Rising Tides-The Humber Estuary Shoreline Management Plan. Black&Veatch for Environment Agency. 2005. Humber Estuary Coastal Habitat Management Plan.