Managed realignment (bank)

Design guidance

Existing guidance

Numerous guidance documents are currently available for the purpose of Managed Realignment, all of which cover the physical and biological factors that need to be considered when adopting this mitigation measure.

Guidance documents of specific relevance to the design of Bank Realignment techniques are:

Coastal and Estuarine Managed Realignment: Design Issues (CIRIA, 2004)

Section 2.8.2 of this document specifically relates to ‘Bank Managed Realignment’.  This section defines what bank realignment is, describes the situations to which it is suited and suggests how sensitive issues can be overcome.

Section 5 of this document provides useful information on ‘Designing and Implementing Managed Realignment’ in general and covers issues such as:

  • Physical processes and morphology (tidal levels, tidal range, tidal prism, tidal discharges, tidal current velocities, tidal asymmetry, waves, sediment dynamics and morphological responses, including the stability of breaches.
  • Engineering requirements (site boundaries, new ‘backstop’ defences, new intertidal zones (including topographic levels, creek systems, vegetation establishment, plant and access, dealing with existing buildings, structures and services, decommissioning, and health and safety).

Whilst dealing with Managed Realignment in general, much of the above is directly relevant to Bank Realignment. 

Managed Retreat: A Practical Guide (Burd, 1995)

This guide provides information on site selection and practical recommendations for implementation of managed realignment in general.  In addition, the section on wave climate (page 19) discusses the option of replacement of the original sea wall / embankment with the more flexible protection of a ridge of coarse sediment within a Bank Realignment setting.

Scheme design

The reduction in defence height may be to ground level or to a slightly higher elevation to provide a sill that erodes down gradually and can encourage sedimentation landward of the defence. The height of a defence may also be reduced, rather than removed entirely, to allow land use to continue for at least part of the time with a reduced flood defence level.

Bank realignment can also comprise the moving inland of soft defences such as shingle ridges or sand dunes. These types of defence (and habitat) should be rolled back inland under natural processes where possible. The majority of currently managed ridges do migrate inland over time, but the rate of movement has been reduced. It may be necessary to modify the profile of a managed ridge to achieve sustainable roll-back. Where the ridge is in an unsustainable position it may be necessary to move the ridge inland to allow this process to take place effectively. This may require de-stabilising the existing ridge by physical means or removal of sand dune vegetation. As with the removal of hard defences, it may be possible to allow the system to function with inundation up to high ground or it may be necessary to provide a new, inland, defence line.

Various tools exist to assist in the assessment of physical processes associated with Bank Realignment schemes. These may be used at different stages of the design process to provide an appropriate degree of certainty in the outcome. Some of these tools are more applicable to estuarine environments, others to coastal environments. Often there will be a requirement to use a range of tools in combination to provide a robust approach (see Leggett et. al., 2004. p91-93).

Shingle ridges or sand dunes

Bank Managed Realignment can also comprise the moving inland of soft defences such as shingle ridges or sand dunes (see Figure 6 of the 'What is Manged Realignment' page). These types of defence (and habitat) should be rolled-back inland under natural processes where possible. Most currently managed ridges do migrate inland over time but the rate of movement has been arrested. It may be necessary to modify the profile of a managed ridge to achieve sustainable roll-back; where the ridge is in an unsustainable position it may be necessary to move the ridge inland to allow this process to function effectively. This may require de-stabilising the existing ridge by physical means or removal of sand dune vegetation. As with the removal of hard defences, it may be possible to allow the system to function with inundation to high ground or it may be necessary to provide a new, inland, defence line.

Case Studies

Project Name:                              Welwick bank realignment

Project Location: Outer Humber Estuary, Yorkshire

Project Proponent(s): Associated British Ports (ABP)

Year(s): 2006

Short Description:

At the Welwick realignment site in the Outer Humber Estuary, the site was re-profiled to increase the area below Mean High Water Neap (MHWN) tides, as mudflat creation was the main objective of the site. The reprofiling included the creation of a gentle slope from the existing mudflats to the rear of the sites. New flood defences were created at the rear of the 54 ha site and designed to withstand a 1 in 50 year design event. An area of saltmarsh was expected to develop in front of the new defences. However, the saltmarsh was not taken into account when determining the embankment height, due to uncertainty surrounding the long term sustainability of the saltmarsh. 70,000m� of material needed for the new set back defences was obtained from a combination of re-profiling and creation of temporary borrow pits. The new set back embankment was seeded and left to stabilise for one year. The existing seawall was removed over a length of 1400m and the 20,000m3 of material gained was used to fill the borrow pits used to create the new set back embankment. The complete removal of the existing defence instead of the creation of solitary breaches was chosen for the following reasons:

  • It improves connectivity with the wider estuary
  • It more closely recreates the type of environments that existed before reclamation
  • It enables the whole cross sectional area of an estuary including the realignment site, to respond to estuary wide changes
  • It increases energy levels within the site, thereby improving the likelihood that mudflat habitat will be maintained.

The old defence was removed in a series of stages and the existing saltmarsh in front of the existing defences were breached to increase wave energy further into the realigned site. The saltmarsh could not be removed completely as it was designated. The breaches were also required to allow the site to drain and flood sufficiently. The width of the breaches in the saltmarsh was determined by calculating the discharge and considering the critical threshold for the erosion of sediment. The selected breach size was seen as being large enough for the velocities to be below the critical threshold for erosion. Key lessons learnt include the need for topographic monitoring over the lifetime of the project. Topographic monitoring results show that there has been an overall accretionary trend - an average increase of 15 cm across the site. The realignment site has developed as a major roosting site for a number of wading birds at high water. A total of 27 different bird species were observed during the survey and all three target species were observed within the survey area adjacent to the newly created realignment site.

 

Project Name:                              Chowder Ness bank realignment

Project Location: Inner Humber Estuary, Yorkshire

Project Proponent(s): Associated British Ports (ABP)

Year(s): 2006

Short Description:

The initial objective of the Chowder Ness scheme was to create 10.5 ha of mudflat and 0.8 ha of saltmarsh to support a variety of invertebrate and bird species.  At the Chowder Ness site, new flood defences were created at the rear of the site to a minimum height of 6.7m above ODN. Material for these defences was obtained from within the site from a combination of reprofiling and creation of temporary borrow pits (these were later infilled with material obtained from the bank removal).The existing seawall was removed over a length of 570m (some 200m remain), to a level of around 1.6 to 2m ODN. This removal, rather than the creation of solitary breaches, was chosen for a number of reasons:

  • It improves connectivity with the wider estuary;
  • It more closely recreates the type of environments that existed before reclamation;
  • It enables the whole cross sectional area of the estuary including the realignment site, to respond to estuary wide changes; and
  • It increases energy levels within the site, thereby improving the likelihood that mudflat habitat will be maintained (as mudflat creation was the main objective of the site).

The old defence was removed in a series of stages: (1) removing the rear of the embankment, (2) the concrete wave return, the bitumen and rock face, and (3) the overall lowering of the embankment (to levels around 1.6-2m OD).

The first monitoring report for this scheme has yet to be published; however, initial results of the topographical monitoring show that there has been an overall accretionary trend - an average increase of 9 cm across the site has been approximated (with the caveat that the relatively high rates of observed accretion in some areas could be a function of errors associated with the surveying methods). Within the pre-existing fronting intertidal areas, the largest changes in elevation at Chowder Ness were associated with the pre-existing island of saltmarsh at the eastern entrance to the realignment site.  As predicted, this island of saltmarsh has been reduced in extent, presumably through increased tidal flows.

Bird monitoring showed that a total of 13 water bird species used the newly created Chowder Ness intertidal area in the 2006/2007 count season. Throughout the survey period, most water bird usage was concentrated on the disturbed ground along the line of the removed seawall and the adjacent area of new mudflats within 10m of that area. Three water bird species (lapwing, dunlin and redshank) were present on the new mudflats during all seven of the monthly surveys. Species observed in the highest numbers on the site included golden plover, lapwing, dunlin, black-headed gull and curlew.

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