Rehabilitation of banks and riparian zone

Design Guidance

Bank rehabilitation may be undertaken using two approaches that both aim to reduce the uniformity of the bank and provide a range of niches for different habitats.  There are several main bank rehabilitation measures, which are described in more detail below. 

Removal of hard bank protection

Whilst there are several sources of information regarding the use of soft engineering solutions and design of new bank protection, there is little guidance focussing specifically on the removal of existing hard bank protection alone.

The method of removal will depend on the material, design and location of the hard bank protection. For example, the removal of wooden toe boarding is likely to be simpler than the removal of full face vertical concrete protection.

It is important to recognise that the removal of hard bank protection alone may not maximise the biological benefits that may be realised in re-establishing natural bank conditions. In low energy rivers, or those that have been significantly modified, the river may not be able to adjust to re-establish natural bank form and processes, particularly following removal of extensive hard bank protection. The vertical bank profile left following removal of full face protection may adjust through geotechnical failure over time.

Removal of hard bank protection may also generate a significant volume of waste. Consideration of how the waste could be used on-site should be considered prior to removal of the protection to avoid potentially large disposal costs.

A key consideration in removal of hard bank protection will be the potential for significant sourcing of sediment in the short-term whilst the bank remains unvegetated. This is likely to be of less concern in channels that have gravel banks, but may be a particular issue in lowland rivers with fine bank material. Fine sediment sourced to the channel during the removal process itself and in the short-term recovery period may adversely impact on local habitat. The potential for sediment sourcing can be reduced using several approaches:

  • Timing of the works to ensure they are not undertaken during or just prior to periods of high flow.
  • Use of methods to control downstream transport of sediment (e.g. sediment traps).
  • Reprofiling of the channel banks following removal of hard bank protection – see bank rehabilitation.
  • Planting of the banks to encourage more rapid colonisation of vegetation – see bank rehabilitation.

Timing of the works should also be planned to avoid sensitive periods for fish, breeding birds and other protected species that may be present (e.g. otter, water vole).

It is recommended that the banks are monitored following removal of hard bank protection to assess the progress of natural recovery and ascertain whether further intervention is appropriate in order to maximise the biological benefits of the work.

Bank reprofiling

River banks can be artificially reprofiled to reduce their gradient and create shallower areas next to the channel edge (WWF, 2000).  For example, a bank with a steep, uniform slope down to the edge of the channel can be physically reshaped to incorporate shallow ledges just under the water line, areas of vertical river cliff, and intermediate ledges that lead to a more stepped profile. 

Detailed guidance for bank reprofiling is provided in the Farming and Watercourse Management Handbook, produced by WWF (2000). 

  • The gradient of the reprofiled bank should be approximately 35�, although this is dependant on the physical properties of the bank material.  An angle of greater than 45� should be avoided. 
  • It may be necessary to undertake some reprofiling upstream and downstream of the target section of bank in order to create a smooth transition.  However, this should be minimised to prevent damaging the existing banks and the habitats they support.
  • If the bank material is particularly easy to erode, it may be necessary to stabilise the bank surface using vegetation or geotextile matting.  This can be pre-planted, and will allow natural vegetation to colonise. 
  • Due to the potential for fine sediment sourcing whilst vegetation is re-establishing it is recommended that where possible, work is only undertaken on one bank at a time. 

Schematic diagram of a bank reprofiled to produce a shallower angle (Source: WWF, 2000, p.35). 

Furthermore, it is recommended that an asymmetric profile which incorporates shallow shelves and low berms should be aimed for.  Material excavated from the top of the bank can be replaced at the bank toe, which narrows the river channel and creates new areas of marginal and aquatic habitats, whilst maintaining overall channel capacity.

Schematic diagram of a bank reprofiled to create a submerged berm (Source: Buisson et al., 2008, p.82). 

Bank reprofiling has been successfully applied on the River Eden, near Appleby in Cumbria (Brock, 2002).  Prior to reprofiling works, erosion rates on the steep banks were very high (up to 2 m per year).  The banks were reprofiled to an angle of approximately 35�, following the existing contours of the eroded section.  A turf mat (sourced from adjacent grazing land) was installed on the reprofiled surface to prevent the soft sand being eroded, and willow toe protection was installed at two particularly vulnerable points. 

Channel widening

An alternative measure is to undertake bank reprofiling in order to widen the river channel (WWF, 2000).  In this method, material is excavated from one bank in order to create a shallow marginal area, on which new habitats can develop.  This measure typically reduces flow energy and erosiveness, and is therefore employed on the opposite bank to where erosion is an issue (WWF, 2000). 

Schematic diagram of a bank reprofiled to reduce flow energy and improve marginal habitats (Source: WWF, 2000, p.35). 

Creation of aquatic ledges

Aquatic ledges or berms consist of narrow ledges of material that are placed at the base of a river bank, close to the low flow river level (RRC, 2002).  These features increase the ecological value of a river channel by providing new habitats for aquatic and marginal communities, and also help to prevent bank toe erosion.  Instead of using material sourced from the bank, new ledges can be created using a combination of soft engineering measures and backfilled material from the river bed (RRC, 2002). 

Aquatic ledges have been used successfully in a number of river systems.  Two differing designs have been used on the River Skerne as it flows through Darlington in County Durham (RRC, 2002) and subsequently applied to the River Cole at Coleshill, Oxfordshire:

  • Wide design (Type A): Timber posts were installed into the bed, and the void between the outermost posts and bank line was filled with material from the river bed.  This was then topped with pre-planted coir pallets to speed up vegetation growth.  This design was used to create ledges with a maximum width of 2 m, although the width can be varied to introduce a more varied planform. 
  • Narrow design (Type B): Timber posts were driven into the channel bed and backfilled, with further reinforcement form planted coir rolls.  This design was used to create narrow ledges for use in situations where the river bed slopes steeply away from the bank. 

Schematic diagram of the aquatic ledges used on the River Skerne (Source: RRC, 2002, Section 3.2)

Coir matting was used in both designs to stabilise the backfilled material until emergent plants could establish (RRC, 2002).  Further details of the use of geotextiles are provided in the Managing bank instability and erosion measures sheet. 

The aquatic ledges installed on the River Skerne in Darlington have been demonstrated to work effectively (RRC, 2002).  The wide (Type A) ledges were installed in late 1997, and survived several floods without damage.  Plant regrowth was noted to be patchy during summer 1998, with siltation smothering some of the pallets (RRC, 2002).  However, dense vegetation cover subsequently established, and a wide range of species were reported.  The narrow (Type B) ledges were installed in summer 1996, and by 1998 became fully vegetated features.  Significant silt deposition (up to 300 mm) has been recorded over the winter flooding period, which is assimilated into the ledges following vegetation regrowth in the spring (RRC, 2002).  The same measures were applied on the River Cole with some success, although only 50% of vegetation planted on the ledges survived flooding during the first winter.  However, the ledges revegetated quickly and now support good quality emergent habitats (RRC, 2002). 

Planting bankside vegetation

Planting on riverbanks may be desirable where natural or man-made processes or human intervention have resulted in bare soil being exposed. In many cases, planting represents the most sustainable option for stabilising such banks. A planting scheme is recommended to determine the methods and materials to be used for both marginal and bankside planting.  It should take into account the existing water quality, water level fluctuations, channel capacity, maintenance needs and existing seed bank.  It should also detail a strategy to ensure successful establishment in the longer term.

The New Rivers and Wildlife Handbook (RSPB, 2001) provides details of the occurrence and distribution of riparian and marginal plant species throughout mainland UK (pp32-46). This can be used to help guide the choice of species for planting. Spring and early summer are ideal times for planting as vegetation has a significant part of the growing season during which to become established.

It is assumed that the bank structure and stability has been assessed and that vegetation planting has been identified as a sustainable solution (see Managing bank instability and erosion).

Marginal planting

Marginal planting can be carried out manually or using machinery. For large scale planting, dredged spoil transfer is effective as long as it has been obtained from nearby sites. For smaller scale planting, good results are often obtained by digging out wild plants in spring and planting immediately into moist ground. Alternatively stock can be obtained from specialist plant nurseries, and should be the same as the species already present at the site. Common methods to avoid this involve the used of stakes and berms or soft engineering measures such as geotextiles and pre-planted coir rolls. Guidance on marginal planting can be found in the Riparian Vegetation Management good practice guide (SEPA, 2009) (pp8-10 and pp18-20) and The New Rivers and Wildlife Handbook (RSPB, 2001) (pp246-249). Marginal vegetation can also be established through the installation of aquatic ledges where marginal habitat is lacking. Details of how these can be used are provided in the Manual of River Restoration Measures (RRC, 2002).

Bankside Planting

Seeding can be undertaken on existing banks and during construction of new banks.  For existing banks the vegetation is stripped back and a thin layer of topsoil (<10cm) is mixed with subsoil before reseeding.  Low maintenance seed mixes containing wild flowers are often used, and tall herbs establish themselves as particularly valuable habitats.

Natural regeneration is desirable, assisted by low levels of grazing, but if the process does not occur quickly, then erosion can become a problem. This can be overcome through the planting of a quick growing “nurse crop” in conjunction with a low maintenance seed mix. Shade is an important factor for riparian and marginal plants, affecting fish and invertebrate assemblages. Usually, a mixture of shaded and unshaded areas provides most benefit to the biota of a watercourse. The New Rivers and Wildlife Handbook (RSPB, 2001) (pp259-265) contains information on how to establish vegetation on riverbanks and floodbanks.

Tree planting

Tree planting should only take place in suitable areas, and consider existing features of the landscape. Wet habitats and herb communities should not be used as they can be adversely affected by tree planting. Planting on the bank allows nutrients to enter the river (e.g. through leaf fall), providing food for invertebrates. It is recommended that native species should be used and where possible, local provenance species. The New Rivers and Wildlife Handbook (RSPB, 2001) (pp282-284) lists the species of trees and scrub that are of native provenance to mainland UK, and are therefore recommended for planting in different areas. Tree and scrub planting guidance is also provided (pp281-286).

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