Managing bank instability and erosion

Description of measure

The use of soft engineering in rivers to manage bank instability can be traced back to the first century BC but has had a resurgence in recent years due to the increased requirement for sustainable river management solutions. Soft engineering or bioengineering refers to the use of living and inert vegetation to stabilise banks by either binding together unstable bank material or by protecting the bank face from erosion due to weathering, fluvial scour or trampling by livestock. 

The technique covers both the construction of new soft bank face protection, and the replacement of hard bank protection with a softer solution (although in some cases it may be possible to remove hard bank protection without replacing it - see Rehabilitation of banks and riparian zone). A variety of different techniques can be used to stabilise banks and are described in more detail in the Design Guidance section. 

Application

Soft engineering techniques can be used to remediate the failure of unstable river banks and can also be used to stabilise exposed or newly reprofiled banks. These techniques are particularly recommended for use in situations where management changes (e.g. restricting livestock access) are unable to address the problem (SEPA, 2002); this may include areas where sediments are non-cohesive.  These techniques are more sensitive to the natural requirements of a river and the habitats it supports compared with hard bank protection and are also less likely to have a significant adverse impact on the ecology and hydromorphology of a water body. 

Before installing a soft engineered solution, it is important to consider why erosion is occurring and whether it is necessary or desirable to prevent it from doing so.  Erosion can be caused by a variety of factors, including the natural action of water on the bank surface, geotechnical failure, changes in sediment supply and/or flow regime, the installation of hard bank protection upstream, and trampling by livestock.  In many cases, it may be possible to address erosion by controlling the factors which cause it, for example by controlling livestock access or through improved riparian management techniques, upstream deflection of flows or increases in sediment load resulting in an increase in channel capacity.  However, where erosion is particularly severe, assets or important habitats are at risk, or banks have been newly reprofiled, it is frequently necessary to employ a soft engineering solution. 

The design and  installation of soft engineering is still an emerging industry with the benefits and limitations of the different techniques not always evident. Hence it is important to recognise that soft engineering is not a panacea for all bank stability issues and professional advice should be sought before installing this measure.

One of the key differences between soft engineering and traditional methods is the lead time required to source, grow and install materials. Mature coir rolls or coir pallets require an 18 month growth period, prevegetated coir rolls and pallets need to sown in March for installation in July. Bespoke planting schedules with local provenance species require a 2 year lead time due to seed collection. Installation of live material (with the exception of willow) should ideally occur between April and September when the plants can set down roots before winter. While a number of suppliers will have a certain amount of standard stock available that can be bought ‘off the shelf’ (e.g. Phragmites coir pallets) which are suitable for small schemes, large schemes and bespoke planting requirements will require significant planning upfront in order to meet the desired installation date. 

Many soft engineering solutions are reliant on vegetation growth for them to be fully effective.  This can take some time, and the use of these techniques can therefore be constrained by requirements for rapid results.  It may therefore be necessary to employ techniques that have an additional structural element (e.g. branches or stakes) if a short-term solution is required. In channels with very high flow velocities, there may be a risk that soft engineering solutions could be vulnerable to being washed away (Environment Agency, 1999).  In addition, techniques which incorporate living vegetation can only be used in areas where suitable conditions for vegetation growth are available (i.e  sufficient light, shallow water depth, minimal water level fluctuations), and may be vulnerable to the effects of livestock grazing and trampling (Environment Agency, 1999). Adaptive management and maintenance is often required during the first three years following installation of soft engineering solutions and some locations may require protective fencing to prevent waterfowl from removing young vegetation.

Benefits for Water Framework Directive

Unstable banks are frequently vulnerable to rapid erosion, surface weathering, and ultimately geotechnical failure.  This can directly contribute to degradation of the geomorphological condition of the river bank and riparian zone, with a corresponding decline in the habitats that these areas support.  Measures to stabilise the river banks and prevent failure occurring could therefore have a beneficial impact on WFD quality elements such as morphological conditions, aquatic macrophytes and benthic invertebrates. 

In addition, the failure of unstable banks can lead to an increase in sediment supply to the river system, which can cause deposition and geomorphological changes further downstream.  Bed deposition and a change in geomorphological structure can have a detrimental impact on fish, aquatic macrophytes and invertebrates, for example by increasing turbidity, reducing light penetration, and physically smothering the coarser substrate required by many species.  Bank stabilisation techniques could therefore help to reduce sediment input and prevent deterioration in many aquatic and marginal habitats.  Furthermore, the reduction in sediment supply can have a positive impact on the physico-chemical condition of a water body by limiting the supply of sediment-related nutrients. 

In addition to preserving geomorphological structure and important river habitats, soft engineering solutions can provide the following benefits:

  • Low whole project life cost and lower long-term maintenance costs than traditional methods;
  • Low maintenance of live plants after they are established (with the exception of willow);
  • Environmental benefits of enhanced wildlife habitat, water quality improvement and aesthetics;
  • Improved strength over time as root systems develop and increase structural stability;
  • Increased sustainability due to the use of locally sourced materials and the ability to self propagate;
  • Compatibility with environmentally sensitive sites or sites with limited access;
  • Increased  macrophyte, morphological and flow diversity (dependant on the technique used);
  • Reduces flow velocities by dissipating energy and encourage sediment to accumulate on channel margins;
  • Pre- vegetated techniques can limit the establishment of invasive species following construction or disturbance; and
  • Retains connection and continuity between the bank and the river (compared to traditional approaches). 

To read more about the effectiveness of the measure within academic literature please click here: Effectiveness for Biological Quality Elements.

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