There are a wide range of soft engineering techniques that have been successfully applied to bank stabilisation. Useful design guidance on soft bank face protection techniques can be found in the Good Practice Guide for Bank Protection: Rivers and Lochs (SEPA, 2008), Waterway Bank Protection: A guide to erosion assessment and management (EA, 1999), Environmentally sensitive channel and Bank Protection Measures (NCHRP, 2005) and Estuary Edges: Ecological Design Guidance (Biodiversity by Design, 2008). In addition to describing a range of techniques in detail, these documents also provide methods for selecting the most appropriate techniques for a given situation. Practical examples of the application of these techniques can be found in the River Restoration Manual, produced by the River Restoration Centre.
Soft engineering techniques can be applied to three different types of design categories:
- bioengineered – designs which rely wholly on plants to provide stability and protection from erosion (e.g. coir pallets, coir rolls and facines).
Bioengineered (e.g. willow)
- biotechnical – designs which use hard techniques to provide structural support for plants or live bioengineered products (e.g. rock rolls to support coir rolls, reinforced vegetated geotextiles). Can also be referred to as composite or hybrid designs.
Biotechnical (e.g. coir roll and rock roll)
- biostructural - those designs that have a hard structure with a soft face (e.g. vegetated gabions).
Biostructural (e.g. vegetated gabions)
Descriptions of commonly used soft engineering techniques are provided below.
Revegetation is a frequently applied bank stabilisation technique, in which vegetation cover is increased through the establishment of young plants and trees or by planting seeds (SEPA, 2002). Vegetation can be established directly on or behind the exposed bank face, or used in conjunction with pre-seeded or pre-planted geotextiles.
When reseeding, it is recommended that standard seed mixes are avoided in favour of a more diverse plant assemblage, which should be representative of locally occurring species (Environment Agency, 1999). Reseeded areas may need to be protected from the damaging effects of trampling, grazing by waterfowl, and consumption by seed-eating birds until young plants become established. Although this technique is able to withstand relatively high flow velocities (1.8 m s-1 for long periods and 4.5 ms-1 for short periods), it generally cannot withstand the effects of wave action and is unsuitable for very steep or vertical banks (Environment Agency, 1999).
As an alternative to reseeding, young trees and shrubs can be established on top of the bank (Environment Agency, 1999; see figure). These can be supplied as plug plants or saplings. Plug plants are grown in nurseries and refer to young plants or seedlings. Plug plants are generally commercially grown using root trainers (110 or 230cc) that allow for easy removal from the container without damaging the roots. Plug plants are often planted directly into bare soil or through jute matting which provide temporary erosion protection of the soil. As these plants develop, their root systems help to stabilise the bank and reduce further erosion. Riparian trees such as alder and voracious shrubbery such as bramble are particularly well suited as saplings, although a more diverse species assemblage that is representative of local species is recommended (Environment Agency, 1999). Trees should be established in small groups, at a distance of between 0.5 and 1 m from the channel edge. If trees are planted closer to the bank, the root systems may lead to further damage. Conversely, if they are planted further away, their root densities may be insufficient to have a beneficial effect (Environment Agency, 1999). Revegetation has been demonstrated to effectively stabilise a river bank. Plant roots bind soil particles and limit geotechnical failure, and the presence of trees and ground-cover vegetation acts as a physical barrier against soil erosion (Thorne et al., 1998). This technique can provide a long-term solution to bank instability, but is not suitable for application in the short term.
Schematic diagram of a bank stabilised using young trees (Source: EA, 1999, p.145).
Where a rapid solution is required, grassed geotextiles can be used in place of reseeding or manual planting. Further information on this technique is provided under Geotextiles.
Geotextiles consist of woven mats, rolls or bags of natural fibres (coir, hessian or jute) or synthetic materials (open weave polythene mesh) that are placed on the bank surface or bank toe to prevent fluvial scour and the removal of fines from the bank face (Environment Agency, 1999; SEPA, 2008). In addition to the direct protection that they provide, geotextiles can also be used as a rooting medium for grasses and marginal plants, and can incorporate pockets for reed planting (Environment Agency, 1999).
Recent developments in the manufacture of geotextiles mean that they can be used in a variety of flow conditions depending on the specific properties of the different products. They must be laid carefully and secured in place to prevent localised scour, and may need to be used in conjunction with toe protection to prevent undercutting.
It is generally recommended that biodegradable geotextiles are used in place of synthetic materials where possible, since they are more able to absorb moisture and provide favourable growing conditions for marginal vegetation. In addition, they typically degrade completely within 5 years, by which time vegetation will have established (Environment Agency, 1999).
Geotextiles have been used successfully at a number of locations on the River Skerne in Darlington and the River Colne in Oxfordshire, in conjunction with landscaping and planting, to create a naturalised bank that is protected from erosion (RRC, 2002).
Brushwood bundles and mattress
Brushwood bundles consist of bundles of wood that are tied together and secured onto the bank surface to slow the flow of water and collect sediment (Figure 2). They are installed parallel to the direction of flow, and are typically set into shallow trenches and staked into position (SEPA, 2008). Live wood (typically willow) can be used for a long-term solution; this will root and continue to grow, further stabilising the banks. Alternatively, dead wood (typically hazel or chestnut) can be installed if new willow growth is not desired (SEPA, 2008).
Schematic diagram of a bank stabilised using brushwood bundles (Source: SEPA, 2008, p.16).
Brushwood mattresses consist of a thick layer of cut branches (typically willow) that are placed on the bank surface and secured using wooden stakes and sheep netting, in order to prevent erosion (Environment Agency, 1999; SEPA, 2008). Willow shoots grow rapidly from the live branches, and the open nature of the mattresses means that they are effective at trapping sediment when inundated.
This technique has been used successfully on the River Skerne in County Durham, in conjunction with a crushed rock revetment below the waterline (RRC, 2002). However, it is possible to use a soil-filled branch mattress in place of the rock revetment to support a brushwood mattress (Ward et al., 1994).
Brushwood mattresses have been demonstrated to work well on steep river banks and in fast flowing streams (Environment Agency, 1999; SEPA, 2008). In some cases, it may be necessary to use techniques such as toe protection to prevent the mattresses becoming undercut (RRC, 2002; SEPA, 2008; see figure).
Schematic diagram of a bank protected using brushwood mattresses and green toe protection (Source: SEPA, 2008, p.17).
Faggots or facines consist of bundles of branches or coppice that are secured in place at the base of the river bank (Figure 4) (Environment Agency, 1999) and are used to provide a flexible solution to many scour problems particularly below low water level. Faggots are typically 0.2 to 0.3 m in diameter and 2 m long and material can be sourced locally or imported to site (typically hazel if imported). Faggots can be installed parallel or perpendicular to the bank, and are generally staked in place and secure with twine.
Faggots should always be installed underwater to limit degradation and can be stacked in deep water. Live wood (willow) faggots can be secured in place at the base of the bank, where they protect it from toe scour and, once regrowth occurs, help to stabilise the bank (Environment Agency, 1999).
Faggots can be used to encourage natural siltation, or can be incorporated with dredged material (Environment Agency, 1999). Live wood faggots (generally willow) are suitable for application in larger channels, while dead wood faggots (typically chestnut) are more suited to smaller channels, where tree growth could impede flow. Faggots are generally suitable for light erosion, and may not withstand stronger forces (Environment Agency, 1999).
Schematic diagram of a bank protected using willow faggots (Source: WWF, 2000, p.33).
Willow faggots have been used successfully to improve steep, eroding banks on the River Cam (Gulickx et al., 2007). Live willow branches from four species (osier Salix viminalis, purple osier S. purpurea, crack willow S. fragilis and white willow S. alba) were cut to 4-5 m in length and tied into bundles. These bundles were secured into place at the bank toe using wooden stakes at intervals of approximately 1 m. In places, the banks behind the faggots were reprofiled (see the Bank rehabilitation techniques sheet for further information), and the arisings were used to create a shallow ledge between the bank and faggot.
Willow spiling is a traditional soft engineering technique used to stabilise eroding banks. It consists of weaving live willow rods between live willow stakes set into the affected bank at regular intervals. The space behind the willow wall to the existing bank is then filled with soil to provide an area for the willow roots to grow. (Figure 5 & 6)
Osier willow is considered to be the most appropriate species to provide poles, due to its pliable nature and the fact that it is an indigenous species (RRC, 2002). Alternatively, commercially available alternatives could be used, but these will require more vertical support posts (RRC, 2002). Willow spiling is normally carried out between November and April during the harvesting of the willow (dependent upon the season) and should be installed within three weeks of harvest.
Spiling is generally considered to be effective in most fluvial environments, with the exception of high-energy gravel bed channels which may be too mobile. The technique can be applied on steep and vertical banks, but cannot be used as a retaining structure (Environment Agency, 1999).
Willow spiling has been successfully applied on the River Skerne in Darlington, County Durham, where it was used in conjunction with rock reinforcement below the waterline, wooden toe boarding at the low water level, and synthetic geotextiles (RRC, 2002).
The RRC (2002) note that this technique does not have the flexibility to accommodate natural bank settlement, and the use of geotextiles or mattressing may be preferable if there is not a plentiful local supply of osier willow. However, this technique can be used in relatively narrow spaces, which may be an advantage in some situations. It should be noted that willow spiling does require regular maintenance and is not suitable for small river channels where vegetation growth could potentially obstruct flow.
Schematic diagram of a bank protected using willow spiling (Source: WWF, 2000, p. 32).
Example of willow spiling used in a river channel (Source: H. Dangerfield, Royal Haskoning.
Root wads are composed of cut tree trunks, complete with the root system (Figure 7). The trunks are pushed into the river bank, and the roots remain exposed (NCHRP, 2005; SEPA, 2008). This reduces the energy of the water flow and deflects water away from the banks which reduces bank erosion. The general rule of thumb is two thirds of the trunk should be keyed into the bank with the remaining third which includes the root wad projecting out into the river. However, this technique can also create scour by locally increasing turbulence and its placement should be carefully considered (SEPA, 2008).
Schematic diagrams of banks protected using root wad revetments (Source: NCHRP, 2005, p. 35).
A coir roll is made of coir fibre (from coconut husks) hydraulically pressed into a polypropylene or coir net bag and tied (Figure 8). Coir rolls are available as either an unplanted, pre-planted or mature planted product and are typically used to protect the toe of the bank from erosion (Figure 9). They are also used to provide instant marginal vegetation in river restoration schemes. The concept behind coir rolls is the plant root system will establish into the bank or bed and persist once the coir degrades and hence become self sustaining.
Coir rolls are typically 300 to 400mm in diameter and can come in 3 m or up to 6 m lengths. Coir rolls can be installed as a single unit or placed on top of faggots or rock rolls in deeper water. They are typically installed using chestnut stakes on either side of the roll and secured with polypropylene twine. They can be also be installed without stakes and secured to the bank using a wire loop technique. Alternatively coir rolls can also be used in ‘hanging baskets’ in front of sheetpile.
Example of unplanted coir rolls. (Source: K White, Royal Haskoning)
They provide an excellent method of introducing instant, marginal habitat and can be used in low to medium energy environments with velocities up to 3.5 m/s. They are best suited to river margin depths of up to 300mm and the river level at which they are placed should be considered carefully – too high and the plants will be left high and dry, too low and the plants will struggle to photosynthesise. Coir rolls are not recommended to be used as terrace features on banks nor do they adapt very well to locations which are heavily shaded. Coir rolls have become a very popular soft engineering technique in recent times with different rates of success, however if installed correctly and used appropriately, they are an excellent option for sustainable bank toe protection.
Preplanted coir rolls used in an urban river enhancement project. (Source: K White, Royal Haskoning)
Coir pallets are made of a thin coir mattress which supports a variety of marginal plant species (Figure 10). The mattress is cut into pallets (typically 2 x 1m or 2 x 1.5 m) and can be placed directly on the channel margin or over other treatments such as brushwood providing instant marginal habitat.
Coir pallets can be supplied unvegetated but are most effective when established with mature vegetation due to the well developed root systems ensuring a quick establishment rate. Coir pallets are typically used to provide instant marginal vegetation in areas that may be vulnerable to scour or to ‘top’ other treatments such as brushwood packing and berm creation. They provide excellent marginal habitat and are a flexible technique that can be installed by hand.
Coir pallets can be installed in depths from 0.1 to 0.5 m and are best suited to low energy environments (velocities up to 2 m/s). Pallets installed on channel margins are fixed in place by wooden pegs and installation should ensure that the first pallet near the waters edge is sufficient secured to prevent water from lifting up the pallet.
Similarly to coir rolls, coir pallets do not establish well in heavily shaded areas and the vegetation will die if the water level drops significantly for a long period of time (months). They are also not recommended for watercourses with a high fine sediment load as excessive deposition can smother young plants.
Mature coir pallets Source: K White, Royal Haskoning
Rock rolls are flexible ‘sausages’ of crushed rock contained with nylon netting that are place at the bank toe to prevent undercutting and fluvial scour. Rock rolls are typically 300 to 400mm in diameter and come in 2 m or up to 5 m lengths.
They are installed at a depth of 0.1 to 0.5 m (although can be stacked) and are effective in velocities up to 4 m/s. They are effective in rivers with wave action up to 0.4 m high.
Coir rolls can be fixed over rock rolls and become homogeneous as roots penetrate downwards into the rock and the adjacent soil. The design provides inbuilt flexibility whilst allowing the plants to develop in stable conditions. However the application of rock rolls can be limited due to the fact that machinery is required to install them and the netting can be damaged through contact with mooring boats. However, they do provide a flexible solution when used in biotechnical designs to address scour issues deep below the water line as well as providing structural support for other techniques such as coir rolls and coir pallets.
Stacked rock rolls placed at the bank toe (Source: K White, Royal Haskoning)