Bank creation and stabilisation

Banks may need creating or stabilising for various reasons, listed below. The method of construction, called a revetment, is basically the same whether the bank is being stabilised, relocated or created. Similar techniques can be used for constructing shelves near water level for marginal and emergent plants, which will greatly reduce bank erosion. Any bank stabilisation work should be linked closely with vegetation establishment and management (Chapter 8 – The planting and management of new ponds and Chapter 14 – Vegetation management). High banks can be stabilised by two or more lines of revetments. A variety of structures are detailed below, any of which may be suitable. The choice will normally be made according to the location, materials available and access.

Reasons for bank creation and stabilisation include:

Erosion of the existing bank. This may be due to pressure of people or animals destroying the bankside vegetation, due to wave action from boats, or from development upstream causing changes in water flow or silt deposition. Protection is particularly important where erosion is threatening local property or bankside paths. Normally protection or rebuilding of the bank along its existing line is required.
Bank realignment or channel narrowing may be needed where reduction in flows due to water abstraction or changes in weather patterns have resulted in the channel being too wide for the new water level. This can cause a particular problem in chalk streams, where the water flow then becomes insufficient to keep gravel beds clear of silt, to the detriment of flora and fauna.
Pond, lake or island creation may require the construction of revetted or stabilised banks, either where there is insufficient space for gently sloping banks, or where steeper profiles are required for pond dipping or visitor access.
Stabilisation or revetment may be needed near bridges or other man-made structures.
Bankside diversification. Many artificial banks, either uniform and steeply sloping earth banks, or vertical revetments, can be improved by the creation of bays, ledges and other features to increase habitat diversity.
Historic features. Mills and other old waterside developments include revetted banks, leats, weirs and other features. As well as their historic value, many of these sites have developed into valuable habitats. Bank repairs and other work may be needed to keep them in working condition, and to maintain their wildlife value.

Factors to consider

The following points should be considered:

Is the work necessary? Bank erosion, together with movement of sediments and deposition, is a natural process. If erosion is not a threat to property, and is not apparently causing adverse changes elsewhere, then it may be best to take no action. After a period of erosion, the bank may settle to a more resistant profile, with consequent vegetation re-establishment.
Some steep and apparently vulnerable banks, in hard clays, cohesive gravels and sandstones, are in fact quite stable. Vertical sandy cliffs may provide nesting sites for sand martins. Eroding areas under mature trees are valuable nesting sites for otters and other mammals. Bare areas of mud, silt and gravel are important for invertebrates, and as loafing and basking sites for other animals. Mature trees that fall into the water due to bankside erosion are especially valuable in providing nesting sites and shelter for young birds and animals.
Is the work feasible? In flowing water, the erosive force may be too great for any work which volunteers could undertake. Changes upstream from building development or other factors may also mean that any balancing work downstream will need to be of an equal scale. Take advice from the Environment Agency and local authority as appropriate.
Where bank stabilisation is needed to protect a bankside path, it may be possible to relocate the path further back from the bank. Where the path is a right of way, a Diversion Order will be necessary. Contact the local authority for advice.
Use local materials wherever possible, both to save transport, and to produce an environmentally sound and aesthetically pleasing result. Even in major engineering schemes, the trend is now away from ‘hard’ revetments, using concrete, steel piling and other reinforced techniques, and towards using ‘soft’ revetment techniques, including willow spiling, vegetation and geotextiles. The type of location will suggest what is appropriate. In upland streams and water bodies, rock is the natural material, either placed informally to reproduce a natural feature, or used in constructed revetments or gabions (see below). Marginal vegetation is usually naturally sparse. In lowland areas, timber revetments or geotextiles, together with marginal vegetation, is the usual solution.
Gaining sufficient bulk material for backfilling behind revetments is often a major consideration. Bank repair work is usually combined with dredging or other works nearby, which produces suitable material for backfilling. Indeed this type of work may be interdependent, as disposal of spoil is also a major factor in the feasibility of dredging and other operations. Bank reprofiling, island creation and other work is nearly always a ‘cut and fill’ operation, involving the localised rearrangement of bulk material.

Wooden revetments

If you have a choice, use oak or elm for durability. Alder remains durable if submerged, so is suitable for making marginal shelves. Otherwise use whatever is available locally, and try to combine stabilisation work with pollarding, coppicing or felling in the vicinity.
Do not apply creosote or other preservative, as it will pollute the water. Old pressure-treated timbers such as sleepers and sluice boards can safely be used, as excess preservative will have weathered off.
Don’t bother to debark timber, unless you want to prevent the sprouting of willow stakes and branches. The ability of willow to regrow can be very useful in stabilising banks (see below).
All wire, staples, nails and other fittings should be galvanised for durability.
Do not drive stakes into puddled clay bottoms of ponds, canals or other artificial waterbodies. Even if you have been told the bottom is naturally impermeable, it is always worth checking for the tell-tale sticky red- brown, yellow, blue or pale grey clay beneath the silt or gravel top layer.

Stake or timber pilings

Wooden pilings can be placed vertically or slanting. The design shown is suitable for revetting banks where erosion is mainly due to trampling along bankside paths, or at access points along waterways or around ponds. It is particularly used where space on the bank is limited, and any further erosion would, for example, cause loss of access. The steep profile does not encourage growth of marginal vegetation, but shelves can be constructed below the pilings to allow vegetation growth as required. Wooden pilings of this type are unlikely to be robust enough to withstand the erosive forces of fast-flowing water. Any marginal vegetation that can be established will help protect the pilings. This type of revetment is easier to construct if the stakes are straight and of an even diameter. Sawn timber such as old railway sleepers or sluice boards can also be used.

Cut stakes of sufficient length that half the stake is driven into the bottom of the waterway. The height above water will depend on the purpose of the revetment and expected maximum water levels.
Start the post holes with a crowbar. Then place the stakes shoulder to shoulder, and drive them in with a mell. Staple a galvanised wire along the face of the revetment to discourage vandalism.
Fill in behind with gravel or stony spoil that will not wash through the gaps, and compact with a tamper. A geotextile can be used behind the stakes to retain the spoil.

Stake and ether revetments

This design can be adapted to use varying sizes of timber, branches, brushwood and coppice material as available. Select the straightest material for stakes, and cut to a suitable length for the height of revetment required.

Drive in the stakes at intervals of about a metre. Drive them tilting slightly towards the bank to allow for some movement when the material behind is compacted.
Remove side branches from trunks, and cut up branched material so that it lies neatly when piled behind the stakes. Use the largest material at the base. Overlap the ends of the poles and branches to avoid creating weak points in the structure.
Wire the upper poles or branches to the stakes to discourage vandalism.
Deposit spoil behind the revetment and compact with a tamper.
If possible, erect temporary fencing to prevent trampling and grazing of the restored bank, and leave in position until vegetation is well established. In most situations vegetation will naturally regenerate from the seed source within the spoil or from nearby, but selected vegetation can be planted as required.
Marginal vegetation can be established on shelves constructed beneath vertical or steeply sloping revetments. Construct the shelf just above normal water level, to encourage the growth of typical marginal vegetation. Intermittent flooding will reduce the incidence of unwanted weedy species. If there is marginal vegetation upstream, the newly created shelf should rapidly become colonised from waterborne seeds and rhizomes. Elsewhere, marginal vegetation can be planted by hand. This technique may be suitable for riverbanks or lakeshores. It can also be used to improve the wildlife value and enhance the appearance of artificial concrete or other channels, where the resulting reduction in flow capacity is not a problem.

Wired log revetment

This design produces a sloping bank which should rapidly vegetate and stabilise. If willow is used, this will sprout and root to provide long-term stabilisation. Use short stakes with wires to secure the material in position. Logs, branches or bundles of brushwood can be used as available.

Willow

Willow spiling

This is a woven structure, using the ancient technique for building wattle walls and fences. If freshly cut willow is used, it will root to produce a ‘living’ revetment.

Crack willow (Salix fragilis) or white willow (Salix alba) produces the best material for stakes, which should be about 65-100mm (2.5-4″) diameter for spilings up to 600mm high. For spilings up to 900mm (3′) high, thicker stakes of 100-130mm (4-5″) diameter will be needed. Withies should be cut from osier (Salix viminalis), goat willow (Salix caprea) or grey willow (Salix cinerea). The withies should have a butt diameter of 10-20mm (.25-.75″), and be of a length equivalent to five times the stake spacing, to give a close, tight weave. Where unavailable, the best alternative material will have to be used.

November to February is the best season for working to ensure sprouting of the willow. Stakes and withies can be stored at this time for up to three weeks, provided they are kept cool and damp. Outside this season, they are best used within 48 hours of cutting.

Pollarding or coppicing of bankside willows, or pruning of earlier spiling will produce suitable material, as detailed above.
Knock the stakes in vertically along the line of the revetment, about half a metre apart, and so that two- thirds of the stake is below soil level, and then cut the tops to the finished height.
Weave the withies between the stakes, alternating the direction of the butt ends, and tucking both ends in so they don’t protrude. At the end of each length of revetment, the spiling should be run into the bank so that the last two to three stakes are completely buried after backfilling.
To deter vandals, wire down the top withies and secure to the stake with a staple.
Where there is a risk of backfill washing out, place faggots (see below) behind the spiling. Backfill to the required bank profile with soil.

The following alternative design of spiling produces a looser weave, and is suitable for revetments above water level.

Knock the stakes in as described above.
To start the weaving, push the end of a short withy into the ground between the second and third stake from one end. Weave it between the stakes. Then push another withy into the ground next to it, and weave along in the same direction. The withies will root at the ends.
Continue weaving up to the top of the stakes, ensuring the top withies are tightly woven to secure the spiling.
To deter casual vandalism, wire along the top or down through the spiling. Erect temporary fencing to exclude browsing stock for two to three years.

Wattle hurdles

A similar weaving technique can be used to make wattle hurdles, which are constructed off-site, and then carried to the bank and positioned to form the revetment. This is useful where working conditions at the bankside are difficult. Wattle hurdles for sheep fencing and garden use are available from hurdle makers, but are normally too expensive for bankside revetment work. On bankside revetments they should anyway become disguised with vegetation, so a neat finish is not so important, as long as the structure is strong enough to survive handling and positioning.

Hazel is the best material for hurdle making, but willow is also suitable, and its capacity to root is useful in most situations. If possible, use split hazel for the uprights or ‘zales’, as they hold the laterals better than do round poles.

Assemble the hurdles wherever convenient, and take them to the site in ready-made sections. A traditional sheep hurdle is 1.8m (6′) long by 1m (3.5′) high, with ten zales 200mm (8″) apart. For bankside revetment work, the dimensions are not critical, but zales should be no more than 250mm (10″) apart to give a sufficiently tight weave. A hurdle of the dimensions shown below is easy to handle, but larger sections can be made as required.

Push five 900mm (3′) zales about 300mm (1′) into the ground, evenly spaced in a straight line as shown.
Weave hazel or willow rods between the zales, tucking each end back under itself to lock the wands in place. Force the wands down with your foot to give a tight weave. It may be necessary to wire or tie the top wands to discourage vandalism.
Lift out the sections for transport to the site. Place the hurdles in position, overlapping as shown, and drive them in as far as the lower wands.
Using two 1.5m (5′) stakes per hurdle, knock the stakes into position on the outer side of the revetment, and wire the hurdles to the stakes.
Backfill with spoil as appropriate.

Faggots

Faggots or fascines are bundles of willow or brushwood, bound together and used for various stabilisation and revetment purposes. Faggoting is an ancient technique, undergoing something of a revival as the advantages of ‘soft’ revetments are realised. The faggots slow the flow of moving water and trap silt and sediment, and the buried branches then help consolidate the sediments against renewed erosion.

Willow faggots will root, which is beneficial in most bankside situations. However, where growth may narrow channels and cause flooding, material other than willow should be used.

Faggots are normally made to the maximum size which can be easily lifted by two people, but smaller ones can be made as necessary. The traditional size is about 2m (7′) long and 300mm (1′) diameter, bound at three points. The binding can be of twisted willow, baler twine or wire.

To stabilise a bank, the faggots can be laid in a single or double row, secured by stakes and wire as shown. As necessary, another layer can be added cross-wise.

Smaller faggots, of about 100mm diameter, can be bound together to form ‘mattresses’, for laying on banks subject to boat wash or strong flows. The faggots should be laid in position and wired together, and then wired to stakes knocked well down into the bank. A large faggot at the base absorbs some of the wave energy.

Geotextiles

The term geotextiles encompasses a vast range of woven and non-woven fabrics used for a variety of purposes in civil engineering. These uses include revetment, ground stabilisation and reinforcement, drainage and revegetation. Geotextiles may be made from natural, biodegradable materials, or from long-lasting synthetics, according to their design and function.

Some of the many designs suitable for revetment, bank stabilisation and revegetation are shown below, but other makes and designs may be suitable. Manufacturers will supply detailed literature and advice on particular sites. Horticulture Week, Landscape Design Extra and other periodicals are the best source of up-to-date information on suppliers.

Lightweight three dimensional matting for surface protection and root reinforcement, suitable for banks above water level, and banks below water level in still or slow moving water. For example, Enkamat.

Woven geotextile for use with timber stakes, to protect the bases of slopes and vertical banks of watercourses. For example, Nicospan.

Woven geotextile with pockets for vegetation or ballast. For example, MMG Pocket Fabric.

Woven polyester/polyamide bags containing silt or sand, for underwater repairs and bank protection. For example, Nicobags.

Coir fabric rolls and pallets ready-planted with native wetland plants, to protect banks from erosion. For example, Bestmann Fibre Rolls and Pallets.

Geogrids of polyester and PVC, or other similar materials, laid in layers as shown, to stabilise steep banks. For example, Fortrac.

See page163 for suppliers.

Dry stone revetments

Dry stone revetments are a traditional stabilisation technique used on the upstream side of earth dams, spillways and banks where there is some current, but not strong continuous erosion or tidal scour. See Dry Stone Walling for full details on all aspects of walling.

The main requirement for stone revetments is a good source of stone, preferably within close reach. You may be able to get waste stone from a nearby quarry or from a demolished stone building.

Clear away roots and debris and clean out the eroded area until you have sufficient space for the foundation layer, which will normally need to be at least 500mm wide. Choose the largest stones for the foundation and base layers.
Place the stones in overlapping layers, with each stone resting securely in position, and slanting slightly into the bank. Occasional long stones can be placed with their long axis locking into the bank.
As you build each layer, fill behind with small stones to prevent the bank washing out from behind. Do not force small stones into gaps in the front of the revetment, as they will simply wash out and leave the wall weakened.
Top the revetment with turf to help protect it both from erosion and from being dislodged by people or animals.

Gradually sloping banks can be reinforced by stone pitching. Large, block-shaped stones make the strongest pitching, but smaller stones of variable size and shape can be used.

Grade the bank as necessary to an evenly sloping profile.
Excavate a trench for the largest foundation stones and place them in position.
Lay the stones in overlapping layers, with their long axis into the bank. Try and fit them together neatly to avoid large gaps, as these may erode and loosen the whole structure from underneath.

Gabions

A gabion, from the old French word for ‘cage’, was originally a wicker basket filled with earth and stones, and used for fortifications. The modern equivalent is a wire mesh box, available in various sizes and designs, widely used for roadside revetments, river banks, retaining walls and other reinforcement purposes. Gabions provide a means of building large revetments with small stones, and provide a strong but flexible and porous barrier. Although the mesh is not visible until close up, the ‘square’ nature of a gabion construction and the appearance of the rubble it contains means that structures built of gabions are easily recognisable for what they are. They can become disguised with vegetation in suitable locations. Manufacturers of gabions are listed on page 163. Gabion-like baskets or rolls can be made on site out of steel or polypropylene mesh.

For low revetments, a single layer of gabions will be sufficient, set on a level foundation. For two layer revetments, the gabions should be staggered, with the face of the revetment angled to 10˚ to increase stability. The face can be stepped or flush. Engineering advice should be sought for design of structures higher than two gabions.

Where scour is likely at the base of the revetment, mattress- shaped gabions may be needed.Alternatively, a foundation layer can be sunk below the base of the bank.

The minimum size for stone to fill the gabions is 10mm larger than the mesh size. Stones larger than 200mm are not suitable for rough filling, as they will pack unevenly. To improve the appearance, the exposed face of the gabion can be carefully constructed like a dry stone wall, with the remainder roughly filled.

Dig out a trench as necessary for the foundation layer, making the base as even as possible. Finish to a 10˚ angle for a two-layer revetment.
Assemble the gabions with ring clips or wire, according to the manufacturer ’s instructions, and join the first layer together in situ. For a straight revetment, the gabions can be tensioned by anchoring one end, and threading crowbars through the other end of the row of gabions and then tensioning with a winch.
Fill the gabions in layers, one third at a time, fitting horizontal wires to tighten.

Where shrubby growth would not be a hindrance to access, it can be encouraged by inserting fresh willow or alder cuttings into the gabion during construction. These will sprout and quickly disguise the gabion. Reeds and other marginal plants can also be inserted into the mesh as required.

As well as bankside stabilisation, gabions can also be used for overflow weirs on single spillway dams (see Chapter 13 – Dams, weirs and sluices), and as current deflectors or ‘croys’. For this, the gabions should be anchored to steel stakes driven a metre or so into the stream bed. Water movements and changes in the stream bed should be carefully monitored to see if the gabions remain stable and have the effect intended.

Chainlink gabions

Galvanised chainlink fencing formed into long cigar- shaped gabions have been successfully used at Lindisfarne National Nature Reserve to protect a tern nesting island from tidal scour. Sand collects behind the gabions and vegetation roots in the mesh, further stabilising the shore. An advantage of this design is that it lacks square corners, which would create points of scour in tidal conditions. Rolls 22m (25 yards) long of 1220mm (48″) width, 12.5 gauge, 50mm (2″) mesh double link fencing are used to make structures 22m (25 yards) long and about 375mm (15″) in diameter, sewn up with wire.

Roll out the chain link fencing where the gabion is to be sited.
Place stones along a central strip, leaving enough margin for ‘wrapping’. Use the largest stones at the base, carefully filling the gaps with smaller stones. Don’t use stones smaller than the mesh size.
Starting at one end, one person wraps the fencing over the stones so the edges meet, while another person sews them together with a length of wire, pulled tight as it is sewn.

As necessary, another gabion can be added in the following year, by which time the first gabion should have stabilised sufficient material to be partly buried. Place a line of boulders as necessary to support the second gabion, and join it to the base gabion with wire ties.

Sandbags

The use of hessian bags (approximate size 450 x 600mm) filled with sand or other material is another traditional technique for building revetments.

The following method has been used for canal bank repairs in Warwickshire:

Dig out collapsed section of bank to fairly firm foundations.
Using a cement mixer, make a dry mix of aggregate: cement in the ratio 6:1. Shovel into hessian bags, filling by no more than one half.
Place a bag lengthwise into the bank, tucking the open end underneath, and flattening the top with the fists or feet. Place the next bag so that it slightly overlaps the first one, and continue to complete the first layer.
Thoroughly wet the bags, but do not flood with water, or the cement will wash out.
Continue with following layers, overlapping the bags so that each touches two bags in the layer below. Fill behind with rubble and earth.
Where three or more layers are below water level, tie the layers together by hammering steel stakes through the bags at 1 metre intervals. The stakes should be 300mm (12″) long, 16mm (3/4″) diameter, and hammered in until the top of the stake is below the surface of the bag.
Finish top with soil and turf.

Where the depth of bank to be repaired is higher than 400mm (16″), the sandbags should be tied into the bank. The ties are made of 3m lengths of 16mm (3/4″)diameter steel bar, bent to the dimensions shown. Two ties are required for every 3m length of revetment.

Place the bags up to the normal water level, fixed with stakes as described above.
Add one further layer of sandbags.
Use a steel tie to mark the correct position for the pit, as shown in the diagram.
Dig the pit to the dimensions shown, and as necessary, a channel for the ties.
Secure the ties into the sandbags with U shaped pins of 16mm (3/4″) steel.
Fill the pit with dry aggregate and cement mix, and add water.
Add further layers of sandbags up to the level of the rest of the bank, and top with soil and turf.