The construction of floodwalls and embankments has been the traditional means of protecting lowlying communities and infrastructure against flooding. Although the primary function of a wall or embankment may be flood defence, such structures also frequently have a secondary function – quite often with the aim of enhancing the environment or improving the amenity or both.
Based on "C E Rickard (2009): 9 Floodwalls and flood embankments 29p. In: UK Environmental Agency (2009): Fluvial Design Guide (Contains public sector information licensed under the Open Government Licence v3.0.)"
Flood embankments
Flood embankments are earthfill structures designed to contain high river levels. They are commonly grass-covered, but may need additional protection against erosion by swiftly flowing water, waves or overtopping. Protection may take many forms, but options include: stone riprap; gabions and gabion mattresses; open-stone asphalt; concrete bagwork; concrete blockwork (which can either be individual blocks or linked to form a mattress); various products that may be categorised as bioengineering such as coir rolls, faggots and fascine mattresses. Geogrids and geotextiles can also be used to reinforce grass on flood embankments.
The basic form of a flood embankment is trapezoidal in cross section, with a horizontal crest and sloping inner and outer faces.
The width of the crest is normally determined by asset management requirements, with widths of 2m to 5m being the normal range. In the absence of more specific guidance, designers are advised to adopt a crest width which is two metres wider than the maximum width of plant that will be used on the crest (allowing one metre safety margin on each side).
The slopes of the inner and outer faces are a function of:
- the strength characteristics of the earthfill material used;
- the type of maintenance equipment used (for grass cutting, for example);
- any landscaping requirements.
Normally the embankment side slopes are between 1:2 (vertical to horizontal) and 1:3. Steeper slopes are very difficult to maintain (grass cutting), while flatter slopes tend to add unnecessarily to the footprint of the embankment and the quantity of fill material required.
An embankment with relatively steep face slopes has a smaller footprint and lower earthfill require-ment than one with more gentle slopes; it may therefore cost less and have a lower environmental impact. Steeper slopes can be achieved by using earthfill with a higher clay content or by a range of soil strengthening techniques, but designers must always take into account the asset management needs and ensure that these can be carried out safely (for example, avoiding the risk of maintenance plant overturning on a steep slope). The designer must be certain that the profile of the embankment selected meets all the service requirements and, in particular, is stable throughout the full range of loading conditions.
Embankments are normally set back from the edge of the river to:
- allow for some flood storage on the floodplain;
- reduce the risk of undermining caused by riverbank erosion.
Set-back embankments are also less prone to erosion of the riverward face due to high velocity flow, but may be more prone to wave damage.
Flood embankments can be constructed from a variety of earth materials. Wherever possible, locally won material should be used, to reduce costs and lessen the environmental impact. The strength of the material used to construct the embankment is increased by compaction, which is a fundamental part of the construction process. The required strength is achieved by constructing the embankment in layers and compacting each layer using mechanical plant appropriate to the type of soil. It may be necessary to add water to each layer to improve the degree of compaction required; this depends on the nature of the soil and its moisture content. The advice of a geotechnical engineer should be sought regarding the appropriate layer thickness and the type of compaction plant required.
Soils with high clay content are best avoided because these crack when they dry out, and such cracks can extend a metre or more into the bank, compromising its function as a flood defence. Soils with a high sand or gravel content can be used, but may have to incorporate some form of cutoff to reduce seepage in flood conditions. Granular soils are less resistant to erosion than cohesive soils once the topsoil layer has been eroded.
Because of the shortage of suitable fill and the adverse environmental consequences of importing large quantities of fill from afar, various alternatives to conventional fill material have been explored. These include the use of recycled tyres compressed into bales to form a central core to a flood embankment. Options such as this need careful investigation before being adopted, with particular emphasis being given to long term durability and stability, environmental risks (such as contamination) and the overall environmental impact.
It is normal to strip topsoil from the foundation of an embankment before construction starts. This helps to key the embankment to its foundation and reduces settlement. It also provides a source of topsoil to encase the embankment and allow the establishment of a suitable grass cover.
Where the foundation soils are weak (for example, a layer of peat), the options are:
- remove the weak layer (if it is near the surface and relatively thin);
- strengthen the foundation (potentially an expensive option);
- accept and allow for the resulting long-term settlement;
- pre-load the foundation to accelerate settlement.
If the foundation is highly permeable (for example, a thick layer of gravel), it may be necessary to take steps to cut off the seepage path through the foundation.
Embankment foundations should always be checked for the presence of buried (agricultural) land drains prior to construction, as any that are left in place could result in excessive seepage and even embankment failure.
Other services may also be present along the route of the flood embankment, and these may need to be diverted or protected to avoid damage. The cost of diverting a gas or water main can be significant, but is normally much less than the costs from accidental damage during construction of the embankment!
Embankments in rural settings are often accessible by livestock and agricultural machinery. Both can cause significant damage, degrading the bank crest where they regularly congregate or cross the defence. Fencing can be used to control livestock movement, and pathways and machine access routes can be surfaced to reduce the likelihood and amount of damage. Cattle can be prevented from grazing flood embankments by providing two strands of barbed wire at the top of fence posts. The height of the lower strand can be high enough to allow sheep to pass under, as sheep do not cause damage to the embankment surface. Stock-proof fencing may be required at field boundaries. Gates or stiles may be required to maintain pedestrian access.
If a high level of burrowing damage is expected, it may be appropriate to incorporate a deterrent (such as wire netting) into the surfaces of the embankment.
Cracks in embankments can create seepage paths. Cracking occurs in clay soils during dry conditions and is best avoided by not using highly plastic clay soils for fill in the top metre of the crest.
Seepage can also occur where structures pass through the embankment (for example, a drainage culvert). The soil–structure interface requires careful attention during construction to minimise this risk, most notably by ensuring good compaction of the embankment fill around the structure. The likelihood of seepage can also be reduced by lengthening the seepage path (for example, by constructing a concrete collar round a pipe passing through the embankment)
Floodwalls
There are two basic types of floodwall:
- those that also form part of the river frontage, such as a wharf, retaining wall, or quay;
- those that are remote from the river, generally with the sole purpose of providing a flood defence.
Defences that form part of the river frontage usually have deep foundations and considerable overall height. Often such walls have been in existence for many years and their flood defence function has increased with time, with progressive heightening of the crest level. Such defences need careful investigation if they are to be upgraded or refurbished to provide an acceptable service life.
The form of construction of such walls includes brick, masonry, timber, sheetpiling and concrete.
The main factors to consider include:
- the type, condition and stability of any existing foundations;
- the presence of historic wall elements that might make driving of new sheetpiles very difficult (old timber piles that have rotted away often leave embedded parts in surprisingly good condition – these can present significant obstructions to the driving of new piles);
- there may be a requirement to conserve historic elements of a wall;
- the need for tie rods or ground anchors to restrain the wall against overturning (commonly used with steel sheetpile walls);
- the need for access ways in the defence to allow the continuation of business and leisure activities on the river frontage;
- traffic loading surcharge on the landward side (these can be particularly onerous at an operating wharf or quayside);
- additional loadings on the wall from mooring or boat impact;
- the need to accommodate diurnal variation in river level for tidal rivers (which may result in daily changes in the hydrostatic pressure direction on the wall).
Should the existing river frontage not be suitable for upgrading or rehabilitation (having reached the end of its service life), the option of setting the floodwall back from the frontage should be considered. This has implications for the flood defence of the land between the river and the floodwall, but may be the only acceptable option if the flood defence is to remain independent of the frontage and thereby not dependent on its stability. Such a situation is likely to arise when the party responsible for constructing and maintaining the flood defence does not have (and does not want to take on) any responsibility for the existing river frontage structures.
For defences remote from the river, construction tends to be more straightforward. Concrete (both precast and insitu) is the most common form of construction, often with some form of cladding or decorative finish. Brick and masonry can be used, but these either have to be massive structures (unless very low in height) or be reinforced with steel bars. Low brick walls can be formed by constructing a tied cavity wall on a concrete foundation, with reinforcing bars extending from the foundation up the cavity. The cavity can then be filled with concrete, during which the brick skins may need external support while the concrete in the cavity hardens.
Where a cutoff is required, a sheetpiling wall offers the advantage of providing both the cutoff and the wall – though it is normal to clad the wall with brick or masonry to improve its appearance. Where space permits, one side of a sheetpile wall can be given a ‘half-bank’, so that it appears to be a flood embankment from that side.
Standard precast wall concrete units offer the advantage of speed of construction, but may lead to wastage if the ground level along the wall alignment is very variable, requiring the wall height to vary. (The advantage of using precast units is reduced if many different sizes are needed or if the largest size required is used throughout.) Cast insitu walling is more often used where there are frequent changes of direction or wall height.
Where a floodwall passes through private land, there may be a need for an easement to ensure the right of access for inspection and maintenance is provided for ever.