Fluvial Design Guide - Chapter 9

Floodwalls and flood embankments

9.8 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 (see Figure 9.4), 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 requirement 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.

General guidance on flood embankments can be found in Management of flood embankments – good practice review (Defra and Environment Agency, 2007). An example is shown in Case study 9.3.

Guidance on the environmental and landscaping aspects of flood embankments can be found in Chapter 5 of this guide and in the Environment Agency’s Landscape and environmental design guide (Environment Agency, 2007).

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).

Figure 9.10 shows a floodbank with a crest wall and Figure 9.11 a flood embankment with a sheetpile wall.

Figure 9.10 Floodbank with crest wall

The addition of the crest wall raises the height of the defence without increasing the footprint of the embankment. It also presents the opportunity to have a crest level that is not subject to erosion damage. However, the wall may restrict vehicular access along and across the bank, and it does detract from the visual amenity of the bank. It is vital to undertake a stability analysis to check the performance of the crest wall under extreme hydraulic loading.

Figure 9.11 Flood embankment with sheetpile wall

This flood embankment has a sheetpile wall driven through the crest. The advantages of this approach include:

  • the sheetpiles can be driven deep enough to provide an effective cutoff;
  • the presence of the sheetpiling allows the embankment to occupy a smaller footprint;
  • the projecting sheetpiles provide additional height to the defence and form a consistently level and erosion-proof crest.

However, visual amenity considerations may discourage the use of this approach. It also has the disadvantage (in this case) of preventing vehicular access for inspection and maintenance.

Embankments offer good opportunities for landscaping flood defences into their surroundings. They do not have to follow straight lines, and can have variable crest widths and side slopes – provided the dimensions are appropriate for the materials used and do not compromise maintenance activities. Trees should not be planted on flood embankments as they accelerate drying out and cracking, and a breach of the bank may result if they are blown down in a storm.

Figure 9.12 River Trent floodbank in Nottingham

Note that the floodbank incorporates a surfaced path. This not only improves access along the river for members of the public, but also helps to define the crest level and makes its degradation less likely.

Flood embankments are often used as public footpaths, either informally or as designated rights of way (see Figure 9.12). Access ways along or across embankments may need surfacing to prevent degradation of the flood defence. Details are available for footpaths, steps and ramps from the Environment Agency’s National Capital Programme Management Service (NCPMS) (see TD09, TD10 and TD13 on the NCPMS website).
 

 

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