Fluvial Design Guide - Chapter 9

Floodwalls and flood embankments

9.2 Fundamentals

This chapter defines a flood defence as any structure that is designed to (or by virtue of its nature and location is able to) contain floodwaters and to reduce the probability of flooding in the defended area. In the UK, such defences rarely exceed 5m in height and more commonly are 1–3m high.

The traditional approach to the design of such flood defences was to estimate a design flood level and then calculate the required elevation of the top of the flood defence by adding on a suitable freeboard. This guide adopts a risk-based approach to the design and assessment of flood defences and therefore considers the performance of the whole flood defence system under a wide range of conditions, including extreme floods. The fundamental principles that underlay this approach are presented in Sections1.2 and 1.3.

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. Indeed, for any works commissioned or consented by the Environment Agency, there is a duty under the Environment Act 1995 to conserve and enhance the natural beauty of rivers and coasts. There are some notable examples of this multi-function approach, such as at Gainsborough and Perth (see Figure 9.2), where the construction of urban flood defences has been used as an opportunity to improve an urban waterfront. Further guidance on landscape improvement as part of fluvial design is given in Chapter 5.

Figure 9.2 Perth flood alleviation scheme

In this scheme, the designers took the opportunity to enhance the riverside through the creation of attractive river walls with a high quality masonry finish. The flood defences have been sympathetically incorporated into a major improvement of the river frontage. This is a really good example of a ‘win–win’ project.

A floodwall can be constructed from brick, masonry, concrete, sheetpiling or a combination of these materials. Steel is the most common material for sheetpiles, though the alternative of plastic should not be overlooked for situations where the lower inherent strength is acceptable. A flood embankment is constructed from earth, and may include a clay core to reduce seepage through the embankment. Both floodwalls and flood embankments may require a cutoff to limit seepage through the foundations (see Section 9.9).

The traditional temporary barrier against flooding is the sandbag wall, which offers flexible and versatile emergency protection, but cannot produce a watertight defence and requires a lot of effort to erect and remove. Temporary and demountable defences (see Section 9.10) can be constructed from timber, steel, aluminium, plastic and various combinations of these materials (Ogunyoye and van Heereveld, 2002).

Very few defences are completely watertight; most exhibit some leakage or under-seepage through the foundation material when holding back a flood. The design should cater for this.

It is fundamental that a flood defence structure remains stable under hydraulic loading, even if the loading is prolonged or if the defence is overtopped. Although some damage to a flood defence may be expected in an extreme flood, this should not impair the serviceability of the structure and under no circumstances should the defence collapse during a flood. The assessment of existing defences and the design of new structures must therefore consider all potential failure modes (see Section 9.6). The greater the height of a flood defence, the more serious would be the consequences of a collapse of the wall (because the flood wave generated would be more damaging). High walls and embankments thus merit particular attention, both in their design and in asset inspection.


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