Flood & coastal defence • Pollution control & water quality • Biodiversity & conservation importance • Palaeoecology & Archaeology • Designations & conservation management

Flood and coastal defence

Flood defences on low-lying open coasts and along estuaries are often fronted by mudflats and saltmarshes which form a first line of defence against wave attack. Wave energy dissipation (also described as wave attenuation) is reflected in a reduction in the wave heights experienced at landward sea defence structures. The presence of fronting marshes thus reduces the risk of overtopping as well as the structural undermining of defence structures and, ultimately, reduces costs for maintaining these defence lines.

The profile of the role natural processes play in contributing to flood defence management has improved in line with increasing costs for sea wall maintenance and repair and due to the influence of sea level rise in accentuating existing problems along low-lying coasts. To this end, the value of saltmarsh (and fronting mudflat) as a functional, natural and potentially sustainable (i.e. self-repairing) wave break has been acknowledged although they may have lesser effect in attenuating extreme water level/wave conditions. The recognition that habitats form an integral part of coastal management has led to alternative and, potentially, more cost-effective and sustainable options for coast protection being developed and implemented (Möller et al., 2001). These include beach nourishment, foreshore recharge and, in the context of saltmarsh management most importantly, managed realignment of flood defences.

The effective attenuation of wave energy by saltmarsh confirms its potential for contributing to shoreline protection. Möller et al., (2001) investigated the likely attenuation and overtopping frequencies for a range of event return periods. This work, determined the effectiveness of saltmarshes in protecting backshore defences, when compared with unvegetated intertidal flat. This study showed that the occurrence of high overtopping discharges (i.e. those that are most likely to cause damage to the defences) is significantly lower when protection structures are fronted by saltmarsh. The main cost savings that can result relate to the lower acceptable specification of the sea wall.

Based on the assumption that a seawall without saltmarsh protection costs £5000/m savings of up to £4600/m could be made with an 80m wide saltmarsh see figure below.

Comparative cost benefits associated with the construction of sea defences in relation to extent of fronting saltmarsh

These values were derived several years ago and, therefore, have to be considered as comparative costs only. It should also be noted that the presence of marsh (and the cost savings obtained) should not be used to justify a sea defence scheme, i.e. this should be economically justifiable in its own right.