Mangroves effectively reduce the height of wind and swell waves over short distances (less than 500 m),udand can reduce storm surge water levels over greater distances (several kilometres of mangroves). Thus mangroves canudcontribute to coastal defence strategies. However, their appropriate use depends on a thorough understanding of theudconditions under which they can provide these coastal defence services. Here we present a literature review of thisudtopic. Small wind and swell waves can be reduced in height by between 50 and 100% over 500 m of mangroves. Waveudreduction largely depends on water depth and vegetation structure and density. However, few measurements areudavailable for the reduction of bigger waves (> 70 cm in height) in deeper water (> 2 m). Storm surge water levels mayudbe reduced by between 5 cm and 50 cm per kilometre of mangrove, based on field measurements and validatedudnumerical models; water level reduction rates depend on the distance from the edge of the mangrove and the forwardudspeed of the cyclone, amongst other factors. Extreme events may severely damage or destroy mangroves, reducing theirudeffectiveness as a coastal defence. The use of mangroves in hybrid engineering can reduce flood risk: for example, audmangrove foreshore in front of a sea wall/dyke will reduce wave impacts on the wall/dyke. The likelihood of wavesudovertopping the sea wall or walls being breached is thus reduced, with an associated reduction in sea defenceudmaintenance costs. Therefore mangroves can contribute to coastal risk reduction, alongside other risk reductionudmeasures such as sea walls/dykes, early warning systems and evacuation plans. Additionally, mangroves can responduddynamically to rising sea levels, in some cases maintaining their surface elevation with respect to local sea level; thusudthey may act as a sustainable coastal defence in the face of rising sea levels and changing climatic patterns.
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