Numerous studies and reports have cited the value of mangroves in protecting shorelines, both from ongoing processes of erosion, and from the more dramatic impacts of storms and tsunamis, but a more detailed investigation reveals relatively sparse data, and some more mixed observations. It is clear that mangroves can support soil accretion both by capturing and holding sediments, and through organic growth and litterfall, however any increase in overall elevation of soil surfaces is compounded by other processes, notably the ongoing compaction of sub-surface sediments. Clearly mangroves attenuate waves, and even at the highest tides their influence remains strong due to the presence of both roots stems and lower branches interrupting water movements. During storms and extreme events mangrove also help to capture and hold debris and reduce wind-speeds at ground levels locally.
Mangroves are a group of trees and shrubs that grow in the upper inter-tidal zone. They are predominantly found in the warm coastal areas from the tropics to some warm temperate regions, where they can form extensive forests. Ideal conditions for mangroves include gently shelving coasts, with soft sediments and low wave action – they are at their most extensive then in estuaries and deltas, and around coastal and coral reef lagoons. Many mangrove species have evolved aerial roots as an adaptation to life in waterlogged soils. These same roots reduce the height of waves in their passage through the mangroves (wave attenuation). They also help to capture and hold sediments and organic matter, encouraging accretion. Mangroves also have a high root biomass, and the growth of sub-surface roots helps to maintain or even increase the elevation of mangrove soils.
Mangroves are highly productive ecosystems, especially in areas where freshwater from rivers or rainfall enhances growth. They provide critical habitat or nursery areas for fish and shellfish and support the lives and livelihoods of millions of people world-wide through the provision of food, timber and fuelwood. Unfortunately large areas of mangroves are being lost through coastal development and the expansion of aquaculture. Although not sufficient to compensate for ongoing losses, mangrove restoration has become an important activity in many countries, and one of the primary drivers for restoration has been for coastal protection.