Natural features, structures and ecosystems can play a significant part in mitigating hazards, and the value of ‘green’, ‘bio’ or ‘soft’ engineering approaches is being recognised. They can also complement structural engineering interventions (this mixture of approaches is sometimes referred to as ‘hybrid’ engineering). In many countries, conventional attempts to control flooding, such as dams or embankments, are being modified in favour of restoring natural flood plains and wetlands that absorb surplus water more effectively (see Case Study 8.2: Room for the River). Green spaces, permeable surfaces, sustainable urban drainage systems and green roofs can all reduce flooding due to rainfall (pluvial flooding) in urban areas.
Planting trees and grass is a well-established method of preventing rapid water run-off and stabilising hillsides. Bamboo, trees and certain kinds of reeds are often grown along river banks to bind soil, collect sediment or protect against water scouring. In Afghanistan, the NGO Concern Worldwide has planted fast-growing trees behind floodwalls to stabilise the soil and reduce erosion as the walls wear.+A. Clark-Ginsberg, Concern’s Approach to Disaster Risk Reduction: Afghanistan (London: Concern Worldwide, 2013). Planting mangroves along coastlines can provide effective defence against the winds and sea surges generated by tropical cyclones and storms by absorbing and dispersing the force of the oncoming water. The mangrove forests in the Sundarbans in southern Bangladesh are said to have played a significant role in mitigating the effects of Cyclone Sidr in 2007, whereas previous human destruction of mangroves contributed to the damage caused by Cyclone Nargis in Myanmar in 2008. Mangroves can also help to control coastal erosion. Mangrove-planting projects give a boost to local livelihoods: mangroves grow quite quickly and the crabs, shrimps and molluscs that live in them can be sold in local markets.+See http://www.wetlands.org/Whatwedo/Mangrovesforcoastalresilience/tabid/174/Default.aspx; http://www.scidev.net/global/policy/news/un-mangrove-loss-intensified-myanmar-cyclone-damag.html; http://www.scidev.net/global/earth-science/news/mangroves-protect-coastal-villages-during-cyclones.html.
Flood management has been practised for many centuries in the Netherlands, which is very exposed to sea and river flooding. Extensive land reclamation and major engineering projects were generally effective in controlling water and reducing flood risks. However, in 1993 and 1995 increased river discharges caused by snow melt and rainwater from upstream countries led to the evacuation of a quarter of a million people. These events, and forecasts of more frequent high river discharges in future, resulted in the Room for the River programme, due to be completed in 2015, which complements the flood defence system by allowing space for controlled flooding in more than 30 locations, creating new floodplains and setting dikes further back from the river.