Responses to flooding
How well a flood hazard is dealt with depends on:
- the level of prepardness at the locatipn of the expected hazard
- the amount of warning there has been
- the level of economic development, which influences such factors as emergency service provision, infastructure and the ability to repair and rebuild.
Flood management stratergies
Flood management seeks to reduce the frequency and magnitude of flooding, and therefore, to limit the damage that floods cause. Flood protection can be achieved by the following hard engineering methods:
- banks and/or channel can be modified to enable the river to carry a larger volume of water. Artifically raised and strenghtened banks form a significant part of this strategy. In some cases, parallel lines of flood banks act as a double form of protection - if the river overtops the first barrier, then it has difficulty rising over the second bank some distance behind. The removal of large boulders from the bed of the river reduces roughness, therefore increasing the velocity of flow.
- dams and weirs can be built to regulate the rate at which water passes down a river.
- diversion channels can be constructed to divert rivers away from areas vunerable to flooding.
- dredging can be used to create a deeper channel so that greater volumes of water pass through.
- the height of the floodplain can be increased by dumping material on it.
- retention basins and balancing lakes can be constructed into which water is diverted at times of high discharge.
Softer approaches to flood management are mainly concerned with flood abatement - changing land use upstream. In recent years conservation and sustainability of management schemes have been taken seriously and water manager now balance the needs of flood protection with those of the environment. Soft engineering is generally more sustainable and can be achieved by:
- Afforestation in the drainage basin. Increased interception slows down the rate at which water reaches a river and greater evapotranspiration by trees reduces the amount that reaches to the channel. Although this method can be effective in the long term, it takes time for trees to mature after they have been planted and large areas much be planted in order to reduce the discharge downstream.
- Contour ploughing and strip farming in semi-arid areas, which reduce the amount of surface runoff and therefore, reduce the liability to flooding.
- Floodplain zoning, which allows certain areas of the floodplain to flood naturally - land uses are limited to grazing and recreation in such areas. This method protects other, more economically valuable areas.
- Wetland and riverbank conservation schemes, which involve protecting existing nautral river channels and thier valleys so that habitats and species diversity can be maintained. In some cases arable land is returned to its former use as natural meadowland.
Soft engineering continued.
- River restoration schemes, which return rivers to thier original state before they were managed. This involves compromise between sustainable environmental gain and social and economic considerations. Such schemes aim to improve the quality of the river water as well as implementing more sustainable, soft flood management schemes, and they aim to work with nature. Many rivers in lowland England have been channelised and dredged, which may mean they are disconnected from thier former floodplains. In many cases artifical embankments, constructed on both sides of the channel, create a sharp break between the river and its floodplain. These are unsightly and support little wildlife but once they are removed the flood water can spill onto the floodplain once again. Functional floodplains upstream act as a buffer against catastrophic flooding downstream. Occastional flooding can be beneficial in improving soil fertility.
- Forecasting floods and warnings, in areas where flooding is regular and unavoidable. In Bangledesh the yearly floods which engulf most of the countryside cannot be prevented, but if people are given enough warning they can take refuge in flood shelters.