Flood simply means inundation of extensive land area with water for several days in continuation. Generally, floods are considered to be associated with rivers and people conceive flood as the outcome of accumulation of huge volume of water coming out of the rivers through overtopping of river banks during peak discharge period. In fact, flood is an attribute of physical environment and thus is a component of hydrological cycle of a drainage basin. It may be pointed out that flood is a natural phenomenon and is a response to rainfall but it becomes hazard when it causes colossal loss to human lives and property. It is also important to note that floods are also aggravated by human activity and thus flood hazard is both natural as well as man-induced rather man-accentuated phenomenon. Floods are very often associated with rivers draining extensive alluvial and flood plains About 3.5 percent of the total geographical area of the word is covered by flood plains which house about 16.5 percent of the total population of the world. The most notorious rivers of the world in terms of devastating floods and resultant damage to natural environment (riparian decay) and loss of human lives and property are the Ganga and its major tributaries such as the Yamuna, the Ramganga, the Gomti, the Ghanghra., the GaiJak, the Kosi, the Damodar etc. (northern India), the Brahmaputra (north-east India), the deltaic segments of the Mahandi, the Krishna, the Godawari, the Tapi, the Narmada, the Lum, the Mali etc. (all in India), the Mississippi and Missouri (U.S.A.) the Yangtze, the Yello (China), the Irrawadi (Myanmar) the Indus (Pakistan), the Niger (Nigeria), the po (Italy), the Euphratus and Trigris (Iraq) etc.
Causes of Floods
Since the floods of rivers are the responses of both natural and anthropogenic factors, the cause of flood of the alluvial rivers become highly complex and their relative importance varies from place to place. Among the natural factors which cause river floods Important are prolonged high intensity rainfall; meandering courses of the rivers; extensive flood plains; break in slope in the long profiles of the rivers. i.e. sudden change in channel gradient at the intervening zones of the foothill slope of the mountains and upper end of the plains; blocking of free flow of the rivers because of enormous debris provided by landslides and due to volcanic eruptions; nature of river valleys and channels etc. Anthropogenic activities such as building activity and eventual urbanizaion, channel manipulation through diversion of its (of the river) course, construction of bridges, barrages and reservoirs, agricultural practices, deforestation, land use changes etc. by man invite several hazards in the river system viz, disastrous floods, landslides and slumping along the banks, massive erosion along the river banks causing large-scale riparian decay. Shifting of channels and even of the river courses, siliting of beds. deposition of sands, silts and clays in the flood plains etc. which pose a serious threat to human society and necessitate river regulation and flood control (Savindra Singh, 1983). The following causes may be held responsible for devastating floods of alluvial rivers. They may be held responsible for devastating floods of alluvial rivers. It may be pointed out that these factors should never be considered separately because it is the cumulative effects of several factors which ultimately cause severe floods.