Riverine floods

Riverine floods are caused by precipitation over large areas, or by the melting of the winter's accumulation of snow, or both. Riverine floods take place in river systems whose tributaries may drain large geographic areas and encompass many independent river basins. Floods on large river systems may continue for periods ranging from a few hours to many days. Flood flows in large river systems are influenced primarily by variations in the intensity, amount and distribution of precipitation. The condition of the ground - amount of soil moisture, seasonal variations in vegetation, depth of snow cover and imperviousness due to urbanization – directly affects flood runoff. Three characteristics of river channels – channel storage, changing channel capacity, and timing – control the movement of riverine flood waves. As a flood moves down the river system, temporary storage in the channel reduces the flood peak. As tributaries enter the main stream, the river gets larger and larger downstream. Tributaries are not of the same size nor are they uniformly spaced; therefore, their flood peaks reach the main stream at different times. The difference of timing tends to modify peaks as a flood wave moves downstream.

Urban development, settlement, drainage of wetlands for agriculture, and canalization of rivers for irrigation or navigational purposes have removed the normal flood plains of numerous rivers throughout Asia, Europe and, in particular, the Americas. In the absence of natural absorption basins, rivers rise higher, flow faster and flood more violently. For example, the flooding of the Odra river in Central Europe in the summer of 1997 is estimated to have cost nearly US$6 000 million in economic losses in affected countries, with the most severe damage in Poland which lost 2 000 km of railway line, 3 000 km of roads, 900 bridges and 100 000 houses (Munich Re 1997). The 1996 flooding of the Yangtze River in China killed more than 2 700 people, left two million people homeless, drowned tens of thousands of animals, destroyed crops over some 20 million hectares of farmland, and resulted in a 4-6 per cent loss in GDP. China experienced severe flooding again in 1998.