Surface Runoff

Abstract

Rainfall, if it is not intercepted by vegetation or by artificial surfaces such as roofs or pavements, falls on the earth and either evaporates, infiltrates or lies in depression storage. When the losses arising in these ways are all provided for, there may remain a surplus that, obeying the gravitation laws, flows over the surface to the nearest stream channel. The streams coalesce into rivers and the rivers find their way down to the sea. When the rain is particularly intense or prolonged, or both, the surplus runoff becomes large and the stream and river channels cannot accept all the water suddenly arriving. They become filled and overflow and in so doing they do great harm to the activities of men. The most serious effect of flooding may be the washing away of the fertile top soil in which crops are grown, and of which there is already a scarcity on the earth. In urban areas there is great damage to property, pollution of water supplies, danger to life and often total disruption of communications. In agrarian societies floods are feared like pestilence because they can destroy crops, cattle and habitations, and bring famine in their wake.