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001156 A. Floods as a Natural Process It is often observed that floods are natural but flood losses are man-made. 'This perhaps oversimplifies the situation, since human actions may greatly influ- ence natural flooding, and natural flood "losses" may occur in the "form of ero- sion and other physical damage. But the distinction does indicate the importance of understanding the natural process of flooding before considering how to deal with its consequences. Flooding may be defined as the occa- sional overflow of surface water onto ad- joining land. It is estimated that about seven percent of the continental land area of the United States is subject to occasional flooding. Floods arise from a variety of causes. . Coastal flooding: In coastal areas, flooding is caused by tropical stonns and hurricanes along the Atlantic and Gulf coasts and by earthquakes and tsunamis on the Pacific coast. Coastal flooding usu- ally takes the fonn of an abnor- mally high tide known as a "storm surge," accompanied by heavy wave action in areas exposed to the open sea. . Flooding of rivers and streams: In- land flooding chiefly occurs along rivers and streams (''riverine flood- ing"), due to rapid snowmelt, severe thunderstorms, or prolonged rain- fall. . Lake flooding: Flooding along lakes can arise from sudden changes in atmospheric pressure ("seiches"), although along the Great Lakes floods are of less concem than shore erosion. But seiches can be a problem: for example, Lake Okee- chobee in Florida in 1928 experi- enced a serious flood induced by seiche. This report will be largely concemed with riverine floods. 1. The Hydrologic Cycle Flooding is one aspect of the "hy- drologic cycle." A brief review of this cycle is therefore a good starting point (Fig. 1). Precipitation falling as rain or snow on land surfaces must go some- where. Much of it is returned to the at- mosphere through evaporation from ex- posed surfaces and as transpiration from trees and other plants. Some of the pre- cipitation is absorbed by the land itself by percolation through the soil. Such groundwater may be stored for consider- able lengths of time in geologic fonna- tions called "aquifers. .. Groundwater gradually moves through its aquifer until it eventually reaches the surface through a spring or other outlet. Still other pre- cipitation may be stored on the surface for varying lengths of time in the fonn of ice and snow; much of the world's fresh water supply is frozen in the polar ice caps. All other precipitation-which is not evaporated, absorbed into the ground, or frozen-begins its descent from wher- ever it fell towards sea level in the fonn of surface runoff. At first, this runoff moves in minute quantities by overland "sheet flow." As gathering quantities of runoff accumulate, more defined path- ways are created in the form of rills, gul- lies, and rivulets. These in turn combine to fonn brooks, streams, and finally rivers whose destination is the ocean. The hydrologic cycle is completed as moisture retums to the atmosphere from the oceans of the world. 2. What Is a Watershed? For floods to be understood, water- sheds (also known as drainage basins) must be understood also (Fig. 2). It was stated above that water flowing across land becomes concentrated in defined channels. 'This is the result of the in- fluence of gravity: water always flows downhill. Thus, the route of surface run- off from rain to ocean follows a gen- erally predictable watercourse, and all land areas contributing their runoff con- stitute the watershed of that water- course. 'The amount of water in the watercourse in turn is directly dependent upon the amount of runoff contributed by its watershed (including groundwater which reaches the surface). All land thus falls within some water- shed (although surface runoff may occur only occasionally during dry times). Watersheds adjoin each other along hy- drologic "divides "-namely, the contours 2