THE PHYSICAL PROCESSES OF FLASH FLOODS

The National Weather Service (NWS) Hydrologic Glossary (NWS, 2004c) defines flooding as the inundation of a normally dry area caused by high flow or overflow of water in an established water channel (e.g., river, stream, drainage ditch) or the ponding of water at or near the location where substantial rain fell. Flooding occurs whenever a drainage system receives more water than it can handle. As noted at the beginning of this chapter, a flash flood is a flood that follows the causative event in a short period of time and often is characterized by a sudden increase in level and velocity of a flowing water body. The term “flash” reflects a rapid response to the causative event, with rising water levels in the drainage network reaching a crest within minutes to a few hours of the onset of the event, leaving extremely short time for warning. A threshold of approximately 6 hours often is employed to distinguish a flash flood from a slow-rising flood (Mogil et al., 1978; Georgakakos, 1986a; Gruntfest and Huber, 1991; Polger et al., 1994; NWS, 2004c). Thus, flash floods are localized phenomena that occur in watersheds with maximum response times of a few hours—that is, at spatial scales of approximately 10,000 km2 or less, depending on the catchment characteristics (Hirschboeck, 1987; Gruntfest and Huber, 1991; O’Conner and Costa, 2004). Most flash floods occur in streams and small river basins with a drainage area of a few hundred square kilometers or less (Kelsch, 2001). Such basins respond rapidly to intense rainfall rates because of steep slopes and impermeable surfaces, saturated soils, or because of human- (i.e., urbanization) or fire-induced alterations to the natural drainage.

Causative events may be either excessive rainfall in a natural drainage basin or human-altered catchment or the sudden release of water impounded by a natural jam (i.e., formed by ice or rock, mud, and wood debris) or human-made dam or levee. This report focuses on flash flood events associated with heavy rainfall. Extraordinary unit discharges (i.e., the rate of water flowing past a stream gauging station divided by the drainage area) from a watershed are related to specific topographic and climatologic conditions (Kelsch, 2001; O’Conner and Costa, 2004). In general, basins producing high unit discharges correspond to areas in which regional climatic patterns can produce extraordinary precipitation, such as within and flanking the Appalachian Mountains along the Atlantic seaboard, the western or southwestern flanks of mountain ranges near the Pacific Coast, and a broad northeast-trending zone in the southern Midwest extending from southwest Texas to southeast Kansas and southern Missouri. Such extraordinary rain-



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