Floods

Causes of Floods

Floods can be understood within the concept of the drainage basin (watershed), often referred to as the fundamental unit of hydrological research. This unit can be viewed as a system, with one part inextricably linked to the next. The basic input to this system is precipitation, and it is how precipitation moves through various storage components that determines downstream discharge, the output to the system. The speed of water moving through the system and the associated “losses” to evaporation, transpiration, and storages in groundwater and lakes will affect the timing and quantity of water in the river channel. If the outcome is flows exceeding bankfull capacity, then there is a flood in that drainage basin (Figure 1).

Determining exactly how individual drainage basins respond to a given input of precipitation is a challenge to hydrologists. The characteristics of drainage basins, including their shape (especially at small scales), geology, soil types, vegetation, and geomorphology, all play a role in determining discharge. Furthermore, when precipitation occurs, it is the antecedent environmental conditions within the drainage basin that control flood severity. For example, snow cover, frozen ground, wet or saturated soils (or even dry soils on occasion), preexisting high river levels, ice-covered streams, and heavy precipitation can all exacerbate hydrological conditions and lead to floods. The response of a drainage basin to a given input of precipitation, therefore, may be relatively fast, or “flashy,” where flows can be high, peaking rapidly, often with a high velocity (Figure 2). In contrast, some systems may take days or weeks to reach peak discharge and inundate the floodplains, conditions that are termed sluggish. The former systems have a short precipitation-to-flood lag time, the latter, a longer lag time. So while all streams flood, they do not necessarily flood for the same reasons or in the same way.

The extensive flooding in the Midwestern United States in June 2008 demonstrates the role of antecedent conditions. These floods were triggered by a series of storms that crossed the [Page 1135]region, dumping in excess of 12 in. (inches) of rainfall on top of already saturated soils following an abnormally wet winter and spring. The result was high streamflows and record-setting floods. Bangladesh, in contrast, regularly experiences flooding during the monsoon season, and consequently, local communities have adapted agricultural practices to these seasonal patterns. The floods of August 2007, however, were exceptional and caused widespread damage even though the total rainfall was somewhat less than normal. The floods occurred because most of the rain fell in just 2 months, June and July.

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