Typically, river flooding begins as a high crest on the upper part of a watershed that takes several days to move downstream. Due to the slow nature of river flooding, ample advanced warning is provided to evacuate people or property in the path of the flooding. Usually, flash floods occur within a few minutes or hours following an excessive rainfall event. They can also be caused by a man-made event, such as a dam or levee failure.
Flash floods cause most of the fatalities associated with flooding events.
Usually, less warning lead time is provided for flash flooding which requires quick action on the part of the public. Urbanization increases runoff by 2 to 6 times over what would occur in natural terrain. Flood waters can fill streets, freeway underpasses, and parking lots and can sweep away cars. Due to the large average annual rainfall totals over southeast Texas, flash flooding is a relatively common event. Tropical systems during the summer and early fall, and strong winter storm systems can cause widespread flooding and flash flooding across southeast Texas.