Population increase demands more land be turned to development. Most of our most common flood events are classified as riverine . This means that rainfall or snowmelt occurring at any point in a watershed will have an impact on waterways and communities located down stream. The further down stream you go, the more water there is pouring into the main river from its many tributaries. This is why flooding in coastal areas, particularly when coupled with storm tides and high winds can be so devastating as the greatest volumes are concentrated there.
Authorities measure flood events by their rate of rise .
It is important to know this in order to predict when a community will be inundated or the time in which it will take the flood to reach the top of a levee. All emergency efforts are centered around the rate of rise in order to schedule evacuations and station crews to shore up levees. The flood crest is the point when the river ceases to rise and begins to gradually recede. Should there be repeated rainfall or similar events within the watershed, there may be more than one crest with each varying somewhat in their final height. Nearly half of all flash flooding deaths are auto related when motorists are caught unaware by flood water, or when they underestimate the danger of shallow flood water.