AGS / Geohazards / Other Geohazards / Floods
by J. Michael Howard
Floods are among the most powerful forces expressed on the surface of the earth. Societies worldwide have developed along and adjacent to rivers where floods have major impacts. The importance of the role of flooding in shaping our cultural and physical surroundings has led to study of the variety of types of floods, their mechanisms and their magnitudes through time and space. Today, stream flow analyses are taken of many major rivers and streams in Arkansas as part of these on-going studies.
Several types of floods are recognized by their area extent and duration. Flash floods occur in Arkansas due to localized heavy rainfall, especially after periods of gentle soaking precipitation. Large magnitude or regional floods, along major rivers, occur after major long duration rains in the given stream or river watershed. Such flooding may not be due to local rain events at all, but rainfall that fell days or months before in other areas of the watershed. Such events are characteristic of rivers like the Mississippi or Arkansas. Annual flooding along major rivers have lead to channelization, lock and dam systems, levees and other man-made attempts to minimize regional flooding. But these are only temporary efforts and must be constantly maintained and improved.
Flash floods in Arkansas are more typical of the Interior Highlands Region, due to its larger number of smaller drainage basins and steeper stream gradients when compared to the Gulf Coastal Plain Region.
Floods are classified according to their frequency and depth. The phrase "100-year flood" still causes confusion to the public, lenders, and insurers. A 100-year flood does not mean that it is a flood that occurs every 100 years. The term is actually is an abbreviated way of describing the magnitude of a rainfall and subsequent flood that has a one percent chance of happening. A 100-year flood occurs less frequently than a 10-year flood, but because it has larger volume and greater depth of water, it is far more destructive and damaging, and is a more serious threat to human safety. A newer designation termed "base flood" is the equivalent of the term "100-year flood". Base flood elevations in a given area indicate the expected depth of water should that magnitude flood occur.