Floods

Floods are controlled by the rate of precipitation, run-off, stream flow, and shape of the land surface. They may occur as water overflows the banks of a standing water body (such as a lake) or flowing water (such as a stream), or when rainwater accumulates in an area that normally has neither standing nor flowing water. Areas near rivers, tributaries, creeks, and streams are likely to experience flooding during periods of heavy rainfall.

Floodplains are areas adjacent to rivers and streams that occasionally flood but are normally dry, sometimes for many years. When storms produce more runoff than a stream can carry in its channel, waters rise and flood adjacent lowlands, leaving behind layers of settled sediment. Significant damage and sometimes loss of human life may occur when buildings and other human infrastructure are built on floodplains, under the assumption that future floods may never occur or will only occur in the distant future. Floods can occur at any time, but major floods are more frequent in spring and fall after periods of heavy or sustained rains when stream levels rise rapidly.

In the South Central US, the greatest human impact from flooding is related to events along the banks of major rivers. Historically, there have been a number of record-setting floods along the Mississippi River (in 1927, 1937, 1945, and 1993), which runs along the eastern edge of the South Central. Many South Central floods have also involved major tributaries of the Mississippi, such as the Missouri River, which intersects the Mississippi near St. Louis, and the Kansas River, which intersects the Missouri near Kansas City. The Great Mississippi and Missouri Rivers Flood of 1993 (Figure 10.22) was preceded by a wet fall and a winter with heavy snowfall, followed by a series of precipitation events in roughly the same locations through the spring and summer of 1993. Many locations near St. Louis were flooded for over half a year. Dozens of individuals lost their lives and costs are estimated to have been in the $15 - 20 billion range. However, even with the massive damage sustained from this flooding event, the impact could have been even worse were it not for a series of levees and reservoirs built in response to a large flood of the Kansas River in 1951—an event called “one of the worst [disasters] this country has ever suffered from water,” by President Truman.

Flash floods—rapid flooding of low-lying areas—are often associated with heavy rain, which can quickly waterlog soil and lead to mudslides on steep terrain, resulting in damage to roads and property. In areas of lower elevation, flash floods can be produced when slow-moving or multiple thunderstorms occur over the same area. When storms move more quickly through an area, flash flooding is less likely. Flash floods can also occur in conjunction with a dam break or levee failure. A special case of flooding due to a failed levee occurred in fall 2005, when Hurricane Katrina forced water over and through the levee holding seawater back from New Orleans, part of which is built below sea level (see the “Storms” segment later in this chapter for more information). In this case, flooding came not from precipitation, but from a “storm surge,” where seawater was transported high onto shore through a combination of low atmospheric pressure and powerful winds (Figure 10.23).

While floods are always considered a hazard to life and property, they present a compound threat when they trigger mudslides or contribute to the conditions that cause expansive soils and karst topography. While there is no way to completely avoid the destructive impacts of flooding, good community planning and informed decision-making can greatly reduce the safety concerns and economic impacts of these events. The Federal Emergency Management Agency (FEMA) provides guidelines for communities that are planning mitigation strategies designed to minimize the impacts of natural hazards such as flooding.

Figure 10.22: Confluence of the Mississippi and Missouri rivers, near St. Louis. A) 2002, during non-flooding. B) 1993, during the Great Mississippi and Missouri Rivers Flood.

Figure 10.23: This house in New Orleans was destroyed when it floated off its foundation during the 2005 flood.