Floods are the most common and widespread of all natural disasters, except fire. Most communities in the United States can experience some kind of flooding after spring rains, heavy thunderstorms, winter snow thaws, waterway obstructions, or levee or dam failures. Often it is a combination of these elements that causes damaging floods.
Flood waters can be extremely dangerous. The force of six inches of swiftly moving water can knock people off their feet and two feet of water can float a car. Floods can be slow or fast rising but generally develop over a period of days. Flooding is a natural and expected phenomenon that occurs annually, usually restricted to specific streams, rivers or watershed areas.
A flash flood is an event that occurs with little or no warning where water levels rise at an extremely fast rate. Flash flooding results from intense rainfall over a brief period, sometimes combined with rapid snow melt, ice jam release, frozen ground, saturated soil, or impermeable surfaces. Most flash flooding is caused by slow-moving thunderstorms or thunderstorms repeatedly moving over the same area. Flash flooding is an extremely dangerous form of flooding which can reach full peak in only a few minutes and allows little or no time for protective measures to be taken by those in its path. Flash flood waters move at very fast speeds and can move boulders, tear out trees, scour channels, destroy buildings, and obliterate bridges. Flash flooding often results in higher loss of life, both human and animal, than slower developing river and stream flooding.
Flooding has been a regular and frequent hazard in Iowa. Iowa has been involved in 32 Presidential Declarations of a major disaster related to flooding since 1953. The Nation Climatic Data Center lists 30 flood and 34 flash flood or urban and small stream flood events from January 1950 through March 2009 that affected communities in Linn County and resulted in over $888 million in property damage and $22 million dollars in crop damage. The unprecedented flooding in 2008 affected100% of the City of Palo, 14% of the City of Cedar Rapids, and damaged 7,289 structures throughout the County. The damage was estimated to be over $4.5 billion.
As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff two to six times over what would occur on natural terrain. Portions of Linn County are very developed with significant amounts of impervious surfaces, as more development occurs in the watersheds; the amount of runoff produced also increases. If measures are not taken to reduce the amount of runoff (or slow its movement), flash floods will continue to occur and may possibly increase. Linn County experiences minor to moderate seasonal flooding from both the Wapsipinicon and Cedar Rivers and their tributaries.
The Federal Emergency Management Agency has delineated the probable extent of the 100 and 500 year flood hazard areas in most areas. These Flood Insurance Rate Maps (FIRMs) show properties affected by the floods that have at least a 1% chance (100 year floodplain) or .5% (500 year floodplain) of occurring in any particular year. Generally, these areas are in the floodplain or adjacent areas. Much of these areas are parkland, agricultural areas, or conservation land, but residential and commercial areas are impacted by river flooding as well. People and property located in areas with narrow stream channels, saturated soil, or on land with large amounts of impermeable surfaces are likely to be impacted in the event of a significant rainfall. Unlike areas impacted by a river/stream flood, flash floods can impact areas a good distance from the stream itself. Flash flood prone areas are not particularly those areas adjacent to rivers and streams. Streets can become swift moving rivers, and basements can become deathtraps because flash floods can fill them with water in a manner of minutes.
Below is an enhanced FEMA Flood Insurance Rate Map (FIRM) that shows the areas of the County that could be could be more susceptible to flooding.