Hazards Where Geology and Weather Interact
Flooding is a natural hazard resulting from the combination of geology and weather. The flat-lying bedrock of the Midwest and the large storm events create an ideal setting for flooding. Much of the southern and western boundaries of Midwestern states border the Mississippi and Ohio Rivers. Additionally, most of the waterways of the Midwestern states drain through the Upper Mississippi and Ohio River Basins (Figure 10.8). Areas near rivers, tributaries, creeks, and streams are likely to experience flooding during periods of heavy rainfall. The floodplain of a river is the land around the river that is prone to flooding. This area can be grassy, but the sediments under the surface are usually deposits from previous floods. For those living near a river, it is important to know both the location of the floodplain boundaries and the likelihood and extent of flooding in that area.
Scientist assess the hazard risk of flooding based on historical data, and they then establish a probability of an extreme flood event taking place over a particular time interval. Recurrence interval is the time between major flood events. The physical extent of the flooding during that interval is the floodplain itself. For example, a 100-year flood would be the most extreme flood event to happen over 100 years and would identify a zone of land in the 100-year floodplain with a 1% chance of recurrence each year. Since this value represents only the probability of flooding, it is possible for multiple 100-year floods to occur within a few years. Most people who live within the 100-year floodplain must carry flood insurance on homes, businesses, and personal property. It is not a matter of if a flood will occur in these floodplains, but rather when.
Flooding has some direct impacts that can themselves be hazards. For example, the increased volume of water flowing through a river channel travels at an increased velocity. This allows for the transport of large amounts of sediment, which can erode supports for roadways, bridges, and buildings. Water traveling at high velocity can also carry large objects like cars and buildings. The water itself can enter buildings (causing permanent damage to homes and businesses) and flood farmland (damaging crops and killing livestock). There are also secondary impacts of major flooding events: Floods can cause transportation problems, loss of electricity for long periods of time, and contaminated drinking water supplies. This can cause long-term economic strain on the towns and industries impacted by flooding.
Figure 10.8: Major river basins of the continental US.
In addition to the direct hazards associated with large flooding events, human land use can greatly impact the type and severity of the natural hazards associated with flooding. Because water cannot infiltrate impermeable pavement, there is much more concentrated runoff during rainstorms as the water flows off roads into drains and directly into rivers. To accommodate this, humans have built extensive storm drain systems to prevent the flooding of roads. Unfortunately, these storm drains empty into already swelling rivers. Although these efforts are helpful in preventing flooding in one area, they amplify the flooding event in the river. Heavily developed areas with extensive buildings and bridges can also put increased weight on the ground surface, causing compaction of underlying sediments. This can decrease infiltration rates and the storage capacity of the ground.
While there is no way to completely avoid these human impacts on the natural system, 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.
