Causes of Flooding • Too much rain: • Lots of rain over short period (flash floods) • Continued rain over long periods • Rain on melting snow • Dam failure • Hurricanes (sea water floods land)
Floodplains • Floodplain – flat areas directly adjacent to streams that is periodically covered with water.
Floodplains • Importance: • Serve as natural storage area for flood waters • Flat, fertile land (floods deposit nutrient-rich sediment) • Wetland ecosystems
Floodplain Construction • Over time, meandering widens a stream valley. • Floodplain is formed by shifting point bar deposits. • Note: point bar deposits can be mined for sand and gravel. Fig. 16.21
Floodplain Construction • Floodplains are also constructed by overbank deposits. • water overflows channel onto floodplain • velocity decreases, sed. deposited (sand, silt, clay) Fig. 16.23
Floodplain Construction • Natural levees are the highest point on a floodplain. That's why the French Quarter wasn't flooded during Katrina.
Floodplain Destruction • Fine-grained overbank sediments are deposited on floodplains during a flood. Fig. 16.23
Floodplain Destruction • Flooding is inevitable. • If we choose to live on floodplains, we must be prepared to deal with the consequences. Fig. 16.26
Should the government restrict development in floodplains? • If not, who should pay for reconstruction when homes/buildings in floodplains are damaged or destroyed?
Human Impacts on Streams • Land use changes: • Natural conditions, high infiltration, low runoff • Urban/agriculture, lower infiltration, higher runoff • Human activities tend to increase runoff.
Human Impacts on Streams • Natural – water stored below ground, slowly released to streams • Urban/ag. – runoff increases stream discharge during storms. Fig. 16.30
Flood "Control" • Flooding cannot be prevented and is a common geologic process. • We can only minimize the damage caused by flooding. • Levees (keeps water in the channel) • Detention basins and dams (store flood water)
Levees failed Levees held Flood "Control" • A levee is an artificial wall that keeps water in the stream channel. • Increased water height can cause more damage when levees fail.
Flood "Control" • Detention basins temporarily store water away from streams. • Required drainage control for new developments.
Flood "Control" • Detention basins often double as open spaces.
Flood "Control" • Tipton Trails, Bloomington – artificial meanders and lakes detain water.