Levees Artificial levees prevent floods Usually made with fine-grained sediments that are easily eroded in floods Higher-quality levees mix coarse gravel or face with riprap to resist erosion
Levees
Recurrence Intervals and Discharge
Floods are more intense when ground is saturated
Floods are more intense in urban areas
Floods occur in areas without storms
Flash Flood
Flash floods are common in deserts Time is < 6 hours Water is not absorbed >50% fatalities occur in vehicles Slot canyons are bad for hikers Can occur via dam failure –Natural dams (ice, quake) –Constructed dams
Levee Failure
Liouzhou, China (1996)
Summer ‘08 20 Levees failed on Mississippi River Mississippi flooding near Meyer, Illinois, 6/21/08
Levee reduces width of flood-flow part of river and raises water level during flooding Levees act to: –Eliminate storage of floodplain –Constrict flow and raise upstream water levels –Increase flood levels upstream and downstream Unintended Consequences of Levees
Case In Point Repeated Flooding in Spite of Levees: Mississippi River Basin Flood, 1993
Case In Point
Iowa 1993
Summary Drainage basins and divides Meandering vs. Braided Deltas and Alluvial fans Discharge calculation Natural/artificial levees Flood recurrence intervals