Levees Artificial levees prevent floods Usually made with fine-grained sediments that are easily eroded in floods Higher-quality levees mix coarse.

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

1800

12

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

Iowa 1993

Iowa 1993

Summary

• Drainage basins and divides• Meandering vs. Braided• Deltas and Alluvial fans• Discharge calculation• Natural/artificial levees• Flood recurrence intervals