5. 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

6. Levees

7. 1800 12

8. Recurrence Intervals and Discharge

9. Floods are more intense when ground is saturated

10. Floods are more intense in urban areas

11. Floods occur in areas without storms

12. Flash Flood

13. 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

14. Levee Failure

15. Liouzhou, China (1996)

16. Summer ‘08 20 Levees failed on Mississippi River Mississippi flooding near Meyer, Illinois, 6/21/08

17. 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

18. Case In Point Repeated Flooding in Spite of Levees: Mississippi River Basin Flood, 1993

19. Case In Point

20. Iowa 1993

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