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Rivers and Flooding

Rivers and Flooding. Rivers and Flooding. Drainage basin or watershed. A River Drainage Basin. Mill Creek Drainage Basin. Walla Walla River Drainage Basin. Mill Creek Drainage Basin. Velocity Variation. Meandering and Floodplains. Rivers and Flooding. Gradient Velocity Discharge

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Rivers and Flooding

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  1. Rivers and Flooding

  2. Rivers and Flooding Drainage basinorwatershed

  3. A River Drainage Basin Mill Creek Drainage Basin

  4. Walla Walla River Drainage Basin Mill Creek Drainage Basin

  5. Velocity Variation

  6. Meandering and Floodplains

  7. Rivers and Flooding Gradient Velocity Discharge Q = V x A ft3/sec (cfs) or m3/sec (cms) V = velocity A = Cross-sectional area = Width x mean depth Base level (ultimate and local)

  8. 2000 Hydrograph for Mill Creek at Walla Walla 1800 Averages for 1942-1990 1600 1400 1200 1000 800 600 400 Maximum 200 Mean Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Discharge (cfs)

  9. Mill Creek Flows 3500 3000 2500 2000 Discharge (cfs) 1500 1000 500 0 0 5 10 15 Day in Feb, 1996 Hydrograph for Mill Creek flood, Feb, 1996 Measured at Kooskooskie (max. was 6000 cfs at Walla Walla)

  10. Load: • Bed load (rolling, saltation) • Suspended load: silt and clay carried in suspension • Dissolved load: ions in solution HCO3- SO42- Ca2+ Na+ Mg2+ typically  90% of dissolved Competence: the amount of load that a particular stream can carry

  11. Equilibrium and the longitudinal profile • Gradient • Velocity • Discharge • Load • Base level

  12. Deltas

  13. Alluvial Fans

  14. Flooding is dependent upon • precipitation in drainage basin • precipitation rate/infiltration rate • topography Flood Discharge = channel discharge when a river overflows its banks Flood Magnitude = discharge above flood discharge Flood Stage = elevation of water surface for a high-water condition likely to  damage Thus depends on use of floodplain

  15. Recurrence Interval as flood indicator Rec. Int. = (Y-1)/R Y = # of years for data R = rank of flood

  16. Upstream floods • In upper parts of drainage basins • Usually due to downpour in small area • Flash floods that may not  flooding in larger rivers downstream • Can still be huge events locally (> 100-yr flood) Downstream floods • Produced by storms of long duration • Saturate soil and cover large area • Flooding on small tributaries may be limited, but cumulative effect is huge

  17. Chattooga = upstream flood Savannah at Clayton = downstream flood

  18. Development and flooding The effect of urbanization: • More rapid runoff • More frequent • Shorter lag-time • Higher peak discharge • Shorter duration

  19. Factors in damage: • Land use on floodplain • Magnitude (depth and velocity) • Rate of rise • Effectiveness of warning systems • Season (crops lost) • Sediment load deposited

  20. Response to Flood Hazards • Physical barriers • Levees • Flood walls with huge steel gates around towns • Retention reservoirs • Channelization to handle increase discharge • Diversion channel system (LA)

  21. Levees protect area behind until flood becomes too great working levee failed levee levee damming water behind it

  22. Floodwalls on Mississippi in St. Louis and New Orleans

  23. Mill Creek - Bennington Lake Project

  24. Mill Creek - Bennington Lake Project

  25. Mill Creek - Bennington Lake Project

  26. US Army Corps of Engineers attempts to control the Mississippi - Atchafalaya River

  27. Corps’ control structures

  28. Floodplain Regulation • Combine restrictions on use and physical barriers • Goal: obtain most beneficial use of the floodplain and minimize flood damage with an overall management plan

  29. Flood Hazard Mapping Delineate risk areas and past floods

  30. Flood Hazard Mapping Satellite images Upper = normal flow 1988 Lower = 1993 flooding

  31. Floodplain Zoning Establish flood-prone areas • Floodway District most susceptible • Allows development that will not increase flood elevation by > 0.3 m • Farms, pasture, wildlife sancturaries, golf, parking areas, bridle trails… • Floodway Fringe Districtless so, but still within 100-yr flood area • Allows some buildings if anchored to prevent flotation

  32. Channelization: a controversy

  33. Channelization: a controversy Adverse effects: • Aesthetics and habitat • Drainage of wetlands • Cutting trees along banks • Destroys diversity of flow patterns & changes peak flows • Destroys aesthetics • In many cases may increase flooding downstream

  34. Mitigating Channelization

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