Rivers and Flooding Rivers and Flooding Drainage basin
Rivers and Flooding
Rivers and Flooding Drainage basin or watershed
A River Mill Creek Drainage Basin
Walla. Basin Walla River Drainage Basin Mill Creek Drainage
Velocity Variation
Meandering and Floodplains
Rivers and Flooding Gradient Velocity Discharge Q = V x A ft 3/sec (cfs) or m 3/sec (cms) V = velocity A = Cross-sectional area = Width x mean depth Base level (ultimate and local)
2000 Hydrograph for Mill Creek at Walla Averages for 1942 -1990 1800 1600 Discharge (cfs) 1400 1200 1000 800 600 400 Maximum 200 Mean Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep
Hydrograph for Mill Creek flood, Feb, 1996 Measured at Kooskie (max. was 6000 cfs at Walla) Mill Creek Flows 3500 Discharge (cfs) 3000 2500 2000 1500 1000 500 0 0 5 10 Day in Feb, 1996 15
Load: Bed load (rolling, saltation) F Suspended load: silt and clay carried in suspension F Dissolved load: ions in solution HCO 3 - SO 42 - Ca 2+ Na+ Mg 2+ typically 90% of dissolved Competence: the amount of load that a particular stream can carry F
Equilibrium and the longitudinal profile l l l Gradient Velocity Discharge Load Base level
Deltas
Alluvial Fans
Flooding is dependent upon F precipitation in drainage basin F precipitation rate/infiltration rate F 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
Recurrence Interval as flood indicator Rec. Int. = (Y-1)/R Y = # of years for data R = rank of flood
Upstream floods F In upper parts of drainage basins F Usually due to downpour in small area F F Flash floods that may not flooding in larger rivers downstream Can still be huge events locally (> 100 -yr flood) Downstream floods F Produced by storms of long duration F Saturate soil and cover large area F Flooding on small tributaries may be limited, but cumulative effect is huge
Chattooga = upstream flood Savannah at Clayton = downstream flood
Development and flooding The effect of urbanization: F More rapid runoff F More frequent F Shorter lag-time F Higher peak discharge F Shorter duration
Factors in damage: · Land use on floodplain · Magnitude (depth and velocity) · Rate of rise · Effectiveness of warning systems · Season (crops lost) · Sediment load deposited
Response to Flood Hazards F Physical barriers · Levees · Flood walls with huge steel gates around towns · Retention reservoirs · Channelization to handle increase discharge · Diversion channel system (LA)
Levees protect area behind until flood becomes too great working levee damming water behind it failed levee
Floodwalls on Mississippi in St. Louis and New Orleans
Mill Creek - Bennington Lake Project
Mill Creek - Bennington Lake Project
Mill Creek - Bennington Lake Project
US Army Corps of Engineers attempts to control the Mississippi - Atchafalaya River
Corps’ control structures
Floodplain Regulation l l Combine restrictions on use and physical barriers Goal: obtain most beneficial use of the floodplain and minimize flood damage with an overall management plan
Flood Hazard Mapping Delineate risk areas and past floods
Flood Hazard Mapping Satellite images Upper = normal flow 1988 Lower = 1993 flooding
Floodplain Zoning Establish flood-prone areas F Floodway District most susceptible s s F Allows development that will not increase flood elevation by > 0. 3 m Farms, pasture, wildlife sancturaries, golf, parking areas, bridle trails… Floodway Fringe District less so, but still within 100 -yr flood area s Allows some buildings if anchored to prevent flotation
Channelization: a controversy
Channelization: a controversy Adverse effects: l Aesthetics and habitat · Drainage of wetlands · Cutting trees along banks · Destroys diversity of flow patterns & changes peak flows · Destroys aesthetics l In many cases may increase flooding downstream
Mitigating Channelization
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