Flooding

New Orleans, Aug. 2005

Flooding

– Varies with intensity and amount of rainfall– Perhaps the most universally experienced natural

hazard• Flood discharge- water overflows the channel• Flood stage- stream elevation

Precipitation and Discharge

Types of Floods

• Flood Types– Upstream -intense

rainfall over a small area

– Downstream -cover large areas and result from persistent storms

Development and Flooding

Urbanization – Increases runoff

• Impervious cover (paving)• Storm water (sewers)

– Decreases sediment load – May increase stream erosion – Mostly a small to moderate scale

effect

Decreases lag time (flashy discharge)

Magnitude and Frequency of Floods

R =( N+1)/M

– R = recurrence interval in years

– M (magnitude) is the rank of an individual flow in an array of flows with time

– N is the number of years of record (9 in the example in the book)

• 2-100-year floods can occur in a single year

• Extrapolation limited to 2X length of record

• Run-off Equation: Q = C x i x A

Developing a Rating Curve

N = 9 years (record length)M = 1 (rank in the array)

Thus, a flood with 280 m3

discharge will occur:

R = (N+1)/ M = (9 + 1)/ 1 = 10 years

Magnitude and Frequency of Floods

Magnitude and Frequency of Floods

• Hydrographs of floods– Chattooga-Savannah River downstream flood

– Buffalo Creek upstream flood (artificial due to dam failure)

Upstream flood

Urbanization and Flooding

– Has little effect on 50-year floods; frequency of flooding increases with urbanization

– Urbanization decreases lag-time between rainfall and runoff

– Urban grass has less runoff than row-cropped agriculture

Developmentand Flooding

Nature and Extent of Flood Hazard

• Floodplain advantages– Rich soils– Abundant water– Ease of waste disposal– Proximity to transport, communications, and

commerce• Westward migration led to land clearing and drainage

that led to more and greater floods• Dams and levees cannot control large floods

Nature and Extent of Flood Hazard

• Factors controlling damage by floods– Land use in floodplain– Magnitude (depth, velocity, and frequency of

flooding)– Rate of rise and duration of flooding– Season– Sediment load deposited– Effectiveness of forecasting and warnings

Nature and Extent of Flood Hazard

• Effects of Flooding– Primary

• Injury and loss of life• Direct property damage• Erosion and deposition of sediment

– Secondary • Short-term pollution of rivers and water supplies• Hunger• Disease• Displacement of people• Fires

Responses to Flood Hazards

• Prevention– Structures (levees and dikes)– Reservoirs– Retention ponds– Channelization– Flood insurance

• Adjustment– Floodplain regulation--best approach

• Design with nature, not against it• Minimize the need for structures

Responses to Flood Hazards

• Flood hazard maps• Based on hydrologic discharge data• Estimate using foliage and soil maps

– Floodplain zoning• Floodway district

– Designated passage for 100-year flood

– Elevation increases of 0.3 meters or less

» Agriculture» Airports» Recreation

Responses to Flood Hazards

• Flood hazard maps (Ventura, CA, RV Park)– Floodway Fringe District--land located between

Floodway District and the maximum 100 year flood elevation

– Uses include anchored residential accessory structures

– Fills protected against erosion

The Channelization Controversy

Channelization

– Methods• Straightening• Deepening• Widening• Clearing• Lining

– Objectives • Controlling floods**• Draining wetlands**• Controlling erosion• Improving navigation

The Channelization Controversy

The Channelization Controversy

The Channelization Controversy

– Results• Not necessarily bad• Inadequate consideration of its adverse

environmental effects

– Adverse Effects of Channelization• Impacts wetlands and wildlife• Destroys aesthetics• Wetlands become a ditch environment

The Channelization Controversy

– Benefits of Channelization and Channel Restoration

• Mississippi River drainage• Kissimmee River in central Florida

– Future of Channelization• Should be applied sparingly to specific problems• New designs with new purposes need to be developed

Perception of Flooding

• Knowledge does not inhibit occupation of floodplain areas

• Flood hazard maps are not an effective means of communication

• Upstream development is blamed for downstream floods

• People react differently to the threat of flooding

Flooding Case Studies

Mississippi River: 1973, 1993, and 1998