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Living with Streams in Flood

Living with Streams in Flood. Dayton, OH March 25, 1913. A Recipe for Disaster. A century of land clearance, drainage and development. Major city in a flood plain at confluence of four rivers. Large Meander downstream = bottleneck Stream channel gets smaller downstream.

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Living with Streams in Flood

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  1. Living with Streams in Flood Dayton, OH March 25, 1913

  2. A Recipe for Disaster • A century of land clearance, drainage and development. • Major city in a flood plain at confluence of four rivers. • Large Meander downstream = bottleneck • Stream channel gets smaller downstream. • Heavy late Spring snowfall to saturate the ground followed by heavy Spring rains.

  3. Physical FloodPrevention • Build Flood-control dams on all major tributaries. • Raise levees and flood walls • Strengthen levees against erosion due to faster stream flow. • Straighten and deepen stream channels through major cities

  4. Dams create storage or retention basins that hold runoff and slow runoff into river channel

  5. Channelization: Highly Controversial

  6. Floodplain Regulation: Keep people out of harm’s way

  7. Typical Zoning Map Before and After the Addition of Floodplain Regulations

  8. How do we calculate regulatory parameters?

  9. Recurrence Interval: RI = N + 1 M Where: N = # of discharge measurements in data set; M = the rank of discharge that you are calculating the RI for.

  10. List from high to low the • discharge events in your data set: • Date Q m3/sec Rank • 1980 3800 1 • 1978 2580 2 • 1975 2500 3 • 1983 2350 4 • 1977 1420 5 • 1979 1300 6 • 1976 1280 7 • 1981 1100 8 • 1982 830 9 RI = N + 1 M RI = 9 + 1 = 10 1 A Q of 3800 m3/secwill occur approximately once every 10 years. OR, more precisely A Q of 3800 m3/sechas a 1/10 chance (10%) of occurring each year

  11. What makes flooding worse? 1) Agriculture – removes absorbent topsoil. Reduces permeability of ground and increases runoff. 2) Urbanization – Blacktop & concrete are impermeable. Storm sewers funnel water into rivers at very rapid rates.

  12. EFFECT OF URBANIZATION

  13. Geologists recognize two different types of Floods • Downstream Floods – cover a large area, caused by large, long lasting weather events. These floods tend get worse downstream. • Great Dayton Flood is a classic example. • 2) Upstream Floods – short, often catastrophic events. Short intense thunderstorms in narrow canyons or dam failures are good examples. Flood tends to dissipate downstream.

  14. A Classic Upstream Flood: The Big Thompson River Canyon Flood of 1976

  15. Flood Occurred in evening of July 31st, 1976. • Our country is celebrating its 200th birthday. • Many people camping and vacationing in on of Colorado’s most popular and scenic spots. • Intense Thunderstorm develops quickly and a massive flood develops in the narrow canyon.

  16. Isohytes (lines of equal rainfall) for the July 31 storm: One year of rain fell in 4 hours!!

  17. What Happened? • 11 –12 inches of rain fell in 4 hours, 8 inches fell between 7:30 and 8:40 pm. • A wall of water 20 feet high travels down the canyon at nearly 20 miles per hour. • At 6 pm Q = 137 cfs • At 9 pm Q = 31,200 cfs – almost 4 X greater than a 100 year flood!!

  18. OUTRUN IT? OR CLIMB TO SAFETY? Those who tried to outrun the flood drowned. Many who were able to climb above the torrent lived.

  19. The Aftermath: • 144 people dead (six never found. • 418 homes destroyed. • Over 400 cars washed away. • The canyon remodeled by the flood. • New river floodplain regulations instituted.

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