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.

Heavy late Spring snowfall to saturate the ground followed by heavy Spring rains.

Physical Flood

Prevention

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

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

Channelization: Highly Controversial

Floodplain Regulation: Keep people out of harm’s way

Typical Zoning Map Before and After the Addition of Floodplain Regulations

How do we calculate regulatory parameters?

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

A) List from high to low the discharge events in your data set:

Date Q m3/sec Rank1980 3800 11978 2580 21975 2500 31983 2350 41977 1420 51979 1300 61976 1280 71981 1100 81982 830 9

RI = N + 1 M

RI = 9 + 1 = 10 1A Q of 3800 m3/sec will occur approximately once every 10 years. OR, more precisely

A Q of 3800 m3/sec has a 1/10 chance (10%) of occurring each year

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.

EFFECT OF URBANIZATION

Geologists recognize two different types of Floods

1) 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.

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

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.

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

What Happened?

1) 11 –12 inches of rain fell in 4 hours, 8 inches fell between 7:30 and 8:40 pm.

2) A wall of water 20 feet high travels down the canyon at nearly 20 miles per hour.

3) At 6 pm Q = 137 cfs At 9 pm Q = 31,200 cfs – almost 4 X greater

than a 100 year flood!!

OUTRUN IT? OR CLIMB TO SAFETY?

Those who tried to outrun the flood drowned. Many who were able to climb above the torrent

lived.

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.