FLOOD04322

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Benefit-Cost Analysis for Urban Drainage Planning
• Date: 3/1/1974
• Prepared For: UDFCD
• Prepared By: CSU
• Floodplain - Doc Type: Educational/Technical/Reference Information

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- 34 -

For each reach, and for the entire stream, construct a

graph of total flood damage versus probability of occurrence in

any given year. The graph will be similar to Figure 2.

From this graph, it is possible to interpolate flood damages

for flood events other than the ones investigated. A damage

versus probability curve will be required for the existing

development situation (if a comparison is desired between the

existing and future flood damages), and for each flood control

alternative. Compute the area under the curves, which is the

average annual flood damage potential in dollars per year.

Figure 2 has a great deal of utility for calculating flood

damages of numerous alternative solutions. If a peak discharge

scale is constructed corresponding to the probability scale,

flood damages can be computed quickly for any sized detention

facility. For example, assume that a certain sized detention

reservoir is being considered just upstream of the reach under

study, and that hydraulic studies have determined that it will

reduce the peak 100-year discharge from 20,000 cfs to 14,000

cfs, the 25-year discharge from 10,000 cfs to 7,000 cfs and

the 10-year discharge from 7,000 cfs to 3,000 cfs. By entering

the curve on the peak discharge scale we can determine the

expected flood damages in the reach downstream of the dam.

The damages would be $90,000 for the 100-year event, $65,000

for the 25-year event, and $30,000 for the 10-year event. A

~ Leonard Rice Consulting Water Engineers. Inc.