FLOOD01647

• County: Statewide
• Title: Optimal Sizing of Urban Flood Control Systems
• Date: 3/1/1974
• Prepared For: US
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

rate of the fourth power; for deviations less than 10% the penalty is reduced
rapidly.
The objective function is a meaningful representation of system performance
only if it is possible to accurately calculate and develop confidence in the individual
components comprising the function, For example, the annual damage at a
control point, AD j' results from economic analyses that define potential damage
and hydrologic analyses that define the exceedence frequency relationships.
In order that this procedure be as nearly acceptable to Corps of Engineers
users as possible, the hydrologic and economic analyses are performed by the
computer simulation model by approximate current state-of-the-art methods in
use by the Corps.
The hydrologic simulation is performed using rainfall-runoff procedures that
consist of: (I) Subdividing the watershed into subbasins; (2) computation of

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FIG. 1.-Rainfall.Runoff Computations for Complex Basin

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subbasin average rainfall; (3) extraction of subbasin losses to yield rainfall excess;
(4) computation of a runoff hydrograph from individual subbasins by use of
the unit hydrograph procedure; (5) routing subbasin hydrographs to concentration
points by application of hydrologic routing procedures; and (6) combining
hydrographs at concentration points. The simulation is performed by the HEC-I
computer program (6) that has been in use by Corps hydrologists for a number
of years. A schematic diagram of the computation of runoff hydrographs at
various points in a complex basin is shown in Fig, 1.
The economic calculation of the expected value of annual damages is performed
using the Corps procedure that consists of: (I) Estimating the economic conse-
quence of a flood from a damage function that relates the damage for a flood
event to the peak flow or stage; and (2) combining this function with the exceedence