FLOOD07007

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Expected Annual Flood Damage Computation - Users Manual
• Date: 6/1/1977
• Prepared For: State of Colorado
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

PROGRAM DESCRIPTION

5. BASIC PRINCIPLES

The basic principle upon which this program is based is that flood
damage to an individual structure, group of structures, or flood plain
reach can be estimated by determining the dollar value of flood damage
for different depths of flooding and by estimating the percent chance
of exceedance of each flood depth. For a single flood event, of known
depth, the damage caused by that event is estimated directly from a
depth-damage relationship. When it is desired to compute the average
damage which can be expected a year,' then the damage corresponding to
each depth of flooding is weighted by the percent chance of each depth
occurring (damage caused by rare events is weighted less). Thesum
of the weighted dilmage represents the expected annual flood damage.
If the damage and frequency relationships remain unchanged each year,
then the expected annual value represents the damage which can be
expected to occur during anyone year. However, in practice, either one
or more relationships are likely to change over time; therefore, the
expected annual value will also change. This results in a non-uniform
annual distribution of expected values over time. To compare the magni-
tude of damage of alternative plans or to compare damage with costs, an
equivalent annual value is computed. This equivalent value represents
a uniform distribution (the same each year) of annual values and is
computed by discounting and amortizing each year's expected annual damage
value over a period of analysis. The discounting and amortization takes
into account the time value associated with damage values.

There are several different combinations in which the stage, flow,
damage and frequency data can be expressed to develop the damage-frequency
relationship. The simplest way is to relate stage or flow to damage and
stage or flow to frequency. The common parameter, stage or flow, can be
used to relate damage to frequency. If the damage and frequency data are
not di rectly rel ated to a CODmon parameter then another rel ationship must
be used. This is commonly a stage-flow relationship. Thus, if damage is
expressed as a function of stage and frequency as a function of flow,
or vice versa, damage can be related to frequency with the stage-flow
funct i on.

Because these stage, flow, frequency and damage relationships vary
along a river it is common practice to divide a river into reaches and let
a set of these relationships represent the stage, flow, frequency and
damage data for a reach. An index location is selected within the reach
and a single stage or flow-frequency relationship and stage-flow relationShip
are applied at that location and considered representative of these

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