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

In the computer program the sequence of computations is the same
as that described above except that three intermediate points are used
between each pair of input frequency values. For example, if the
frequency input data values were those shown in Table 1, the computer
would interpolate between .1 and .2 and compute three intermediate
points, .125, .150 and .175. Each of these points would also be used
to compute damage values. A "spline-fit" routine uses the input and
intermediate data to compute additional points prior to integration.
The use of intermediate points allows for a more accurate determination
of the damage-frequency relationships. This point is illustrated in
Figure 4 which shows both the input data and the intermediate points
computed by the computer.

SINGLE EVENT FLOOD DAMAGE

Using Figure 4,f100d damage can be estimated for any flood event
of known frequency simply by picking the damage off the curve at known
frequency values. For example, the 100-year event (1.0 exceedance
frequency in events per 100 years) would cause an estimated $2.39
million damage with 1980 conditions. The 50-year event (2.0 exceedance
frequency) would cause an estimated $1.5 million damage. The computer
program can compute damage for any single event uSing stage or flow as
its input values instead of frequency.

EXPECTED ANNUAL FLOOD DAMAGE

Expected annual damage is the damage which can be expected to occur
in anyone year assuming conditions remain unchanged. It is computed by
weighting each damage value in Figure 4 according to its probability of
occurrence. Graphically this amounts to finding the area beneath the
damage-frequency curve over the entire range of damaging events. It
needs to be emphasized that the correct computation of expected damage
includes the full range of probabilities from initial threshold to zero.
Any truncation, that is, not going to 0 or to the threshold will result
in an error which can be significant depending upon the shape of the
function. The hand integration or weighting of the damage values is
commonly performed by planimeter or rectangular area computation. The
latter approach is illustrated in Table 2.

EQUIVALENT ANNUAL FLOOD DAMAGE

The expected annual damage computed from Figure 4 is for 1980
'conditions. That is, the damage, stage, flow and frequency data
are characteristic of the flood plain and flood hazard in 1980.
If these conditions were to remain the same in the future, then the
expected annual damage value would be the damage which could be expected
to occur during anyone year. However, it is common for conditions to
change--damageab1e property in the flood plain may increase or decrease,
urbanization upstream may cause increased runoff, or the channel itself may

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