FLOOD07643

• County: Statewide
• Basin: Statewide
• Title: Physical and Economic Feasibility of Nonstructural Flood Plain Management Measures
• Date: 3/1/1978
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

APPENDIX A
SENSITIVITY ANALYSIS OF FLOOD DAMAGE DATA

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As part of this investigation into nonstructural measures it was desired to investigate the
n'lagnitude and sensitivity of flood damage to various hydrologic, hydraulic, and economic
parameters, and to evaluate the effectiveness of selected nonstructural measures to reduce
flood damage. The parameters investigated included the elevation-frequency relationship,
frequency "skew", depth-damage relationship, value of structure contents as a percentage of
the value of a structure, type structure, and location in the flood plain. The effectiveness of
selected nonstructural measures was evaluated by computing the damage reduced for several
levels of protection. The damage values shown in graphs and tables in this Appendix were
computed using generalized elevation-frequency and depth-damage data. While these values
are useful for screening alternative measures they should not be used in lieu of more detailed,
site specific data required for surveyor Phase I GDM Studies.

Elevation-Frequency Relationships
In 1970 the Federal Insurance Administration (FIA) published a series of generalized
elevation;frequency curves which were derived from stage-frequency curves taken from FIA
Flood Plain Information Reports and Corps of Engineer's Survey Reports gathered nationwide
(1). It is estimated some 150 to 200 curves were used. While the 1970 curves do not show the
actual field data, they were constructed to represent an envelope 01 these data. Each curve was
identified by what is termed a flood hazard lactor (FHF) which is the difference in elevation
between the 10-year and 100-year exceedance interval event. The flood hazard factor varies
from 0.5 feet to 20.0 feet in increments of 0.5 feet. Each flood hazard factor cu rve, has associated
with it several additional curves of different "skew". This "skew" is not the same as the skew
associated with log-Pearson Type III discharge-frequency analyses, rather it is developed by
taking percentages of the flood hazard factor and adding or subtracting these values at the 25-
and SOO-year exceedance intervals from a straight line through the 10- and 1M-year events.
Constructing the maximum positive and negative skews (skew D and I respectively) in this
manner gives an envelope of curves within which most of the field data lie.

A 1974 study by an FIA consultant used a series of elevation-frequency curves and depth-
damage data (provided by the FIA) to compute flood insurance rates (4). The elevation-
frequency curves were nearly the same as the median curves in the 1970 FIA report. These are
shown in Figure A-1. They were extrapolated to the annual event and are the ones used in most
of the analyses of this study. For purposes of identification they have been designated skew M
(median) in this study. A range of flood hazard factors - 1.0 to 20.0, In increments of 1.0 foot -
were analyzed, although only selected ones are reported. Some inconsistencies in expected
annual damage were observed and these are discussed more fully under "Computational
Accuracy" in this Appendix.

Depth-Damage Relationships
In addition to the frequency information published in 1970, the FIA also presented
information on depth-percent damage relationships for different type structures, with damage
being expressed as a percentage of the structure and contents value (1). These data use total

A-1