FLOOD01141

• County: Statewide
• Community: Nationwide
• Title: Approximate Method for Quick Flood Plan Mapping
• Date: 11/7/1975
• Prepared By: ASCE National Convention
• Floodplain - Doc Type: Educational/Technical/Reference Information

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The discharge can be converted to stage either by computing backwater

curves or, more approximately, by applying uniform flow equations. Both

approaches require a cross section at the site, and backwater computa-

tions require additional sections downstream. Numerous handbooks (4)

and textbooks (1) describe how to compute water surface elevations for

open channel flow and computer programs are available for the computa-

The tlstage index slope" (518) was defined as

SIS

S25 - SIO .........................(6)

tionS (7).

where 525 is the stage of the 25-year flood and 510 is the stage of the

lOryear flood. These two floods are frequent enough for ready estimation

from regional flood studies or historical data but large enough to exceed

the channel capacity of most natural streams. If floods of less than bank-

full stage are used, values of the "stage index slope" vary over a wider

range and in a more unpredictable pattern.

For each of the 51 stations, stages for the 10, 25. 50, 100, and

500-year floods were tabulated and normalized by dividing by the SIS as

shown in Table 2. The plotted data showed that most of the curves follow~

ed the same general shape but that those with computed skews either sub-

stantially larger of substantially smaller than those for most of the

data were generally steeper or flatter respectively.

For the 100 and 500-year flood frequencies, normalized stages are

plotted versus the computed station skews on Figs. 3 and 4. Least squares

regressions for the four frequencies showed the relationship to be signi-

ficant and to improve for rarer events. The results were used to plot

normalized stage curves for flood frequencies between 25 and SOD-years

on Fig. 5.

Normalized Stage Curves

A method that is even quicker but somewhat more approximate is to

use stages directly in the extrapolation. In order to develop a normal-

ized curve for this purpose, it is necessary to incorporate the effects

of the shape and roughness of flood plain cross sections, stream slope,

and backwater effects and other factors that influence the stage-discharge

relationship (rating curve). The data base used for this study was the

USGS gaging stations for which long-term records are available. These

stations generally have fairly well established rating curves and their

computed skew is more reliable.

The 67 selected stations have records ranging from 52 to 104 and

averaging 64 years. Frequency curves were developed for each gaging

station by using the log Pearson type III distribution with computed

skews. Using these frequency curves and rating curves obtained from the

USGS, stage frequency curves were developed for 51 gaging stations. Six-

teen of the 67 gaging stations were not used because their stage-discharge

curves were inadequately defined for extrapolation to the 100-year flood.

Rating curves for the other 51 stations were plotted on 10g.10g paper

and extrapolated where necesssry and if the points fell on a straight line.

Use of the "StalZe Index Slooell Method

In order to use Fig. 5 to estimate the lOO-year flood stage, -it

is necessary to determine the stages for the lO-year and 25-year floods.

One approach would be to convert the flows from regional hydrologic

studies to stages by using open channel flow equations or backwater

curves. Sometimes known stages for historical floods can be used direct-

ly by assigning a frequency to the events by some means. For a desired

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