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

The average absolute errors in per cent and feet along with the correla-

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tion coefficients between the published stages and the estimated stages

are shown in Table 4.

TABLE 4 . Accuracy of Results
Correlation Ave. ABS. Ave. ABS. Number of
Flood Coefficient Error in % Error in Ft. Comparisons
50 -year 0,99 5.2 0.15 37
100-year 0,95 10.4 0,57 19

Aoolication of Aooroximate Method to Selected Site

Dischar2e Method:

In order to demonstrate the approximate procedure for flood plain

mapping, an example flood plain reach on Sweetwater Creek, a tributary

to the Chattahoochee River near Austell, Georgia. about 25 miles west of

Atlanta, was selected (Fig. 11). A USGS gage is located about 6 miles

downstream from the site. and a detailed Flood plain Information Report

was completed by the U. S. Army Corps of Engineers (6). However this

exercise assumes that no gage records or flood information are available.

The most recent USGS regional frequency study for this region

(Table 1) and the tribucary drainage area of 233 sq. mi. were used to esti-

mate QlO as 9,400 cfs .and Q25 as 12,000 cfs. The "discharge index slope"

(log Q25/QlO) is thus 0.1061. The regional skew coefficient for the

Atlanta area (2) is +0.05. According to Fig. 2, the normalized dis-

charges are thus N050 ~ 1.66, NOlaO = 2.24, and NDsao = 3.43.

The discharges as estimated from Eq. 5 are on Table 5.

These discharges must now be converted to stages or elevations

based on surveyed cross sections or cross sections taken from a detailed

topographic map. Sections surveyed at State Highway 6 and at u. S.

25