FLOOD06431

• County: Statewide
• Basin: Statewide
• Title: Estimating Design-Flood Discharches for Streams in Iowa Using Drainage-Basin and Channel-Geometry Characteristics
• Date: 1/1/1993
• Prepared For: Iowa City
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

Table 2. Statewide drainage-basin characteristic equations
for estimating design-flood discharges in Iowa

[Q, peak discharge, in cubic feet per second, for a given recurrence interval, in years; CDA,
contributing drainage area, in square miles; RR, relative relief, in feet per mile; DF, drainage
frequency, in number of first-order streams per square mile; ITF, 2-year, 24-hour precipitation
intensity, in inches]

Estimation equation

Average
Standard standard error
error of estimate of prediction
LoglO Percent (percent)

Average
equivalent
years of
record

Number of streamflow-gaging stations = 164
Q2 = 53.1 CDAo.799 RRo.643 D~.381 (ITF _ 2.5)1.36 0.171 41.0 42.2 3.9
Q5 = 98.8 CDAo.755 RRo.652 D~.380(ITF _ 2.5p.985 .156 37.2 38.6 5.4
QIO = 136 CDAo.733 RRo.654 D~.384(TTF _ 2.5)0.801 .160 38.2 39.8 6.5
Q25 = 188 CDAo.709 RRo.655 D~.393 (ITF _ 2.5p.61O .172 41.3 43.2 7.8
Q50 = 231 CDAo.694 RRo.656 D~.401 (ITF _ 2.5)0.491 .185 44.5 46.5 9.5
QIOO = 277 CDA 0.681 RRo.656 D~.409 (ITF _ 2.5p.389 .198 48.0 50.2 11.5

Note: Basin characteristics are map-scale dependent. See Appendix A and Appendix B for
basin-characteristic descriptions, computations, and scales of maps to use for
manual measurements.

frequency are the number of first-order streams
(FOS) and contributing drainage area (CDA).
The value of FOS is determined by using
Strahler's method of ordering streams (Strahler,
1952). A description of Strahler's
stream-ordering method is included in Appendix
B. The 2-year, 24-hour precipitation intensity
(TTF) is a primary drainage-basin-
characteristic measurement of the maximum
24-hour precipitation expected to be exceeded on
the average once every 2 years.

Additional information pertaining to the
characteristics used in the drainage-basin
equations (table 2) is included in Appendix A.
Techniques on how to make manual
measurements from topographic maps for the
primary drainage-basin characteristics used in
the equations are outlined in Appendix B.
Several of the primary drainage-basin

~

,

characteristics are map-scale dependent. Use of
maps of scales other than the scales used to
develop the equations may produce results that
do not conform to the range of estimation
accuracies listed for the equations in table 2.
The scale of map to use for manual
measurements of each primary drainage-basin
characteristic is outlined in Appendix A and
Appendix B.

Examination of residuals, the difference
between the Pearson Type-III and multiple-
regression estimates of peak discharge for the
drainage-basin equations, indicated no evidence
of geographic bias. The drainage-basin
equations thus were determined to be
independent of hydrologic rcgionalization
within the State.

ESTIMATING DESIGN-FLOOD DISCHARGES USING DRAINAGE.BASIN CHARACTERISTICS 17