FLOOD04543

• County: Statewide
• Community: Harrison County, Iowa
• Basin: Statewide
• Title: California Bend Missouri River Fish and Wildlife Mitigation
• Date: 11/1/1994
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Data ‎(general)‎

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Table 2. Statewide drainage.basin charaderistic equations
for estirnatiTlll 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 atreams per square mile; 1TF. 2-year, 24-hour precipitation
intensity, in inches]

Estimation equation

Average
Standard standard error
error of estimate of prediction
Log10 Percent (percent)

Average
equivalent
years of
record

Number of streamftow-gaging stations = 164
Q2 = 53.1 CDA 0.799 RRO.643 DFJ.381 (1TF _ 2.5)1.36 0.171 41.0 42.2 3.9
Q5 = 98.8 CDAo.755 RRO.6I;2 DFJ.380 (1TF _ 2.5)0.985 .156 37.2 38.6 5.4
QI0 = 136 CDA O. 733 JmO.654 DFJ.384 (1TF _ 2.5)0.801 .160 38.2 39.8 6.5
Q25 = 188 CDAo.709 JmO.6I;5 DFJ.393 (1TF _ 2.5p.610 .172 41.3 43.2 7.8
Q50 = 231 CDAo.694 JmO.6I;6 DFJAOl (1TF _ 2.5)0.491 .185 44.5 46.5 9.5
QlOO = 277 CDAo.681 JmO.656 DFJ.409 (1TF - 2.5)0.389 .198 48.0 50.2 11.5

Note: Basin characteristics are map-lICale dependent. See Appendix A and Appendix B for
basin-characteristic descriptions, computations, and lICales of maps to 1188 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
(1TF) 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 regionalization
within the State.

ESTIMATING DESIGN-PLOOD DIlICIIAIlGBS USING D_E-1lA81N C1IAIlACTERI8T1C8 17