FLOOD08373

• County: Statewide
• Community: Rock Rapids, Iowa
• Basin: Statewide
• Title: Problem Identification Report and Appendixes at Rock Rapids
• Date: 4/1/1995
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

I :
I-
t -
I
I
I
I
I
I
I
I
,
I
,
,
I
~
I
1
I
1
i I
I.
I r
y ,
-
n
I
I

.........

ralil.'..- Stau"widc di-ii1ia6,.boIiIa c1uaia&riItic 'fuotwrv
, fo;'"ii~tu'i6n~~'bi'oUa' oC.::-.
,..... - , ..- -- - .....

[Q, peak discharge, in cubic feet per eec:ond, for a liven recurrence interval, in yean; CDA,
contributing drainage area, in square miles: RR, relative relief, in feet per mile; DF. drainage
frequency, in number of firat-order atreams per square mile; TTF, 2.year, 24.hour precipitatiOD
intensity, in inches]

Eatimation equation

Average
Standard ltancl4rd error
error of eatimate of ptediction
LoIl0 Percent (percent)

Avenp
equivalent
years of
record

Number of atreamftow.gaging statiODl . 164
Qa .53.1 CDAo.799 RRO.643 D~.381 (TrF. 2.5)1.38 0.111 41.0 42.2 3.9
Q, = 98.8 CDAo.7M RRO.662 D~.380(TrF. 2.6)0.986 .156 37.2 38.6 5.4
Ql0 .136 CDAo.733 RR'!.664 D~.3M (TrF - 2.6)0.801 .160 38.2 39.8 6.5
,it
Q. .188 CDAo.709 RR'!.656 D~.393 (TrF. 2.6f.610 .112 4U 43.2 1.8
~
Q60 .231 CDAO.694 RR'!.656 D~.401 (TrF - 2.6f.491 .185 44.5 46.5 9.5
!
QUM) = 277 CDAo.681 RR'!.656 D~,409 (TrF - 2.5)0.389 .198 48.0 50.2 lL5

Note: Baain characteristics are map-scale dependent. See Appenclis A and Appenclis B lor
baain-characteristic descriptions, computations, and acalea 01 mape to w lor
manual measurement&. . .

frequency are the number of firat-order atrelUDB
(FOS) and contributing drainage area (CDA).
The value of FOS ia determined by using
Strahler'a method of ordering streams (Strahler,
1952). A description of Strahler'1
stream-ordering method is included in Appendix
B. The 2-year, 24-hour precipitation intenaity
(TTFl ia a primary drainage-basin.
characteristic measurement of the maximlUll
24-hour precipitation expected to be exceeded on
the average once every 2 yeara.

Additional information pertaining to the
characteristics used in the drainage-baain
equations (table 2) ia included in Appendix "-
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
mapa of scalea other that! the scales used to
develop the equationa ma, produce results that
do not conform to the range of eatimation
accuracies listed for the equationa 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 dift'erence
between the Pearson Type-In and multiple-
regression estimates of peak discharge for the
drainage-basin equations, indicated no evidence
of geographic bias. The drainage.baain
equatiollS thus were determined to be
independent of hydrologic regionalization
within the State.

IlSTDIATlNG DESlGN.FLOOD DISCIL\IlGES VSING DIWIlAGE-_ CIIAJIACIDIII'l 17