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

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The mode! error (co) is estimated by

Co' mulmum ro: (SEI1-cl( 1 D'
I L ERL

where SE is the standard error of the
estimate from the OLS equation;

C 1 is a c:onatant; and

\
\ EHL is the mean effective record
length, in yean, for all gaging
stationa used in each respective
regression-model data set.

,

The time-sampling error (ti) ia estimated by
'i . c1 (ER~ ), (6)
(I)

where ERL(i) is the effective record length, in
yeBrll, for the ith gaging station
used in each respective
regression-model data set.

The constant, Cl' is related tD the recurrence
interval of the response variable and tD the
weighted skew coefficient (g) of the observed
annual-peak discharges. It is determined by

C1 . muimum [0:;2(1+ ~2 (I +~i21 +i, D, (7)

where i

is the mean standard deviation of
the logarithms (base 10) of the
observed annual-peak dia-
charges;

1

(5)

streamnow-,acina stationa analyzed using
either the drainage-bain or channel-geometry
flood-estimation technique.. These estimation
methods are based on the a.sumption that i and
i are approximately c:onatant for all the gagina
.tationa in the State.

The effective record length (ERL) of a gagina
station is based on an empirical analysis made
by Gary D. Taker (U.S. Geological Survey,
written commun., March 1992) of resulta
reported ill Tuker and Thomas. (1978)and,_
Stedinger and Cohn (1986). It is determined by ~u

ERL. LS+ (HST-LS).,

(8)

where LS is the systematic record length of
a ,agina station, in yean, the
number of water yean durinc
which the gaging station wu
operated;

HST is the hiatDric record length of a
gaging station, in yean, u used
in a Pearson Type-II1 hiatDrical
nood-frequency analysis; if a
systemalie flood-frequency
analysis was performed, HST =
LS; if (HST-LS) > 200, set
(HST- LS) = 200: and

. . O.55-G.lrloc.(~ll.
L: (I-pn) J

(9)

In the lut equation, ph = 1.0 - (np I HSTJ, and
np is the number of historic and extremely large
discharge (high-outlier) peab.

The ERL used in the weighted least-squares
regression analyses for each gaging station is
listed in table 8 (at end of this report).

is the mean standardized
Pearsoo Type-III deviate for
selected T-year recurrence ESTIMkruJGDESIGN:Fi.oOD
interval and !!lean weighted ..DISCHARGES USING. .
skew coefficient g (IACWD, 1982, _" DRAINAGE-BAsIN
p. 3-1 - 3-27): and _.~ CHARAl;I'MilSTIcS.~.

i is the mean weighted skew
coefficient of the logarithms (base
10) of the observed annual-peak
dischaips (IACWD, 1982, p.
12-15).

The values 8 and i are statewide estimates
determined by the averages of the 188

The drainage-basin nood-estimation method
uses selected drainage-basin characteristics to
estimate the magnitude and frequency of floods
for stream sites in Iowa. Multiple-regression
equations were developed by relating significant
drainage.basin characteristics to Pearson
Type-III, design.flood dischargea for 164

ESTIMATING DESIGN.FLOOD DISCHARGES USIIIG DJlAJNAGE-IIASIN CHARAo; J dIlIlICS S