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

basin and channel-geometry flood-estimation
methods or on the Pearson Type-III estimate
and only one of the regcression-equation
estimates. The design-flood discharge estimate
is a weighted average of these values in which
the Pearson Type-III estimate for the gaged site
is weighted by the effective record length (ERL)
at the gaged site, and the regression-equation
estimates are weighted by the average
equivalent years of record associated with their
respective regression equations.

Calculation of Estimates

The- weighted design-flood discharge
estimate for a gaged site as outlined by the
Interagency Advisory Committee on Water Data
(lACWD, 1982, p. 8-1 - 8-2) is calculated as

regression-equation design-flood
discharge for a gaging station, in
cubic feet per second, (listed as
either method BFRI, ACRI,
ACRII, or BFRII in table 8); and

Eicgl

is the average equivalent years of
record for the channel-geometry
regression equation used to
determine QTigcgl (table 4 or 5).

If both the drainage-basin regression-
equation estimate QTigdbl and the channel-
geometry regression-equation estimate QTIgcgl
are not available for a gaged site, then equation
16 used to calculate the weighted design-flood
discharge estimate QTiwg) is simplified to the

QT(wg)

IQrig)) (ERL) + IQTigdl.)l iEidb)) + iQT(gcg)1 iE(cg)l
ERL+E(db) +E(cg)

, (16)

where QT(wgl is the weighted design-flood
discharge for a gaging station, in
cubic feet per second, for a
selected T-year recurrence
interval;

QT(gl is the Pearson Type-III
design-flood discharge for a
gaging station, in cubic feet per
second, as determined by the
analysis of the observed
annual-peak discharge record
(listed as method B17B in table
8);

ERL is the effective record length for a
gaging station, in years,
representing the QTig) analysis
(table 8);

QTigdb) is the drainage-basin regression-
equation design-flood discharge
for a gaging station, in cubic feet
per second, (listed as method
GISDB in table 8);

Eidbl is the average equivalent years of
record for the drainage-basin
regression equation used to
determine QTigdb) (table 2);

QTIgcgl IS

channel-geometry

the

weighting of two estimates based on QT(g) and
ERL and either QTigdb) and Eidb) or QTigcg) and
Eicgl' An example of weighting a gaged site with
only one regression-equation estimate is
illustrated in "Example 7."

By including both the drainage-basin and
channel-geometry regression-equation esti-
mates, or only one of these estimates, with the
computed Pearson Type-III estimate for a gaged
site, design-flood histories for a relatively long
period of time are incorporated into the
weighted estimate for the gaged site and tend to
decrease the time-sampling error (Choquette,
1988, p. 41). Climatic conditions during a short
gaged period of record often are not indicative of
the longer term climatic variability associated
with a particular gaging station. Such
time-sampling error may be particularly large
when the observed gaged period of record
represents an unusually wet or dry climatic
cycle compared to the longer term average
climatic conditions. Time-sampling error thus is
minimized for a gaging station by weighting the
design-flood discharge estimate QT(wg)'

Examples ofWeighting--Examples 6-7

Examole 6.--Calculate a weighted 100-year
peak-discharge estimate for the discontinued
Black Hawk Creek at Grundy Center
crest-stage gaging station (station number

36 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA