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

05463090; map number 73, figs. 1 and 2). An
inspection of table 8 lists regression-equation
estimates for both the drainage-basin and
channel-geometry flood-estimation methods.
The 100-year Pearson Type-III estimate is
8,320 ft3/s, and the effective record length is 24
years (table 8). The 100-year drainage-basin
regression estimate is 7,740 ft3/s (table 8), and
the average equivalent years of record for this
regression equation is 11.5 (table 2). The
100-year Region I, bankfull channel-geometry
regression estimate is 8,860 ft3/s (table 8), and
the average equivalent years of record for this
regression equation is 16.1 (listed in the first set
of equations in table 4). The weighted 100-year
flood estimate for this gaging station is
calculated using equation 16 as

Estimating Design-Flood
Discharges for an Ungaged Site on a
Gaged Stream

Design-flood discharges for an ungaged site
on a gaged stream can be estimated if the total
drainage area of the ungaged site is between 50
and 150 percent of the total drainage area of the
gaged site by an adjustment procedure
described by Choquette (1988, p. 42-45) and
Koltun and Roberts (1990, p. 6-8). This
procedure uses flood-frequency information
from the Pearson Type-III and
regression-equation estimates at the gaged site
to adjust the regression-equation estimate at
the ungaged site.

QIOO (wg) =

(Ql00(g)l (ERLl + (Ql00(gdb)) (E(db)) + (Q!OO(gcg)) (E(Cg))
ERL+E(db) +E(cg)

(8,320) (24) + (7,740) (11.5) + (8,860) (16.1)
24 + 11.5 + 16.1

= 8,360 ft'is.

Example 7.--Calculate a weighted 50-year
peak-discharge estimate for the discontinued
Fox River at Bloomfield gaging station (station
number 05494300; map number 133, fig. 1),
located in Davis County, at a bridge crossing on
a county highway, about 0.5 mi north of
Bloomfield, in the SEl/4 sec. 13, T. 69 N., R. 14
W. Table 8 lists a regression-equation estimate
for only the drainage-basin flood-estimation
method. The 50-year Pearson Type-III estimate
is 10,600 ft3/s, and the effective record length is
21 years (table 8). The flood-frequency curve
developed from the Pearson Type-III analysis
for this gaging station is shown in figure 3. The
50-year drainage-basin regression estimate is
7,600 ft3/s (table 8), and the average equivalent
years of record for this regression equation is 9.5
(table 2). The weighted 50-year flood estimate
for this gaging station is calculated using a
simplified version of equation 16 as

(QsO(g)l (ERL) + (QSO(gdb)) (E(db))
Qso (wg) = ERL + E
(db)

(10.6001 (211 + (7.600) (9.5)
21 +9.5

= 9,670 ft'ts.

Calculation of Estimates

The regression-equation estimate for the
ungaged site is determined as one of the
following: (1) the weighted average QT(dbcg)
calculated from both the drainage-basin and
. channel-geometry regression-equation esti-
mates using equation 14 or (2) the regression-
equation estimate of QT(db) or QT(cg) calculated
from either one of these flood-estimation
methods. The calculation for this adjustment
procedure is

QT(au) = QT(ru)[AF-(2:;~:)(AF-1)} (17)

where QT(au) is the adjusted design-flood
discharge for the ungaged site, in
cubic feet per second, for a
selected T -year recurrence
interval;

QT(ru) is the regression design-flood
discharge for the ungaged site, in
cubic feet per second, determined
as one of the following: (1) the
weighted average of both the
drainage-basin and channel-

APPLICATION AND RELIABILITY OF FLOOD-ESTIMATION METHOD8 37