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

equations in table 4, is determined to be the
most applicable. The only channel -geometry
characteristic used for the Region I bankfull
equation is the bankfull width tBFW). Appendix
C describes the procedure for conducting this
channel-geometry measurement.

Step 2. Three bankfull widths measuring 52,
50, and 52 ft, measured along a straight channel
reach about 0.75-1.0 mi downstream of the
gaging station, were used to calculate an
average bankfull width (BFW) of 51 ft. Figure
7 B shows the bankfull reference level at one of
these channel measurement seetions.

Step 3. The 100-year flood estimate for the
Region I bankfull equation (table 4) is calculated
as

Q,OO = 122 (BFWlL09,
= 122 (511LO.,
= 8,860 ft';s.

Examole 3.--Use a re~jonal channel-
geometry equation to estimate the 50-year peak
discharge for the Big Cedar Creek near Varina
continuous-record gaging station (station
number 05482170; map number 108, fig. 2),
located in Pocahontas County, at a bridge
crossing on County Highway N33, 5.5 mi
northeast of Varina, in the NEI/4 sec. 24, T. 91
N., R. 34 W.

Step 1. This gaging station is located in
Region II, and an inspection of the USGS
1:100,000-scale Pocahontas County map and a
visit to the site show evidence of channelization.
Therefore, the 50-year active-channel equation
for Region II, listed in the second set of
equations in table 5, is deterr,ined to be the
most applicable. Features that. are character-
istic of channelized streams are illustrated in
figure 7D, which shows the straightened and
leveed channel reach downstream of the gage.
The only channel-geometry characteristic used
for the Region II active-channel equation is the
active-channel width (ACW). Appendix C
describes the procedure for conducting this
channel-geometry measurement.

Step 2. Three active-channel widths
measuring 25.6, 25.3, and 24.2 ft, measured
along a straight channel reach about 0.25-0.5 mi
downstream of the gaging stat.ion, were used to
calculate an average active-"hannel width

(ACW) of 25.0 ft. Figure 7D shows the
approximate active-channel reference level for
the channel reach measured to calculate an
average active-channel width.

Step 3. The 50-year flood estimate for the
Region II active-channel equation (table 5) is
calculated as

Q,o = 59.5 (ACW)u"
= 59.5 (25.0)'14,
= 2,330 ft';s.

Example 4.--Use a statewide channel-
geometry equation in table 3 to estimate the
100-year peak discharge for the gaging station
used in example 2.

Step .1. Because a statewide equation is to be
used and no evidence of channelization is
evident, as determined in example 2, the
100-year bankfull equation listed in the first set
of equations in table 3 is applicable. Bankfull
width (BFW) and bankfull depth (BFm are the
channel-geometry characteristics used for this
equation. Appendix C describes the procedure
for conducting these channel-geometry
measurements.

Step 2. The average bankfull width (BFW)
calculation of 51 ft for this stream channel is
outlined in example 2.

Step 3. The average bankfull depth (BFm
for this stream channel was calculated to be 6.0
ft. The bankfull depth measurements used to
determine this average are listed in the
"Bankfull-Depth (BFD) Measurements" section
of Appendix C, and they are illustrated in figure
10.

Step 4. The 100-year flood estimate for the
statewide bankfull equation (table 3) IS
calculated as

QIOO = 104 (BFW)o_766 (BFD)o.747,
= 104 (51)0766 (6.0)0747.
= 8,060 ft'/s.

Examples 2 and 4 illustrate the use of
bankfull measurements in computing 100-year
flood estimates for this gaging station using
regional and statewide multiple-regression
equations. The regional estimate was
determined to be 8,860 ft3/s, and the statewide
estimate was determined to be 8,060 ft3/s.

30 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA