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

Table 4. Region I channel-geometry characteristic equations for estimating design-flood discharges in
Iowa outside of the Des Moines Lobe landform regionl

[Q, peak discharge, in cubic feet per second, for a given recurrence interval, in years; BFW, bankfull
width, in feet; ACW; active-channel width, in feet]

Estimation equation

Standard
error of estimate
LoglO Percent

Average
standard error
of prediction
(percent)

Average
equivalent
years of
record

Bankfull equations
Number of streamflow-gaging stations = 78

Q2 = 4.55 BFW1.45
Q5 = 15.6 BFW1.32
QlO = 29.2 BFW1.25
Q25 = 55.7 BFW1.18
Q50 = 84.2 BFW1.13
Q100 = 122 BFW1.09

0.160
.140
.146
.162
.176
.192

38.1
33.1
34.5
38.5
42.3
46.4

38.9
33.8
35.4
39.8
43.9
48.3

4.8
7.4
8.8
9.8
12.6
16.1

Active-channel equations
Number of streamflow-gaging stations = 120

Q2 = 45.6 ACW1.07 0.213 52.1 53.0 2.4
Q5 = 118 ACWO.982 .180 43.2 44.2 4.0
QlO = 190 ACWO.937 .175 41.9 43.0 5.4
Q25 = 312 ACWO.889 .179 43.1 44.5 7.0
Q50 = 427 ACWO.858 .188 45.3 46.9 8.9
Q100 = 566 ACWO.828 .198 48.2 50.0 11.0

lThe Des Moines Lobe landform region is delineated as Region II in figure 2.

Note: Bankfull equations may provide improved accuracies over active-channel
equations for channels unaffected by channelization. For channels affected by
channelization, the active-channel equations only are applicable when the active
channels have stabilized (approximately 5 to 10 years after channelizatiou). See
Appendix C for a discussion of stabilized channels.

ESTIMATING DESIGN-FLOOD DISCHARGES USING CHANNEL-GEOMETRY CHARACTERISTICS 25