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 5. Region II channel-geometry characteristic equations for estimating design-flood discharges
in Iowa within 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; BFD, bankfull depth, 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 = 33

Q2 = 2.77 BFW0844 BFD1.48 0.123 28.8 30.3 6.5
Q5 = 7.42 BFWO.783 BFD1.43 .131 30.8 33.6 6.1
QlO = 12.1 BFWO.748 BFD1.4l .143 33.9 37.7 6.3
Q25 = 19.7 BFW0715 BFD1.38 .162 38.6 43.4 6.6
Q50 = 26.7 BFWO.694 BFD1.37 .176 42.3 47.8 7.9
QlOO = 34.9 BFWO.675 BFD1.36 .190 45.9 52.1 9.3
Active-channel equations
Number of streamflow-gaging stations = 37
Q2 = 7.80 ACW1.30 0.236 58.5 59.7 1.9
Q5 = 19.1 ACW1.23 .235 58.4 60.1 2.1
QlO = 29.6 ACW1.l9 .240 59.7 61.8 2.6
Q25 = 45.6 ACW1.l6 .248 62.0 64.8 3.3
Q50 = 59.5 ACW1.l4 .255 64.2 67.4 4.4
Q100 = 75.0 ACW1.l2 .262 66.4 70.0 5.7

lThe Des Moines Lobe landform :"egion is delineated as Region II in figure 2.

Note: Bankfull equations may provide improved accuracies over active-channel
equations for channells unaffected by channelization. For channels affected by
channelization, the active-channel equations only are applicable when the active
channels have stabili2:ed (approximately 5 to 10 years after channelization). See
Appendix C for a discussion of stabilized channels.

26 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA