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 6. Statistical summary for selected statewide drainage-basin and channel-geometry
characteristics. and for selected regional channel-geometry characteristics at streamflow-gaging
stations in Iowa

[CDA, contributing drainage area, in square miles; RR, relative relief, in feet per mile; DF, drainage
frequency, in number of first-order streams per square mile; TTF, 2-year, 24-hour precipitation inten-
sity, in inches; BFW, bankfull width, in feet; BFD, bankfull depth, in feet; ACW, active-channel width,
in feet]

Statewide drainage-basin characteristics
Statistic CDA RR DF TTF
Maximum 1,060 48.7 2.95 3.26
Minimum .338 1.57 .043 2.82
Mean 209 6.48 .520 3.11
Median 80.7 4.45 .510 3.14
No. of sites 164 164 164 164
Statewide channel-geometry characteristics
Statistic BFW BFD ACW
Maximum 523 17.1 510
Minimum 9.6 1.7 4.2
Mean 110 7.0 77.0
Median 82.7 6.7 49.8
No. of sites 111 111 157
Regional channel -geometry characteristics
Region I Region II
Statistic BFW ACW BFW BFD ACW
Maximum 523 510 361 12.5 339
Minimum 9.6 4.2 19.3 2.0 6.9
Mean 106 73.7 120 6.6 87.4
Median 71.0 46.1 106 6.6 73.3
No. of sites 78 120 33 33 37

E = r2(s~J'

(13)

mean weighted skew coefficient
of the logarithms (base 10) of the
observed annual-peak discharges
at the gaging stations used in
each respective regression-model
data set and the recurrence
interval relating the standard
error of a T-year peak discharge
to the index of variability (s) and
the number of observed annual-

interval of T-years is calculated as (Hardison,
1971, p. C231)

where E

is the average equivalent years of
record, in years;

r

is a factor that is a function of the

APPLICATION AND RELIABILITY OF FLOOD-ESTIMATION METHODS 33