| Abstract: | This report describes a statewide study
conducted to develop main-channel slope (MCS)
curves for 138 selected streams in Iowa with
drainage areas greater than 100 square miles.
MCS values determined from the curves can be
used in regression equations for estimating floodfrequency
discharges. Multivariable regression
equations previously developed for two of the
three hydrologic regions defined for Iowa require
the measurement of MCS. Main-channel slope is
a difficult measurement to obtain for large streams
using 1:24,000-scale topographic maps. The
curves developed in this report provide a simplified
method for determining MCS values for sites
located along large streams in Iowa within hydrologic
Regions 2 and 3.
The curves were developed using MCS
values quantified for 2,058 selected sites along
138 selected streams in Iowa. A geographic
information system (GIS) technique and
1:24,000-scale topographic data were used to
quantify MCS values for the stream sites. The
sites were selected at about 5-mile intervals along
the streams. River miles were quantified for each
stream site using a GIS program. Data points for
river-mile and MCS values were plotted and a
best-fit curve was developed for each stream.
An adjustment was applied to all 138
curves to compensate for differences in MCS
values between manual measurements and GIS
quantifications. The multivariable equations for
Regions 2 and 3 were developed using manual
measurements of MCS. A comparison of manual
measurements and GIS quantifications of MCS
indicates that manual measurements typically
produce greater values of MCS compared to GIS
quantifications. Median differences between
manual measurements and GIS quantifications of
MCS are 14.8 and 17.7 percent for Regions 2 and
3, respectively. Comparisons of percentage differences
between flood-frequency discharges calculated
using MCS values of manual measurements
and GIS quantifications indicate that use of GIS
values of MCS for Region 3 substantially underestimate
flood discharges. Mean and median
percentage differences for 2- to 500-year recurrence-
interval flood discharges ranged from
5.0 to 5.3 and 4.3 to 4.5 percent, respectively, for
Region 2 and ranged from 18.3 to 27.1 and
12.3 to 17.3 percent for Region 3.
The MCS curves developed from GIS
quantifications were adjusted by 14.8 percent for
streams located in Region 2 and by 17.7 percent
for streams located in Region 3. Comparisons of
percentage differences between flood discharges
calculated using MCS values of manual measurements
and adjusted-GIS quantifications for
Regions 2 and 3 indicate that the flood-discharge
estimates are comparable. For Region 2, mean
percentage differences for 2- to 500-year recurrence-
interval flood discharges ranged between
0.6 and 0.8 percent and median differences were
0.0 percent. For Region 3, mean and median
differences ranged between 5.4 to 8.4 and 0.0 to
0.3 percent, respectively.
A list of selected stream sites presented
with each curve provides information about the
sites including river miles, drainage areas, the
location of U.S. Geological Survey streamflowgaging
stations, and the location of streams crossing hydrologic region boundaries or the Des
Moines Lobe landform region boundary. Two
examples are presented for determining river-mile
and MCS values, and two techniques are
presented for computing flood-frequency
discharges. |
| Genre: | USGS Numbered Series |
| ProdID: | 52914 |
| Citation Author: | Eash, David A. |
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| Citation Language: | ENGLISH |
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| Citation Phsyical Description: | vi, 215 p. : ill., maps. (some color) ; 28 cm. |
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| Citation Series: | Water-Resources Investigations Report |
| Citation Series Code: | WRI |
| Citation Series Number: | 2003-4120 |
| Citation Search Results Text: | Main-channel slopes of selected streams in Iowa for estimation of flood-frequency discharges; 2003; WRI; 2003-4120; Eash, David A. |
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| Citation Year: | 2003 |
| Type: | citation/reference |
| Text: | Main-channel slopes of selected streams in Iowa for estimation of flood-frequency discharges; 2003; WRI; 2003-4120; Eash, David A. |
| URL (THUMBNAIL): | http://pubs.usgs.gov/wri/2003/4120/report-thumb.jpg |
| URL (DOCUMENT): | http://ia.water.usgs.gov/pubs/reports/WRIR_03-4120.pdf |
| URL (DOCUMENT): | http://pubs.usgs.gov/wri/2003/4120/report.pdf |
| Date Other: | Wed, 1 Oct 2003 00:00 -0500 |
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