FLOOD08373

• County: Statewide
• Community: Rock Rapids, Iowa
• Basin: Statewide
• Title: Problem Identification Report and Appendixes at Rock Rapids
• Date: 4/1/1995
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

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Rock Rapids, Iowa
Section 205 Reconnaissance Study

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HYDROLOGIC ANALYSIS

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Hydrologic analyses for this project were done for the pwpose of computing discharge-frequency values
for the Rock River and two of its tributaries, Tom Creek and Moon Creek. The three methodologies used
for this analysis along with the stream names and locations to which they were applied are shown below.

Methodoloe;y

Stream Name and Location

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Iowa Regression Equations (USGS, 1993)
and Minnesota Regression Equations (USGS, 1987)

Rock River just dls of Tom Creek
Rock River between Moon and Tom Creeks
Rock River just ufs of Moon Creek
Tom Creek at Mouth
Moon Creek at Mouth

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HEC-FFA (Bulletin 17B)

Rock River just dls of Moon Creek

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Computed peak discharges obtained from all methodologies, along with those values published in the Rock
Rapids FIS are presented in Table I in the main text of the Problem Identification report. Gaging records
for the gaging slation at Rock Rapids were available from 1960 to 1974. Although this is not a very long
period of record, a statistical analysis was perfonned using the Corps of Engineers HEC-FF A computer
program which reflects techoiques described in Bulletin 17B (Water Resource Council, 1981). The gaging
station in Rock Rapids is located on the Rock River just downstream from the Moon Creek confluence.

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The effective FIS for Rock Rapids, Iowa contains frequency-discharge data for two locations on the Rock
River within Rock Rapids. The methodology used to develop these peak discharges is described in Section
4"0 of the FIS as follows:

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"Using a historic period of 60 years, station statistics were adjusted using guidelines set by
Bulletin No. 17B. The resulting frequency curve was significantly lower than the curve obtained
without historic adjustment. Regional U.S. Geological Survey equations were also used to
estimate flow frequency values."

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Results produced by the above techoiques were compared to the applicable peak discharges published in
the Rock Rapids, Iowa FIS. The 1993 Iowa Regression Equation results compared very well to the FIS
values. while the Minnesota regression equation results produced values significantly lower than those in
the FIS. The HEC-FF A model produced results which were higher than the FIS values which could be due
to the short period of record available for the Rock Rapids gaging station. The 1993 Iowa regression
equation results were chosen by the Corps for use in this study after reviewing the hydrologic analysis
results. The items shown in the list below are included in this appendix as supporting documentation for
the hydrologic analysis. Additional infonnation can be found in the text of the overall problem
identification report.

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I) Peak Discharges Computed from 1993 Iowa Regression Equations
2) Discharge-Probability Curves for Rock River
3) Discharge-Probability Curves for Moon Creek and Tom Creek
4) Calculations and Backup Data for 1993 Iowa Regression Equations
5) HEC-FF A Output for Rock River
6) Calculations and Backup Data for 1987 Minnesota Regression Equations
1.l Annual Peak Discharges for Rock River Gaging Station at Rock Rapids (1960-1974)

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