FLOOD01032

• County: Statewide
• Community: Pipestone Minnesota
• Title: Problem Identification Report and Appendixes at Pipestone Minnesota
• Date: 4/1/1995
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

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Pipestone, Minnesota
Section 205 Reconnaissance Study

HYDRAULIC ANALYSIS

The hydraulic analysis for the problem identification phase of this study involved estimation of existing
conditions water surface profiles for the Main Ditch through Pipestone, Minnesola.

The HEC-2 backwater model from the February, 1991 Flood Insurance Study (FIS) for Pipestone,
Minnesota was used exclusively for the computation of Main Ditch water surface profiles within the study
area. Revised peak discharges for the 10-. 2-, 1-, and 0.2 percent chance exceedance floods as computed in
the hydrologic analysis (appendix A) were used in the FIS model. Peak discharge values used for this
study were similar to but slightly higher than the ones used in the FIS model.

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Two of the bridges within the study area have been structurally modified since the publication of the FIS.
Bridge plans for the Burlington Northern (BN) railroad bridge were obtained from BN railroad personnel,
and bridge plans for the Hiawatha Avenue bridge were obtained from Pipestone County Highway
Department. The bridge roulines in the HEC,2 study model were revised to reflect the latest bridge
geometry as shown on the bridge plans.

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The FIS HEC-2 input file and the one used for this study were run using a version of the HEC-2 model
which was developed by the Omaha District. The computer model known as HEC2BPR was required to
run the input files due to special coding of the bridges in the study reach,

Manning's "n" values are typically set at 0.033 for the main channel and 0.075 for both overbank areas.
The Manning' s "n" values were reviewed and appear to be within the nonnal range of values which would
be applicable to the existing physical conditions. A field inspection during a site visit in 1994 did not
indicate the need for revisions to the Manning's "n" values. Contraction and expansion coefficients for
most of the cross-sections were set at 0.3 and 0.5 respectively. however some cross-sections have values of
0,4 and 0.6, respectively, The use of the previously seclected contraction and expansion coefficients for
h~ HEC,2 model seems to be reasonable based upon a reconnaissance-level review of the channel and
rbank conditions as found during the 1994 site visit and as shown on available topographic mapping.
e HEC,2 cross,section contraction and expansion coefficients are within the recommended range for the
existing site conditions, and therefore no modifications were made to them.

Starting water surface elevations in the study model were computed by HEC-2 using the slope-area method
for detennining nonnal depth at the most downstream cross-section. The estimated energy slope was based
on the channel slope between the fIrSt two cross,sections in the HEC-2 model. The area of interest for this
study is approximately one mile upstream of the starting section, and therefore it was assumed that the
slope-area method was adequate for this level of study.

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The city of Pipestone supplied surveyed cross,sections along the Main Ditch at the northern end of the city.
The cross-sections are about 150' wide on average, and do not extend into the left and right overbank
areas, The city cross.sections were incorporated into the existing HEC-2 cross-sections at stations 7550,
8750, 9575, and 10280. The surveyed cross-sections and the FIS cross,sections appear to be on the same
datum. The revised cross-sections reflect the city-surveyed main channel with left and right overbank
geometry as detennined during the FIS study.

The results of the hydraulic analysis using the revised HEC-2 input file were similar to the results obtained
from the FIS analysis. The lOO'year water surface elevations computed by the study model are compared
with the FIS values in a table which is included in this appendix.