FLOOD02774

• County: Statewide
• Community: Pipestone Minnesota
• Basin: Statewide
• Title: Technical Assistance Report Main Ditch at Pipestone Minnesota - Volume 1
• Date: 9/1/1995
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

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Table 1
Existing Conditions Peak Discharges
Used in Hydraulic Model

Flooding Source
and Location

Peak Discharge (c,f.s.)
10- YR 50- YR 100- YR 500- YR

Main Ditch just downstream from
County Ditch A
Main Ditch just upstream from
County Ditch A

1,490 3,200 4,130 7,100

390 870 1,130 2,000

HYDRAULIC ANALYSIS

Water surfaces profiles for the hydraulic analysis were computed using the Corps of Engineers
HEC-2 backwater computer model. The existing conditions HEC-2 input file developed for the
previous Section 205 Reconnaissance Study was used as the basis for modeling the proposed
improvements for this project. The 100-year and 5oo-year peak discharges previously presented
in table I were used in the proposed conditions HEC-2 model.

It is very important to note that the bridge structures crossing the Main Ditch in the Pipestone
area have capacities that are well below the 100-year discharges. The constrictions at the bridges
cause significant backwater effects and are partly responsible for the flooding problems at
Pipestone. A cursory analysis was performed to determine if channels and levees could be
conceptually designed (for 100-year protection) with the existing bridges in place. It was
determined that a 100-year level of protection could not be achieved by means of channel and
levee improvements alone. The roadway and railroad bridges through the City would have to be
upgraded, in addition to the channel and levee improvements, in order to achieve a 100-year level
of protection,

All of the proposed channel and levee facilities for this study were sized assuming that the
undersized bridges would be upgraded to 100-year capacity. Construction of the proposed levees
upstream of 9th Street NE is not recommended unless the existing bridges are also upgraded,
Construction of the channel improvements and diversion channel downstream of 9th Street NE
would be acceptable without upgrading the existing bridges. Further analyses would be required
to determine the sizes and costs for the necessary bridge replacements.

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