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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undisturbed, but a new overflow channel would divert some of the flows at the 90 degree bend,
The diversion channel is described in the next section,

Diversion Channell!pstream of Hiawatha Avenue
A proposed overflow diversion channel near the downstream end of the study reach was
conceptually designed for this study, The diversion channel would begin downstream of 9th
Street NE where the Main Ditch turns 90 degrees to the west. It would continue in a
northwesterly direction across existing agricultural land and through a box culvert under
Hiawatha Avenue, The channel would then continue in a northwesterly direction, through the
old railroad embankment, where it would tie into existing ground just to the north of the
Pipestone National Monument land.

The diversion channel was designed for 100-year capacity using the following procedure, The
capacity of the Main Ditch between the Hiawatha Avenue bridge and the 90 degree bend was
estimated to be 1,550 cfs using the HEC-2 study model. That discharge was then subtracted
from the 100-year peak flow of 4,235 to obtain the required diversion channel capacity of 2,685
cfs. At the 90 degree bend, the diversion channel invert was set at 2 feet above the Main Ditch
invert so that low flows in the Main Ditch would continue along the historic flow path. The depth
of flow at the diversion channel inlet was then estimated as the difference between the computed
100-year water surface elevation (HEC-2) and the proposed diversion invert. The diversion
channel bottom width was then sized using normal depth calculations assuming a 0.3 percent
slope, a Manning's un" value of 0.035, and side slopes of3H to IV. A trial procedure yielded a
channel bottom width of 36 feet, with a normal flow depth of 7 feet. A typical cross-section for
the diversion channel is shown on plate 4,

An RCB culvert with 45 degree wingwalls at Hiawatha Avenue was sized for the loo-year
design flow assuming inlet control with a headwater to depth (HWID) ratio of 1.2, Channel
protection at the inlet and outlet of the culvert would be required, but was not specifically
addressed for this study.

The diversion channel was originally intended to be a shallow overflow channel with gentle side
slopes that could be traversed by farm equipment. Consequently, the shallow channel would
allow for easy access to the agricultural land on both sides of it. However, the preliminary
evaluations indicate that the diversion channel would have to be quite deep due to the existing
ground elevations versus the required channel invert. Portions of the channel would be
excavated to depths of more than 16 feet, which is more than twice the depth required for the

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