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 105 Reconnaissance Study

HYDROLOGIC ANALYSIS

Hydrologic ana1yses for this project were doDe for the purpose rA ~ discharge-frequency values
for the Main Ditch and one of its tributaries, County Ditch A The tMI methodologies used for this
ana1ysis along with the stream names and locations to which they were applied arc shown below,

MethodolO2V

Suam ~ and I"""tinn

Main Ditch u1s rA County Ditch A
Main Ditch at Hwy 23/ Hwy 7S
County Ditch A at mouth

Minnesota Regression Equations (USGS, 1987) Main Ditch uIs rA County Ditch A
. L.. J .I Main Ditch at Hwy 23/ Hwy 7S
y~ 'l IV""- County Ditch A at mouth

( ~C: discharges obtained from both methodologies, along with those values published in the
, Flood Insurance Study (FIS) arc presented in Tillie I in the main text of the Problem
Identification report, There is no USGS gaging station along the Main Ditch in Pipestone, however theIe
is a station on Pipestone Creek near Pipestone, Minnesota. 'I'hcIl: seems to be very limited data available
for this gaging station. and therefore a statistical ana1ysis was not "",fuuaod for this study.

Iowa Regression Equations (USGS, 1993)

The effective FlS for Pipestone, Minnesota contains frequcncy-dschargle dIU for two locations on the
Main Ditch within Pipestone. The methodology used to develop these peak discharges is described in
Section 4.0 of the FlS as follows:

"Discharge-prollability relationships for the Pipestone area were dcvdopcd for the 10-, SO-, 100-,
and SOO-ycar floods using the Environmental Protection AgCDI:'J Storm Water Mallagemcnt
(SWM) model, as Modified by the COE ..."

Results produced by the techniques used for this study were compared to the applicable peak discharges
published in the Pipestone, Minnesota FlS. The 1993 Iowa Regression ElpJation resu1ts were slightly
higher but compared favorably to the FIS values, while the M'm_ Regression Equations produced
values significantly lower than those in the FIS.

The discrepancies created by the Jater method can only be expIaiJm by the different data sets, parameters,
and empirical equations which were used in the development of the regression equations. 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 appcncix as SUwuo!iug documentation for the
hydrologic ana1ysis. Additional information can be found in the text of the cm:rall problem identification
repon.

1) Peak Discharges Computed from 1993 Iowa Regression EqwIlions
2) Dischargc-Probability Curves for Main Ditch
3) Discharge-Probability Curve for County Ditch A
4) CalcuIations and BadaIp Data for 1993 Iowa Regression Equations
~ CalcuIations and Backup Data for 1987 Minnesota Regression Equations