FLOOD09132

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Floodplain Study
• Date: 10/1/1999
• Prepared For: Greeley
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

8.U )<'LOODW A YS

A floodway with a maximum of a one-foot-rise above existing conditions was developed

for the Cache la Poudre River in the study area. In the reaches where new hydraulic models were

develope:!, an equal conveyancE' floodway was deJinea~ed. Bccause the existing conditions

hydraulic models included split flow computations, sepal'8te floodway models were developed

without split flow, using the with the split Hows computed in thc cxisting conditions models.

Where the current effective FIS maps for the City and county have Hoodway, the

floodway for this study was delineated following the flood way boundaries in the current

effective floodways. Because of the increased discharges, at some locations floodway widths

had to be increased from those ill the current effective Hoodway.

At the locations where split flow occurred, the I lXI-year split flow discharges were added

to the How in the Cache la Poudre River reaches bypassed by the split How to determine the

increase in Hood water elevations above existing cOllditions. If the addition of the split flow

dischargE to a bypassed reach did not result in an increase in discharge greater that 1 foot, then

that condition was used for the fIoodway delineation. lflhe addition of the split flow discharge

to the bypassed reach in itself cLUsed an increase in flood water surface elevations of more than I

fO'Jt above existing conditions, then the split flow discharge was not added to the main river flow

and a floodway was delineated along the split flow reach.

For the split now reach along 8th Street, adding the split flow discharge to the total

discharge in the main channel reach bypasses by the split !low resnlted in a greater than I foot

increase in some water surface elevations above existing conditions. A floodway was delinc:ated

for the split flow path. Because a natural stream channel does not exist along east 8th Street, thc

fl(lodway location was based on an assumed How baseline Downstream of Ash A venue, the

existing conditions IOO-year flood flows spilI back across:m 8th Street and return to the Cache

L2. Poudre River. This return spill occurs along East 8th Strcet from Ash Avenue to near the

airport. 11v'hen the floodway was delineated, it was designed to return the split now to the river

immediately east of Ash Avenue to Cross Section 9. Between East 8th Street and the north bank

of the Ca~he La Poudre River the floodway width Vias maintained at the width of the overflow

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