FLOOD08945

• County: Larimer
• Community: Fort Collins
• Stream Name: Dry Creek
• Basin: South Platte
• Title: Hydrologic Analysis Major Drainageway Planning Dry Creek
• Date: 7/1/1979
• Prepared For: Larimer County
• Prepared By: Gingery Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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infiltration in the upper basin, loss rates were determined by a
second method, using the SCS curve number tables for a 100-year
event. Assuming the Corps' depression storage of 0.3 inches to
be good, an infiltration rate was determined. This constant value
was then input into the s~mM model in place of the permeability
range, and the model was rerun. Peak outflows from Douglas Reservoir
increased slightly from cfs to cfs. This method
amounts to using the SCS loss rates with the more detailed SWMM
model, and this model setup was selected for use above Douglas Reser-
voir. Again, the flow is consistent with observations regarding
the spillway.

Reservoir Routing. Reservoir routing through Douglas Reservoir and
Park Creek was completed using the Modified PuIs Method as contained
in the SWMM version developed by the Corps of Engineers.

- HYDROLOGY RESULTS -

The SWMM model was used to determine the 2-, S-, 10-, 50-,
100-, and SOD-year frequency discharges at selected locations in the
study reach. In addition, the 100-year event was determined neglecting
Douglas Reservoir. This discharge was determined to show the effect
of a by-pass channel around Douglas Reservoir on discharges in the
Fort Collins area.
The study reach for this project extends from the Dry Creek con-
fluence with the poudre River upstream through the SW~ of Section 26,
approximately one-half mile above U.S. Highway 287.
For the 2-year and 5-year flood events, the Eaton Ditch inter-
cepts all flows reaching its channel. Therefore, for these events,
separate runs were made above and below the ditch. The run above
the ditch included only tributary area below the Larimer County
Canal since this canal would intercept all flows from the area
above its Dry Creek crossing.
For the 10-year, 50-year, 100-year, and SOD-year events, the
model was run to include the entire basin. Because of uncertainties
in the response of the canal systems to large flood peaks, the canals
were ignored in the analysis. The canal embankments are likely to
fail under these extreme conditions, and the effect on the flood

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