FLOOD00961

• County: Larimer
• Community: Fort Collins
• Stream Name: Spring Creek
• Title: Hydraulic Evaluation of the Centre Avenue Bridge at Spring Creek
• Date: 11/22/2000
• Prepared For: Fort Collins
• Prepared By: Anderson Consulting Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III, PRE-PROJECT CONDITION ANALYSES

3.1 Duplicate Effective Hydrologic Model

As reported in the effective FIS [FEMA, 1996], discharges for Spring Creek were
computed using the V,S, Environmental Protection Agency's (EPA's) Storm Water Management
Model (SWMM) based on a 2-hour storm duration, Rainfall information was taken from the 1973
NOAA Atlas, The SWMM model utilized 97 subbasins, The level of urban development, and
corresponding percent impervious estimates for each subbasin were determined by field inspection
and were modeled for existing basin development for 1988, Individual flood hydrographs were
combined at various design points along each stream segment to determine peak discharges at
designated design points, Values for infiltration, overland roughness coefficients, and surface
storage depths recommended by the Urban Drainage and Flood Control District (UDFCD) and
EPA were used for both pervious and impervious areas, The SWMM model accounted for the
effects of irrigation canals, detention ponds and inflow hydrographs into the Spring Creek Basin,
No modifications were made to the SWMM model as part of the current project. However,
the model was used to generate baseline operating conditions and inflow hydro graphs to the BNRR
detention pond for use in the EXTRAN model. Discussion and presentation of inflow hydrographs
to the BNRR pond is reserved for Section 4,\,
The effective SWMM model provided by the City was used to define pre-project condition
release rates through the BNRR embankment and water surface elevations in the BNRR detention
pond, The results of the model indicate a 100-year release of 1,465 cfs and a 100-year water
surface elevation of 4993.47 ft, NGVD, corresponding to a maximum storage volume of 202.4
acre-feet.
The effective HEC-2 model for Spring Creek, obtained from the City of Fort Collins,
indicates a 100-year discharge of 1,465 cfs at Cross Section 16293, located directly downstream
of the BNRR, The 100-year water surface elevation imposed at Cross Section 16430 in the HEC-
2, located directly upstream of the BNRR, is 4993.47 ft, NGVD, From this comparison, it is
apparent that the results of the duplicate effective SWMM model match the discharge and water
surface elevations utilized in the effective HEC-2 model.

3,2 Duplicate Effective Hydraulic Analyses

The effective HEC-2 floodplain and floodway models were provided by the City of Fort
Collins, As noted in the previous section, water surface elevations imposed in the HEC-2 models

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ANdERSON CONsulTiNG ENGiNEERS, INL