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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4.2 Post-Project Condition Hydrologic Model

Although the Centre A venue Roadway Extension Project significantly alters the internal
operation of the detention ponding area between the BNRR and the Arthur Ditch, the critical
operating parameters associated with the I DO-year event (maximum ponded water surface elevation
and peak release at the BNRR embankment) are not significantly changed, Consequently, no
change to the effective SWMM model is proposed as part of the current project or the analyses
documented in this report, Since the effective SWMM model provides a reasonably accurate
representation of the critical post-project condition operating parameters for the l00-year event.
further calibration of the SWMM model is not meaningful, This issue was discussed with the City
of Fort Collins Floodplain and Master Drainage Planning Managers who concurred with the
decision to continue using the effective SWMM model as the regulatory hydrologic model.

4.3 Post-Project Condition Hydraulic Analyses

Post-project condition floodplain and floodway models were prepared by incorporating the
effects of the Centre A venue road extension and the Spring Creek bridge into the corrected
effective hydraulic models primarily by entering known water surface elevations (from EXTRAN
analysis) for the given return periods in the HEC-2 models upstream of both the BNRR and Centre
A venue embankments,
In support of the post-project condition hydraulic analyses, the following changes were also
incorporated into the floodplain and floodway models: (a) Cross Section 17380 was modified to
reflect the grading associated with the compensatory excavation site located directly upstream of
the BNRR; (b) Cross Section 18625 was removed, as it transected the Centre Avenue
embankment; (c) Cross Section 18770 was added directly upstream of Centre A venue; and (d) the
south end of Cross Section 19337 was modified to reflect grading for Centre A venue and the
Sherwood Lateral. It is noted that the corrected effective Manning's roughness coefficients were
not modified for the post-project hydraulic analyses,
Due to the dynamic movement of the flood wave and the interaction of the two ponds, all
of which was accounted for in the EXTRAN model, the actual Centre A venue bridge configuration
was not incorporated into the hydraulic models, Attempting to model the bridge in a steady-state
model such as HEC-2 would generate erroneous results, as HEC-2 cannot account for either the
time-dependent nature of both the inflow hydrographs and water levels in the two detention ponds,
or the combined storage effects of the two ponds,
Results of the post-project condition floodplain analysis were compared to the corrected
effective modeling results, as shown in Table 4.4, No increase in base flood elevations between

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ANdERSON CONsulTiNG ENqiNEERS, INC.