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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3.3 Corrected Effective Hydraulic Analyses

In general, the duplicate effective hydraulic models did not consider conveyance within the
two irrigation ditches, Arthur Ditch on the north and Sherwood Lateral to the south; this appears
to be an accurate representation of the physical operating condition of the system, However, as
defined, the left end of three cross sections (Nos, 18625, 19337 and 19830) would allow effective
conveyance within Arthur Ditch for either the 100- or 500-year event, while the main portion of
the Spring Creek cross section (exclusive of the ditch) could actually contain these flows,
Consequently, a corrected effective HEC-2 model was prepared which encroached the left end
of these three cross sections in order to preclude flow in the ditch, Inspection of the duplicate
effective model also revealed that Cross Section 19337 passes through, or in close proximity to,
two spot elevations which were not coded into the model. These two spot elevations (4993,6 and
4995,0 ft, NGVD) were also included in the corrected effective model. No other changes were
made in the corrected effective HEC-2 models,
Several other spot elevations shown on the work maps, located in close proximity to the
cross sections downstream of Cross Section 19337, were not included previously in the duplicate
effective HEC-2 model. Unlike Cross Section 19337, these spot elevations were not incorporated
into the corrected effective model as it appeared they would not materially affect the analysis,
This determination was made based on the results of the duplicate effective hydraulic analysis
which indicated that the reach within the BNRR detention pond from the upstream face of the
railroad embankment to Cross Section 18625 is entirely influenced by backwater during the 100-
year event. It is noted that the 100-year water surface profile through this reach is horizontal at
the 100-year ponded water surface elevation of 4993,5 ft, NGVD,
Results of the corrected effective floodplain analysis were compared to the duplicate
effective modeling results, as shown in Table 3,1. Base flood elevations computed by the
duplicate and corrected effective models matched at all but two cross sections, At Cross Sections
19337 and 19830, the corrected effective base flood elevations differed by -0,1 and +0,1 feet,
respectively, compared to the duplicate effective base flood elevations, It is noted that the O,l-foot
rise is located at Cross Section 19830 and does not impact any insurable structures, Graphical
representation of the corrected effective flood profiles through the current study reach is provided
in Appendix B, In addition, graphical representations of the cross sections through the study reach
which were modified for the corrected effective analysis are provided in Appendix C.
For the corrected effective floodway analysis, floodway encroachments were not modified
from those used for the duplicate effective analysis, Results of the corrected effective floodway
analysis are summarized in Table 3,3, The table indicates that the floodway water surface
elevations are identical to the base flood elevations, thereby meeting the intent of the original
floodway analysis through the BNRR ponding area, HEC-2 output for the corrected effective
flood plain and floodway analyses is provided in Section C to the Technical Addendum,

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ANdERSON CONsulTiNG ENGiNEERS, INC,