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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The outlet facilities in the BNRR embankment were not modified from the pre-project condition
model. However, the stage-area rating curves were modified to reflect modifications to the
grading which accompany the project. The Centre A venue embankment removes approximately
8,0 acre-feet of storage volume below elevation 4994 ft, NGVD, The area of compensatory
excavation is located directly west of the BNRR, as shown on Sheet 1, As part of the creation of
compensatory storage, the large fuel storage tank and surrounding berm within the excavation area
were removed, In total, the compensatory excavation efforts resulted in the addition of
approximately 16 acre-feet of storage below elevation 4994 ft, NGVD,
As noted in Section 2,2, two large span culverts were installed at the Spring Creek
crossing, The northern three-sided concrete box culvert has an average height of 11.6 feet and
is 20,2 feet wide, which accommodates the low flow channel. The bottom of the culvert is a
trapezoidal riprap-lined channel with approximately a 13-foot wide bottom and side slopes of
2H: 1 V, The southern three-sided concrete box culvert has an average height of 9 feet and is 18.4
feet wide, which provides access under Centre A venue for the City's bicycle trail along Spring
Creek, In order to model the culverts in EXTRAN, an idealized rectangular section had to be
developed for each culvert. A diagram of the proposed and idealized culvert configurations is
provided in Figure 4,1. Calculations which support the definition of the idealized culverts are
provided in Section A of the Technical Addendum,
For the northern culvert, the height of the opening varies from 11.3 to 11. 9 feet. Because
of this an average height of 11,6 feet was utilized, To account for the reduced area in the lower
trapezoidal section, the idealized culvert width was reduced to maintain the same open area,
Consequently, the culvert is modeled as a 1 1.6-foot high by 19.4-foot wide culvert, A composite
Manning's n value for the northern culvert was calculated to be 0,023,
For the southern culvert, the height of the opening varies from 8,6 to 9,5 feet. Because
of this an average height of 9,0 feet was utilized, The idealized culvert width was reduced to
maintain the same open area, Consequently, the culvert is modeled as a 9,O-foot high by 18,O-foot
wide culvert, Commensurate with the concrete floor/trail surface in the southern culvert, a
Manning's n value of 0,015 was used,
Similar to the pre-project condition model, an equivalent Manning's n value was calculated
to account for the entrance and exit losses for the Spring Creek culverts, For the northern culvert,
and equivalent Manning's n value of 0,0662 was calculated, For the southern culvert, a value
of 0,0557 was calculated, All calculations associated with determining composite and equivalent
n values are provided in Section A of the Technical Addendum,
Results of the EXTRAN analysis indicate a 100-year post-project condition water surface
elevation of 4993.4 ft, NGVD at the upstream face of the BNRR embankment which is 0,2 feet
lower than the pre-project condition 100-year water surface elevation at that location, In addition,

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ANdERSON CONSulTiNG bGiNEERS, INC.