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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II. PHYSICAL SETTING

2.1 Pre-Project Conditions

Prior to the Centre A venue roadway extension project. the BNRR embankment formed one
large detention pond which effectively attenuated flood peaks from storm events which exceed that
associated with the SOO-year return period, The single detention ponding area was bounded by
Arthur Ditch on the north, Hill Pond on the west. and Sherwood Lateral on the south, Spring
Creek entered the BNRR detention pond from the west at Hill Pond and flowed from west to east
through the pond, As identified in the analyses associated with the 1993 FEMA revisions for
Spring Creek, the outlet from the BNRR detention pond consists of four culverts, a 36-inch RCP
and three 84-inch RCP's. through the embankment which are described in the following
paragraphs,
The northern outlet pipe is a 36-inch RCP which transitions into a 48-inch RCP and
discharges directly to Spring Creek immediately upstream of the College Avenue bridge, The 36-
inch RCP passes through the original 12-foot high by 14-foot wide RCB culvert through the
railroad embankment, which was abandoned and plugged by the City in conjunction with the
installation of the triple 84-inch RCP's, as well as the completion of other significant floodplain
improvements, The entrance to the 36-inch RCP is in a concrete headwall, The entrance elevation
is 4979,0 ft, NGVD, which is 0.4 feet above the lowest outlet invert elevation for the pond and
17,0 feet below the minimum railroad embankment overtopping elevation.
The northern 84-inch RCP outlet is located adjacent to the other two 84-inch RCP's.
approximately 800 feet south of the 36-inch RCP outlet. Both the entrance and exit of the culvert
terminate in a concrete headwall. The entrance elevation is 4980,1 ft, NGVD, which is I.S feet
above the lowest outlet invert elevation for the pond and IS,9 feet below the minimum railroad
embankment overtopping elevation,
The middle 84-inch RCP outlet is the lowest outlet for the detention pond, Both the
entrance and exit sections for the culvert are located in a concrete headwall. The entrance
elevation is 4978,6 ft. NGVD. which is 17.4 feet below the minimum railroad embankment
overtopping elevation,
The southern 84-inch RCP outlet is intended to convey pond releases during major storm
events only, Although the upstream invert elevation is 4980,2 ft. NGVD. which is 1.6 feet above
the lowest outlet invert elevation for the pond, a concrete entrance box was constructed around the
culvert inlet to prevent low flows from entering the culvert. The entrance box consists of a
concrete weir at elevation 4988,0 ft, NGYD, which is 9.4 feet above the lowest outlet elevation
for the pond, and 8,0 feet below the minimum railroad overtopping elevation, The open area of

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ANdERSON CONSulTiNG ENGiNEERS, INCo