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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

of the hydrograph from CE 221 to obtain an inflow hydrograph to the lower portion of the BNRR
pond, Preparation of the EXTRAN inflow hydrographs is presented in Section A of the Technical
Addendum,
As previously described in Section 2,1, outlet facilities from the BNRR detention pond
consist of one 36-inch RCP and three 84-inch RCP's, The concrete inlet box which serves as the
entrance to the southern 84-inch RCP, to prevent low flows from entering the culvert, has an inlet
grate which is perched 9.4 feet above the minimum outlet elevation for the BNRR pond, Culvert
information and capacity calculations associated with the effective SWMM model are provided in
Section A of the Technical Addendum,
EXTRAN does not allow for the direct input of entrance and exit losses for culverts,
Consequently, culvert losses were considered by calculating an equivalent Manning's n value using
the following equation:

{ , (R(413)) } 1/2
n, = n- + 29,L (ken/ranee + kexil)

where ne is the equivalent Manning's n, kenrmnce and kexu represent the entrance and exit loss
coefficients, n is the actual culvert Manning's n, L is the pipe length in feet, and R is the
hydraulic radius in feet.
The horizontal inlet grate which restricts inflows to the southern 84-inch RCP was analyzed
for the effective SWMM model as an orifice with an open area of 14 square feet and a discharge
coefficient (C) of 0,63, EXTRAN does not provide a straight forward method for analyzing a
drop (horizontal) orifice, when the drop orifice is elevated above the bottom of the associated
junction (in this case the minimum outlet elevation in the BNRR pond), Consequently, the inlet
grate was modeled as three small, perched orifices each with a diameter of 12 inches, The size
of each orifice was limited to 12 inches in order to keep the centroid of the modeled orifices to
within 6 inches of the actual inlet grate elevation, Consequently, the total open area of the three
equivalent orifices is 2,36 square feet. In order to compensate for the reduced area ofthe modeled
orifice opening, the discharge coefficient was adjusted to keep the product of C.A equivalent to
that indicated in the effective SWMM model calculations, The product of C'A for the SWMM
model was 8,82; the discharge coefficient for the EXTRAN model was initially set at 3,74 in order
to maintain the 8,82 value for C'A,
Initial modeling parameters used in EXTRAN were based solely on the physical
characteristics of the various facilities, Initial modeling results indicated a slightly lower water
surface elevation, and a correspondingly larger discharge than computed by the effective SWMM
model. Consequently, Manning's n values for the pipes were modified to provide better

15

ANdERSON CONSulTiNG ENGiNEERS, 'NC.

T:'..QPEN\cocsrlDl "cocsrlOl fema-LO!'vIR !inal-t'Pt.\\pJ