FLOOD05064

• Designation Number: 205
• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainageway Planning - Cooper Slough, Boxelder Creek, Fort Collins, Larimer County, Colorado
• Date: 8/1/1981
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Rainfall volumes fop the 2-, ]0-, 25-, 50- ]00- and 500-yeap recurpence
intepval stopm ~epe assumed to deJ~ne hydpogpaphs of the coppeeponding
recuppence intepval flood from the Stop~atep Management Model (S~).

EXECUTIVE SUMMARY

This report presents the results of the mastep d~inageway planning study
for Boxelder Cr€!ek and Cooper Slough. The pepopt was prepared fop ,the City of
Fopt ColZins, La'l'imer. County and Colo~do Water Consepvation Board. The
repopt defines the basic data used in the hydpologic, hyd~ulic and economic
analysis and presents possible altePnatives to solve the potential flooding
problems. One altePnative is selected, based p'l'imapily :m the economic analy-
sis, as the recommended imppovement plan. For this plan, a detailed
discussion of the preliminary design and plan implementation is presented.

Basin Charactepistics
The t'l'ibuta7'1J area fop the Coopep Slough basin is apppoximately 10.2
squape miles. The basin is apppoximately six miles long and 1.75 miles wide.
The basin is fairly flat with poorly defined d~inage pattePns. The Cooper
Slough basin is undergoing development at ppesent, and in the future will be
almost completely urbanized. Coopep Slough flo~s in a southerly dipection and
joins Boxelder Creek in the vicinity of Prospect Street and I~25 east of Fort
Collins, Colo~do. The t'l'ibutapy area of the Boxelder Cpeek is apppoximately
60.3 square miles with an additional ]90.7 squape miles contpolled by SCS
Flood Contpol reservoips neap Wellington. The ,~tershed is about 32 miles
long and eight miles wide. The Boxeldep basin is mostly fapmland, except the
area within the urban growth boundary, "mere some urbanization is expected.
Boxelder Creek flows south and .;oinll the Cache la PoudPB River in the vicinity
of Prospect street and I-25.

Hydm.uZic Analysis
Hydm.ulic analyses of Boxelder Creek and COOpBP Slough ~ere carpied out
to provide estimates of the watep supface elevations fop floods of 2-, 10-,
25-, 50-, 100-, and 500-year recurrence intepvals. The Apmy Corps of
Engineers' (CaE) HEC-2 ~ater surface profile computer model was used to deter-
mine the water surface elevation (WSR) at selected locations along the stpeam
channels. Cross sections ~ere measuped from topogmphic maps (J" = 100' with
2-foot contour intepvals) provided by La'l'imer County, Colorado, May, 1979.
Additional cposs sections ~ere field surveyed for a one-half mile reach above
Vine Drive on Cooper Slough. Field surveyed cross sections ~ere used in con-
junction ~ith map-interpolated crOss sections for the apea south of State
Highway 14 on Cooper Slough.
The results of the hydm.ulic analysis show a wide, shallo~ floodplain
~ith ponding behind T'Oa.dJJay crossings for Cooper Slough thpough the study
area. The floodplain for Boxelder Creek is confined to the channel valley.
Floo~tep fpom Boxelder Creek leaves the floodplain at Ppospect Stpeet and
I-25. The potential flooding caused hy this excess watep was not defined in
this study.

HydrQlogic Analysis
A hydrologic analysis was pepformed to detePmine peak flo~ rates and
flood volumes at variouis design points fop sevepal selected fpequencies. The
hydrologic analysis of Boxeldep Cpeek assumed developed basin conditions
~ithin the Urban Growth Area and existing basin conditions outside the Urban
GrOlJth Area bounda7'1J. Coopep Slough was analyzed for existing and fully deve-
loped basin conditions.

Economic Analysis
The economic analysis involved estimating flood damages, costs for dif-
ferent imppovements and the amount of reduced flood damages as a result of the
improvements. Three (Jeneral. improvement altePnatives ~ere consideped.
AltePnative 1 considers no improvements to the existing drainage system.
Flood damage reduction is accomplished by floodplain zoning, restpictions to
encroachments or development, evacuation of the floodplain, flood ppoofing
individual str>uctures or flood insumnce. AltePnative 2 consideped chan-
nelization and enlapged openings at channel crossings. AltePnative 3

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