HomeMy WebLinkAboutDrainage Reports - 04/29/1986II
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DRAINAGE REPORT
CLARENDON HILLS
FORT COLLINS, COLORADO
James H. Stewart and Associates, Inc.
Clarendon Hills is a proposed single family residential subdivision
situatedin the West 1/2 of Section 2, Township 6 North, Range 69
West of the Sixth P.M.. It contains 161 acres and borders South
Shields Street from the South line of the Larimer County Vo-Tech
School South to Fossil Creek Drive. The overall density of the sub-
division is 2.1 units per acre. The entire subdivision is located
in the Fossil Creek Drainage Basin which has a Master Drainageway
Planning Study by Simons, Li & Associates dated August 1982.
The subdivision is broken down into three subbasins. Drainage
Basin "A" contains 10.1 acres and is situate in the Northwest cor-
ner of Clarendon Hills and drains Northeasterly to an existing
drainageway in the Larimer County Vo-Tech School property. That
drainageway flows Easterly along Harmony Road to Mail Creek at the
Burlington Northern Railroad. It continues Easterly as Mail Creek
to its confluence with Fossil Creek West of Lemay Avenue. All
of Drainage Basin "A" is located in future filings and will have
onsite detention designed at the time the final plat is made.
The amount of detention required is 94,176 cubic feet and the
allowable outlet rate is 2.57 cubic feet per second. The three
drainage basins are depicted on the enclosed drainage plans.
The drainage plans show existing contours, two year developed
runoff quantities, direction of flows and discharge points.
Drainage Basin "B" contains 82.3 acres and is generally the South
1/2 of the Northwest 1/4 of said Section 2. It drains Easterly
in an existing thalweg thru Brookwood Subdivision, across the Bur-
lington Northern Railroad, thru Cameron Park and enters Fossil
Creek on the West side of South College Avenue. Drainage Basin .
"B" will have 10.58 acre feet of detention in a series of five
ponds located in Tracts "B", "D" and "E" of Clarendon Hills, First
Filing. The ponds were designed using a mass diagram that required
10.46 acre feet of detention to detain the 100 year developed
storm and release at a two year historic rate which is 14.3 cubic
feet per second. Two of the ponds and perhaps a third one will be
overexcavated to provide a permanent pool of water. The deten-
tion capacity will be above the permanent pool There will be
a provision for irrigation water to supplement the permanent pool
and thereby alleviate any stagnation. Some ground water surfaces
in this thalweg presently and feeds several existing ponds down-
stream.
Drainage Basin "C" contains 68.6 acres and is located in the West
1/2 of the Southwest 1/4 of said Section 2. Fossil Creek traverses
Easterly along the Southerly portion of Clarendon Hills. The Burns
Tributary of Fossil Creek enters Fossil Creek in this area. Be-
cause of the close proximity to the Fossil Creek and Burns Tribu-
tary it was determined that no onsite detention would be required.
James H. Stewart and Associates, Inc.
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The Fossil Creek Drainage Basin Master Drainageway Planning Study
also states that no detention is required.
A drainage easement that encompases the 100 year flood plain limits
with future basin conditions was dedicated on the plat to prohibit
any improvements being built in the flood plain. The rear lot
lines of the lots backing onto the creeks follow the centerline
of the creeks. The steep banks along the creeks are going to be
flattened and reseeded by the developer prior to sale of any lots.
The slope of the banks will not exceed a 3:1 slope and will be
reseeded with the mix shown on the drainage plan. The seeding
is to be drilled and crimp mulched to ensure no washing during
the start of the growth. The flattening of the banks will widen
the flood plain boundaries but will not encroach on the 100 foot
wide area we have designated on the subdivision plat. The max-
imum depth, in this reach, from the stream bed to the 100 year
flood elevation is 9.6 feet at the confluence of Fossil Creek and
Burns Tributary. The average depth is approximately 4 feet. The
capacity of the creek will not be affected because the tops of
the banks are higher than the 100 year flood elevation. The velo-
cities and the water surface elevation will be decreasing.
The runoff quantities were calculated using the Rational Method.
The storm drain inlets were sized to take the two year storm.
The runoff is to be carried in the streets based on the criteria
of the two year storm not topping the top of curb. Inlets and
storm drains will be provided where the runoff cannot be carried
in the street.
The drainage calculations and mass diagram are enclosed and two
year developed runoff quantities are shown on the drainage plan.
�.. a. G?AZV,`-'
Richard A. Rutherford, P•E. & L.S.
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James H. Stewart and Associates, Inc.
COMPUTATIONS
'James H. Stewart & Associates, Inc. 214 N. Howes St. Ft. Collins, CO 80521 (303) 482-9331
By: )2 A E Date 4 1z 3 C• Client: M i -rc N F c.z- r CO. Sheet No. of
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1fffl COMPUTATIONS
Lames H. Stewart & Associates, Inc. 214 N. Howes St. Ft. Collins, CO 80521 (303) 482-9331
By: R. A• ko. Date 6-n / Client: M i TcNP_r_[. e Ld. Sheet No. of
thkd By: Date Project: CLA e,-A)nOnI N ._. Job No.
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