HomeMy WebLinkAboutDrainage Reports - 04/11/1988EWART&O SSOCIATE S
Consulting Engineers and Surveyors r,v" T'-1
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APR 11 19a��
STORM DRAINAGE REPORT
SAN CRISTO P.U.D.
San Cristo P.U.D. is a ten acre site lying just east of Anderson Place,
and south of the Burlington Northern Railroad. The future Lemay Avenue
by—pass will lie along the easterly side of San Cristo P.U.D. Tenth
Street, which runs along the east side of Anderson Place, will be the
west street of the San Cristo P.U.D. Tenth street is presently paved
to a half width, and the pavement has a straight slope to the east.
There are -no curbs in Anderson Place, and the east —west streets have
inverted crowns which drain to the east. It appears that most of the
Anderson Place development will drain to the east side of Tenth Street.
This water presently drains south along the east edge of the existing
pavement, although the slope along this east edge is approximately 0.2
percent so the run—off is very slow and there are puddles. When the
east half of Tenth Street is paved. and the east curb is constructed,
this off —site run—off from Anderson Place will drain through the proposed
San Cristo P.U.D. into the new detention pond. The attached map shows
the proposed drainage patterns and flow quantities at critical locations.
There will be no off —site drainage from the north because of the railroad
embankment.
We propose to provide detention for.the San Cristo.P.U.D. in the normal
manner of detaining the difference between the 2 year historical run—
off and the 100 year developed volume, as calculated on page ¢ of this
report. Since the Anderson Place development is draining into the
detention pond, we plan to increase the out —flow quantity by the amount
of the 2 year flow from Anderson Place. These calculations are shown P'
on page 5 of this report. /006
James H. Stewart and Associates, Inc.
214 N. Howes Street
PO. Box 429
Ft. Collins, CO80522
3031482-9331
•
The 100 year flow from Anderson Place shall be routed through San Cristo
in the streets, and will enter the detention pond. These flow quantities
are shown on the map on sheet 11.
The calculated composite C factor for this site is 0.5 as shown on sheet
3 of this report. The required detention quantity for the San Cristo
development is 52154 cubic feet as is calculated volume of the proposed
detention pond is 67861 cubic feet volume. There will be an outlet
structure to let the allowable outflow of 6.02 c.f.s. out from the bottom
of the pond and to let the overflow through a grated opening when the
pond has been filled to a depth of 3.7 feet.
Sheet 6 is a mass diagram of the combined areas of San Cristo and Anderson
Place, and it shows both the 10 year and 100 year volumes. From this
mass diagram, we see that the pond will hold the 10 year flow, and will
activate the overflow after 20 minutes of the 100 year storm. The pond
will not overflow the 0.8 foot freeboard during the 100 year storm.--.,
The 6.02 c.f.s. outflow and a maximum overflow volume of 15.8 c.f.s.,
as calculated on sheet 7, will be carried under future Lemay Avenue in
a 24 inch R.C.P. After leaving the pipe, the 21.82 c.f.s. will be carried
in an open ditch to the drainage way along Lincoln Avenue. This ditch
will be temporary until such time the property it crosses is developed.
There shall be a twenty foot wide easement provided for this ditch and
flow calculations are shown on sheet 11.
Phillip I. Robinson, P.E. & L.S. A
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Sheet 2 of 11
STEWA &kSSOCIATES
COMPUTATIONS Consulting -Engineers and Surveyors
214 N. Howes St. Ft. Collins, CO 80521 (363) 482-9331
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