HomeMy WebLinkAboutDrainage Reports - 08/27/1987August 27, 1987
City of Fort Collins
Storm Drainage Utility
P.O. Box 580
Fort Collins, CO 80522
ATTN: Mr. Glen Schlueter
RE: Drainage Report
Congregation of the Living God Building
4820 South College Avenue Project No.: 206.23
GENERAL
The project is a remodel of an existing building at 4820
South College, converting the facility from a commercial building
to one more appropriate for religious pursuits. Aside from the
removal of an existing deck on the northeast corner of the
building, the remodel is internal. The onsite parking available
will be expanded considerably due to the change in use. This
report addresses the drainage improvements needed to comply with
City Criteria.
The site is located on the east frontage road of College
Avenue approximately a quarter mile south of the intersection
with Harmony Road, in the northwest quarter of Section 1, T6N,
R69W. The property is 0.574 acres in extent, bordered on the
north by a similar commercial building, on the south by an empty
lot, and by residential property to the east. The westerly
portion of the site drains to the frontage road; the easterly
portion drains to a swale running southerly to Fossil Creek.
The western portion of the site consists of lawn and paved
parking, and will receive only minor modification. The eastern
portion of the property, as well as a 5 foot access easement on
the south boundary, is covered with scrub grass which will be
replaced with paved parking and landscaping. Slopes are 4 to 15%.
HYDROLOGY
The property is zoned BL which indicates a minor design
frequency of 10 years, however as the site involves no
conveyances to be sized the minor design storm evaluated was the
2 year event. The Rational method was utilized; rainfall
intensities were taken from the Criteria curves. The western
basin is 0.274 acres in extent including some offsite lawn to the
north. Proposed improvements to the western basin include
widening the onsite drive by 5 feet to the south and the addition
of a landscaped island on existing pavement. Existing composite C
was taken to be 0.575 and the developed composite C is 0.600
based on the proposed site plan. Existing peak flows are 0.43 CFS
ENGINEERING PROFESSIONALS INC.
2000 Vermont Dr., Fort Collins, CO 80525 (303) 226-3852
Plaza of the Rockies,111 South Tejon, Suite 300, Colorado Springs, Colorado 80903 (303) 634-5006
and 1.38 CFS for the 2 and 100 year events. Developed flows are
0.44 CFS and 1.44 CFS respectively. These flows historically
drain to the eastern double gutter in the frontage road and
thence southerly to the draw behind the Fred Schmid building and
into Fossil Creek. Due to the nature and grades of the existing
onsite improvements an onsite detention facility is impractical.
Since the proposed improvements do not appreciably increase the
historic flows, no detention is proposed for the western basin,
and an attempt was made to provide additional detention volume
for the eastern basin to compensate.
The eastern basin encompasses 0.403 acres including some
offsite land to the north. Existing composite C was taken to be
0.312 and the developed composite C is 0.668 based upon the
proposed site plan. The 2 year existing flow is 0.29 CFS, and the
100 year existing flow is 1.10 CFS. Developed peak flows are 0.77
CFS and 2.36 CFS. Detention is in the parking lot as shown on the
grading plan. The grading plan was developed with the intent of
maximizing the detention volume available without overly
affecting the function of the parking lot. The available volume
for detention is 2185 CU FT at a maximum pool depth of 12 inches.
The inflow curve was plotted and the release rate determined to
be 0.27 CFS, slightly less than the existing 2 year flow of 0.29
CFS. While this does not provide for any appreciable excess
detention, it does appear to be the maximum the site will allow.
IMPROVEMENTS
The eastern portion of the site will be paved for parking.
The grading plan depicts the lowpoint in the southeast corner of
the proposed parking. The parking detention as shown on the plan
contains a total volume of 2185 CU FT. Discharge is through an
steel orifice plate with a diameter of 3-7/16 inches feeding a
concrete chute accessing the irrigation lateral. The design
discharge rate is 0.27 CFS, appreciably the same as the 2 year
existing flow of 0.29 CFS. Excess flow will overtop the plate, if
necessary. This lateral ditch lies in the natural thalweg and
currently receives runoff from the site and surrounding property.
Construction of the proposed parking lot will not change the
historic runoff patterns or volumes. The irrigation lateral is
owned by the West Fairway Estates Water Association and lies in a
dedicated bridle path and utility easement as shown on the plat.
Negotiations have commenced concerning acceptance of the proposed
improvements by the Water Association and a signoff block has
been added to the grading and drainage plan.
No grading or drainage improvements are proposed for the
western basin since it is impractical to regrade the existing
parking lot and lawn to provide detention based on the historic
conditions prior to the initial development of this property. An
attempt was made during the design process to provide additional
detention volume in the eastern basin to compensate; however, the
rear parking lot cannot accommodate any additional detention
without creating overly flat slopes in the parking lot and
ponding depths in excess of 12 inches. This problem could have
been addressed when the site was initially developed, but was
not a requirement at the time it was platted under County
jurisdiction. Therefore a variance is requested concerning
detention for the western basin. Calculations are attached;
please refer to the grading and drainage plan for particulars.
ENGIIN��EEyyRING PROFESSIONALS, INC.
42!I/��
Robert W. Schildg�.
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GRADING AND DRAINAGE NOTES
1. NO GRADING OR DRAINAGE IMPROVEMENTS ARE PROPOSED FOR THE
WESTERN BASIN DRAINING TOWARDS THE U.S. 287 FRONTAGE ROAD.
EXISTING FLOW PATTERNS AND VOLUMES REMAIN APPRECIABLY UNCHANGED.
2. HYDROLOGIC DATA ARE AS FOLLOWS:
EVENT ACRES L, ft S8 Tc, min I, in/hr C Q,cfs
WEST 2 EXIST 0.274 145 3.4 7.9 2.7 0.575 0,43
100 EXIST 0.274 145 3.4 5.7 7.0 0,719 1.38
WEST 2 DEV 0.274 145 3.4 7.5 2.7 0.600 0.44
300 DEV 0.274 245 3.4 5.3 7.0 0.750 1.44
EAST 2 EXIST 0.403 152 3.6 11.9 2.3 0.312 0.29
100 EXIST 0.403 152 3.6 10.7 7.0 0.390 1.10
EAST 2 DEV 0.403 154 2.9 7.1 2.85 0.668 0.77
100 DEV 0.403 154 2.9 4.3 7.0 0.835 2.36
3. PARKING LOT DETENTION IS PROVIDED FOR THE EASTERN BASIN FOR
BASICALLY THE 100 YEAR DEVELOPED - 2 YEAR HISTORIC DIFFERENCE.
RELEASE RATE IS 0.27 CFSI VOLUME IS 2185 CU FT; MAXIMUM POOL
DEPTH IS 12 INCHES.
4. ALL CURB IS 60 VERTICAL, EXCEPT AT DETENTION POND WHERE
MAXIMUM CUR
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SECTION A - A
P2OFILE-TUebUC-Ji 0I5C-IC¢6E CHUTE
N.T.S.
OPIFICE RATE IS '/1' STEEL. -�C'T d. • '/I* ANCACIZ P)OI.TS
IN VErzT. 6u¢6 GAZE TO ANNO¢ RATE TO CHUTE.
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