HomeMy WebLinkAboutMIDTOWN ARTS CENTER (3750 S. MASON ST.), CHANGE OF USE - PDP - 1-10/A - CORRESPONDENCE - DRAINAGE REPORT3750 S. MASON ST., FORT COLLINS, CO
u
DRILL 1/4" DIA. HOLE IN
FLOATABLES BAFFLE TO
PREVENT SIPHONING x
SECTION A - A
INLET MODIFICATIONS FOR STORMWATER QUALITY IMPROVEMENT
B
t-
A E7
A <-1
PLAN VIEW OF INLET
PROPOSED
FLOATABLES BAFFL
1/8" GALVANIZED PL
PROPOSED
SEDIMENT BAFFLE
1/8" GALVANIZED PLA
EXISTING PRECAST
CONCRETE INLET
BOX
B
T
SECTION B - B
EXISTING
DISCHARGE
PIPE BEYOND
Ms. Deanne Frederickson
RE: 3750 S. Mason St. Property — Storm Drainage and Water Quality Enhancement
January 26, 2010
Page 2 of 2
floating pollutants. The attached sketches show the recommended configuration of the baffles
to be installed inside the inlet box.
The inlet is to be inspected semi-annually and the accumulated sediments and floatables
are to be removed from the inlet, using a vacuum truck or other acceptable means, when the
depth of sediments reaches one half the height of the lower baffle. Removed sediments and
other pollutants are to be property disposed of in an approved waste management facility.
I trust that the information presented in this letter report is sufficient and satisfactory. If there
are further questions or comments regarding the findings and/or recommendations presented
here, please do not hesitate to contact me.
Yours truly,
Owen Consulting Group, Inc.
Larry C. Owen, P.E.
Attachment
CG OWEN
CONSULTING
OUP, INC.
January 26, 2010 Project File #: 10-345
The Frederickson Group, LLC
7711 Windsong Drive
Windsor, CO 80550
Attention: Ms. Deanne Frederickson
RE: 3750 S. Mason St., Fort Collins, CO
Site Drainage Assessment
Dear Deanne,
Per your request, I have examined the existing drainage regime at the site of the referenced
project and have considered what improvements might be necessary or appropriate to enhance
the water quality of the runoff leaving the site.
The site, which comprises an overall area of 2.68 acres, is currently fully developed. The
existing building accounts for 0.43 acres and the landscaped areas account for an aggregate of
0.38 acres. The balance of the site, amounting to 1.87 acres, consists either of paved parking
lot or concrete sidewalks. Thus, approximately 85.8% of the surface area of the site is
considered to be impervious and the remaining 14.2% is considered to be pervious. All runoff
migrates via overland flow to the southwest corner of the property. Storm drain utility plans for
the vicinity of the property indicate that an area inlet in the southwest corner of the parking lot
discharges to a nearby street inlet at the east flow line of Mason St., which discharges to a
storm main flowing southward in the Mason St. right-of-way.
It is my understanding that under the proposed redevelopment plan for the property, the
improvements outside the building will be limited to construction of a trash enclosure on a
currently paved surface, and addition of four new landscaped islands within the existing parking
area, to interrupt the continuity of several existing long banks of parking stalls. The trash
enclosure will have no effect on the overall percent imperviousness of the site, and the addition
of the new landscaped islands will actually improve the composite runoff coefficient of the site
by reducing the percent imperviousness of the developed site by approximately 0.2%. Thus,
the amount and rate of stormwater runoff from the existing site will not be exacerbated by the
proposed site redevelopment, but will actually be slightly improved.
This situation was discussed with Basil Harridan and Wes Lamarque of the City of Fort
Collins Department of Public Works and Utilities, and it was agreed that it would not be
necessary to provide on -site detention for stormwater runoff. However, the City did request that
some form of permanent Best Management Practice (BMP) be implemented to improve the
quality of the storm runoff entering the public storm drain system and ultimately the natural
water courses in the vicinity of the site. It was agreed that modifications should be made to the
existing inlet near the southwest corner of the parking area, to enhance the capture and
retention of sediments and floating pollutants conveyed by the runoff from the parking areas.
The consensus reached during our discussions was that installation of a pair of baffles in the
existing inlet structure would provide appropriate barriers to off -site transport of sediments and
3715 Shallow Pond Dr., Fort Collins, CO 80528 Phone (970) 226-0264 Fax (970) 226-3760