HomeMy WebLinkAboutCITY UTILITIES SERVICE CENTER PUD PRELIMINARY - 16 90 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT40 •
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Engineering Consultants
2900 South College Avenue
Fort Collins, Colorado 80525
303/226-4955
April 2, 1990
Ms. Susan Hayes
Stormwater Utility Dept.
City of Fort Collins
P.O. Box 580
Fort Collins, CO 80522
RE: CITY SERVICE CENTER
PRELIMINARY DRAINAGE REPORT
Dear Susan:
RBD Inc. is pleased to submit this letter as the Preliminary
Drainage Report for the City of Fort Collins Service Center.
PROPOSED IMPROVEMENTS
This City of Fort Collins project consists of a Building Addition
to the current onsite facilities and the addition of more parking
facilities. The Addition will be located between the two main
buildings currently occupying the site, namely the Streets
Department facility (Bldg. B) and the Water & Sewer Department
and Light & Power Department facility (Bldg. A). Other proposed
site improvements include additional parking facilities south of
the present site, realignment of the main entrance to the
facility and realignment and repavement of approximately 280 feet
of Wood Street along the site's frontage. Wood Street improve-
ments will also include the installation of curb and gutter along
the west side to the improved street section. Although the en-
tire 18.8 acre site is being platted for the P.U.D. process site
improvements will be limited to the western third of the site.
EXISTING DRAINAGE CONDITIONS
This site is located along east side of north Wood Street at its
intersection with West Vine St. The site is bounded by the
Colorado & Southern Railroad right of way on the south, Wood
Street on the west, the Nauta property on the north and an unused
Other Offices: Vail, Colorado 303/476-6340 9 Colorado Springs, Colorado (719) 598-4107
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street right of way and City owned property on the east.
The City owned property on the east extends to the Poudre River.
The vast majority of the site is paved parking and storage areas.
Most of the site sheet flows to the east and north with a small
area (Basin H5) draining to the northwest. Area inlets gather
limited flows on site in various locations (See Overall Drainage
and Utility Plan). These inlets are tied to various storm drain
pipe systems which cross the site along six different alignments.
Three of these systems outfall along the north property line into
the Nauta property while the remaining systems outfall to the
property to the east.
Offsite flows drain toward the site from the west and south.
Flows from the west are intercepted by Wood Street. These flows
drain to the north and south from the high point of Wood Street
which corresponds to its intersection with West Vine Street.
Flows from the south are limited by the railroad embankment and a
30 inch X 12 inch wooden culvert.
PROPOSED DRAINAGE APPROACH
No previous drainage analysis has been done in this area. On site
drainage facilities appear to have been designed as interim solu-
tions for drainage problems that may have been occurring at the
time of their construction. Offsite issues particularly relating
to the discharge of storm water flows onto adjacent properties
have never been adequately addressed. Offsite areas draining
toward the site are existing neighborhoods, therefore, regardless
of the City's normal drainage requirements the options for han-
dling undetained flows in the area are extremely limited.
Based on these observations the design approach taken for this
site is based on the following assumptions:
1. The existing conditions on site and in the area surrounding
the site are treated as the historic conditions.
2. Because on the site's proximity to the Poudre River no on -
site detention will be required.
3. Analysis is limited determining the effect of proposed im-
provements to existing drainage flows and the facilities
downstream.
4. Offsite drainage problems such as the impact of undetained
flows ponding against the existing railroad embankment will
not be addressed in this approach.
5. During final design the effect of the offsite drainage dis-
charges and the acceptance by adjacent properties will be
formally resolved.
These assumptions clarify the overall drainage approach. A
detailed explanation of the actual drainage design follows.
As stated earlier, the proposed improvements will only affect the
western third of the site. The only exception to this statement
is a increase in the flows which are directed toward the outlet
of the railroad culvert at Design Point A.
Within the western third of the site flow patterns will change
slightly due to the additional parking which is proposed along
the south edge of the site. This additional parking is developed
through the acquisition of a portion of the railroad right of way
and a small triangular property south of the existing railroad.
These areas which historically drained offsite (Basins Hla and
Hlb) will now be directed eastward as part of a larger basin
(Basin D1). Drainage from this proposed basin will drain to a
low point in the new parking area and then will flow in a
drainage swale to Design Point A.
The Building Addition will block off drainage patterns which cur-
rently flow between Buildings A and B (Basin H3). A pipe system
is proposed to carry flows from the south side of the Addition
through the Addition's foundation to an existing pipe system
north of the new Addition. Since extending a storm drain through
a building foundation is rarely done several safeguards are
proposed:
1. The area draining into this pipe system has been minimized
to reduce the extent of any possible drainage problem.
2. The inlets in this area and pipe extended through the build
ing will be oversized to handle over 100 year undetained
flows.
3. In the event of the pipe system failure this area is
designed to overflow into the adjacent basin at Design Point
B. The pavement elevation at Design Point B is 0.3 feet
lower than the finish floor elevation of the Addition and
Building A.
These safeguards should address the concerns raised by routing
the storm drain through the addition.
Throughout the remainder of the site historical drainage patterns
are closely approximated. Offsite discharge rates do increase
slightly by the proposed changes. As stated earlier, the effect
of these offsite discharges and their acceptance by adjacent
properties will be formally resolved during final design.
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By proceeding with this approach the drainage will flow through
this site in a controlled manner. When the final design is
implemented, this approach will satisfy the hydrologic and
hydraulic concerns and current City of Fort Collins standards for
storm water management.
Sincerely,
'_�'0- 114-
Stan A. Myers, P. .
CC: Jack Gianola, City of Fort Collins ��;�sttrtiur,rrrr
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George Brelig, RB&B
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