HomeMy WebLinkAboutDrainage Reports - 08/03/1990City of Fort Collins
CITY SERVICE CENTER
REVISED FINAL DRAINAGE REPORT
JUNE91990
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' Engineering Consultants
2900 South College Avenue
Fort Collins, Colorado 80525
' (303) 226-4955
June 29, 1990
Revised July 25, 1990
' Ms. Susan Hayes
Stormwater Utility Dept.
City of Fort Collins
P.O. Box 580
Fort Collins, CO 80522
' RE: CITY SERVICE CENTER
FINAL DRAINAGE REPORT
Dear Susan:
RBD Inc. is pleased to submit this letter as the Final 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 improvements will also
include the installation of curb and gutter along the east side of
the improved street section. Although the entire 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 the 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 street
' Other Offices: Vail, Colorado (303) 476-6340 • Colorado Springs, Colorado (719) 598-4107 • Longmont, Colorado (303) 678-9584
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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
H4) 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. These six historical
outfall points have been labeled on the enclosed Overall Drainage
' and On -site Utility Plan as points 1H thru 6H. The westernmost of
these outlets is point 1H. This line; historically used by the
City of Fort Collins as a storm drainage pipe; is a water right
which is owned by the property Owner to the north. This line has
been designated as the Nauta drain line. As a part of this
project, all storm drain connections to this line will be removed
and incorporated into other drainage facilities.
' Within this same basin (basin H3), just north of Bldg. A is an
existing sump. This sump contains a catch basin which collects
' local drainage and is also fed by an 8" RCP storm sewer located at
the northwest corner of Bldg. A. The outfall from this sump is
piped through an 18" CMP onto the property just north of the
service center. outfall point 2H.
' The property to the north also receives water through another drain
line; Griffin Drain; which bisects the easterly most parking lot.
' outfall point 3H. The remaining storm drain pipe systems (4H thru
6H) outfall to the east into the unused street right of way and
City owned property to the north. All the site flows, with the
exception of the basin mentioned, eventually outfall into the City
' property north and east of this site. On this property they drain
through a rather ill-defined channel directly to the Poudre River,
approximately 60.0 feet Northeasterly. The area through which this
' channel flows exists as bottomland and no structures exist between
the outfall point and the Cache La Poudre river. A hydraulic
analysis of the channels capacity was not performed as a part.of
this project. However, it is our opinion that the minor
modifications described herein to the historical drainage basins
will not significantly change the existing hydraulic conditions of
the channel.
' 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, the options for
handling 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 of the site's proximity to the Poudre River no onsite
' detention will be required.
3. Analysis is limited determining the effect of proposed
improvements to existing drainage flows and the facilities
' downstream.
4. Offsite upstream drainage problems such as the impact of
' undetained flows ponding against the existing railroad
embankment will not be addressed in this approach. The
proposed improvements will not affect this area except to
reduce some flows which may have been contributing to this
' area from historic basin H1. It is anticipated that this
potential problem will be investigated in depth when the
City or another developer choose to develop the property to
the south of the railroad.
During investigation of the site, it was determined that the
Nauta's hold water rights on drainage lines that lie within the
City Service Center Property and that the City of Fort Collins was
utilizing these lines as a part of their storm sewer system. This
phase of the Service Center Improvements has focused on eliminating
' these tie-ins and further, to reduce off -site flows onto the Nauta
Property. As such, any existing known tie-ins to the Nauta Drain
have been eliminated and transferred to storm drain lines A and C.
Storm drain line A is planned to service the sump condition at the
' new turnaround drive and a portion of the revised parking lot
(Basin D2). Because this sump drains under the building addition,
three safety features have been designed into the system. Concrete
' encasement is required under the building, it has been designed to
carry twice the 100 yr. event and 2 inlets have been incorporated
as a backup. In addition, an outfall swale has been designed to
' carry the 100 year event offsite in the unlikely event this system
is overtaxed. Storm drain line A has been designed to carry the
combined flow of lines B & C. Storm drain line C, has been
installed to pickup those flows which previously were connected
' into the Nauta drain line. At the intersection of storm drain line
A & C, a catch basin has been installed. It is intended for this
catch basin to capture nuisance flows directed to the inlet by an
' asphalt curb at the north property line, after regrading is
complete as a part of the second phase of these improvements.
' These assumptions clarify the overall drainage approach. A
detailed explanation of the actual drainage design follows.
As stated earlier, the proposed improvements 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 Dl).
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. Inflow curb & gutter has been designed in this area to
' minimize nuisance flows.
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 improved pipe system north of the
' new Addition. Several safeguards are proposed for this system:
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 under the building
will be encased and oversized to handle over 100 year
tundetained flows.
3. In the event the pipe system is overtaxed, 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 under the addition.
' Throughout the remainder of the site historical drainage patterns
are closely approximated. Offsite discharge rates do increase
slightly by the proposed changes.
1
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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, `\`o„"""pn EG. If",,,,,
^tea
' ,o: ;25074 ^w
Cort D. Nickel, P.E.
' CC: Jack Gianola, City of Fort Collins
George Brelig, RB&B
344-001, 022-1006.344
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