HomeMy WebLinkAboutDrainage Reports - 03/01/1984STORM DRAINAGE
FOR
STUDY
PLATTE RIVER
POWER AUTHORITY
MAINTENANCE FACILITY
FORT COLLINS, COLORADO
MARCH 1984
qm,NC
Engineering Consultants
TDINC
Engineering Consultants
Drake Terrace Suite 240
343 West Drake Road
Fort Collins, CO 80526
303/226-4955
021-005
March 2, 1984
Mr. Phil Waite
City of Fort Collins
Storm Drainage Utility Department
P.O. Box 580
Fort Collins, CO 80522
RE: Platte River Power Authority Office Maintenance Facility
Dear Phil:
Please accept this report and enclosed calculations for review and accep-
tance of the PRPA drainage plan.
SITE DESCRIPTION
Platte River Power Authority proposes to construct a maintenance facility
with office space on the northwest corner of the PRPA office complex at
Horsetooth and Timberline Roads. The portion of the PRPA property consti-
tuting the site is presently vacant land adjacent to the Union Pacific
Railroad and to the Collindale Industrial Park. A parking area that is to
serve as an addition to an existing parking area abutting the site to the
south is also to be constructed at the west end of the site. Minor modifi-
cations to the existing parking area.are to be made. An access road to
serve the site is to be constructed at the southwest corner of the PRPA
property.
EXISTING SITE DRAINAGE
Drainage from the site has previously been considered in a report, "Revised
Drainage Report for Platte River Power Authority, Fort Collins, Colorado"
prepared by Resource Consultants, Inc., February 1980. A copy of this
report was obtained from the City for the present drainage investigation.
As described in the Resource Consultants report, a pond was constructed at
the east end of the present site to accommodate the runoff from this
undeveloped portion of the PRPA property. This pond is still existent, and
will be bermed and shaped to handle the detention requirements of the
proposed project. The pond is drained through a 10" RCP at the northeast
corner of the site.
Contributing off -site flows are limited to the portion of the PRPA property
south of the project site. Flows from the existing parking lot and the
existing and proposed access roads contribute to the site. Because of the
presence of berms at the Collindale Industrial Park and the Union Pacific
property boundaries, no off -site contributions are made from these proper-
ties. Topography on the site is flat, with slopes less than two
percent from south to north. Soil is the Nunn clay loam, for which runoff
is slow to medium and water erosion is slight to moderate, according to the
Soil Conservation Service. The Hydrologic Soil Group is C.
Calculation of storm water runoff was determined by the rational method.
The volume of detention storage was determined by comparison of the 100-
year developed cumulative runoff and the existing 2-year cumulative runoff,
referred to as a mass -diagram analysis. The methodology follows that
presented in the "City of Fort Collins Storm Drainage Criteria", dated
January 1980. The rainfall intensity duration curves presented in Figure 1
of that report were used for the present investigation. The capacities of
channels and piping were determined from the Manning equation.
The total drainage area contributing to the site is 156,220 square feet, or
3.59 acres. The site itself comprises 87,910 square feet, or 2.02 acres,
all of which is presently bare ground with a runoff coefficient of 0.20.
The proposed development would have the following land uses:
Building
10,190
S.F.
Sidewalk
30130
S.F.
Parking and Drives
100340
S.F.
Landscaped
640,250
S.F.
West Access Road
7,200
S.F.
The existing east access road and parking lot south of the proposed build-
ing contains 56,360 square feet of impervious area. The existing drainage
basin thus contains 87,910 square feet of bare ground and 68,310 square
feet of impervious area. The proposed development would have_91.972 square
feet of impervious area and 64,248 square feet of landscaped area. The
runoff coefficients are 0.20 for the historic condition and 0.64 O K
for the proposed condition. /
Given the 2-year runoff of 1.00 cubic feet per second, the site will
require a stormwater detention storage of 23,560 cubic feet during t
year storm event., Maximum detention pond capacity is 2-L2M cubic feet.
The maximum water surface elevation for the 100-year storm is 52.0 feet
The time to empty the detention basin of the accumulated volume from the
100-year storm is computed to be 6.5 hours at the allowable discharge rate.
The actual time to empty the detention would be different than this,
because the orifice plate is designed to pass the allowable dischargpw.hen_
the water surface_is at 51.3 feet; i.e. when the pond is about half -full.__
On -site drainage will be directed as shown by the arrows on the drainage
plan. The new parking lot will have an inverted crown. As this handles
local flow only, there will be sufficient capacity to contain major storm -
water flows safely.
The existing parking area adjacent to the site has two 3-foot curb openings
for drainage of parking lot flows. These will be replaced by a sidewalk
chase and a metal curb chase.
The detention pond is to be landscaped to be made aesthetically attractive.
The outflow from the detention pond connects to an existing storm runoff
system. There is an existing detention pond on the northeast corner of the
PRPA property which will be the receiver for the flow.
The existing 10-inch RCP pipe will be cut in order to install a control
catch basin. The control catch basin will have an orifice plate attached
to a riser pipe. This system is designed to reduce the flow blockage
problems that commonly occur when orifice plates are used at pipe
entrances. It also serves as an oil/water separator and should improve the
water quality of the runoff. The orifice is set in horizontal position so
that flow through it is vertically upwards. The orifice has a diameter of
4.6 inches and is set at an elevation of 48.2, so that maximum head on the
orifice is 3.8 feet. At maximum head, the discharge through the orifice is
1.00 cubic feet per second. Should water surface elevation exceed the
design 100-year level, flow would also enter through the top of the riser
pipe.
THe riser pipe and control device are made with CMP pipe in order to
minimize weight and avoid reinforcement problems. The Contractor will be
given the option of replacing the existing 10-inch RCP with 10-inch CMP at
matching inverts in order to reduce costs.
The control catch basin is located as close as practicable to an existing
parking lot for ease of maintenance.
With the on -site detention pond as planned, and the existing downstream
detention pond, there will be no stormwater quantity impact from the pro-
posed development.
RECOMMENDATIONS
1. Grading and construction of drainage facilities be in accordance with
the grading and drainage plan.
2. Finished grade at the proposed building to be sloped away from the
structure on all sides.
3. Detention basin, drainage swales and berms to be seeded and sodded as
part of the landscape plan.
4. All grading and construction of drainage facilities be accomplished
under observation of an engineer.
Should there be comments or questions concerning this drainage report and
plan, please contact us.
Very truly yours,
RBD, INC.
Mark Siegenthaler
(3d F. ar, .E.
MS/DFS:kac QP'UU
PV0 F•
22306
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OFFSET FR,&,ML GRATE SO TIIAT
RESTRI C,wmR/SEPARATOR DEVICE IS
VISIBLE AT EDGE OF OPENING /4ND
DIRECTLY OVER THE L DDEP.
(LOCKING LID REQUIRED)
-7„—
t2"
STD. GALV. CB
LADDER STEPS-
D104 (w « \
H /1
IIM
PIPE
MIN.X; shi+9
SLOPE
0.5% �d„RAP
INVERT ELEVATION —�
RESTRICPOR PLATE
APPROPRIATELY
SIZED ORIFICE CUT
IN PLATE. NOT
REQUIRED IF SYSTEM
IS FOR POLLUTION —
CONTROL ONLY
Grate not allowed on
resrrictor manhole.
Must be round, solid
selflocking lid. .
ELEVATION = $-9� i
sc, t, 6 04, fo
/ CHAIN - 200Yt CAPACITY
SLACK WHEN GATE IS DOWN
WELD CHAIN TO CB FRAME
0.5'MIN.
E4. sto
--w—OVERFLOW ELEVATION - SET TO
PROVIDE REQUIRED DETENTION
AND ADEQUATE OIL SEPARA-
TION PROTECTION
OVERFLOW PIPE & OUIM
PIPE MUST HAVE CAPACITY
EQUAL TO OR GREATER THAN
o . $ [ ' COMBIlIED TNi .Fr SYSTEM
OUTLET PIPE MINIMUM
SIOPE = 0.5%
x : nti1 6;, io " �'LP
2'MIN.
OK ,2�{'L19GE WrH /O"cn-[P
to GHP
To MarcMEk[an.�G nve2r=.
0
FLOW RESTRICTOR/OIL SEPARATOR
CONTROL DEVICE/CATCH BASIN
— EL. -4s7
NOTE - ALL STEEL PARTS AND'
SURFACES MUST BE GALVANIZED
AND COATED WITH ASPHALT
(TREATMENT 1 OR BITTER)