HomeMy WebLinkAboutDrainage Reports - 10/29/1993August 12, 1992
Mr. Steve Humann
TST, Inc.
L STONE & ANDERSON, I -c..
Water Resources and Environmental Consultants
736 Whalers Way, Suite F-200
Fort Collins, Colorado 80525
(303) 226-0120
748 Whalers Way, Building D
Fort Collins, CO 80525
Re: Engineering Services for the City of Fort Collins: Buckingham Park Drainage
Investigation (IA Project No. CO-TST-05.13)
Dear Steve:
We have completed the drainage investigation for the proposed improvements to
Buckingham Park and are pleased to submit the following letter report summarizing our
findings and recommendations. The documentation concerning hydrologic analyses, and
curb inlet and pipe designs for the project are attached to this letter.
Based on the survey completed by TST, Inc. of Buckingham Park along First Street, it was
determined that a north-south drainage divide is located approximately 75 feet north of the
existing restroom facility. Consequently, the drainage area contributing to the north end of
the park/or existing conditions consists entirely of a 120-foot length of the west side of First
Street and the adjacent sidewalk (refer to Basin A indicated on page 4 of 8 of the
accompanying material). The existing basin area is 0.06 acres and, based on the rational
method, generates a peak discharge of 0.4 cfs during the 100-year storm.
For with-project conditions, the drainage divide along the centerline of First Street would
not be altered but the entire enlarged parking area would drain to a sump to be located
approximately 85 feet north of the existing restroom facility (Basin B on page 5 of 8). This
would move the north-south divide 75 feet to the south thereby enlarging the contributing
drainage area to about 0.17 acres and increase the 100-year peak runoff to 1.2 cfs.
A Fort Collins standard 4-foot curb inlet would be sufficient for draining the sump in the
parking area; the use of this inlet would result in a water depth of 3 inches above the inlet
invert or 1 inch above the non-depressed gutter flow line. A 15-inch ADS pipe placed at
a 0.2 percent slope would be adequate for conveying the 100-yea:r peak runoff from the
sump area. The computations for sizing these facilities can be found on pages 1 through 3
of 8 in the attached material.
Branch Office : Box 27, Savery, Wyoming 82332
Steve Humann
Page 2
Although the flow depth and velocity at the pipe outlet would be 0.5 feet and 2.5 fps,
respectively, it is recommended that a small riprap apron be placed at the outlet. A buried
triangular apron 5 feet long (in the direction of flow) and 5 feet wide constructed of Class
6 rock is suggested. The apron should be covered with suitable soil material and vegetated.
For both existing and with-project conditions, runoff from this area would be conveyed to
the north, and enter onto a low-lying parcel of land owned by Poudre Pre-Mix, Inc. It is our
understanding that the owner is willing to accept this small increase (0.8 cfs) in stormwater
runoff to the property and that a letter from the owner stating the acceptance of this flow
will accompany the final design drawings.
It has been a pleasure working with you on this project. Please contact us if you have
questions concerning the enclosed material or are in need of further assistance.
Sincerely,
1::iil~
President
~ti!
Senior Engineer
Enclosure
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·s.3.5 Grates for Pipes
Where a clear and present danger exists such as a siphon, a drop in elevation adjacent to a
sidewalk or road. a long pipe with one or more manholes, or at pipes which are near play-
grounds , parks, and residential areas, a grate may be required. For most culverts through
embankments and crossing streets, grates will not be required . r'
When called for on the plans , grates shall meet the following requirements:
a. Grating shall be constructed of steel bars with a minimum diameter of 5/8". Reinforcing
bars shall not be used .
b. Welded connections shall be 1/4" minimum.
c . Spacing between bars shall normally be 6" unless site conditions are proh ibitive.
d . All exposed steel shall be galvanized in accordance with AASHTO M 111 .
e. Welded joints shall be galvanized with a rust preventive paint.
f. Grates shall be secured to the headwall or end section by removable devices such as
bolts or hinges to allow maintenance access, prevent vandalism, and prohibit entrance by
children .
·s.4 Inlets
Storm inlets shall be installed where sump (low-spot) conditions exist or street runoff-carrying
capacities are exceeded.
The curb inlets shown in the Standard Details, pages D-7, 8, 12 & 13, shall be used in all City Streets.
If larger inlets are required, the Colorado Department of Highways Type R Curb Inlet, Standard M-604-
12, shall be used. For drainageways other than streets (for example, parking lots, medians , sump
bas ins) an Area Inlet similar to the detail on page D-9 shall be used .
The outlet pipe of the storm inlet shall be sized on the basis of the theoretical capacity of the inlet, with
a minimum diameter of 15 inches, or 12 inches if elliptical or arch pipe is used.
All curb openings shall be installed with the opening at least 2 inches below the flow line elevation. The
minimum transition length shall be 3'6" as shown on the standard details previously listed.
Because of debris plugging, pavement overlaying, parked vehicles, and other factors which decrease
inlet capacity, the reduction factors listed in Table 5-4 shall be utilized .
Tab le 5-4
INLET CAPACITY REDUCTION FACTORS
Drainage Condition
Sump or Continuous Grade .......................................... .
Street -Sump ............................................................. .
Street -Continuous Grade ......................................... .
Inlet Type
CDOH Type R-Curb
Opening
5 '
10'.
15 '
Percentage of
Theoretical Capacity
80%
85%
90%
80% -...r:::.,...__ __
80%
.f arking Lots, Medians .................................................. .
4' Curb Opening
4' Curb Opening
Area Inlet 80%
MAY 1984
The theoretical capacity of inlets in a low point or sump shall be determined from Figures 5-2 and 5-3.
The theoretical capacity of curb openings on a continuous grade shall be determined from Figures 5-4,
5-5 and 5-6 .
The standard curb-opening is illustrated by Figure 5-4 and is defined as having a gutter depression
apron W feet wide at the inlet opening which extends W feet upstream and downstream from the open-
ing, has a depression depth (a) equal to W /12 feet at the curb face, and a curb opening height (h) of at
least 0 .5 feet. The graph as presented by Figure 5-5 is based on a depression apron width (W) equal to
2 feet and depression width (a) equal to 2 inches . The pavement cross-section is straight to the curb l
5-8 DESIGN CRITERIA
1.0 12 5 l:!
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NOMOGRPAH FOR CAPACITY OF CURB OPENING INLETS IN SUMPS, DEPRESSION DEPTH 2" ( Adapted from Bureau of Public Roads Nornograph
MAY 1984 5-10 DESIGN CRITERIA