HomeMy WebLinkAboutDrainage Reports - 03/22/1982W.
STORM
WATER DRAINAGE REPORT
FOR
DRAKE CROSSING SHOPPING CENTER
IN THE
CITY OF FORT COLLINS, COLORADO
MARCH 1982
,o1® TARANTO, STANTON Ac TAGGE
CONSULTING F:NCINF.ERS
PORT C O I. L I N' C O L O R A D O
_.
March 17, 1982
RE: Storm Water Drainage Report
Drake Crossing Shopping
Center
City of Fort Collins
Department of Engineering -Drainage
300 Iaporte Avenue
Fort Collins, CO 80522
Attn: Phil Waite
Dear Phil:
This Storm Water Drainage Report has been prepared for Drake
Crossing Shopping Center. Sole variations have been made in the
development of the flows from this area, from that which was
assumed and outlined in the Valley Forge P.U.D. Study. I believe
that you will find these variations self-explanatory. Mark
Engwoen reviewed the Valley Forge P.U.D. Report and should be
able to assist you.
If you should have any questions, please do not hesitate to
contact our office.
Respectful su�
^� S
Tarry Stanton, P.E.
Robert H. Schen¢r>`rling, P.
TAP,;N=, STANTON & TAGGE
CONSULTING ENGINEERS
LES:RHS:lla
400A
112 W. 11TH AVE. 6412 S. COLLEGE AVE. P.O. BOX 703
HOLDREGE. NE 68949 FORT COLLINS. CO 80525 HAYDEN. CO 81639
STOPM WATER DRAINAGE REPORT
FOR
DRAKE CROSSING SHOPPI14G CENTER
Drake Crossing Shopping Center is located in the Southeast Quarter of Section
21, Township 7 North, Range 69 West of the Sixth Principal Meridian, City of
Fort Collins, Larimer County, Colorado. This development is surrounded to the
West by RP zoned, unplatted property and to the North, South and East by the
following:
A. North - Valley Forge (A Planned Unit Development)
B. South - Drake Road and Unplatted Property
C. East - Taft Hill Road and the Peoples Church of Fort Collins
The average undeveloped slope of this property is one (1%) percent falling
from West to East (Exhibit "A").
This development has been previously studied in the Valley Forge P.U.D.
Drainage Study. This report expands on the drainage which will be developed
from the Shopping Center and explains how this development will affect the RP
zoned, vacant property to the West.
Drake Crossing Shopping Center consists of 12.11 acres. The land is presently
undeveloped and made up of well drained soils. Since the area is less than
200 acres, the City of Fort Collins Drainage Regulations allow a modified form
of the Rational Formula ( Q = CCfiA) to be used to determine runoff rates
where:
Q = Discharge is cubic feet per second.
C = Unitless coefficient estimating ground perviousness.
Cf Unitless rainfall frequency adjustment factor.
i = Estimated rainfall intensity in inches per hour.
A = Basin design area in acres.
PLEASANT B
LAKE VALLEY BROWN FARM
CANAL 3rd FILING
FA
DRAKE i
ROAD
CROSSING
/ALLEY FORGE
P.U.D_
DRAKE CROSSING
SHOPPING CENTER
EXHIBIT A
LEXINGTON
GREEN
OVERALL
DRAINAGE
BOUNDARY
DRAINAGE
COLLECTION
POINT
9929Tro.e e.a .• aoa e. eo a
.f■917�{CRY■1 uv<<nw<
The City of Fort Collins has specified the unitless coefficient values based
on zoning. The zoning of this area is BP and the coefficient values are as
follows:
2 & 10 YEAR STORMS
Undeveloped Conditions Developed Conditions
C Cf C Cf
.2 1.00 .85 1.00
100 YEAR STORM
C Cf C Cf
.2 1.25 .85 1.25
Rainfall intensity is determined by estimating the time it takes to
concentrate the maximiun discharge a storm can create at a point of collection.
This is known as the Tine of Concentration. The City of Fort Collins Drainage
Requirements estimate the Time of Concentration by the following equation:
Tc = 1.87 (1.1 - C C€) D" where:
S 1/3
Tc = Time of Concentration in minutes.
C, Cf = Unitless coefficient previously described.
D = Maximum length of design basin in feet.
S = Average basin slope in percent.
Rainfall intensity is correlated to Time of Concentration by the
Rainfall -Intensity -Duration Curve (Exhibit "B"). Rainfall Intensity is
measured in inches per hour.
All runoff from the area shown on Exhibit "A" presently drains to a existing
drainage swale and 40' easement which parallels the West right-of-way of Taft
Hill Road from Drake Road to the north property line of Valley Forge P.U.D.
INTENSITY -INCHES PER HOUR
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Flow in the swale drains from North to South an enters a storm drain inlet at
the south end. Upstream of the inlet is a 24-inch R.C.P. storm drain which
carries runoff from Brawn Farm Filing No. 3. At the inlet, the storm drain
increases to a 36-inch R.C.P. It has been calculated that the increased
capacity of the storm drain at this point is 50 c.f.s. (Appendix "A").
Drainage figures for Valley Forge P.U.D. have been developed in the Valley
Forge Drainage Study and are presented on Table 1. The developed flows
include all of Valley Forge Drive and that part of Hampshire Road shown on the
Valley Forge design drawings.
The Drake Crossing Shopping Center consists of 14.66 acres. This includes the
remainder of Hampshire Foad from Valley Forge Drive to Drake Road. Tracts B
and C are also included in this study. Since the entire area is zoned, BP,
developed runoff figures for Drake Crossing Shopping Center will include
tracts B & C as if they were to be developed. The drainage from tracts B & C
described above will flow directly into the drainage swale. This should
preclude the need for an additional drainage study for these areas.
The only area which will contribute drainage to this site is the RP zoned area
to the West. This area consists of approximately 20 acres. Drainage from
this area will have to flaw to the existing 36-inch storm drain. We have
allowed for this by sizing the storm drain that goes through the Shopping
Center so that the estimated 2-year developed flow of 14 c.f.s. (Table 2) can
also flaw through the pipe. This capacity will also be able to handle the
100-year undeveloped flow. Once developed, the 100 year developed flow will
have to be retained on site. This retention is estimated to be 1.60 acre-feet
(Appendix "B") .
Die have evaluated developed flaws for the Drake Crossing Shopping Center for
the 10-year and 100-year storms as required. Also evaluated was the 2-year
storm. The reason for this was to correlate the findings of the Shopping
Center to Valley Forge P.U.D. and the RP zoned area to the West-.
Undeveloped, the flaw from the Drake Crossing Shopping Center, the RP zoned
area, and Valley Forge P.U.D. flows from West to East. The entire undeveloped
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area, Exhibit "A", consi-sts of approximately 40 acres. Table 3 outlines the
expected undeveloped flow data for the 2, 10 and 100-year storms. It is found
that the extra capacity of the 36-inch storm drain is more than adequate to
handle the 100-year undeveloped flow from the entire undeveloped Exhibit "A"
area.
The developed flows from Valley Forge P.U.D. will drain totally into the
retention area provided along Taft Hill Road. No intermediate on -site
retention is provided.
For the Drake Crossing Shopping Center, the project will consist of five (5)
drainage areas in the developed state plus tracts B & C. These areas and the
direction of flows are shown on Exhibit "C". The flaw figures for these areas
are given on. Table 4.
The drainage plan for the area is to discharge the 100-year flows from the
site as quickly as possible. Based upon a release rate of 50 c.f.s. in the
36-inch R.C.P. for the area shown on Exhibit "A", the following plan for its
use is advised.
At the time the Drainage Study for Valley Forge was prepared, it was assumed
it
that all developed drainage from the Shopping Center would also drain directly
0
into this swale. As can be seen on Exhibit "C", this is not longer true.
' Only that drainage from Area 4 and tracts B & C flow into the drainage scale
overland. These areas combined with Valley Forge P.U.D. have a 100-year
developed flow of 62.9 c.f.s.
The new storm
drain which collects drainage
from the Shopping Center and the
RP zoned area
has been designed to let another
25
c.f.s. enter the exi sti.-ig
36-inch storm
drain. The drainage from Area
5 and
Drake and Taft Hill Roads
h will flow on an existing inlet on Drake Read.
This
inlet will allow a inflow
uof 6 c.f.s. Retention for all flaws over 6
c.f.s.
from Area 5 and Drake and
_ Taft Hill will
be on the street.
Subtracting 6 c.f.s. from the 50 c.f.s. capacity available, a remainder of 44
c.f.s. can be allowed into the storm drain from the drainage swale. The new
•'1 .
storm drain flows directly to the 36-inch storm drain. Therefore, a 19 c.f.s.
Iinflow is allowed for Area 4, tracts B & C and Valley Forge P.U.D. The
retention for this area has been calculated using a mass flow diagram. This
is shown in Appendix B and it was found that retention of 1.40 acre-feet is
needed. The existing drainage swale can easily handle this retention.
I Retention will be required in Areas 2, 3 and the RP zoned ara. The storm
sewer has been designed to drain a 2-year storm for areas 1, 2 and 3 and the
2-year storm for the RP zoned area. It is impossible to drain the 10-year
storm as required because of the size of the existing storm drain under Drake
IRoad. We have been able, though, to provide adequate retention.
It has been calculated that the RP zoned area will generate 14 c.f.s. during
the 2-year developed flood. The retention required for this area is the
difference between the 100--year and 2-year developed storms. At the time
' retention is built for this area, careful design will have to be done so as
not to allow more than 15 c.f.s. from entering the drain. For the present
100-year undeveloped flood of 15.5 c.f.s, the water will flow onto Iiampshire
Road and enter the rain through a inlet at the low point. This inlet should
i have a throat opening of 8-feet. There will be a slight backup of water, but
I it will occur only during the 100-year undeveloped flood and be eliminated
when the RP area is developed.
Calculations for Area 1 show that the 100-year developed storm will generate
14.9 c.f.s. The combination of the two 8-foot inlets on either side of
Hampshire Road will provide enough capacity for this drainage. Since the
Time of Concentration for this area and Areas 2 and 3 are approximately 1/3
that of the fastest Time of Concentration for the RP zoned area in the
developed state, water from these areas should be out of the storm drain and
well down Spring Creek by the time the concentrated flews from the RP zoned
area enter the storm drain. Therefore, the entire 100-year developed flcod
fran Area 1 can be totally drained and no retention will be required.
For both Areas 2 and 3, the storm drain has been designed
to handle
the 2..year
developed storm. Retention is greater than
that 10-year
developed
storm, but
this can be adequately provided as shaven on
Exhibit "C".
The inlet
for Area :.
t
will have to have an 8-foot throat opening and the inlet for Area 3 will be
adequate by using a standard 4-foot opening.
Appendix "B" also shows the mass diagrams developed for retention design for
Areas 2 and 3. The retention required for each area is as follows:
Area 2 = .40 acre-feet
Area 3 = .25 acre-feet
The retention perimeters for Areas 2 and 3 are shown on Exhibit "C". The
worst depth of water expected to occur during the 100-year flood which would
be in Area 2, will not be more than one (1) foot.
Because of the changes made in the location of retention in the Shopping
Center, retention should not be needed on the Valley Forge site. Roan is now
available within the existing drainage swale along Taft Hill to do sane
landscaping and still maintain adequate retention.
If there are any questions regarding this study, please contact our office.
Respectfully submitted,
Robert H. Schemparling, P.E.
Tarry E. Stanton, P.E.
TARANTO, STANTON & TAGGE
CONSULTING ENGINEERS
RHS:LES:lla
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(h 14-YI MVM pq,T"t�NnoN O«VRI 3U 'MIN = �Of--,<7 FT