HomeMy WebLinkAboutDrainage Reports - 09/14/2010Revision No. 1 To The Drainage Report For
Drake Crossing Shopping Center
Our office has partially revised the grading plan for Drake
Crossing Shopping Center. The reason for this revision is
because we discovered that a stockpile of fill material intended
for this project was not of the quantities originally assumed
to be available. To avoid importing more expensive fill to this
site, we have lowered grades to generate additional on -site
materials.
The grading changes made have slightly changed the size of
developed drainage areas 1, 2 and 4, but the drainage patterns
outlined in the original report have remained the same. The
areas of the developed drainage areas are as follows:
Area 1 - 2.80 Acres
Area 2 - 4.40 Acres
Area 4 - 1.90 Acres
The outline of the new drainage areas can be seen on revised
Exhibit "C". As shown on this exhibit, Area 2 now includes the
Safeway Store. This means that the overall additional drainage
area which is proposed to be drained by the storm drain running
through the project is increased by the area of the Safeway Store.
With the help of ZVFK Architects, we have calculated the 100-year
storm flows which will be developed from the Safeway Store roof
and have provided for retainage on the roof and a discharge from
the roof to the storm drain of 1 c.f.s. The roof area is 1.1
acres, and therefore, the revised flow figures for Area 2 were
calculated based on a area of 3.30 acres. The revised flow
figures for revised developed drainage Areas 1, 2 and 4 are
shown on Table 3-A. These flows will be drained by a 10"
diameter storm drain which is shown on the utility plans for
.Drake Crossing.
As described in the original drainage report, the existing 36-
inch R.C.P. culvert under Drake Road, which will carry storm
waters from this project, Valley Forge P.U.D., and the RP zoned
area to the West to Spring Creek, has 50 c.f.s. capacity. We
designed this to allow the full 100-year developed drainage
of approximately 15 c.f.s. from Valley Forge P.U.D. to enter
this 36-inch R.C.P.. We also allowed 10 c.f.s. from Tracts "A"
and "B" and Area 4 to enter this same pipe right-of-way. These
flows will enter the pipe at the Southeast corner of the property
through an existing inlet. Detention for the remainder of the
100-year flows from these areas is provided in an existing det-
ention area along the East boundary line of this project.
The remaining 25 c.f.s. capacity of the existing 36-inch R.C.P.
pipe is made up of flows provided by a proposed storm drain.
We are not proposing any changes to this system. We merely
provide for slightly different retention schemes on the shopping
center sites. The storm drain system to the first inlet, which
drains Area 3, will remain basically the same. See Exhibit "C"
for revised volume and depth anticipated.
Area 2 drainage will now flow to two (2) 4-foot inlets instead
of one (1) 8-foot inlet. The flow entering these inlets from
the parking areas is slightly less than what it used to be,
therefore, retention in the parking lot itself will also be
slightly less than outlined in the original report. The
controlled discharge from the Safeway Store will be tied into
the storm drain at the downstream inlet in this area. Enclosed
are the architectural plans for the Safeway design. This 1 c.f.s.
brings the total discharge allowed into the storm drain from
Area 2 to the same flow as provided for in the original report,
11.7 c.f.s.
Finally, in Area 1, as originally designed, allowed.that the
entire 100-year flow to enter the storm drain. The reason
for this was because the storm drain was designed to eventually
drain the RP zoned area to the West. Old Area 1, developed the
same 100-year developed flow as the 2-year developed flow for
the RP zoned area and it was proved that the time of concentra-
tion of area was substantially shorter than that of the RP zoned
area and developed waters from Area 1 would be well down the
storm drain before the 2-year developed concentrations from the
RP zoned area entered the pipe. This flow was calculated to
be 15 c.f.s.
The 100-year developed flow has increased to 16.7 c.f.s. in
this area. A small amount of retention will now occur. This
volume has been calculated as 3,540 ft3 and the mass flow dia-
gram is attached with the revised work sheets. This volume will
not provide over .5-foot of depth and should not hinder public
movement. In addition, even with the small amount of retention
now provided, the water will still drain and vacate the storm
drain before the volume is required for the RP zoned area.
We still need and would like to have intact, the variance
allowing the 5-year developed flow to drain from this site
for the same reasons it was originally requested. Slopes for
the storm drain will remain the same and the length of the throat
openings are sized the same as before, therefore, inflow into
the storm drain is the same as originally planned. It is
important to note, the discharges carried by the piping shall
remain the same as the slopes have remained the same.
If you have any comments, please do not hesitate to contact
our office.
Respectfully submitted,
#-5. b7 Ll g. EL-0
Robert H. Schemmerling, P.E.
TARANTO, STANTON & TAGGE
CONSULTING ENGINEERS
RHS:llf
DATA
TABLE
3-A
Developed
Area
Length
Avg. Slope
C
Cf
To
I
O = CCfIA
Drainage
(Acres)
(Ft.)
M
(Min)
(In/fir)
(cfs)
Area 1
2 yr
2.8
360'
2.0
.85
1.0
7
3.0
7.1
5 yr
2.8
360
2.0
.85
1.0
7
3.7
8.8
10 yr
2.8
360
2.0
.85
1.0
7
5.0
11.9
100 yr
2.6
360
2.0
.85
1.25
10
7.0
16.7
Area 2
2 yr
3.3
330
2.0
.85
1.0
6.7
3.0
8.4
5 yr
3.3
330
2.0
.85
1.0
6.7
3.8
10.7
10 yr
3.3
330
2.0
.85
1.0
6.7
5.1
14.3
100 yr
3.3
330
2.0
.85
1.25
10
7.0
24.5
Area 4
2 yr
1.9
620
1.0
.85
1.0
11.6
2.4
3.9
5 yr
1.9
620
1.0
.85
1.0
11.6
3.4
5.5
10 yr
1.9
620
1.0
.85
1.0
11.6
4.2
6.8
100 yr
1.9
620
1.0
.85
1.25
10
7.0
11.3
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