HomeMy WebLinkAboutWPO202000058 VSMP - SWPPP 2020-12-23SHIMP ENGINEERING, P.C.
Design Focused Engineering
December 23, 2020
John Anderson, PE
County of Albemarle
Department of Community Development
401 McIntire Road, North Wing
Charlottesville, Virginia 22902
RE: WPO 2020-OOXXX Albemarle Business Campus SWM Calculation Packet
A Revision of WPO 2018-00084 Royal Fern Phase 2 VSMP
Dear John,
Enclosed is the stormwater calculation packet for Albemarle Business Campus (ABC). This is
development will occupy a site that has an area of 8.55 acres. The disturbed area is 9.10 acres. The
proposed stormwater design meets the runoff requirements set forth in 9VAC25-870-95 as the Part IIC
regulations are applicable to the project per 9VAC25-870-47. This VSMP plan is intended to revise the
approved plans for Royal Fern Phase 2, also prepared by Shimp Engineering.
In effect, the strategies for ABC and Royal Fern Phase 2 are the same. All onsite runoff is routed
to the existing pond built as part of the site plan for Sterling University Housing (SDP 2001000129). The
narrative from Royal Fern Phase 2 is copied below, but to achieve approval, all that remains to be shown is
that the impervious area / Curve Number for the ABC plan remains lower than what the pond was
designed for, and that the runoff to the offsite storm pipes remains lower than the original design capacity
for those pipes. Both of these conditions are still met with the ABC development.
COPIED FROM WPO2018-84: After research of the approved site plan for Sterling University
Housing (SDP 200100129), it was determined that the downstream sewer system empties into the wet
pond on the Cavalier Crossing Apartments property (TMP 76-54A1). This pond was designed to manage
and treat runoff from a fully -built Sparrow Hill Phase 2 [Now ABC] site, listed in the approved calculation
packet for SDP 2001000129 as the "western residue of the Heischman property". The pond was designed
under VA Part IIC regulations and constructed before 2014 while these regulations were the current VSMP
ordinance. Per 9VAC25-870-47 (B), "Land -disturbing activities that obtain an initial state permit or
commence land disturbance prior to July 1, 2014, shall be conducted in accordance with the Part H C
(9VAC25-870-93 et seq) technical criteria of this chapter. Such projects shall remain subject to the Part H
C technical criteria for two additional state permit cycles." Only one permit cycle has completed since the
pond was approved, so Part IIC regulations are applicable to the Royal Fern Phase 2 project.
Since Phase 2 [Now ABC] falls under Part IIC regulations, and since the pond to which all site
runoff shall drain was designed for a fully developed site, the Royal Fern Phase 2 project [Now ABC] must
fall within the original design assumptions for the pond to achieve SWM compliance. Royal Fern Phase 2
[Now ABC] has less onsite impervious area than the assumed drainage area used for the pond design, so
nutrient treatment and downstream natural channel requirements are satisfied. Royal Fern Phase 2 [Now
ABC] outlets runoff into the SDP 2001000129 storm sewer at rates that do not exceed the capacity of the
system, so manmade channel requirements are satisfied.
SWMComparison: SDP2001000129Pond/SewerDesign Limits Vs. ABCAchieved Conditions
Design Assumption
Achieved Condition
Land Cover
63% Impervious (total pond
47% Impervious (total pond
contributing Drainage Area)
contributing Drainage Area
Original Inlet El
10 yr runoff:
10 yr runoff:
ABC Str. A2
51.11 cfs
50.30 cfs
Original Inlet Cl
10 yr runoff:
10 yr runoff:
ABC Str. B2
45.58 cfs
40.17 cfs
If you have any questions please do not hesitate to contact me at: keane@shimp-en ing eering com or by
phone at 434-299-9843.
Contents:
Post-Dev Runoff Calculations:
Post-Dev Drainage Spreadsheet
Post-Dev Structure Drainage Map
Post-Dev Pond Drainage Map - Aerial
Post-Dev Pond Drainage Map — Topo
Post-Dev Onsite Land Cover Map
VDOT LD-229 Culvert Capacity
ALBEMARLE BUSINESS CAMPUS
Inlet Drainage Area Summary
C for Disturbed Soil during Construction 0.6
Impervious C 0.9
Pervious C 0.3
Str. Type
Str.
Number
Total Area
(ft)
Impervious (ft)
Turf (ft)
C
Area (ac)
DI-3C
A2
337,600
202,560
135,040
0.66
7.75
DI-313
B5
385,070
192,535
192,535
0.60
8.84
(Offsite DA)
DI-3C
132
113,200
79,240
33,960
0.72
2.60
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ROYALFERN
POST-DEV INLET DRAINAGE MAP
80 160 240
IMAGERY PROVIDED BY BING MAPS, ACCESSED APRIL 24, 2019. 02019
ROADS, BUILDINGS, & DRIVEWAY LOCATIONS PROVIDED BY ALBEMARLE
COUNTY GIS, ACCESSED APRIL 24, 2019.
JSINESS CAMPUS
AERIAL PHOTOGRAPHY -
POND DRAINAGE MAP
LEGEND
Building:
Road:
Driveway:
New Imp: _
Pond DA:
Pond:
Stream: --
11 1 11 •11 •11
r
N
N
k
IN
O�
5-54A1
VA LLC
WADS) PLAT
RESIDENTIAL
L - MULTIFAMILY
6 AC
LD-229 Storm Drain Design Computations
Albemarle Business Campus
Fro;T-2
Catch.
Runoff
Increment
Accum.
Total
Total 10-yr
Total 10-yr
Up
Down
Pipe
Invert
Pipe
Pipe
Velocity
Flow time
Structre
Area
Coef
AC
AC
TOC
Intensity
Flow
Invert
Invert
Length
Slope
Diameter
Capacity
Increment
(ac)
(min)
(in/hr)
(cfs)
Elev.
Elev.
(ft)
%
(in)
(cfs)
(ft/s)
(min)
1
3
4
5
6
7
8 9
10
11
12
13
14
15
16
17
A8
A7
0.00
8.63 0.00
#DIV/01
18
8.0
0.00
A7
A6
0.00
8.63 0.00
#DIV/01
18
8.0
0.00
A6
A5
0.00
8.63 2.20
#DIV/0!
18
8.0
0.00
A5
A4
0.00
8.63 0.00
#DIV/01
18
8.0
0.00
A4
A3
0.00
8.63 0.00
#DIV/0!
18
8.0
0.00
A3
A2
0.00
8.63 0.00
#DIV/01
24
8.0
0.00
C6
C5
0.00
8.63 0.00
#DIV/01
15
8.0
0.00
C5A
C5
0.00
8.63 0.00
#DIV/0!
15
8.0
0.00
C5
C4
0.00
8.63 0.00
#DIV/01
18
8.0
0.00
C4
C3
0.00
8.63 0.00
#DIV/0!
18
8.0
0.00
C3
C2
25.79
#DIV/01
18
8.0
0.00
C2A
C2
0.00
8.63 0.00
#DIV/0!
15
8.0
0.00
C2
C7
0.00
8.63 25.79
#DIV/01
18
8.0
0.00
C1
A2
0.00
8.63 25.79
#DIV/0!
24
8.0
0.00
A2
Al
7.75
0.66
5.12
5.12
15.00
4.79 50.30
471.25
470.25
59.92
1.670%
36
93.4
10.2
0.10
B3D
133C
0.00
8.63 0.00
#DIV/0!
15
93.4
8.0
0.00
133C
B3B
0.00
8.63 25.79
#DIV/0!
15
97.5
8.0
0.00
B3Z
13313
0.00
8.63 0.00
#DIV/01
15
93.4
8.0
0.00
B3B
133A
0.00
8.63 0.00
#DIV/01
15
8.0
8.0
0.00
B3Y
B3A
0.00
8.63 25.79
#DIV/0!
18
93.4
8.0
0.00
B3A
B3
0.00
8.63 0.00
#DIV/01
18
13.9
8.0
0.00
B4'
B3
8.84
0.60
5.30
5.30
10.00
5.62 29.83
463.00
462.70
31.95
0.94%
15
6.8
4.5
0.12
B3
B2
2.60
0.72
1.87
7.18
10.12
5.60 40.19
462.50
462.00
8.04
6.22%
15
17.5
10.0
0.01
B2
B1
0.00
0.00
0.00
7.18
10.13
5.60 40.17
463.00
462.70
31.95
0.94%
15
6.8
4.5
0.12
'From Existing Culvert. Includes offsite drainage area