HomeMy WebLinkAboutWPO202100001 Correspondence 2021-04-29 (2)SHIMP ENGINEERING, P.C.
Design Focused Engineering
April 29, 2021
David James
County of Albemarle
Department of Community Development
401 McIntire Road, North Wing
Charlottesville, Virginia 22902
RE: Variance Request for 9VAC25-870-66-B(3) for 999 Rio Road
Dear David,
The 999 Rio Road site plan is subject to water quantity regulations that are set forth in 9VAC25-870-66-
B(3) which states:
"[W]hen stormwater from a development is discharged to a natural stormwater conveyance
system, the maximum peak flow rate from the one-year 24-hour storm following the land -
disturbing activity shall be calculated either:
a. In accordance with the following methodology:
QDeveloped -" I.F. *(QPre-developed* RVPre-Developed )/RVDeveloped
Under no condition shall QDeveloped be greater than QPre-Developed nor shall QDeveloped be required to be
less than that calculated in the equation (QF.,est * RVF..A)/RVDeveloped; where
I.F. is the improvement factor and =0.8 for sites > 1 acres or 0.9 for sites < 1 acres."
In order to evaluate the requirements set forth in 9VAC25-870-66-B(3), The site was divided into
two drainage areas, each drainage area flows into an underground detention system. The first drainage
area is 1.779 acres which flows into the Stormtech MC-3500 underground detention chamber system. The
second drainage area is 0.169 acres flows into the Stormtech SC-310 underground chamber system. The
site runoff from the development is directed to a Point of Analysis (POA) at the western comer of the
development.
The SWM Quantity requirements for 999 Rio Road can be satisfied if the 1-yr orifice for the
Stormtech MC-3500 chamber system is 1.7-inches and the 1-yr orifice for the SC-310 chamber system is
1.8-inches. Due to the small size of these orifices, they are susceptible to clogging and therefore would
not flow the required flow rate. This would cause the Post Development Peak Runoff (Qp�t-d,) to increase
as the system would hold the water and not function as designed for the 1-yr and 10-yr stones.
If the design of the detention systems were modified to 3-inch orifices, the Post Development
Peak Runoff (Q�.&v) will increase more than the Maximum allowable Peak Runoff (Q�) but remains
below the Pre-Dev Peak Runoff (Qp,e.dev). Below is table 1 to compare the runoff calculations based on the
orifice sizes:
Table 1: Peak Runoff Calculations per Orifice size
Orifice Size
inches
Storm
Year
Pre-dev
Peak Runoff
Max. Allowed
Peak Runoff
Post-dev
Peak Runoff
1.7 & 1.8
1-Yr
0.64 cfs
0.14 cfs
0.13 cfs
1.7 & 1.8
10-Yr
3.32 cfs
3.32 cfs
2.95 cfs
3.0 & 3.0
1-Yr
0.64 cfs
0.14 cfs
0.28 cfs
3.0 & 3.0
10-Yr
3.32 cfs
3.32 cfs
2.44 cfs
SHIMP ENGINEERING, P.C.
Design Focused Engineering
As shown above, the 1-year Post-dev (Qwst_d,) exceeds the allowable flow (Q.�) of 0.14cfs,
however it remains approximately 50% of the pre -development level and well below the 1 cfs threshold
that has been considered a point at which these variances should be considered. By increasing the
minimum orifice size to 3" the intent of the regulations is better served as the system is far less likely to
clog and in -turn fail to even meet the pre -development runoff levels for the project.
Energy balance calculations showing the results of the various orifice dimensions are attached for your
review and consideration.
If you have any questions about this calculation packet please do not hesitate to contact me at:
steohanie@shimp-en ing eerieg com or justin nAshimp-en ing eerieg com or by phone at 434-299-9843.
Regards,
Stephanie Paul
Shimp Engineering, PC
999 Rio Road
Energy Balance Equation Summary: POA
24 Hr 1 year Storm Comparison:
Q,,,,m<I.F.*(Qercd loped*RVr' D-n k MYRV&-[. d
Q&e :5 (Qr :gym*RV,.re: )/RVde.el
I.F. 10.81
0.64
0.051 Max Q&1d,M = 0.14 cfs
0.192
SITE AREA:
Qpred Iopm
aC
=
RVp ,. pm
=
RVoe�,a
=
Not Achieved Post-Dev Max Runoff 01= 0.28 cfs. Site complies with 9VAC25-870-66- 3 A
Maximum Qloyr = 3.32 cfs (predev) Achieved Qloyr = 2.44 cfs
LF:
1 0.8
FOR LDA > 1 AC
LF:
1 0.9
FOR LDA < 1 AC
LDA= AC
Predev Flowrates
Storm I Cf$
Postdev Plowrates
Storm
Cfs
I-Yr
0.28
10-yr
2.44
Flow Reduction Check
Energy Balance Equation Compliance:
Not Achieved
999 Rio Road
Energy Balance Equation Summary: POA
24 Hr 1 year Storm Comparison:
Qderempm<I.F.*(Qpre eloped*lzv' DevkVMYRVdevdoped
Qdere :5 (Qr :gym*RV,.re: )/RV&e
LF. = 0.8
0.6
n0MaQnva = 14 cfs5100.RVpD,
0.192
SITE AREA:
Qpred I.ped
ac
=
=
RVDeempm
Achieved Post-Dev Max Runoff Qlyr = 0.14 cfs. Site complies with 9VAC25-870-66- 3 A
Maximum Qtoyr = 3.32 cfs (redev) Achieved Qtoyr = 2.95 cfs
LF:
1 0.8
FOR LDA > 1 AC
LF:
1 0.9
IFOR LDA < 1 AC
LDA= AC
Predev Flowrates
Storm
cfs
1-yr
0.
0-y
3.3
Posidev Flowrates
Storm cfs
l-yr
]0-yr
Flow Reduction Check
Energy Balance Equation Compliance:
Achieved
Predev Reduced