HomeMy WebLinkAboutWPO201700048 VSMP - SWPPP 2021-07-22OLD TRAIL CREEKSIDE III, PHASE II STORMWATER
MANAGEMENT CALCULATIONS PACKET
(WPO 201700048, Amendment #1 to WPO 201200008)
Date of Calculations
JUNE 81 2020
;COTT/ft.I�INS'Z
Lic. No. 035791 .b.
7/14/21wQ
�.n
PREPARED BY:
COLLINS CivGINEERING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
Stormwater Management Amendment #1 Narrative:
Overall:
The approved WPO 201200008 plans specify for the conversion of a sediment basin to a
bioretention basin. However with this amendment, and the stabilized site and basin, the
proposed conversion is no longer necessary and limited modifications to the facility are needed.
These modifications include proper installation and maintenance of the riprap lined outfall
channel, converting the riser to have an inlet top and stabilizing the sediment basin with
positive drainage to the new inlet. This plan also shows continued compliance with stormwater
requirements. Please see this sheet and the attached calculations for additional information.
Water Quality:
The approved plan achieved water quality compliance through a contribution to the Lickinghole
basin fund and a biofilter treating the first 1/2" of rainfall. This amendment however proposes
to eliminate the conversion to a biofilter and achieves water quality compliance through the
previously paid contribution to the Lickinghole basin fund and through a new nutrient credit
purchase.
Water Quantity:
The approved plans utilized an adequate riprap lined channel that was not erosive during the 2-
year design storm and was contained within its banks during the 10-year design storm. This
channel, labeled 'MS-19 X-Section' on the approved plans, extends to the Lickinghole Creek
bank where the analysis ends since the site's contributing drainage area is 1% of the creek's
overall watershed. This methodology is per the approved plans and this amendment does not
modify the adequate channel or the concept. This amendment does however confirm the
approved 'MS-19 X-Section' continues to not be erosive during the 2-year design storm and
continues to not overtop the riprap channel's banks during the 10-year design storm. This
design also removes the existing temporary sediment basin by modifying its riser to have an
inlet top and to ensure the facility remains dry, stabilized and sloped towards the newly
installed inlet top.
Please note, this project has an approved stormwater quantity waiver dated 9/26/05. Though
this waiver was not utilized with the approved plan, this waiver will no longer be valid with the
approval of this amendment since the waiver relied partially on the installation of a biofilter.
This is in accordance with prior County Engineering direction dated 3/30/17.
In keeping with the approved plan, water quantity compliance for this amendment will be
addressed through the use of the riprap lined channel (i.e. the MS-19 X-Section). Channel
protection is met through 9VAC25-870-66, subsections B.1.a and B.4.a and flood protection is
met through 9VAC25-870-66, subsections C.2.a and C.3.a. The flows used in these analyses
were taken from the approved post -development inflow hydrographs and assumed zero
attenuation occurs onsite. Please see this report and the accompanying VSMP Amendment #1
for details.
DEQ Virginia Runoff Reduction Method
Water Quality Calculations
(COMPLIANCE IS MET THROUGH THE BEST MANAGEMENT PRACTICES OF A CONTRIBUTION TO THE
LICKINGHOLE BASIN FUND AND THE PURCHASING OF NUTRIENT CREDITS.)
Impervious Area Breakdown for the Approved
Old Trail Creekside III, Phase II Subdivision
(Values taken from Approved Sheet 1 of 1)
Roadway 50,375
sf
Sidewalk 7, 575
sf
Lots 57,200
sf
115,150
sf
Water Quality Notes:
1. A contribution to the Lickinghole Basin Fund was made with the originally approved WPO plan.
2. The approved plans, as shown on sheet 1 of 1, require a phosphorous removal rate of 4.98
Ibs/yr.
3. The following DEQ VRRM water quality calculations require a phosphorous removal rate of 5.14
Ibs/yr, when entering the approved impervious areas and the development's total acreage.
4. A required phosphorous removal rate of 5.14 Ibs/yr is the more conservative of the two rates
listed above. And per the County's guidance on 9/19/14 stating "the Lickinghole Basin may be
used to account for 30% of the pollutant load leaving the site", this amendment requires a
nutrient credit purchase of 3.601bs/yr (0.70 x 5.14).
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Verification of the Approved Outfall Channel's Adequacy
to Lickinghole Creek
* Summary of the Approved Post -Development Contributing
Watershed for the Old Trail Creekside III. Phase II Subdivision
Drainage Area 9.16 ac.
Weighted CN Value 70.6
Time of Concentration 0.31 hrs.
2-yr. 24hr. SCS Design Flow 7.36 cfs
10-yr. 24hr. SCS Design Flow 16.03 cfs
*These values were taken from the approved WPO 201200008
biofilter routing calculations shown on sheet 1 of 1. These values
represent the inflow hydrographs for the contributing watershed
to the biofilter that will not be installed.
Adequate Channel Confirmation- 2 Year
Complex Channnel: 2-yr design storm
Verification of Channel Adequacy for the Approved Flows through the Approved
Channel
Input:
Slope
Flow (given)
line
x
y
n
0
0.000
1.000
0.095 (first n value not used)
1
3.000
0.000
0.095
2
6.000
0.000
0.095
3
9.000
1.000
0.095
Output:
Depth 0.552
(y) ( 0.552)
Channel bed segment output:
line A P
0.46 1.75
1.66 3.00
0.46 1.75
Q
V
1
0.92
2.02
2
5.51
3.33
3
0.92
2.02
Page 1
Adequate Channel Confirmation- 10 Year
Complex Channnel: 10-yr design storm
Verification of Channel Adequacy for the Approved Flows through the Approved
Channel
Input:
Slope
Flow (given)
line
x
y
n
0
0.000
1.000
0.095 (first n value not used)
1
3.000
0.000
0.095
2
6.000
0.000
0.095
3
9.000
1.000
0.095
Output:
Depth 0.821
(y) ( 0.821)
Channel bed segment output:
line A P
1.01 2.60
2.46 3.00
1.01 2.60
Q
V
1
2.67
2.64
2
10.69
4.34
3
2.67
2.64
Page 1