HomeMy WebLinkAboutZMA200200008 Other Stormwater Management Plan 2002-08-19 I _ Stormwater Management
BMP Plan
ZMA-98-20 Pantops PD-MC
Tax Map 78, Parcels 20A, 73, 73A, 75, and 76
prepared for
Virginia Land Trust
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prepared by
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Wilbur Smith Associates
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WSA 340170
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Index
Introduction
Purpose
Site Location
General Description
Determination of Water Quality Facility Type
Water Quality Calculations
Existing Hydrology
Proposed Hydrology
Hydrograph Routing
Water Quality Volume Calculation
Sediment Forebays Design
Drainage Area Map
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Introduction
Purpose
The Albemarle County Board of Supervisors approved an application for the general
development of this site on October 28, 1998. This approval was subject to the
requirement that the Engineering Department make an administrative determination of
the adequacy of the proposed water quality measures.
This report describes the proposed water quality measures for the site. These are
rig designed in accordance with the Interim Design Manual for the Water Protection
Ordinance of Albemarle County (Updated February 11, 1999).
Site Location
The site is located in the Pantops Area of Albemarle County on the south side of Route
250 East. It is designated on Tax Map 78 as Parcels 20A, 73, 73A, 75 and 76, with a
�.. total site area of 49.34 acres.
qThere is an offsite area of 9.30 acres just east of the site that is included in this plan
because it drains towards the subject parcel and water quality treatment can be
accommodated in the extended detention facility proposed for this site.
a For the purposes of this report, the "site" will be defined as the summation of the onsite
area (49.34 acres) and the offsite area (9.30 acres), for a total site area of 58.64 acres.
General Description
Along with the site location described above, other pertinent site characteristics are as
follows:
• The site was determined to be entirely within a "development area" as designated by
the county land use plan.
• No regional stormwater basins or master plans are known to impact this design.
• The site was previously determined to be exempt from the stormwater detention
requirements. This is stated in Item (b) of the approval letter from the county, dated
October 28, 1998 (See Supporting Information).
• There are no known stream buffer requirements or associated mitigation measures
for this site, since there are no designated streams on or adjacent to the site.
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Stormwater Management/BMP Plan Wilbur Smith Associates/340170
ZMA-98-20 Pantops PD-MC
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DETERMINATION OF WATER QUALITY
FACILITY TYPE
IThe following is the supporting information prepared to determine the type of
water quality facility will be required as discussed in the Albemarle County
Interim Design Manual. The target pollutant removal percentage is determined. 1
The appropriate BMP is selected and the type of facility is chosen.
-- An enhanced extended detention pond is chosen to best meet the target
.. phosphorus removal rates. Grass swales will also be constructed on the site
where feasible.
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Water Quality Calculations
1 _ Determination of Impervious Areas
The basis of this design is the proposed site plan, which is shown on Figure 1. This site
plan was used to determine impervious areas for most of the site. A maximum
percentage of impervious area (90%) was then used for two outparcels and the offsite
area, since no site plan has been developed as yet. A summary of these calculations is
shown below.
Table 1 - Summary of Impervious Areas
Site Description Total Impervious Percentage
Area (ac) Area (ac) Impervious
Onsite: Proposed Development shown on the Site Plan 40.67 16.05 39.5%
Onsite: 2 Outparcels Proposed for Future Development 8.67 0.0%
Offsite Area Draining to the Pond 9 30 90.0%
TOTAL SITE AREA= 58.64 32.22 54.9%
TOTAL AREA DRAINING TO THE POND= 51.59 32.22 62.5%
Determination of Pollutant Removal Required
Calculations of the site's pollutant removal requirement were performed using the
"modified simple method" worksheets provided by the county. These are included in the
supporting information. The pollutant removal efficiency requirement based on these
calculations was determined to be 57.8% of the 195.9 pounds per year of total
phosphorus generated by the proposed site.
Determination of Pollutant Removal Provided
In order to provide 57.8% of pollutant removal efficiency, the site was designed to
include grassed swales (with check dams) and an enhanced extended detention basin
for water quality treatment. Based on the percentage of site area treated, the pollutant
removal provided for the site is calculated below.
Table 2 — Summary of Pollutant Removal Provided
Description of Selected BMP Options Removal Site Area Total %
Efficiency Treated Removed
Onsite Areas draining through Grassed Swaies (w/Check Dams) 20% 72.1% 14.4%
Onsite and Offsite Areas draining through an Enhanced 50% 88.0% 44.0%
Extended Detention Basin.
TOTAL POLLUTANT REMOVAL PROVIDED= 58.4%
Since the removal amount provided (58.4%) exceeds the removal amount required
(57.8%), this plan meets the required water quality treatment for the site. The plan also
provides for the treatment of stormwater-runoff for a 9.3 acre offsite area to the east,
which may warrant pollutant removal credits.
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Stormwater Management/BMP Plan Wilbur Smith Associates/340170
ZMA-98-20 Pantops PD-MC