HomeMy WebLinkAboutStormwater Report - 919 GREAT POND ROAD 6/20/2003 RECEPNED
JUN 2 0 20003
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GROUNDWATER
RECHARGE
CALCULATIONS
F
ACM DEVELOPMENT INC
200 SUTTON STREET
NORTH ANDOVER, MA 01845
JUNE.<20r.;2Q03
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BERG lULlffY, P.E.
MERRIMACK ENGINEERING SERVICES, Inc.
66 Park Street
Andover, Massachusetts 01810
planners • engineers • surveyors
TABLE OF CONTENTS
ACCOMPANYING
APPLICATION FOR WATERSHED SPECIAL PERMIT
• U.S.G.S. TOPO MAP
• ASSESSOR'S MAP
• SOILS MAP
• NARRATIVE
• CALCULATIONS FOR RETENTION BASIN DESIGN
• SITE PLAN
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NARRATIVE
The project site is located at#919 Great Pond Road in North Andover. It is a corner lot with
frontages on the east side of Great Pond Road and on the south side of South Bradford Street.
The existing dwelling is currently serviced with town water and town sewerage. The
proposed additions to the dwelling is a two-car garage with a great room above, and a
covered front porch. The driveway will be expanded to allow for two vehicles to be parked
beyond the driveway access to the garage from the street. The area along side the garage will
be regraded toward a lower portion of the rear yard. The existing dwelling does not have
gutters at the roof eaves. The proposed addition will not have gutters. The storm runoff
from these roofs will be discharged to the ground surface. During heavy rainfall (high
intensity rain events) the excess runoff will flow over the grassed and landscaped surface
toward the lower grassed area in the rear of the house. At this low area there is a proposed
small retention basin with an overflow. This basin will collect and retain the storm runoff
from the impervious surfaces (roofs and driveway surfaces) as well as overland flow from the
more upland areas that are grass (established lawn) and landscaped areas. The storage
capacity of the basin is significantly larger than the calculated minimum required volume for
recharge to groundwater criteria. However,this method of recharging to groundwater is
preferred in comparison to roof drainage leaching pits or trenches, which requires that the
proposed or existing roof have gutters.
The proposed retention basin shall be situated in the rear of the existing house. The shape of
the basin will be fitted within the natural grass Swale at this location. The depth of the basin
will be established by the height of a proposed berm across the lower portion of the grass
swale. The berm will have an overflow swale lined with rip-rap. The top of the berm will be
6" higher than the bottom of the overflow swale through the berm. Refer to the site plan for
details and additional information.
CALCULATIONS FOR RETENTION
BASIN DESIGN
I. SOIL TYPES ON SUBJECT PROPERTY
Based on Natural Resource Conservation Service (N.R.C.S.) Soils Map, the property
has CmC—Charlton and StB —Sutton series soil types. These soils are in the
Hydrologic Soil Group `B" category. For Standard#3 —Recharge to Groundwater,
the required minimum volume to be recharged is calculated, based on `B" Soils.
V=0.25" X Total Impervious Area
The impervious area is based on the additions to the building and the net difference
in pavement area near the garage. It is noted here that the garage addition will be
constructed where the existing paved driveway terminates adjacent to the existing
house. The net change in impervious surface area has been determined from the Site
Plan to be 1,000 square feet, approximately, as follows.
New front porch roof and steps = 342 sq. ft.
New garage addition minus
existing driveway area = 198 sq. ft.
New driveway pavement area = 414 sq. ft.
TOTAL ADDITIONAL
IMPERVIOUS AREA = 954 sq. ft.
II. REQUIRED RECHARGE VOLUME CALCULATIONS
0.25" x 1000 sq. ft. = 21 cu. ft.
III. The total recharge volume provided by the proposed retention basin is calculated
Elev Area Volume
(sq. ft) (cubic feet)
Incremental -
Cumulative
*135.38 -0- -0- -0-
**135.7 12' x 18' = 216 35 35
*Bottom of Retention Basin
** Invert Elevation at Overflow Rip Rap Weir
IV. CONCLUSION
With 35 cubic feet provided for within the small retention basin, which exceeds the
Required Recharge Volume of 21 cubic feet, this design is in compliance with the
intent of the North Andover Zoning By-Law associated with permitted uses and
activities within the Watershed Protection District(per Section 4.136)