HomeMy WebLinkAbout1985-01-21 Hydrology & Impact Statement i
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ASSOCIATE S,. C4a
Planning Mard
Town Hal I
North Andover, MA 01845
RE : Spring Hill , Hydrology and Environmental Impact Statement
Dear Board Members :
In accordance with your Rules and Regulations Governing the Subdivision
of Land in the Town of North Andover, and more specifically Section III,
B, 5, please find herewith additional information regarding Hydrology
and Environmental Impact. The subdivision has been designed to create
no adverse impact on the environment or the residents of the area.
As you may be aware, the Highway Department as made improvements on Summer
Street between Lots 1 through 8. New drainage, in accordance with these
subdivision plans, has been installed. New pavement has also been in-
stalled, on Summer Street. We are intending. to connect the drainage
from "Spring Hill" into this drainage system and construct a detention,
basin to mitigate the increase in runoff.
I look forward to presenting this project to our Board on Monday,
November 26, 1984. If there are any questions regarding this project,
please do not hesitate to contact this office.
Very truly yours, 'k� OF ` OF'hfR�s�.,
THOMAS E. ,N VE ASSOCIATES, INC. 1110 5 �� °� `HAS
GENE
NLVE
No. 30138 31724 o
Thomas E. Neve, P.E. , R.L.S. 9�GlsTE�`�° ��ti� �vN �STV 1�1
President Fs�l�NAL ����� � suRV
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ENGINEERS 447 OLD SOSTON LOAD
LAND SURVEYORS TOPSFIELD, MA 01983
LAND USE PLANNERS (617) 887-8586
NI Mcm
Ponc.
Site Description
This site is located on a hill , bounded by Summer Street to the west and the
State Forest to the east . There are no existing access trails to the State
Forest from Summer Street across the site . The site is typified by moderate
to fairly steep slopes and is entirely forested , though not of a high density
of trees . Shrub undergrowth is sparse. As mapped by the Soil Conservation
Service (SCS) , the southerly half of the site consists of Charlton - Rock
Outcrop - Hollis soils which range from a 'moderately well drained' (B) to a
' poorly drained ' (D) classification . The northern half of the site consists
of Sutton , Charlton, and Canton soils which are all 'moderately well
drained. ' Pere tests throughout the site have exhibited pert rates of 8 to 10
minutes/inch .
A drainage divide bisects the site creating two principal drainage basins .
The western basin concentrates runoff flow to a culvert under Summer Street .
The eastern basin does not concentrate flow to any point but provides
dispersed overland flow in the general direction of the State Forest .
Because of the hilltop location , there is very little drainage area tributary
to the site from off--site. There are no streams , wetlands or wetland buffer
zones on the site . The headwaters of Mosquito Brook are about 1/2 mile
downstream of the Summer Street culvert .
Hydrology
The site and drainage areas are currently undeveloped. The only existing
impervious surface is Summer Street along the front of the site. Although
sparsely developed (one-acre lots ) , the subdivision would generate more
runoff discharge than existing due to the added impervious surface of the
access road, driveways , and roofs . Therefore , controls have been
incorporated into the subdivision deign to reduce the peak discharge of the
developed site to below existing rates . These include continuous
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gravel-filled swale--drains on both sides of the proposed access road and two
detention ponds , one for each of the two major drainage areas .
The swale-drains will attenuate stormwater runoff by storage and induced
infiltration 'i.nto the ground and will replace the recharge value of the land
under the access road. They will be capable of infiltrating the entire
volume of runoff from the access road during all event storms up to and
including the 100-year storm. Their effectiveness will be enhanced by the
predominance of moderately well drained soils ,
The detention ponds will reduce discharge peaks by storing stormwater runoff
and controlling its outflow. Although the ponds have been designed to reduce
the peak discharge of the 100-year storm, computerized flood routing for the
10-year and 25-year storms has shown that the ponds' will also reduce the peak
discharge rates of these storms to below existing rates . The coordination of
this project with Summer Street improvements permits the proper sizing nd
design of the outlet of Detention Pond 2 (DP2) which will be located on
frontage Lots b and 7 . • Detention Pond 1 (DP1 ) will be located on Lot 19 , its
outflow controlled by a culvert under the proposed Access road.
The DRAINCALC Computer Model , an intensive flood routing program which
incorporates SCS methods and standard hydraulic relationships , has been used
to model existing and post-development hydrology and to size and design the
storm drain system and detention pond outlet structures . The model develops
hydrographs for each subcatchment area within the watersheds , routes them
through channel reaches , pipes and detention areas with controlled outlets
and integrates them to generate to composite hydrograph. The model also
prints out summaries of subcatchment input data and tabulations of detention
(storage) pond inflow, outflow, stage , and storage volume at small time
increments during the modelled storms ,
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Composite Curve Numbers (CN) were derived for each subcatchment from digital
planimetry of overlain SCS hydrologic soils groups and W' 40' ) site plans
showing surface cover . Some developed CNS have been synthetically reduced to
credit the infiltration ability of the swale drains . Other subcatchment data
such as hydraulic length and slope were also measured from the site plan,
The time of concentration for each subcatchment was calculated by the model
(SCS Lag method) based on CN, slope , and length data. Stage vs . Storage
relationships for each detention ponding area were developed using the NEH-4
method based on digital planimetry of the contours on the site plans . Reach
(channel) data (e.g. , slope , bottom width, configuration) were also measured
from the plans .
The existing forested character of the site will be maintained to the maximum
extent possible. The area of roof and driveway per 'lot was conservatively
assumed to be 3000 square feet . For impact modelling and mitigation design
purposes , all Summer Street Form A Lots (1 through 8) were assumed to be part
of the subdivision. The proposed storm drain system was designed according
to Planning Board criteria to meet all specifications of slope, spacing,
velocity limits , and capacity for the 10- year storm.
The modelling procedure and results are fully documented by the attached data
tabulations and model printouts . For summary purposes , the peak discharge
comparisons are tabulated below. As shown, the peak discharge rates to the
State Forest area will be effectively reduced by DPI . The very substantial
reductions at the Summer Street culvert will be achieved because the existing
ponding area is shallow and would result in the street being overtopped by
flow, without the creation of DP2, during all three modelled storms . The
excavation and improved outlet for DP2 will prevent the overtopping of the
street during all three storms ,
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DISCHARGE SUMMARY
Peak Discharge (cis ) to State forest
Storm Event Existing Post--Development,
100-yr 40.8 33.4
25-yr 30.3 24 .9
10-yr 22 .4 18.9
Peak Discharge (cfs ) at Summer Street Culvert
Storm Event Existing Post--Development
100-yr 40.4 7. 1
25-yr 24.0 5 .9
10-yr 8.8 5 .0
Erosion and Sedimentation Control
Although there are no wetlands or wetland buffer zones on the site , every
effort will be made to stabilize disturbed areas and intercept siltation
prior to outflow from the site . Stockpiles of erodible material will be
moistened or covered . Disturbed slopes of 3: 1 or greater will be compacted
and seeded as quickly as possible within the limitations of the growing
season. Staked hay bales and/or silt fence will be placed at the Sumner
Street culvert and elsewhere as necessary during construction. Culvert
inlets will be protected by concrete headwalls and outlets by concrete
endwalls and rip--rap aprons to prevent scour .
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MI No= Pi n c.
The two runoff detention ponds will be excavated prior to access road and
building construction. After construction is complete , they will serve only
to detain runoff from major storms and will normally contain no standing
water . At the discretion of the Planning Board, the ponds may be used as
sedimentation basins during construction. This could be accomplished by
either installing risers on the pond outlets or by excavating the ponds to
below the inverts of their outlets .
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December 13, 1984
Planning Board
Town Nall
No. Andover, MA 01845
RE: "Spring Hill", Subdivision of lots from Summer Street No. Andover,
Applicant, Mr. John Tuttle
Dear Board Members:
At the request of your planner Ms. Karen Nelson, please find herewith additional
information in accordance with your Subdivision Rules and Regulations, Section
III , B, 5, Environmental Impact.
The Specific items requested are as follows.
1 . Percolation Tests, see raw notes attached, since all lots shall be on
Subsurface Sanitary Disposal Systems.
2. Town Services:
Traffic flow: 13 lots based on a traffic volume of 12 vechicles per day
per dwelling unit shall generate 156 vechicle trips per day, well under the
300 figure establishing minor streets.
Police services :
Additional police services shall be required, but no service over and
above the usual .
Fire Protection:
The Subdivision shall have town water and hydrants as required, suitable
for fire protection. The end of the road shall have a cul-de-sac suitable
for turning with fire apparatus.
Public works:
The public works department shall be required to maintain all new Streets
and drainage and also should maintain all detention basins which are proposed
on the plans.
Educational Services:
Estimating a maximum of 2 children per household which may use town
educational services, this subdivision may add 26 new children to the system.
ENGINEERS 447 OLD BOSTON ROAD
LAND SURVEYORS TOPSFIELD, MA 01983
LAND USE PLANNERS (617) 887--8586
i r
December 13, 1984
Planning Board
- Page 2 -
Town Water Supply:
Town water mains shall be extended within this subdivision and accord-
ing to DPW are adequately located and sized. Please note that the water main
was Qxtended along Summer St. in front of Lots 1-8 which served as a major
link to "loop" the existing system. This work should increase both the
volume and pressure in the existing mains.
Generally this subdivision will not significantly impact the services provided
by the Town. The area around this subdivision is being developed however, Town
roads and utilities are also being improved as a result of this development.
I hope this information furnishes you with the information you requested.
I would be happy to meet with you if there is a need.
Very truly yours, OF
THOMAS NEVE ASSOCIATION, INC. THOMA �� � EUGENEI N . � r.
J w ' REVE 14
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Thomas E. Neve, P.E. ,R.L.S.
President ^ '
Enclosures
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