[Amended 11-3-1994; 4-3-2003; 3-22-2007; 2-18-2016]
The Planning Board encourages the use of low-impact development (LID) techniques when feasible. Please refer to Chapter
352, Earth Removal Stormwater Advisory Committee Regulations, for LID requirements. If LID techniques are not used, the following regulations apply to conventional drainage systems:
A. General approach. Storm drains, culverts, swales,
detention basins and related facilities shall be designed to permit
the unimpeded flow of all natural watercourses, to ensure adequate
drainage at all low points along streets, to control erosion and to
intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area being drained. The peak
rate of runoff at the boundaries of the development shall not be increased
from the predevelopment rate during the two-, ten-, twenty-five or
one-hundred-year storm event, unless an increase is authorized by
the Planning Board, following consultation with the Conservation Commission
and consideration of the ability of receiving wetlands or water bodies
to absorb the increase and the consequences of providing detention
capacity. The proposed lot grading shall be shown on the plans so
that the proposed stormwater management system can be properly evaluated.
B. Design basis.
(1) Facility design shall accommodate the two-year and
ten-year storms, as well as:
(a)
Storm sewers, swales: ten-year storm.
(b)
Detention basins: one-hundred-year storm.
(c)
Culverts, other stream crossings: fifty-year
storm.
(2) All tributary areas shall be assumed to be fully developed in accordance with Chapter
218, Zoning, unless publicly owned or deed restricted. Storm drainage systems shall be designed to handle all runoff water from the tributary watershed. The Rational Formula shall be used to determine pipe sizing for the piped drainage system. The United States Soil Conservation Service (USSCS) TR20 or TR55 Methodology, where applicable, shall be used to determine no net increase in the off-site rate of runoff for detention and infiltration systems. Adequate physical access for maintenance purposes shall be provided to detention and infiltration facilities. Water velocities in pipes and paved gutters shall be between two and 10 feet per second and not more than five feet per second on unpaved surfaces.
C. Storm sewers.
(1) All drain pipes shall be at least 12 inches' inside
diameter and made of reinforced concrete conforming with Massachusetts
Department of Public Works specifications for Class III pipe or such
higher class as may be required by the depth of cover, which shall
be not less than 36 inches where the pipe is subject to vehicular
loads. Manhole covers and grates shall be in conformance with Massachusetts
Department of Public Works Specifications 201, designed and placed
so as to cause no hazard to bicycles. No catch basins shall serve
as manholes. Trash racks shall be installed at both ends of all culverts.
(2) Interceptor drains. Interceptor drains shall be designed
and constructed and incorporated into the drainage system where warranted
by groundwater elevations.
D. Structures. Catch basins will be required at all low points and on both sides of the roadway on continuous grade at intervals of not more than 300 feet. Drainage patterns at intersections shall be evaluated and catch basins designed and constructed so as to prevent any flooding at the intersection. Any catch basins used shall be at least six feet deep and four feet in diameter (inside measurements), with a thirty-six-inch or greater sump below pipe invert and shall be constructed of precast concrete units. Manholes used shall be at least four feet diameter (inside measurements) and shall be constructed of precast concrete units, with formed inverts (unless waived by the Board to allow the use of concrete blocks). Oil and grease traps shall be installed on all structures that discharge to a resource area as defined by 310 CMR, the Wetlands Protection Act, and where required by the Board. Manhole covers and catch basin grates shall be in conformance with Massachusetts Department of Public Works Section 201, with catch basin grates designed and placed so as to cause no hazard to bicycles. Standard catch basin frames and grates and manhole rims and covers are specified in Part
2, Construction Appendix. Granite curb inlets shall be provided at all catch basins located within the roadway.
E. Swales. Drainage swales shall have cross-sectional
areas adequate to carry a ten-year storm and be treated as follows:
|
Slopes
(percent)
|
Treatment
|
---|
|
0.75 to 4
|
6 inches of loam seeded
|
|
4 to 10
|
6 inches' minimum dimension angular stones for
a water depth of 1 foot, with a 12-inch gravel subbase (SSH&B
M 1.03.0 type A)
|
F. Waterways. Open brooks or tributary ditches which
are to be altered shall be shaped to a cross section and gradient
and provided with stream bottom hardening, all acceptable to the Planning
Board. Safety fencing may be required where important because of slope,
bank instability, stream depth or flow rate or other reasons. Proposed
finished grade for detention basins shall not be more than seven feet
above or below existing grade unless specifically authorized by the
Board in unusual topographic circumstances.
G. Connections. Proper connections shall be made with
any existing drains in adjacent streets or easements which prove adequate
to accommodate the drainage flow from the subdivision. In the absence
of such facilities or the inadequacy of the same, it shall be the
responsibility of the developer to extend drains from the subdivision
as required to properly dispose of all drainage from said subdivision
in a manner determined to be proper by the Board and to secure for
the Town any necessary drainage rights.
H. Excavations. See Part
2, Construction Appendix, of this chapter.
I. Floodplains.
(1) District. All subdivision proposals and other proposed new development shall be reviewed to determine whether such proposals will be reasonably safe from flooding. If any part of a subdivision proposal or other new development is located within the Floodplain District established under Chapter
218, Zoning, it shall be reviewed to assure that:
(a)
The proposal is in compliance with § 218-29B(2) of Chapter
218, Zoning.
(b)
The proposal is designed consistent with the
need to minimize flood damage.
(c)
All public utilities and facilities, such as
gas, electrical and water systems, shall be located and constructed
to minimize or eliminate flood damage.
(d)
Adequate drainage systems shall be provided
to reduce exposure to flood hazards.
(e)
Base flood elevation (the level of the one-hundred-year
flood) data shall be provided for that portion within the Floodplain
District.
(2) Identification. All one-hundred-year floodplains will
be calculated and identified on the definitive subdivision where not
identified on the Federal Emergency Management Agency (FEMA) maps.