§ 381-11. Stormwater management; use of low-impact development techniques., Article IV. Design Standards, Part 1. Subdivision Rules and Regulations, Chapter 381. Planning Board, 4: Miscellaneous Regulations, Code of Ordinances, Groton

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.

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.

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.

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.

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)

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.

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.

Excavations. See Part 2, Construction Appendix, of this chapter.

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.