Town of Marion, MA: Stormwater management.

	Local Street	Secondary Street	Sidewalk (on one side)
Right-of-way	50 feet	60 feet
Traffic lanes
	1 to 10 home sites	2 10-foot lanes	2 12-foot lanes	Not required
	11 to 20 home sites	2 10-foot lanes	2 12-foot lanes	Required
	21 to 30 home sites	2 11-foot lanes	2 12-foot lanes	Required
	Over 30 home sites	2 12-foot lanes	2 12-foot lanes	Required
Paved shoulders	None	2 6-foot shoulders
Park strips (minimum)	2 5-foot strips	2 6-foot strips
Sidewalks/Bike path	1 6-foot path	1 8-foot path
Utility strips (at sides of pavement)	2 1-foot strips	2 1-foot strips
Maximum gradient	8% to 10%	8% to 10%
Minimum horizontal center line radius	200 feet	400 feet

[1]

Editor's Note: Former § 300-4.5, Floodplain protection, was repealed as part of the recodification of the Town's General and Zoning Bylaws adopted 10-23-2017 STM by Art. S7. The specific changes made are noted in a document entitled "Town of Marion Final Draft," dated August 16, 2017, which document is on file in the Town offices.

§ 300-4.6 Stormwater management.

General.

(1)

Management of stormwater runoff from all developments reviewed by the Marion Planning Board shall meet the standards and design criteria contained in this section for both flood control and non-point source pollution reduction. All assumptions, methodologies, and procedures used to design the stormwater management system components shall accompany all site plan review and subdivision of land applications to the Planning Board.

(2)

Strict adherence to these standards may be waived by the Planning Board upon a clear demonstration by the proponent that full compliance with any specific standard would not serve the public interest or would not be practical due to physical site constraints.

(3)

All applicable standards of the Department of Environmental Protection (DEP) Stormwater Management Policy, dated March, 1997, amended January 2008, and all subsequent amendments thereto, and the (DEP) Hydrology Handbook for Conservation Commissioners dated March, 2002, as amended from time to time, shall apply as minimum standards to all plan submittals, except where this section requires more stringent standards.

(4)

The applicant shall be responsible for compliance with the Marion Board of Health and Conservation Commission requirements governing the pertinent aspects of the stormwater management system.

(5)

Prior to the release of any portion of the performance guarantee, the stormwater management system must be substantially complete, stabilized, and operational. The functionality of the system shall be evaluated by the consulting engineer and the Department of Public Works Superintendent. The applicant shall submit an as-built plan of the constructed stormwater management system prepared by a registered land surveyor and certified by a registered professional engineer. If the system is found to be inadequate by virtue of physical evidence of operational failure even though it was built as called for in the definitive plan, it shall be corrected before the performance guarantee is released. Examples of inadequacy shall include errors in the infiltrative capacity, errors in the maximum groundwater elevation, failure to properly define or construct flow paths, or erosive discharges from the basins.

[Amended 10-17-2022]

Performance standards.

(1)

All design and construction shall be done in a manner such that the post-development stormwater runoff will not exacerbate or create flooding conditions, or alter surface water flow paths, resulting in impacts to the receiving wetland resource area or any adjacent properties to the site for the two-, twenty-five-, and one-hundred-year twenty-four-hour storm events. The pre-development standard shall be those conditions prevailing prior to abandoned projects or previously disturbed terrain.

(2)

The stormwater management systems shall be designed to attenuate the peak rate of the runoff for the two-, twenty-five-, and one-hundred-year twenty-four-hour storm events at or below the pre-development levels. Additional stormwater volume attenuation in excess of the required recharge volume under the DEP Stormwater Policy for these storm events may also be required for any stormwater discharges into floodprone areas, off-site culvert inlets, isolated topographic depressions, adjacent properties, and any receiving wetland and/or water body which may be sensitive to increases in runoff volume.

(3)

Stormwater management systems that may eventually be owned and maintained by the Town of Marion shall be designed and constructed to provide the required level of treatment at the least cost to the Town.

[Amended 10-17-2022]

(4)

Stormwater management systems shall be designed and constructed so that they do not negatively impact groundwater quality or elevations adjacent to or downgradient of the system area. Upon review of the specific site conditions, the Planning Board may require a groundwater mounding analysis based on the Hantush Method and a groundwater water quality evaluation to determine the potential impacts to any adjacent sensitive receptors (i.e., drinking water supplies and basements of existing dwellings).

(5)

Stormwater management systems shall be designed and constructed so that they do not represent safety hazards or nuisances to public health as determined by the Planning Board in consultation with the Board of Health.

(6)

Stormwater management systems shall be designed and constructed so that they do not visually detract from the neighborhood. A landscape design shall be prepared that provides appropriate screening from the adjacent properties and roadways, while providing the degree of access necessary for O&M activities. Landscape plans shall be submitted that appropriately address visibility issues through proper placement, preservation of existing natural vegetation and supplemental plantings where necessary. Rain garden BMPs or other LID BMPs are encouraged.

[Amended 10-17-2022]

Submittal requirements.

(1)

All site plan and definitive subdivision submittals shall be accompanied by a stormwater management system report. At a minimum, the report shall consist of the following:

(a)

Hydrologic calculations for the two-, twenty-five-, and one-hundred-year twenty-four-hour storm events based on the TR-20/55 Methodology for the pre- and post-developed conditions for the overall project as well as specific calculations for the two-, twenty-five-, and one-hundred-year twenty-four-hour storm events also based on the TR-20/55 Methodology for each specific area subject to flooding, including but not limited to isolated depressions, culvert inlets, and ponding areas for the pre- and post-developed condition.

[Amended 10-17-2022]

(b)

Supporting water quality calculations, specific BMP sizing calculations, and a stormwater management form stamped by a registered professional engineer shall also be provided.

(c)

Discussion of the environmental and hydrological conditions of the site for the pre- and post-developed condition, as well as the proposed alterations of the site, all proposed components of the stormwater management system and low-impact development (LID) consistency summation outlined in Subsection D(8)(c).

(d)

Soil evaluation logs, permeability test results and predicted maximum groundwater levels at each component of the stormwater management system validated by an agent of the Board of Health and/or the Planning Board. On-site permeability tests may be required to determine the appropriate infiltration value from Table 2.3.3 of the DEP Stormwater Management Policy. Rawls rates may also be used for the exfiltration analysis.

[Amended 10-17-2022]

(e)

An operation and maintenance plan for the stormwater management system. The plan shall include a maintenance schedule for each component of the stormwater management system, an outline of responsible parties and owners, and all pertinent agreements to be executed to insure proper maintenance of the facilities.

(2)

Pre- and post-development watershed plans at a preferred scale of one inch equals 40 feet shall be submitted with the stormwater management system report and shall clearly depict the following information:

(a)

Pre-development watershed plan.

[1]

The location of all surface waters, wetland resource areas, and all other state/federal jurisdictional resource areas on or within 100 feet adjacent to the site. The boundaries of all such resource areas shall be verified to be accurate by the Marion Conservation Commission.

[2]

The delineation of the one-hundred-year flood elevation as indicated on the Federal Flood Insurance Rate (FIRM) maps. If FIRM maps do not exist or if the one-hundred-year flood elevation of the water body or watercourse is not indicated on the FEMA map, the elevation shall be calculated, utilizing the U.S. Army Corps of Engineers' HEC floodwater modeling methodology or the SCS TR-20.

[Amended 10-17-2022]

[3]

Existing topography at a two-foot contour interval within the watershed study area. Areas with less than a 1.0% grade shall be shown at a one-foot contour interval with existing spot grades.

[4]

Delineation of the existing watershed boundaries on the property, inclusive of all off-site areas contributing runoff to the property.

[5]

Boundaries of existing surficial ground cover conditions within the watershed study area in order to verify the runoff curve number (Cn).

[6]

Prevailing soil types on the site and the hydrological soil groups based on the most current Natural Resource Conservation Service soils map. The provisional soil mapping at the NRCS office in Wareham shall be used in place of the published 1967 Plymouth County mapping.

[7]

Flow paths, lengths, slopes, and design points for each watershed with each segment of the flow path defined.

[Amended 10-17-2022]

[8]

Areas subject to flooding, including but not limited to isolated topographic depressions, culvert inlets, and ponding areas with the calculated one-hundred-year flood elevation associated with each area.

[9]

The location of any public or private water supplies on the property or within 100 feet of the property location, including any mapped Water Resource Protection Overlays (i.e., Zone II, IWPA.)

[10]

Location of soil test pits and groundwater elevations.

(b)

Post-development watershed plan.

[1]

Existing and proposed topography at a two-foot contour interval within the watershed study area. Areas with less than a 1.0% grade shall be shown at a one-foot contour interval with proposed spot grades.

[Amended 10-17-2022]

[2]

Delineation of the proposed watershed boundaries on the property, inclusive of all off-site areas contributing runoff to the property.

[3]

Boundaries of proposed surficial ground cover conditions within the watershed study area, including roadway areas, building footprints, driveways, lawn/landscaped areas and areas to remain in their natural condition in order to verify the runoff curve number (CN).

[Amended 10-17-2022]

[4]

[5]

Flow paths, lengths, slopes, and design points for each watershed with each segment of the flow path defined.

[Amended 10-17-2022]

[6]

[7]

Location of soil test pits, permeability tests, and groundwater elevations for each component of the proposed stormwater management system.

[8]

The proposed development layout, including the locations of roadways, parking areas, limits of land alteration, undisturbed areas, state/federal jurisdictional wetlands, floodplains, drainage collection systems and stormwater management facilities.

Design criteria.

(1)

General.

(a)

The criteria presented herein are provided to assist the designer by identifying minimum requirements, general procedures to be followed as well as specifying any limitations regarding the types of systems to be allowed. The designer is responsible for ensuring that the design complies with the performance standards enumerated in Subsection B above as well as conformance with state and federal requirements and with accepted engineering practice.

(b)

All runoff from storms up to the one-hundred-year storm event must flow through the stormwater management/treatment systems. Systems for the proper conveyance of the predicted one-hundred-year storm to the stormwater management/treatment systems shall be provided.

(2)

Location.

(a)

Stormwater management systems (exclusive of the conveyance system) shall be located on a separate and segregated parcel specifically designated for such use, to be conveyed to the Town should Town acceptance of the subdivision roadway and infrastructure be contemplated. Provisions for vehicular access shall be provided at the circumference of each system. Components of the collection system such as drainage piping, ditches, swales, and certain LID components etc. may be located within access/utility easements. Provisions for vehicular access shall be provided along the entire length of storm drain lines and swales within all designated access/utility easements.

(b)

The following minimum setback requirements shall apply to all stormwater management systems. Any of these setback distances may be waived by the Board upon a clear finding that the proposed stormwater management system will utilize the preferred LID structural stormwater best management practices on a LID scale through decentralization and dispersion.

[1]

Property line: 25 feet.

[2]

Street line: 50 feet.

[3]

Underground utilities: 20 feet.

[4]

Private well: 100 feet.

[5]

Subsurface sewage disposal system: 100 feet.

[6]

Dwelling: 100 feet.

[7]

Surface water body: 50 feet.

[8]

Bordering vegetated wetland: 25 feet.

(3)

Soil testing.

(a)

Soil testing to determine the maximum groundwater elevation and soil profiles shall be performed by a Massachusetts approved Soil Evaluator or Certified Soil Scientist and witnessed by the Board of Health and/or an agent of the Planning Board. At the discretion of the Board, soil permeability testing may be required if the initial soil logs exhibit variable soil conditions and inconsistent groundwater profiles.

[Amended 10-17-2022]

(b)

Investigations.

[1]

Subsurface investigations for infiltration practices are required to define the suitability of soils for subsurface disposal of stormwater runoff. These explorations are necessary to determine the textural characteristics of the various soil strata, restricting layers, location of the estimated seasonal high water table elevation and depth to bedrock in the location of the proposed system.

[Amended 10-17-2022]

[2]

The soil investigation must include an identification of the soil through examination of the soil profile in the location of the proposed system. In addition, the following determinations must be included in the investigation:

[a]

Soil textures, measurement of soil permeability rates, soil hydrologic group, estimation of seasonal high water table elevation by identifying soil gleying and mottling, and identification of any restricting layer(s).

[b]

Acceptable testing methods to determine the soil suitability for infiltration practices are the falling head permeameter test (ASTM D5126-90) or the double ring infiltrometer (ASTM D3385-03/D5093-02). Soil test pits shall be excavated to a depth no less than five feet below the design bottom of the system or to the static water level, to inspect and describe the soil profile. A minimum of one inspection pit must be dug per 5,000 square feet of system bottom area and must be located within the perimeter of the system.

(c)

The infiltration values utilized in the TR-20 calculations to determine the size of the infiltration structure shall be based on the DEP Stormwater Management Policy Volume 3, Chapter 1, Recharge Requirements Table 2.3.3.

(4)

Calculations.

(a)

Rainfall data shall be as determined from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 dated September 30, 2015, providing precipitation data for New England and New York, or the document with the most recent date.

(b)

The use of the Rational Method for volume-related calculations is not permitted.

(c)

In areas where the actual on-the-ground soil evaluations exhibit subsurface conditions inconsistent with the NRCS mapping, or in cases where the site has been extensively reworked, the hydrologic soil group (HSG) curve number (CN) values utilized in the TR-20 calculations should be adjusted to reflect the actual on-the-ground cover conditions based on the determination by the Massachusetts-approved Soil Evaluator or Certified Soil Scientist.

[Amended 10-17-2022]

(d)

The analysis points for the hydrologic study shall be at the edge of the wetland resource area boundary, adjacent downgradient property line, existing storm drain system, or other sensitive receptors such as adjacent agricultural uses. For each pre-development analysis point there shall be a corresponding post-development point.

[Amended 10-17-2022]

(e)

For all infiltration facilities where the bottom is proposed to be within two feet zero inches of the Estimated Seasonal Highwater Table, a groundwater mounding analysis based on the Hantush method for the required design storms shall be prepared by a professional engineer or hydrologist. The applicant shall assess the potential effects from the subsurface disposal of stormwater on adjacent road surfaces, private wells, building foundations, embankments and any other site features that may be sensitive to groundwater flow.

[Amended 10-17-2022]

(f)

The sediment forebay volume below the elevation of the spillway to the detention/retention stage cannot be utilized as available flood storage volume or recharge volume for purposes of sizing the detention/retention basin.

(5)

Treatment required.

(a)

The discharge of untreated stormwater runoff from the developed areas of the property is prohibited.

(b)

Stormwater management systems shall be designed to treat the first flush volume of the entire project site at full build out and shall achieve an eighty-percent TSS removal rate of the total suspended solids at the point of discharge. The first flush volume is a function of the initial 1.25 inches of stormwater runoff from impervious surfaces. The first flush treatment volume in cubic feet (V_t) is determined by the following formula:

V_t = (1.25/12 inches) (R_v)(Site Area in Square Feet)

Where R_v = 0.05 + 0.009(I) and I = the % impervious area. "Impervious area" is defined as any man-made cover that is not vegetated.

(c)

Any development in the Sippican Harbor and Wings Cove watershed areas shall incorporate a physical treatment processes to remove nitrogen at a minimum efficiency rate of 40%. Development in the Mary's Pond watershed shall incorporate phosphorus removal at a minimum design rate of 50%.

(6)

Aboveground basins.

(a)

All water quality basins/ponds shall have a sediment forebay consisting of a separate cell separated from the detention/retention stage by a rock fill filter berm to allow lateral flow into the lower stage. The top elevation of the filter berm shall be at or below the elevation of the inlet piping.

(b)

A gravel access drive not less than 12 feet wide shall be provided around the outer perimeter of all basins to allow for vehicular access.

(c)

The maximum depth of the sediment forebay shall be no greater than two feet.

(d)

Where appropriate, the interior side slopes of all basins shall be no greater than 4:1 but in no case less than 3:1. Exterior side slopes of the embankment shall be no greater than 3:1.

(e)

The bottom elevation of the basin shall be no less than two feet above the maximum groundwater table elevation or bedrock and shall be at least one foot above the elevation of the receiving wetland.

(f)

The calculated peak water levels within the basin for all design storms shall be no greater than three feet and shall be no less than 12 inches below the elevation of the containment berm.

(g)

The entire detention basin area shall be treated with a four-inch layer of screened organic soil borrow conforming to Massachusetts Department of Transportation (MassDOT) Standard Specifications M1.05.0. The side slopes shall be seeded with an erosion seed mix conforming to MassDOT Specifications M6.03.0. The basin bottom shall be seeded with a New England erosion control/restoration mix as manufactured by New England Wetland Plants, Inc., or approved equal, applied at a rate of 35 pounds per acre and supplemented by landscape plantings. Infiltration basins shall be treated with four-inch layer of screened organic soil borrow conforming to Massachusetts Department of Transportation (MassDOT) Specifications M1.07.0 and planted with a water-tolerant grass seed mix.

[Amended 10-17-2022]

(h)

The maximum allowable ponding or storage time for infiltration basins for design storms up to the twenty-five-year storm event is 48 hours. For the one-hundred-year storm event, the maximum drain time shall be 72 hours.

(i)

The bottom elevation of the infiltration basin shall be no less than two feet above the Estimated Seasonal Highwater Table or bedrock and shall be at least one foot above the elevation of the receiving wetland.

[Amended 10-17-2022]

(j)

Infiltration basins may be constructed in fill, providing that a minimum four-foot layer of naturally occurring soil meeting the infiltration requirements of this section is available below the bottom of the basin and that the fill material is a clean granular fill conforming to 310 CMR 15.255(3). Construction in fill shall mean any system where fill is required to replace topsoil, subsoil, peat, or unsuitable soil layers above the requisite four feet of naturally occurring soil.

(k)

Recharge structures shall be provided at the bottom of the infiltration basin to ensure adequate recharge is provided under frozen ground cover conditions.

(l)

All infiltration basins must be provided with an overflow mechanism to a receiving wetland or water body. Access/Utility easements must be provided along the designated overflow path to the receiving wetland or water body. All infiltration basins must have an outlet structure with an accessible valve to allow manual drainage of the basin in an emergency condition, nonerosive flows at the outlets, inlet splash pads and emergency spillway weirs. Outlet structures and all inlet and outlet piping 18 inches or greater in diameter shall be fitted with trash racks.

[Amended 10-17-2022]

(m)

Infiltration practices are limited to soils in Hydrologic Soil Groups A, B and C only, as identified by the most recent NRCS Soil Survey mapping. The provisional soil mapping at the NRCS office in Wareham shall be used in place of the published 1967 Plymouth County mapping. Dry wells for individual residential dwellings and small LID technologies such as rain gardens, bioretention cells and infiltration strips may be permitted in Hydrologic Soil Group C soils, providing the minimum infiltration rate of 0.50 inch per hour can be achieved.

(n)

Detention and infiltration basins may not be located within the VE Zone as depicted on the map entitled, "Flood Insurance Rate Map, Town of Marion," which is in effect at the time of application.

(7)

Subsurface recharge systems.

(a)

Subsurface recharge systems may be used on subdivisions consisting of five lots or less, commercial and industrial site developments. All infiltration systems including aboveground basins and subsurface recharge must be provided with a pretreatment system capable of removing 80% of the total suspended solids (TSS) loading from the contributing watershed area. Subsurface recharge systems are not allowed on residential subdivisions in excess of five lots or land uses with a high-potential pollutant load as defined in the DEP Stormwater Management Policy.

(b)

(c)

The bottom elevation of the subsurface recharge system shall be no less than two feet above the Estimated Seasonal Highwater Table elevation or bedrock.

[Amended 10-17-2022]

(d)

The maximum allowable ponding or storage time for recharge systems for design storms up to the twenty-five-year storm event is 48 hours. For the one-hundred-year storm event, the maximum drain time shall be 72 hours.

(e)

Subsurface recharge systems may be constructed in fill, providing that a minimum four-foot layer of naturally occurring soil meeting the infiltration requirements of this section is available below the bottom of the basin and that the fill material is a clean granular fill conforming to 310 CMR 15.255(3). Construction in fill shall mean any system where fill is required to replace topsoil, subsoil, peat, or unsuitable soil layers above the requisite four feet of naturally occurring soil.

(f)

Subsurface recharge systems located under paved parking areas shall be designed to an H-20 vehicle loading.

(g)

The entire area of the proposed subsurface recharge system shall be roped off during construction to prevent compaction of the underlying soils by heavy equipment. The basin shall be excavated with light earth-moving equipment to prevent compaction of soils beneath the basin floor or side slopes. Light earth-moving equipment does not include bulldozers or standard-size pay loaders, but may include Low Ground Pressure Equipment (LGP).

[Amended 10-17-2022]

(h)

Proper soil erosion and sediment control methods must be used during and after development of the site. Stormwater runoff shall not be allowed into any infiltration basin or recharge structure until the entire contributing watershed area has been stabilized with vegetation and other soil erosion and sediment control techniques.

(i)

Under no circumstances shall any infiltration basin or subsurface recharge system be utilized as temporary sediment traps or stormwater management during construction.

(j)

Subsurface recharge systems shall consist of precast concrete or HDPE galleys, or large-diameter perforated HDPE pipe. The systems shall be encompassed with a 0.75-inch to 1.5-inch double-washed stone conforming to 310 CMR 15.247(1) and wrapped in filter fabric. An individual recharge system for each catch basin inlet or pair of inlets is preferred over a single recharge facility serving multiple inlets. Direct connections from the catch basin inlets to the recharge systems are not permitted. Proprietary treatment systems may be considered in order to satisfy the 80% TSS removal target prior to discharge into the subsurface recharge system.

[Amended 10-17-2022]

(k)

For each line of subsurface galleys or trenches, a minimum of two inspection manholes with cast iron frames and covers to grade for access and maintenance shall be provided at opposite ends of each line. The maximum length of each trench/galley line shall not exceed 50 feet. When multiple trench/galleys are used each row of trenches or galleys shall be separated by a distance not less than three times the effective width or depth, whichever is greater.

[Amended 10-17-2022]

(l)

Individual recharge systems shall be interconnected with an overflow pipe to a downgradient outfall device to prevent flooding of the roadway and adjacent properties in the event the design storm is exceeded or hydraulic failure of the infiltration structure.

(8)

Low-impact development techniques.

(a)

Low-impact development (LID) techniques to manage stormwater shall be considered for new construction and redevelopment of projects for all commercial, industrial, and residential site development projects. Designs that reduce impervious surfaces and employ decentralized stormwater management systems that involve the use of small treatment and infiltration devices and LID best management practices (LID-BMPs) throughout the site in place of a centralized system of closed pipes and a single large facility are preferred.

[Amended 10-17-2022]

(b)

Effective low-impact development includes the use of both nonstructural and structural stormwater best management practices (LID-BMPs). The use of these BMPs is governed by certain principles, objectives and requirements. The maximum practical use of the following seven nonstructural strategies shall be considered:

[1]

Protect areas that provide water quality benefits or areas particularly susceptible to erosion and sediment loss.

[2]

Minimize impervious surfaces and break up or disconnect the flow of runoff over impervious surfaces through the use of vegetative filter strips and buffers.

[3]

Minimize the decrease in the pre-construction time of concentration.

[4]

Minimize land disturbance activities including clearing and grading and preserve naturally vegetated areas.

[5]

Provide low-maintenance landscaping that promotes retention and planting of native vegetation and minimizes the use of lawns, fertilizers, and pesticides.

[6]

Provide vegetated open channel conveyance systems which discharge into and through stable vegetated filter strip areas.

[7]

Provide preventative source controls.

(c)

The applicant shall prepare a low-impact development (LID) consistency statement showing how the above strategies have been incorporated into the developments design in the stormwater management report. For each of the above strategies that were not able to be incorporated into the design due to physical site constraints, engineering, environmental, or safety reasons, the applicant must provide a basis for this contention.

(d)

Preferred structural stormwater best management practices such as rain gardens, bioretention areas, sand filters, and infiltration strips provide storage, infiltration, and treat runoff close to its source. Other standard best management practices such as dry wells, infiltration systems, surface and subsurface detention basins can all be done at an LID scale by downsizing and addressing stormwater runoff close to its source and dispersing these systems throughout the development site.

(e)

Commercial and industrial site development projects shall provide preventative source controls to prevent pollutants from being part of the stormwater runoff. Source controls such as the prevention and containment of spills and other harmful accumulations of pollutants as well the management of trash and debris shall be incorporated into all commercial and industrial site development plans.

(9)

Proprietary treatment devices.

(a)

A proprietary treatment device is a prefabricated stormwater treatment structure utilizing settling, filtration, absorptive/adsorptive materials, vortex separation, and/or other appropriate technology to remove pollutants from stormwater runoff.

(b)

These devices are allowed for new construction and redevelopment projects on privately owned commercial/industrial land development sites. The operation and maintenance of these proprietary treatment devices will be the obligation of the owner of the facility being served. These devices may also be considered on real estate subdivisions upon a clear finding by the Board that no other practical alternative is available to achieve the water quality treatment goals of these rules and regulations.

(c)

On-line devices must be fitted with an overflow bypass for storm events exceeding the stormwater quality design storm.

(d)

The specified devices shall be furnished by a manufacturer regularly engaged in such work and who has furnished similar installations in the Commonwealth of Massachusetts and had them successful and continuous operation for a minimum period of five years. Devices which have been evaluated and assigned a TSS removal efficiency by the DEP through the Massachusetts Strategic Envirotechnology Partnership (STEP) program are approved for use in the Town of Marion. Other proprietary technologies will be evaluated by the Board on a case-by-case basis in accordance with the DEP Technical Guide for Compliance with the Massachusetts Stormwater Management Standards Volume 2, Chapter 4.

(e)

Manufacturers' documentation on similar systems, including but not limited to data on performance testing, service history, TSS removal efficiency, sizing criteria, and operation/maintenance requirements of the specified devices, shall be submitted to the Planning Board.

(f)

Approval of proprietary devices will be based on the following:

[1]

Optimal TSS removal efficiency.

[2]

Minimal operation and maintenance costs.

[3]

Compatibility with existing infrastructure, other BMP devices and physical site constraints.

[4]

Frequency of maintenance and special handling or installation techniques.

[5]

Special equipment required for maintenance and the capability of the Department of Public Works to provide maintenance service.

[6]

Reliability of performance data and potential failure rates.

General landscaping guidance.

(1)

All submittals shall be accompanied by a stormwater management system landscaping plan prepared by a landscape architect registered in the State of Massachusetts. At a minimum, the landscaping plan shall consist of the following:

(a)

Plan views of each stormwater management system with detailed planting locations identified by species, size, and count. The wetted hydrologic zones within and around the basins should be identified and noted on the plan views. The planting species should be selected based on the frequency and depth of inundation within the hydrologic zones.

[Amended 10-17-2022]

(b)

A detailed planting schedule table identifying the size and type of species planted and individual plant counts. The planting schedule shall identify the botanical and common name and spacing.

[Amended 10-17-2022]

(c)

Notation specifications describing the site preparation activities, soil amendments, and procedures for plant installation. Specifications should also address the type of materials (e.g., balled and burlap, bare root, containerized); time of year of installations, sequence of installation of type of plants; fertilization, stabilization seeding, watering and general care.

(d)

Maintenance program consisting of inspection intervals, mulching frequency, removal and replacement of dead and diseased vegetation, watering schedule, repair and replacement of staking and wires, removal and eradication of invasive species.

(e)

A two-year guarantee on all new plant material requiring replacement of any dead plant material within this period is required.

[Added 10-17-2022]

(2)

Planting plan design considerations.

(a)

Use of native plant species is required. Invasive species as defined by the Massachusetts Invasive Plant Advisory Group (MIPAG) are prohibited for use. Existing natural vegetation is to be preserved where possible and enhanced with native plant species. Plantings requiring routine or intensive chemical applications are not permitted.

[Amended 10-17-2022]

(b)

Appropriate plantings should be selected based on the zone's hydric tolerance. Planting locations should be random and consistent with the surrounding native vegetation.

(c)

Trees, shrubs and/or any type of woody vegetation is not allowed on basin embankments. Herbaceous embankment plantings should be limited to 10 inches in height. Trees and shrubs should be planted at least 25 feet away from any perforated pipes and principal spillway structures. Trees and shrubs known to have long taproots should not be planted within the vicinity of any earth embankments or subsurface drainage facilities.

(d)

Inflow and outflow channels and southern exposure areas of any permanent pool areas should be shaded to reduce thermal warming.

(e)

Aesthetics and visual characteristics should be a prime consideration in the landscaping plan. Desirable views should be framed and maintained while unattractive views should be effectively screened from any adjacent residences and roadways.

Stormwater collection systems.

(1)

Design criteria.

(a)

The quantity of stormwater carried by storm drains shall be determined by the Rational Method on the basis of a twenty-five-year-frequency design storm. The inlet capacity and spacing for catch basins shall be designed to limit the flow in the gutter during a twenty-five-year design storm to a maximum of four feet in width as calculated utilizing methodologies described in Drainage of Highway Pavements, Hydraulic Engineering Circular No. 12, as published by the U.S. Department of Transportation, Federal Highway Administration. In any event, water shall not be allowed to run for more than 300 feet on paved surfaces. Computations for drainage requirements shall be prepared by a registered professional civil engineer and submitted with the drainage report prepared for the project.

[Amended 10-17-2022]

(b)

Supporting data for the sizing of the storm drain collection system shall include the following:

[1]

Subcatchment area plan(s) at a clear legible scale showing the following information.

[Amended 10-17-2022]

[2]

Existing and proposed contour grading at the predicted full build out of the subdivision.

[3]

Existing and proposed ground cover conditions.

[4]

Predicted flow paths and delineation of subcatchment areas to each inlet.

[5]

Rational Method calculations based on the twenty-five-year storm event and the one-hundred-year storm event where the collection system is expected to convey the one-hundred-year storm flows to the receiving stormwater management facility.

[6]

Time of concentration (Tc) worksheets based on TR-55 methodology.

[7]

Weighted average ground cover coefficient (C) calculations.

[8]

Inlet grate capacity calculations based on Hydraulic Engineering Circular #12.

[9]

Rainfall intensity based on local rainfall/intensity/duration data.

[Added 10-17-2022]

(c)

The system may make use of gutters, inlets, culverts, catch basins, manholes, subsurface piping, surface channels, and open detention basins. Leaching catch basins will not be permitted. The Board will not approve any design or component which, in its opinion, does not meet the standards of good engineering practice, will not function without frequent maintenance, or is unsuited to the character of the subdivision.

(d)

Where feasible, stormwater should be directed to enter the nearest open stream channel. At all outfalls of drainage systems, a reinforced concrete headwall or reinforced concrete flared end shall be provided. Stormwater shall not be permitted to cross any roadway upon the surface but must be piped underground.

(e)

In general, the design of pipes shall be such as to provide for a flow of water at velocities between two feet per second and 10 feet per second under full flow conditions than that required to achieve the velocities specified. The minimum grade shall be not less than 0.5% or less than that required to achieve the velocities specified and the minimum pipe diameter shall be 12 inches, designed to flow full with the hydraulic gradient at the crown.

[Amended 10-17-2022]

(f)

Storm drains with Class III RCP pipe shall have a minimum of three feet of cover. Drains with less than three feet of cover shall use Class V RCP pipe. In no event shall drains have less than 2.0 feet of cover. All changes in pipe class shall be noted on the plan. In determining the capacity of the pipe drains, the Manning formula shall be used with coefficient of friction "n" equal to 0.013 for RCP.

(g)

Catch basins shall have a minimum four-foot sump below the invert. Catch basins or inlets shall be spaced along both sides of a street at no greater than 300-foot intervals, and located at all low points and corner rounding at street junctions. Drain manholes shall be located at every change of direction and/or elevation but in no cases greater than 300 feet apart. Catch basins shall not serve as manholes. All pipes from catch basins shall flow to manholes.

[Amended 10-17-2022]

(h)

Roof drains, cellar drains or any other "private" non-preexisting drainage systems will not be allowed to connect to the storm drainage system, unless specifically waived by the Planning Board.

[Amended 12-19-2019]]

(i)

Cross culverts and drainage control facilities shall be based on all storms up to a one-hundred-year-frequency storm. At cross culverts, drainage easements shall be established upgradient of the culvert and delineated on the definitive plan based on the projected one-hundred-year headwater elevation. The determination of the headwater elevation shall be based on TR-20 model calculations and the Federal Highway Administration Hydraulic Engineering Circular No. 5 (HEC 5).

(j)

In some cases, earth and stone-paved open channels should be used. The typical section of the earth channel should have a flat bottom and side slopes of one vertical on three horizontal with the top of the slope at least one foot higher than the design water surface. The maximum velocity allowed in an open earth channel at design flow should be 6.0 fps. A coefficient of friction "n" equals to 0.025 maximum should be used for both the earth and stone-paved channels. Detailed calculations, plans and profiles showing proposed channels and treatment of channel base and side slopes shall be submitted for Planning Board approval.

[Amended 10-17-2022]

(k)

Wherever drainage systems within the subdivision are located in or terminate in lands owned by others, proper easements in a form and content acceptable to Town Counsel shall be taken for their access and maintenance by Town personnel.

(l)

Granite curb inlets per Section M9.04 of the Standard Specification shall be installed adjacent to all catch basins at low points and any other location granite curbing is required. If located within an area of Cape Cod berms, a minimum six feet transition curbing (granite, Type VA4) shall be installed along the curbline on both sides of the curb inlet. The Cape Cod berms shall be constructed to blend with the transition curb.

[Amended 10-17-2022]

(m)

Components of the collection system such as drainage piping, ditches, swales, etc. may be located within access/utility easements. Provisions for vehicular access shall be provided along the entire length of storm drain lines and swales within all designated utility easements.

(n)

Rip-rap spillways shall be provided at all pipe outfalls and critical areas within drainage swales or ditches.

[Amended 10-17-2022]

(o)

The drainage design in its entirety shall minimize long-term safety issues, maintenance, and/or reconstruction requirements to the satisfaction of the Planning Board.

(p)

All pipe crowns in manholes must match or the crown of the inlet pipe must be higher than the crown of the outlet pipe.

(q)

No catch basins shall be installed in front of driveway or handicap ramp openings.

(r)

All runoff from storms up to the one-hundred-year storm must flow through the drainage control facilities (detention ponds, etc.) and be mitigated prior to flowing beyond the site. Although the pipes are designed for the twenty-five-year storm, the runoff for storms up to the one-hundred-year storm must reach the drainage control facility either through the pipe systems, swales or overland with easements.

(s)

Stormwater drainage pumping stations are not allowed.

(t)

When in the opinion of the Planning Board, and confirmed by the reviewing independent registered professional engineer, the existing street drainage and/or downstream drainage systems are inadequate, the stormwater management system design shall maintain both rate and volume controls at pre-development levels.

(2)

Storm drainage construction requirements.

(a)

Drainage facilities shall be provided as indicated on the plan and in conformity with these regulations and the requirements of Sections 100, 200, 220, 230, 258, and 260 of the Commonwealth of Massachusetts Department of Transportation Standard Specifications for Highways and Bridges, herein referred to as the "Standard Specifications."

[Amended 10-17-2022]

(b)

The drainage system shall be in place and functional and approved by the Highway Surveyor and the Planning Board at the time of the installation of the binder course pavement. Utility as-built plans of the installed drainage system must be submitted and approved by the Planning Board and the Department of Public Works prior to the installation of the binder course pavement.

(c)

Unsuitable material below normal pipe invert shall be removed and replaced with suitable material. Unsuitable material shall not be used for trench backfill. Pipe and conduits shall be surrounded by six inches of compacted screened gravel if set in earth, and 12 inches if set in rock.

(d)

The standard depth of catch-basins shall be four feet below the invert of the outlet. Manholes shall be constructed to the required depth at each junction point and as shown on the plan. Pipe culvert and pipe drains shall be in conformity with the requirements of Section 230 of the Standard Specifications for installation of pipes.

[Amended 10-17-2022]

(e)

All drain pipes except subdrains shall be reinforced concrete pipe (RCP) with bell and spigot gasketed joints and shall be installed according to the size as shown on the plans. No backfilling of pipes shall be done until approval of the Department of Public Works. All drainage trenches shall be backfilled per Section 201 of the Standard Specifications.

[Amended 10-17-2022]

(f)

Where subdrains are required they shall be constructed in conformance with Section 260 of the Standard Specifications. Such subdrains may be required by the Board following clearing and grubbing operations. No drainage pipes from roof drains, driveway drains, or other on-lot sources shall be connected to the street drainage system. Cast iron manhole covers and catch basin grates shall be as manufactured by or equivalent to EJ OMA 5540 frame with EJ OMA 5520 grate. Frame and grate type to suit. Manhole frames and covers. EJ OMA 1240 solid cover marked "DRAIN" cast into the manhole for catch basins. The word "DRAIN" shall be cast into the drain manhole covers.

[Amended 10-17-2022]

(g)

No more than four pipe openings shall be allowed in any one manhole. Four-foot-diameter manholes will be used for drains up to 30 inches in diameter. Five-foot-diameter manholes shall be used for pipe diameters between 36 inches and 48 inches. Pipes shall not enter a manhole less than 90° of the direction of flow. All connecting lines shall have bricked inverts rounded into the direction of flow.

Notes:
a.	Greater width may be required by the Board where necessary to meet present or future traffic demands.
b.	Minimum gradient of any street shall be one foot in 100 feet.
c.	Efforts shall be made to minimize the gradient of street by locating streets so that they follow contours rather than crossing contours. The gradient shall never exceed 10%.
d.	Sidewalks and/or bicycle paths and curbs will be required only in areas of the Town where the Planning Board determines them to be necessary for public safety. At the discretion of the Planning Board, bike paths may be created by delineating a special lane on a paved way. The center line grade of the proposed road at any point shall be not more than five feet above or below the existing center line grade.
e.	Cut or fill side slopes shall not be steeper than 3:1.
f.	All above-grade features, such as hydrants, utility poles, etc., shall be confined to the utility strip adjacent to the property line and are not to be placed in the park or green strip, no greater than 40 feet on center of both sides of the street.
g.	The number of curb cuts into the streets should be minimized by the use of common driveways serving more than one lot. Such common driveways shall have at least a ten-foot width of traveled way with suitable additional spaces provided for vehicles to be parked on each lot without blocking the common driveway.
h.	All utilities shall be underground. Cable TV shall be included.
i.	A one-hundred-foot-long leveling area shall be provided at all intersections. The leveling area will have a maximum slope of 3%.
j.	Guard rails shall be installed where slopes from the right-of-way exceed 1:4 and the slope exceeds five feet in height, or where required by the Board to protect vehicles from obstructions or to maintain public safety.
k.	Where changes in roadway grade exceed 1%, vertical curves shall be provided.