Town of North Andover, MA Stormwater Management Plan

Purpose;

Methodologies and assumptions;

Existing and proposed uses and conditions;

Existing zoning at the site and abutting properties;

Project impacts and mitigation techniques including:

Summary of proposed land area to be disturbed, existing and proposed impervious area, work within proximity of regulated wetland resources, aquifer protection zones, earthwork within four feet of seasonal high groundwater elevations, and other sensitive environmental areas;

Low-impact development (LID) techniques considered for this project and an explanation as to why they were included or excluded from the project;

Identifying the watershed basin that the project is located in and the immediate down-gradient water body(ies) that stormwater runoff from the project site discharges to, EPA's watershed and water body assessment and TMDL and/or impairment status of the watershed and water body(ies), and the LID techniques and BMPs included in the project to address the pollutant(s) of concern;

Summary of pre- and post-development peak rates and volumes of stormwater runoff demonstrating no adverse impacts to down-gradient properties, stormwater management systems and wetland resources; and

Proposed best management practices and summary of how the project meets stormwater management criteria.

Plans. Site plans included with the stormwater management plan shall be prepared on twenty-four-inch by thirty-six-inch sheets and shall include a title, date, North arrow, names of abutters, scale (one inch equals 20 feet or one inch equals 40 feet), legend, and locus map (one inch equals 800 feet). Revised plans shall contain a notation listing and describing all revisions, additions, and deletions made to the originally submitted plans and the date of each. Plans shall include:

Portion of the USGS Map indicating the site locus and properties within a minimum of 500 feet of the project property line;

Surveyed property lines showing distances and monument locations, all existing and proposed easements, rights-of-way, utilities and other encumbrances;

Lines of existing abutting streets showing drainage and driveway locations and curb cuts;

Existing conditions and proposed design plans showing:

Buildings and/or structures, including materials, approximate height;

Utilities, including size, material and invert data;

Existing and proposed topography on the site, at two-foot intervals, with areas of steep slopes between 15% and 25%, and over 25% specifically delineated, and with spot elevations provided. Existing topography 50 feet beyond the perimeter of the parcel as it appears on the most current Town of North Andover topographic mapping shall also be shown;

Location, delineation and description (including pertinent hydrology and water quality) of all existing and proposed watercourses, water bodies, and wetland resource areas on or entering the site, or adjacent to the site, or into which stormwater from the site flows, collects or percolates. Wetland resource areas shall include those protected under the Massachusetts Wetlands Protection Act (MGL c. 131, § 40) and/or the Town of North Andover Wetlands Protection Bylaw (Chapter 190 of the Town Code);

(e)

Location of the 100-year flood boundary on or within 100 feet of the project site, based upon the most recent Flood Insurance Rate Map (FIRM) or as calculated by a professional engineer for areas not assessed on these maps;

(f)

Location, delineation and description of habitats mapped by the Massachusetts Natural Heritage and Endangered Species Program as endangered, threatened or of special concern, estimated habitats of rare wildlife and certified vernal pools, potential vernal pools, and priority habitats of rare species within 500 feet of any proposed area of disturbance;

(g)

The general outline of existing vegetation, wooded areas, significant trees, unique species and tree clusters and the extent of all vegetation, wooded areas, significant mature trees, unique species and/or tree clusters to be removed;

(h)

Locations of existing and proposed wells and septic systems on or within 100 feet of the site;

(i)

A description of existing soils on the site (type, hydrologic soil group, erodibility), and the volume and nature of any imported soil materials; and

(j)

Estimated seasonal high groundwater elevation in areas to be used for stormwater retention, detention, or infiltration, and the basis for determination.

Stormwater management design plan(s) and details showing:

Location, size, material, inverts data and details for all existing and proposed stormwater management system components, including structures, pipes, swales, detention, retention, and infiltration systems and any other LID techniques or BMPs; and

Profiles of drainage trunk lines.

Separate pre- and post-condition watershed plans indicating:

A drainage area map showing pre- and post-construction watersheds, subwatersheds and stormwater flow paths, including municipal drainage system flows;

Structures, pavements, surface vegetation and other ground cover materials;

Topography sufficient to delineate watershed areas;

Point(s) of analysis;

(e)

The size of the entire parcel, each drainage area on or to the parcel, and the delineation and number of square feet of the land area subject to disturbance;

(f)

Watershed areas including upgradient areas that contribute stormwater flow onto the project site, labeled to be easily identified in calculations. Total pre- and post-watershed areas should be equivalent;

(g)

Breakdown summary of various surface conditions by soil hydrologic group rating; and

(h)

Flow path for time of concentration (Tc) calculation.

Calculations.

Hydrologic calculation to determine pre- and post-peak rates and volumes of stormwater runoff for two-, ten-, twenty-five-, and 100-year twenty-four-hour storm events;

Groundwater recharge calculations and BMP drawdown (time to empty);

Water quality calculations, including (if applicable):

TSS removal calculation for each watershed;

Specific BMPs utilized in critical areas;

Specific BMPs utilized for land uses of higher potential pollutant loads (LUHPPL); and

Specific treatment for pollutant causing impairment of down-gradient water body identified by the U.S. Environmental Protection Agency and the Massachusetts Department of Environmental Protection.

Peak runoff rates and total runoff volumes for each watershed and subwatershed area under existing and proposed conditions;

Infiltration rates, where applicable;

Hydraulic calculations to size drainage pipes, swales and culverts; and

Supplemental calculations for sizing BMPs and addressing impairments to water bodies.

Soil mapping and test data;

Massachusetts Department of Environmental Protection Checklist for Stormwater Report completed, stamped and signed by a registered professional engineer (PE) licensed in the Commonwealth of Massachusetts to certify that the stormwater management plan is in accordance with the criteria established in the Massachusetts Stormwater Management Standards, Stormwater Management and Erosion Control Bylaw and these regulations; and

Any other information requested by the Planning Board.

At a minimum, all projects subject to a land disturbance permit shall comply with the criteria, specifications, and performance standards of the most recent version of the Massachusetts Stormwater Management Standards and accompanying Stormwater Management Handbook, as well as the criteria contained herein. The following general performance criteria shall be applicable to all stormwater management plans, unless otherwise provided for in these regulations:

General performance standards for all sites.

Low-impact development (LID), as defined in § 165-2 of the Stormwater Management and Erosion Control Bylaw, site planning and design strategies must be implemented to the maximum extent feasible. LID employs principles such as preserving and recreating natural landscape features, minimizing effective imperviousness to create functional and appealing site drainage that treats stormwater as a resource rather than a waste product. LID techniques include but are not limited to bioretention facilities, rain gardens, vegetated rooftops, rain barrels, permeable pavements and other green infrastructure.

Landscape design.

Landscape designs shall be developed based on soil, light and other site-specific conditions. Plant species shall be chosen for their ability to thrive in the post-development soil, water and use conditions of the site without significant supplemental water or fertilizer, once established.

Plant species shall be native to inland Essex County or shall be cultivars of these native species.

Wildflower meadows and shrubs are advisable to reduce the amount of lawn or turf on a site.

For landscape areas adjacent to roadways, salt-tolerant plans shall be used.

(e)

Irrigation shall be provided by the use of a rainwater harvesting system to the extent feasible.

The selection, design and construction of all pretreatment, treatment and infiltration BMPs shall be in accordance with the Massachusetts Stormwater Handbook and shall be consistent with all elements of the Massachusetts Stormwater Standards, including but not limited to those regarding new stormwater conveyances, peak runoff rates, recharge, land uses with higher potential pollutant loads, discharges to Zone II or interim wellhead protection areas, sediment and erosion control, and illicit discharges.

For other water quality BMPs not included in the Massachusetts Stormwater Management Handbook, or for which pollutant removal rates have not been provided, the effectiveness and pollutant removal of the structural control must be documented through prior studies, literature reviews, or other means and receive approval from the Planning Board before being included in the design of a stormwater management system.

To the extent that the project will discharge, directly or indirectly, to a water body subject to one or more pollutant-specific total maximum daily loads (TMDLs), implement structural and nonstructural stormwater best management practices (BMPs) that are consistent with each such TMDL.

To the extent that the project will discharge, directly or indirectly, to an impaired water body not subject to a TMDL, implement structural and nonstructural stormwater BMPs optimized to remove the pollutant or pollutants responsible for the impairment.

Tree protection and preservation. Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots. The Town therefore deems that the preservation and protection of certain trees on public and private property, and the requirement to replant trees to replace those removed, are public purposes that protect the public health, welfare, environment and aesthetics. At the discretion of the Planning Board, existing trees on private property with a diameter at breast height of 10 inches or greater and existing trees within the right-of-way or on Town property may be considered protected trees to be retained on the property. Such trees shall be protected and delineated within the submitted erosion and sedimentation plan and described in the land disturbance permit. The area surrounding a tree, which includes at a minimum the critical root zone (CRZ) and dripline of all protected trees must be enclosed within a fence prior to land-disturbing activity and remain undisturbed until work is completed on the property so as to prevent damage to the tree. The Planning Board may require tree replanting either on the applicant's land or on land abutting the applicant's land, with the express written approval of the owner of such abutting land, where protected trees cannot be saved.

(8)

Protection of riparian buffers.

Riparian buffers, also known as a "vegetated buffer" or "forest buffers," are vegetated areas along a stream, usually forested, which help shade and partially protect a stream from the impact of adjacent land uses. Where possible, establish and protect a naturally vegetated buffer system along all perennial streams and other water features that encompass critical environmental features such as the 100-year floodplain, steep slopes (in excess of 15%), lake shorelands, and wetlands.

Riparian stream buffers should be preserved or restored with native vegetation. Buffers are most effective when maintained in an undisturbed condition; mowing and brush hogging should not take place within a buffer.

(9)

Mitigation of thermal impacts of stormwater runoff. Stormwater BMPs must mitigate potential temperature impacts of development and land use conversions within the Lake Cochichewick watershed. Elevated temperatures are caused by reduced shading in developed riparian areas, warming of stormwater as it runs over hot roofs and pavement, and heating of water stored in stormwater management ponds. Traditional peak reduction outlet structures and simple spillway outlets do nothing to cool the water before discharge. The Town's Planning Department has current maps of the watershed. Land disturbance permit sites located within the Lake Cochichewick watershed shall address the following additional design considerations.

To mitigate thermal impacts to the Lake Cochichewick watershed from stormwater, alternative BMPs to stormwater ponds, such as buffers, infiltration or under-drained filters should be used, or, if ponds are required, under-drained outlet structures can provide effective cooling.

Equally important to maintaining cool stream temperature is preservation and/or restoration of riparian trees and shrubs to provide shade. To the maximum extent feasible, trees and other existing vegetation shall be conserved. To the extent that existing vegetation cannot be conserved, new natural areas shall be established by planting additional vegetation, establishing no-mow zones, clustering tree areas, and using native plants in revegetation.

Performance standards for new development.

Stormwater management systems on new development shall be designed to meet an average annual pollutant removal equivalent to 90% of the average annual load of total suspended solids (TSS) related to the total post-construction impervious area on the site and 60% of the average annual load of total phosphorus (TP) related to the total post-construction impervious surface area on the site. Average annual pollutant removal requirements shall be achieved through one of the following methods:

Installing stormwater BMPs that meet the pollutant removal percentages required in Subsection B(1) based on calculations developed consistent with EPA Region 1's BMP Accounting and Tracking Tool (2016) or other BMP performance evaluation tool provided by EPA Region 1, where available. If EPA Region 1 tools do not address the planned or installed BMP performance, then any federally or state-approved BMP design guidance or performance standards (e.g., state stormwater handbooks and design guidance manuals) may be used to calculate BMP performance; or

Retaining the volume of runoff equivalent to, or greater than, 1.0 inch multiplied by the total post-construction impervious surface area on the new development site; or

Meeting a combination of retention and treatment that achieves the above standards.

Performance standards for redevelopment sites.

Stormwater management systems on redevelopment sites shall be designed to meet an average annual pollutant removal equivalent to 80% of the average annual post-construction load of total suspended solids (TSS) related to the total post-construction impervious area on the site and 50% of the average annual load of total phosphorus (TP) related to the total post-construction impervious surface area on the site. Average annual pollutant removal requirements shall be achieved through one of the following methods:

Installing BMPs that meet the pollutant removal percentages based on calculations developed consistent with EPA Region 1's BMP Accounting and Tracking Tool (2016) or other BMP performance evaluation tool provided by EPA Region 1, where available. If EPA Region 1 tools do not address the planned or installed BMP performance, then any federally or state-approved BMP design guidance or performance standards (e.g., state stormwater handbooks and design guidance manuals) may be used to calculate BMP performance; or

Retaining the volume of runoff equivalent to, or greater than, 0.8 inch multiplied by the total post-construction impervious surface area on the redeveloped site; or

Meeting a combination of retention and treatment that achieves the above standards; or

Partial redevelopment. If both new development and redevelopment are proposed for a project site, the redevelopment work shall be conducted in accordance with the provisions in this section and the new development work shall be conducted in accordance with the provisions in § 250-23B.

Performance standards for critical areas.

Public drinking water supply sources (Class A and Class B surface waters used for drinking water) or their tributaries are considered critical areas, and projects which discharge to these areas must provide pretreatment and spill control measures to stormwater discharges to the extent feasible.

Zones A and B of Lake Cochichewick, the Watershed Protection District surrounding the lake in accordance with the Town of North Andover Chapter 195, Zoning Bylaw and Zoning Map, vernal pools, watersheds of impaired waters, and waters subject to a TMDL are considered critical areas.

Direct discharges to Class A waters should be avoided to the extent feasible.

For projects within the Lake Cochichewick, Merrimack River, and Shawsheen River watershed areas, suitable water quality BMPs must be selected in consideration of the sensitivity and/or impaired status of the receiving water body.

Stormwater management design standards.

Projects must be designed to collect and dispose of stormwater runoff from the project site in accordance with Massachusetts Stormwater Management Standards, the Small MS4 General Permit, Town of North Andover Department of Public Works requirements, and the Subdivision of Land Regulations, recognized engineering methodologies and these regulations with an emphasis on including LID techniques in the design. In case of conflicting requirements with applicable federal, state, and local regulations, the more restrictive or more protective of human health and the environment shall take precedence.

Projects must incorporate source controls of contaminants and employ best management practices (BMPs) to minimize stormwater pollution.

Projects must manage surface runoff so that no proposed flows are conducted over public ways, nor over land not owned or controlled by the applicant unless a drainage easement in proper form is obtained permitting such discharge.

Projects must use LID where adequate soil, groundwater and topographic conditions allow. These may include but not be limited to reduction in impervious surfaces, disconnection of impervious surfaces, bioretention (rain gardens), and infiltration systems.

The use of one or more LID site design measures by the applicant may allow for a reduction in the water quality treatment volume required by these regulations. The applicant may, if approved by the Planning Board, take credit for the use of stormwater LID measures to reduce some of the requirements specified in these regulations. The site design practices that qualify for these credits and procedures for applying and calculating credits are identified in the Massachusetts Stormwater Handbook.

Basins shall follow natural landforms to the greatest extent possible or be shaped to mimic a naturally formed depression. The perimeter of all surface basins shall be curvilinear so that from most edges of the basin, the whole basin will not be in view. A more traditionally shaped (oval or rectangular) basin may be permitted when conditions such as topography, parcel size, or other site conditions warrant.

Inlets and outlets of surface basins shall be placed to maximize the flow path through the facility. At a minimum, the flow path shall be twice as long as wide. Baffles, pond shaping or islands can be added within the permanent pool to increase the flow path. If there are multiple inlets, the length-to-width ratio shall be based on the average flow path length for all inlets.

(8)

Low flow outlets shall be designed to prevent clogging.

(9)

Annual groundwater recharge rates shall be maintained by promoting infiltration through the use of structural and nonstructural methods. At a minimum, annual recharge from the post-development site shall mimic the annual recharge from the predevelopment site condition.

(10)

Any site that was wooded within the last five years must be considered undisturbed woods for all preconstruction runoff conditions, regardless of clearing or cutting activities that may have occurred on the site during that preapplication period.

(11)

Projects must use TR-55 and TR-20 methodologies to calculate peak rate and volume of runoff from predevelopment to post-development conditions.

(12)

The stormwater runoff volume to be recharged to groundwater should be determined using the methods prescribed in the latest version of the Massachusetts Stormwater Management Handbook.

(13)

The recharge volume criteria do not apply to any portion of a site designated as a stormwater hotspot. "Hotspots" are defined as sites with higher potential pollutant loads, based on the Massachusetts Stormwater Management Handbook.

(14)

The Planning Board may alter or eliminate the recharge volume requirement if the site is situated on unsuitable soils (i.e., marine clays), karst or in an urban redevelopment area. In this situation, nonstructural practices (filter strips that treat rooftop or parking lot runoff, sheet flow discharge to stream buffers, and grass channels that treat roadway runoff) should be implemented to the maximum extent practicable and the remaining or untreated volume included in the water quality volume.

(15)

Stormwater management systems shall be designed to avoid disturbance of areas susceptible to erosion and sediment loss to the greatest extent practicable, including: the damaging of large forest stands; building on steep slopes (15% or greater); and disturbing land in wetland buffer zones and floodplains.

(16)

Watershed area for hydrologic analysis and BMP sizing calculations must include at a minimum the site area and all upgradient areas from which stormwater runoff flows onto the site.

(17)

For purposes of computing runoff, all pervious lands in the site are assumed prior to development to be in good hydrologic condition regardless of the conditions existing at the time of the computation. Off-site areas should be modeled as present land use condition in good hydrologic condition.

(18)

Length of sheet flow used for times of concentration is to be no more than 50 feet.

(19)

Utilize the twenty-four-hour rainfall data taken from National Oceanic and Atmospheric Administration Atlas 14, Precipitation-Frequency Atlas of the United States (Vol. 10, Northeastern States, published 2015, revised 2019), as it may be amended.

(20)

Soils tests to be conducted by a registered professional engineer or Massachusetts soil evaluator, performed at the location of all proposed LID techniques and BMPs, to identify soil descriptions, depth to estimated seasonal high groundwater, depth to bedrock, and soil texture.

(21)

The design infiltration rate shall be determined from the on-site soil texture and Rawls rates as published in the Massachusetts Stormwater Handbook or saturated hydraulic conductivity tests.

(22)

Size drainage pipes to accommodate the twenty-five-year storm event and maintain velocities between 2.5 and 10 feet per second and provide calculations using the Mannings Equation.

(23)

Size drainage swales to accommodate the twenty-five-year storm event and velocities below four feet per second.

(24)

Size culverts to accommodate the fifty-year storm event and design adequate erosion protection. Design stream crossing culverts in accordance with the latest addition of the Massachusetts Stream Crossing Handbook.

(25)

Size stormwater basins to accommodate the 100-year storm event with a minimum of one foot of freeboard.

(26)

All drainage structures are to be able to accommodate HS-20 loading.

(27)

Catch basins structures are to be constructed as required by the Town of North Andover Chapter 255, Subdivision of Land Regulations, and spaced a maximum of 300 feet apart in roadways.

(28)

Catch basins on grades over 6% and at the intersections with arterial streets shall be built with vertical granite curb inlets with curb transition sections as required by the Town of North Andover Chapter 255, Subdivision of Land Regulations.

(29)

Supplementary catch basins shall be provided as required adjacent to catch basins at low points to prevent ponding if the low point basin grate and inlet become clogged as required by Chapter 255, Subdivision of Land Regulations, Appendix II, Drainage Installation Specifications.

(30)

All drainpipes are to be reinforced concrete pipe and have a minimum diameter of 12 inches.

(31)

Outfalls are to be designed to prevent erosion of soils, and pipes 24 inches or larger are to be fitted with grates or bars to prevent ingress.

(32)

Drainage easements are to provide sufficient access for maintenance and repairs of system components and be at least 20 feet wide.

(33)

Minimize permanently dewatering soils by:

Limiting grading within four feet of seasonal high groundwater elevation (SHGWE);

Raising roadways to keep roadway section above SHGWE; and