[HISTORY: Adopted by the Earth Removal Stormwater Advisory
Committee 4-14-2015.[1] Amendments noted where applicable.]
A.
Stormwater management permits are separate from other development
permits granted by other Town authorities. Projects that must obtain
permits from the Planning Board, Conservation Commission, Zoning Board
of Appeals, or other Town approving authority may also be required
to obtain a stormwater management permit.
B.
Full stormwater management permits. You must file an application
for a full stormwater management permit for any of the following activities:
C.
Limited stormwater management permit. You must file an application
for a limited stormwater management permit for any of the following
activities:
A.
Two copies of a completed application form signed by the applicant
and the landowner.
E.
Operation and maintenance plan for temporary and permanent erosion
control measures.
F.
Full stormwater management permit application shall contain, at a
minimum:
(1)
Two copies of the site plan, prepared and stamped by a registered
professional engineer. The site plan shall include the following information:
(a)
A locus map, legend, and North arrow;
(b)
A scale showing one inch equals 40 feet or less;
(c)
Title blocks identified by project name or parcel;
(d)
Delineation of the total land area to be disturbed;
(e)
Boundary lines of lots and delineation of land area where activity
is to be performed;
(f)
Contours showing existing and proposed elevations and/or contours
at one-foot intervals;
(g)
Symbols for sediment and erosion control practices, including
detail and elevation;
(h)
Existing and proposed roadways;
(i)
Delineation of wetland resources pursuant to the Code of Groton
and the Wetlands Protection Act regulations;
(j)
Delineation of the one-hundred-year floodplain, as shown on
the Federal Emergency Flood Insurance Rate maps;
(k)
Street profiles showing proposed and existing grades and elevations;
(l)
Proposed stormwater management systems;
(m)
All bodies of water, including streams, rivers, lakes and ponds,
within 100 feet of the proposed activity;
(n)
Location of all existing underground utilities [Reminder: The
applicant must call Dig Safe at (888) 344-7233 prior to commencement
of construction.];
(o)
A description of construction activities, in sequence, which
specifies the expected date of soil stabilization and completion;
(p)
Temporary and permanent soil erosion and sediment control measures;
(q)
Temporary and permanent seeding and other vegetative controls;
and
(r)
A survey of existing vegetation, including the following information:
[1]
Major upland vegetation located on the site, including trees,
shrub layer, ground cover and herbaceous vegetation;
[2]
Location of all trees with a caliper 12 inches or larger, noting
specimen trees and forest communities; and
[3]
Line clearly showing the limit of vegetation clearing and specimen
trees to be saved.
(2)
If permanent or temporary structural soil erosion control, sediment
control, or stormwater BMPs are required, the detailed engineering
plans shall include information on these measures, including but not
limited to:
(a)
Detailed location in plan view;
(b)
Spillway or outlet control designs showing calculated stage
elevations;
(c)
Emergency spillway designs showing a section with one-hundred-year
storm stage elevation;
(d)
Notes and construction specifications;
(e)
Type and size of sediment basin or trap;
(f)
Volume of storage required;
(g)
Pipe lengths, materials, and sizes;
(h)
Storage depth below outlet or cleanout elevation;
(i)
Embankment height and elevation; and
(j)
Location of any portable safety fences surrounding any basin
or trap.
G.
Limited stormwater management permit. Two copies of the plan showing
the parcel or parcels on which the activity is to take place. The
plan shall be drawn to scale and show, at a minimum, the following
information:
(1)
A survey of the parcel or parcels on which the activity is to take
place. If a survey plan is not available, a copy of the Assessors
Map or sewage disposal system plan may be used.
(2)
Any proposed soil erosion and sediment control measures.
(3)
Any existing or proposed roadways, driveways or parking areas.
(4)
Any areas subject to protection under the Wetlands Protection Act, MGL C. 131, § 40, or Chapter 215, Wetlands, of the Code of the Town of Groton.
(5)
Existing building or buildings to be constructed.
(6)
Any proposed grading or changes in topography.
(7)
Location of a stabilized construction entrance.
(8)
Proposed limits of vegetation clearing.
(9)
Location of all existing underground utilities. [Reminder: The applicant
must call Dig Safe at (888) 344-7233 prior to commencement of construction.]
[Amended 3-21-2017]
A.
Limited stormwater management permit. The filing fee for administration
(public hearing notice, mailings, etc.) of a limited stormwater permit
is $100, check made payable to the Town of Groton. This fee is deposited
into a revolving fund established for this purpose. There is no project
review fee unless the Committee determines that technical expertise
is required to review the application.
B.
Full stormwater management permit. The filing fee for administration
(public hearing notice, mailings, construction inspections, etc.)
of a full stormwater management permit is $500, check made payable
to the Town of Groton. This fee is deposited into a revolving fund
established for this purpose.
C.
Project review fee. The cost for the consultant to review the project
shall be the amount deposited into the revolving fund; the applicant
is responsible for the cost of the project review. The Committee shall
utilize the following procedure:
(1)
Request a written estimate from the consultant on the cost to review
the project, including the cost to attend meetings with the design
engineer and public hearing(s), if necessary;
(2)
The Committee shall vote to accept the estimate with a provision
that the cost to review the project shall not exceed the estimate
without written approval from the Committee; and
(3)
The Committee shall inform the applicant of the cost estimate for
project review and of any additional costs that may be incurred.
(4)
The applicant shall submit a check made payable to the Town of Groton
for the project review fee prior to the commencement of the project
review by the consultant.
A.
A full or limited stormwater management permit shall be valid for
two years from the date the permit is issued. If a project's
construction is not completed within one year from the date the permit
is issued, it is the responsibility of the applicant to request an
extension. The Committee may grant extensions for additional time,
provided that the applicant submits a written request for renewal
no later than 30 days prior to expiration of the permit.
B.
All projects that receive a permit must obtain a certificate of completion
at the completion of construction and BMP installation; however, the
permit will remain in effect for projects with long-term operation
and maintenance conditions.
A.
When the construction of a project is completed, the applicant shall request that the Committee conduct a final inspection. For full stormwater management permits, the applicant must submit a statement from a registered professional engineer certifying that the project was completed in accordance with the approved plans and construction conditions of the permit. The applicant shall also submit an on-the-ground surveyed as-built plan prepared by a professional land surveyor. The Committee shall determine whether the project complies with the approved plans, construction conditions of the permit, and Chapter 198, Stormwater Management. If completion is satisfactory, the Committee shall issue a certificate of completion.
B.
Operation and maintenance of stormwater BMPs is critical to the intended post-construction performance of the stormwater management system and is therefore critical that the completion of a project include ongoing operation and maintenance. To ensure that the stormwater BMPs are operated and maintained according to the long-term operation and maintenance plan pursuant to Chapter 198, Stormwater Management, the permit conditions may include annual submission of inspection maintenance logs for all stormwater BMPs. The inspection maintenance logs shall be submitted to the Committee no later than June 1 annually.
A.
It is critical to the performance and function of stormwater BMPs
that they are maintained properly; therefore, long-term operation
and maintenance permit conditions may be required to ensure the ongoing
maintenance of the stormwater BMPs according to the long-term operation
and maintenance plan.
B.
All projects that receive a full stormwater permit will include long-term
operation and maintenance conditions that are separate from construction
conditions unless the project is:
(1)
A single-family house.
(2)
A housing development and redevelopment projects comprised of detached
single-family dwellings on four or fewer lots, provided that there
are no stormwater discharges to a critical area.
(3)
A multifamily housing development and redevelopment projects with
three or fewer units, including condominiums, cooperatives, apartment
buildings and townhouses, provided that there are no stormwater discharges
to a critical area.
C.
Construction permit conditions and operation and maintenance conditions
shall be separately listed on the permit document. The certificate
of completion only applies to completion of construction and construction-related
conditions. It does not apply to the long-term operation and maintenance
conditions.
D.
After a project is issued a certificate of completion, the responsible
party or owner as listed in the long-term operation and maintenance
plan shall submit annual BMP inspection and maintenance logs to the
Committee and if applicable an updated long-term operation and maintenance
plan on or before June 1.
Massachusetts promulgated regulations that include Stormwater
Management Standards (formerly the Massachusetts Stormwater Policy),
through amendments to 310 CMR 10.00, Wetlands Protection Regulations,
and 314 CMR 9.00, 401 Water Quality Certification for Discharge of
Dredged or Fill Material, Dredging, and Dredged Material Disposal
in Waters Within the Commonwealth. Recognizing that the Massachusetts
Stormwater Management Standards shall be met for all projects within
the jurisdiction of these regulations, and that the Massachusetts
Stormwater Handbook, which includes additional guidance to these regulations,
may change over time, all stormwater management designs must meet
the design criteria or standards in the Massachusetts Wetland Protection
Regulations and 401 Water Quality Certification Regulations or within
this manual, whichever is more stringent in the protection of the
Town's environmental and infrastructure resources and as authorized
through any permitting agencies under whose purview the project falls.
These regulations apply to land-disturbing activities as outlined in Chapter 198, Stormwater Management — Low-Impact Development. The land-disturbance thresholds apply to development or redevelopment involving multiple separate activities in discontinuous locations or on different schedules that all together exceed the disturbance thresholds under § 198-4, Applicability.
A.
Low-impact development (LID)/green infrastructure techniques must be incorporated into development and redevelopment projects in the Town unless it can be shown per § 352-9A(1) that the use of LID techniques is not feasible. Applicants must use decentralized systems that involve the placement of a number of small treatment and infiltration devices located close to the various impervious surfaces that generate stormwater runoff in place of a centralized system comprised of closed pipes that direct all drainage from the entire site into one large detention basin.
(1)
The applicant must demonstrate to the Committee that the use of LID
techniques is not feasible given existing site constraints. The Committee
or its designated agent will determine if the documents submitted
are adequate to demonstrate the use of LID techniques is not feasible.
B.
The site planning process shall be documented and submitted to the
Committee and shall include the following steps:
(1)
Perform site analysis: identify and map important natural features
such as streams and drainageways, floodplains, wetlands, recharge
groundwater protection areas, high-permeability soils, steep slopes
and erosion-prone soils, woodland conservation areas, farmland, and
meadows.
(2)
Layout preferred development scenario: prepare preferred site development
layout that minimizes total impervious area, reflects the existing
topography, and uses existing hydrologic features. Potential layout
may consider cluster development, parking garages, taller buildings,
reduced road widths, smaller parking areas, permeable paving, and
green roofs. Roadway layouts shall minimize disturbance of natural
drainage patterns by following existing grades.
(3)
Create a decentralized stormwater system: manage runoff at the source
to the extent practical through the use of small decentralized structures,
such as swales, bioretention areas, infiltration structures, filter
strips, rain barrels, cisterns, dry wells, and vegetated areas. Increase
the time of concentration (average time for rainfall to reach a point)
by using open, vegetated drainage systems and maximizing overland
or sheet flow.
No stormwater runoff generated from land development and redevelopment
shall be discharged directly to a wetland, local water body, municipal
drainage system, or abutting property, without treatment in compliance
with these criteria.
A.
Impervious and disturbed surfaces from development alter the natural
hydrologic cycle by discharging stormwater directly to streams, rather
than allowing it to infiltrate through the soils and into groundwater
as it did before development. This increases flooding and reduces
the base flow to streams that is needed in the summer months when
there is little precipitation. The increased runoff from impervious
surfaces also increases stream temperatures, since pavement and other
impervious surfaces absorb substantial amounts of heat in the summer
due to their dark coloring and lack of shade, which is transferred
to runoff passing over the surface. The result is runoff that is dramatically
warmer than natural groundwater inflow would have been under a natural
hydrologic cycle. The purpose of these criteria is to maintain existing
recharge rates to preserve existing groundwater levels and stream
base flows and temperatures.
B.
The volume of water to be recharged shall be based on the site soils.
The volume of water to be retained from the developed site shall be
calculated using the following equation:
Rev = [(S)(TIA)]/12
|
Where:
| ||||
Rev
|
=
|
Recharge volume (cubic feet).
| ||
TIA
|
=
|
Total impervious area (square feet).
| ||
S
|
=
|
Soil-specific recharge factor (inches).
|
Hydrologic Group
|
Soil-Specific Recharge Factor
| |
---|---|---|
A
|
0.60
| |
B
|
0.35
| |
C
|
0.25
| |
D
|
0.10
|
C.
The following criteria shall also apply:
(1)
All recharge systems must receive pretreatment prior to recharge. All pretreatment devices must meet the criteria outlined under § 352-12.
(2)
The applicant must submit all recharge calculations. The capture
area adjustment calculations described in the Massachusetts Stormwater
Management Standards must be applied if runoff from all impervious
surfaces is not directed to the treatment BMPs. In no case shall runoff
from less than 65% of the site's impervious cover be directed
to the BMPs intended to infiltrate the required recharge volume.
(3)
Compaction of soils in designated recharge areas must be minimized
during and after construction.
(4)
If more than one soil type is present at the site, a composite soil-specific
recharge factor shall be computed based on the proportion of total
site area within each soil type. The recharge volume provided at the
site shall be directed to the most permeable soil available.
(5)
To qualify as a treatment BMP, a recharge system must discharge to
soils with infiltration rates less than or equal to 2.4 inches per
hour when used as a treatment BMP.
(6)
At least 44% of the TSS must be removed prior to discharge to an
infiltration structure used for treatment if the discharge is:
(a)
Within a Zone II or interim wellhead protection area;
(b)
Near an outstanding resource water or special resource water;
(c)
Near a shellfish growing area, cold-water fishery, or bathing
beach; or
(d)
From a land use with higher potential pollutant loads as defined
in the Massachusetts Stormwater Handbook.
(7)
At least 80% of the TSS must be removed prior to discharge to an
infiltration structure used for recharge if the discharge is within
an area with a rapid infiltration rate greater than 2.4 inches per
hour.
(8)
The Committee may alter or eliminate the recharge volume requirement
if the site is situated on unsuitable soils (i.e., marine clays) or
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.
(9)
The system should be designed based on the infiltration rates in
Table 1, using the soil evaluation process outlined in Volume 3 of
the Massachusetts Stormwater Handbook:
Table 1. Hydrologic Soil Properties Classified by USDA
Soil Texture
| ||||
---|---|---|---|---|
Texture Class
|
Hydrologic Soil Group
|
Design Infiltration Rate (f)
(inches per hour)
|
Remarks
| |
Sand
|
A
|
8.27
| ||
Loamy sand
|
A
|
2.41
| ||
Sandy loam
|
B
|
1.02
| ||
Loam
|
B
|
0.52
| ||
Silt loam
|
C
|
0.27
| ||
Sandy clay loam
|
C
|
0.17
| ||
Clay loam
|
D
|
0.09
|
Use Group D soils when estimating pre- and post-development
annual recharge. However, do not locate recharge BMPs within areas
with Group D soils.
| |
Silty clay loam
|
D
|
0.06
| ||
Sandy clay
|
D
|
0.05
| ||
Silty clay
|
D
|
0.04
| ||
Clay
|
D
|
0.02
|
Source: Rawls, Brakensiek and Saxton, 1982
|
(10)
Underground recharge systems may only be used to recharge stormwater
runoff directly from rooftops. They may not be used to recharge stormwater
runoff from other surfaces due to sediments in the runoff that may
cause clogging of the recharge system and difficulty to rehabilitate
these systems once they have failed.
(11)
All infiltration structures used for treatment must be designed
using the "Static" Method, which requires the structure to hold the
entire water quality or recharge volume (as applicable) and does not
allow credit for exfiltration as the structure is filling.
(12)
The "Dynamic" Method of sizing infiltration structures assumes
stormwater exfiltrates into the groundwater as the structure is filling
and may be used for roof runoff devices and recharge devices that
follow full treatment to remove 80% TSS from the stormwater runoff.
(13)
All units/devices shall be designed to drain within 72 hours
from the end of the storm.
(14)
A mounding analysis must be performed when the vertical separation
from the bottom of an exfiltration system to seasonal high groundwater
is less than four feet and the recharge system is proposed to attenuate
peak discharge from a ten-year or higher twenty-four-hour storm. The
mounding analysis must demonstrate that the recharge volume is fully
dewatered within 72 hours and that the groundwater mound that forms
under the recharge system will not break out above the land or water
surface of a wetland. The Hantush or other equivalent method may be
used to conduct the mounding analysis.
(15)
Recharge shall not be concentrated to one area. It shall be
distributed to multiple areas throughout the site.
A.
Pretreatment basins must be designed and located to be easily inspected and accessible to facilitate maintenance. Pretreatment devices must also be sized to accommodate a minimum of one year's worth of sediment and debris per § 352-12B(6).
B.
The following criteria shall be followed to ensure that the device
will permit sufficient treatment of stormwater and allow for a reasonable
required maintenance frequency for the stormwater treatment system
(STS):
(1)
Pretreatment devices shall be provided for each STS; and
(2)
Pretreatment devices shall be designed to accommodate a minimum of
one year's worth of sediment; and
(3)
Pretreatment devices for bioretention areas should include a grass
buffer or transition zone between pavement and bioretention (e.g.,
crushed stone entrance, cobbles) that allows sediment to drop out
before entering the treatment device; and
(4)
Pretreatment devices shall be designed to capture anticipated pollutants,
such as oil and grease; and
(5)
Pretreatment devices shall be designed and located to be easily accessible
to facilitate inspection and maintenance; and
(6)
Pretreatment structures shall be sized to hold an annual sediment
loading. An annual sediment load shall be calculated using a sand
application rate of 500 pounds/acre for sanding of roadways, parking
areas and access drives within the subcatchment area, a sand density
of 90 pounds per cubic foot and assuming a minimum frequency of 10
sandings per year. To obtain an annual sediment volume, perform the
following calculation:
Area to be sanded (acres) x 500 pounds/Acre-Storm ÷ 90
pounds/ft3 x 10 storms/year = cubic feet
of sediment/year
|
(7)
The developer shall maintain any STSs used to trap sediment during
construction to prevent sediment from leaving the site and shall remove
all sediment from all STSs when construction is finished and the site
is stabilized.
The following criteria shall be followed to control peak discharge
rates and improve the overall effectiveness of the stormwater treatment
systems. These are minimum design criteria.
A.
At each analysis point, the post-development peak discharge rate
shall be equal to or less than the pre-development peak discharge
rate (based on a two-year, ten-year, twenty-five-year, twenty-four-hour
storm); and
B.
The one-hundred-year, twenty-four-hour storm event must be evaluated
to demonstrate that there will not be increased flooding impacts off
site. If this evaluation shows that increased off-site flooding will
result from peak discharges from the one-hundred-year, twenty-four-hour
storms, BMPs must also be provided to attenuate these discharges;
and
C.
The site shall be designed to ensure that all runoff from the site,
up to the maximum design storm for the particular structure, will
actually enter the control structure. For example, the control structure
may be designed for the twenty-five-year storm, while the drainage
system may only be sized to handle a ten-year storm, with larger storms
flooding the distribution system and traveling overland. This overland
flow, or overflow, must be directed into the peak control structure;
and
D.
For each design storm, the applicant shall account for all run-on
and run-off (including off-site impacts) in both pre- and post-development
conditions; and
E.
Emergency spillways shall be designed for the full range of design
storms assuming the primary outlet structure is not functioning; and
F.
Use SCS methods (TR-20 or TR-55) to develop hydrographs and peak
flow rates for the proposed development site. The hydrograph time
interval (dT) in TR-20 should be no greater than 0.1 hour. All areas
shall be accounted for in the pre-/post-runoff calculations. The total
tributary area that contributes flow from the proposed site, including
runoff entering the site through piped drainage or surface runoff
from off-site sources, shall be included even if a portion does not
contribute flow to the BMP. The objective is for the development's
storm drain design to account for total runoff leaving the site; and
G.
Use curve number (CN) values as provided in Table 2 to calculate
stormwater runoff rates for pre-/post-construction ground surface
conditions; and
H.
Any site that was wooded within the last five years shall be considered
undisturbed woods for all pre-construction runoff conditions, regardless
of clearing or cutting activities that may have occurred on the site
during that preapplication period; and
I.
Off-site areas should be modeled as "present land use condition"
in good hydrologic condition for the design storm events for both
pre- and post-development calculations; and
J.
The length of overland sheet flow used in time of concentration (tc)
calculations shall be limited to no more than 50 feet for pre- and
post-development conditions.
Table 2
Approved CN Values for the SCS Methods (TR-20, TR-55)
| |||||
---|---|---|---|---|---|
Hydrologic Soil Group
| |||||
Pre-Construction Runoff Curve Number (CN Values)
|
A
|
B
|
C
|
D
| |
Open space, such as lawns, parks, and cemeteries2
|
68
|
79
|
86
|
89
| |
Woods and forest3
|
30
|
55
|
70
|
77
| |
Impervious areas, such as paved parking lots, driveways and
roofs
|
98
|
98
|
98
|
98
| |
Gravel roads (processed, dense graded)
|
76
|
85
|
89
|
91
| |
Dirt roads
|
72
|
82
|
87
|
89
| |
Newly graded pervious areas (no vegetation)
|
77
|
86
|
91
|
94
|
Post-Construction Runoff-Curve Number (CN Value)
|
A
|
B
|
C
|
D
| |
---|---|---|---|---|---|
Open space, such as lawns, parks, and cemeteries2
|
68
|
79
|
86
|
89
| |
Woods and forest that is selectively cleared3
|
43
|
65
|
76
|
82
| |
Impervious areas, such as paved parking lots, driveways and
roofs
|
98
|
98
|
98
|
98
| |
Gravel roads (processed, dense graded)
|
76
|
85
|
89
|
91
| |
Dirt roads
|
72
|
82
|
87
|
89
| |
Newly graded pervious areas (no vegetation)
|
77
|
86
|
91
|
94
| |
Source: TR-55, 1986
|
NOTES:
| ||
---|---|---|
1.
|
The runoff curve numbers are for use in calculating runoff with
TR-55 or other approved models.
| |
2.
|
The open space CN values for lawns, parks, and cemeteries assumes
a "poor" condition for grass cover since the post-construction amount
of grass cover cannot be predicted or guaranteed.
| |
3.
|
The pre-construction CN value for woods and forest is based
on a "good" condition where the woods are undisturbed and brush adequately
covers the soil. The post-construction CN value for woods and forest
is based on a "fair" condition if any selective cutting is conducted
since the soils typically become compacted due to the equipment used
to remove the large white pines and there may be post-cutting wind
damage to the remaining unsupported canopy. If the applicant can demonstrate
that no disturbance will occur during construction, then the pre-construction
CN value for woods may be used for the post-construction runoff calculations.
A note should be placed on the plan indicating where selective cutting
will occur.
|
A.
Many stormwater management practices focus on controlling peak flow rates for larger storms, including the two-, ten- and one-hundred-year storms. This does not address the increased duration at which those high flows occur because of the increased volume of water from development compared to pre-development. For example, although the peak flows are kept the same, there is a much greater volume of water leaving the site under developed conditions and the streams have higher flows for longer durations than they did under predevelopment conditions. In addition, because the impervious development has limited recharge, base flow during non-storm event times is lower. The purpose of this criterion is to limit the total amount of time that a receiving stream exceeds an erosion-causing threshold based on pre-developed conditions. The following method shall be used: 24 hours extended detention of the post-development one-year, twenty-four-hour return frequency storm event pursuant to § 352-14A(1). This is not required for direct discharges to a lake or fourth-order stream. However, the recharge and water quality treatment requirements outlined above will still be required for these discharges.
(1)
The one-year, twenty-four-hour return frequency storm event (one-year
storm) shall be detained for an additional 24 hours longer than the
predevelopment runoff discharge time.
A.
Stormwater treatment devices shall be used to handle water quantity
as well as treat water quality. The water quality volume shall be
calculated as:
WQV = [(1 inch)*(TIA)]/12
|
Where:
| ||||
WQV
|
=
|
Water quality volume (ft3).
| ||
TIA
|
=
|
Total impervious area (square feet).
| ||
12
|
=
|
Conversion factor (inches per foot).
|
B.
If discharging to a cold-water fishery, the stormwater water quality
volume must be treated through either infiltration, an underdrain
discharge system, or gravel wetland that allows the water to cool
before being discharged. An underdrain system works by forcing the
stormwater through a soil filter located above a gravel-packed drain
and allowing for slow release of the water. The cool soils and gravel
help to cool the water before it is discharged.
A.
Stormwater management systems shall be designed to remove 80% of
the average annual post-construction load of total suspended solids
(TSS).
B.
If a stormwater discharge is located within the watershed of a water
body with a total maximum daily load (TMDL), pollution prevention
and structural BMPs that are consistent with the TMDL must be selected.
The applicant shall submit a narrative describing how they will meet
the requirements of the TMDL.
Stormwater discharges to critical areas, as defined in the Massachusetts
Stormwater Handbook, shall use specific source control and pollution
prevention measures and the specific structural stormwater best management
practices outlined in the Massachusetts Stormwater Handbook.
Redevelopment projects must meet the same criteria as new development
to the maximum extent practicable. At a minimum, existing stormwater
conditions must be improved, including reduction of peak rates, reduction
of discharge volume, increased recharge, and increased water quality
treatment.
The following criteria shall be met for erosion control, prior
to any land disturbance activities commencing on the site:
A.
Development shall be oriented to the site so that the cutting and
stripping of vegetation and grading are minimized.
B.
Prior to any land disturbance activities commencing on the site,
the developer shall physically mark limits of no land disturbance
on the site with tape, signs, or orange construction fence, so that
workers can see the areas to be protected. These areas must be inspected
by a representative of the Committee before clearing commences.
C.
Appropriate erosion and sediment control measures shall be installed
prior to soil disturbance. Measures shall be taken to control erosion
within the project area. Sediment in runoff water shall be trapped
and retained within the project area. Wetland areas and surface waters
shall be protected from sediment. Erosion controls must be inspected
by a representative of the Committee before clearing commences.
D.
Runoff shall be controlled and conveyed into storm drains and other
outlets so it will not erode the land or cause off-site damage; sediment
in runoff shall be trapped by using staked straw bales, silt fencing,
or sedimentation traps, or other approved erosion control devices.
E.
Sediment basins shall be constructed where necessary to detain runoff
and to trap sediment during construction.
F.
Sediment shall be removed once the volume reaches 1/4 the height
of the silt fence or straw bale.
G.
Off-site runoff shall be diverted from highly erodible soils and
steep slopes to stable areas.
H.
Erosion and sediment controls shall be coordinated with the sequence
of grading, development and construction operations; control measures
shall be in effect prior to commencement of each increment/phase of
the process.
I.
Land disturbance activities exceeding two acres in size shall not
be disturbed without a sequencing plan that requires stormwater controls
to be installed and the soil stabilized, as disturbance beyond the
two acres continues. Mass clearings and grading of the entire site
should be avoided. Prior to any construction on the site, the applicant
shall submit a construction phasing plan to the Committee for review
and approval. The plan shall show how construction will proceed with
a minimum of disturbance at any one time and shall specify approximate
dates for initial disturbance and final stabilization for each phase.
J.
Soil and other materials shall not be stockpiled or redistributed,
either temporarily or permanently, in locations or in such a manner
as would cause suffocation of tree root systems.
K.
Topsoil shall be stripped from disturbed areas, stockpiled in approved
areas and stabilized with temporary vegetative cover if it is to be
left for more than 30 calendar days; perimeter sediment controls shall
be installed around each area of stockpiled topsoil.
L.
Soil stockpiles shall be stabilized or covered at the end of each
workday.
M.
The area of disturbance shall be kept to a minimum. Disturbed areas
remaining idle for more than 14 days shall be stabilized.
N.
Grading shall be kept to a minimum; tree removal shall be minimized.
O.
For active construction areas such as borrow or stockpile areas,
roadway improvements and areas within 50 feet of a building under
construction, a perimeter sediment control system shall be installed
and maintained to contain soil.
P.
A stabilized construction entrance shall be constructed at all entrance/exit
points of the site to reduce the amount of soil carried onto roadways
and off the site.
Q.
Dust shall be controlled at the site.
R.
On the cut side of roads, ditches shall be stabilized immediately
with rock riprap or other nonerodible liners or, where appropriate,
vegetative measures such as sod.
S.
All graded areas beyond the street right-of-way shall be covered
with four inches of topsoil and planted with a native species of vegetative
cover, sufficient to prevent erosion.
T.
Temporary seeding, mulching or other suitable stabilization methods
shall be used to protect exposed soil areas during construction; as
feasible, natural vegetation shall be retained and protected; during
the months of October through March, when seeding may be impractical,
an anchored mulch or sod shall be applied as approved by the Committee
or by the Committee's designee; diversions and/or prepared outlets
may be required in critical areas during construction.
U.
Permanent seeding should be undertaken in the spring from March through
May and in late summer and early fall from August to October 15. During
the peak summer months and in the fall after October 15, when seeding
is found to be impractical, an appropriate temporary mulch shall be
applied. Permanent seeding may be undertaken during the summer if
plans provide for adequate mulching and watering.
V.
Permanent vegetation and erosion control structures, as necessary,
shall be installed, preferably immediately after construction is completed
but otherwise no later than the first full spring season immediately
thereafter; they shall comply with the erosion and sedimentation vegetative
practices recommended by the U.S. Soil Conservation Service.
W.
Temporary ground cover or erosion/sedimentation controls shall be
established on any unbuilt lots as required by the Committee.
X.
Native species shall be used for revegetation.
Y.
All slopes steeper than 3:1 (h:v, 33.3%), as well as perimeter dikes,
sediment basins or traps, and embankments shall, upon completion,
be immediately stabilized with sod, seed and anchored straw mulch,
or other approved stabilization measures. Areas outside of the perimeter
sediment control as shown on the Erosion and Control Plan shall not
be disturbed.
Z.
Monitoring and maintenance of erosion and sediment control measures
throughout the course of construction shall be required. The applicant
shall submit to the Stormwater Committee a complete operation and
maintenance plan for temporary and permanent erosion control measures,
as part of the application package.
AA.
Temporary sediment trapping devices shall not be removed until permanent
stabilization is established in all contributory drainage areas. Similarly,
stabilization shall be established prior to converting sediment traps/basins
into permanent (post-construction) stormwater management facilities.
All facilities used as temporary measures shall be cleaned prior to
being put into final operation.
BB.
All temporary erosion and sediment control measures shall be removed
after final site stabilization. Disturbed soil areas resulting from
the removal of temporary measures shall be permanently stabilized
within 30 days. The applicant's engineer shall submit written
certification that this condition has been met.
Prior to the start of construction, the applicant must submit
a narrative addressing pollution prevention measures to be taken at
the site during the construction period. If the proponent is required
to have a NPDES permit, a copy of the SWPPP can be filed in lieu of
the narrative. The narrative must include emergency contact information
during construction activities.
There shall be no illicit discharges from the site. The applicant
shall submit an illicit discharge compliance statement verifying no
illicit discharges exist on the site. For redevelopment projects,
the applicant must provide a summary of the steps taken to verify
no illicit discharges.
A.
A long-term operation and maintenance plan (O&M plan) for the
permanent stormwater management system is required at the time of
application for all projects. The long-term O&M plan shall be
prepared by a professional engineer and at a minimum identify:
(1)
The names and addresses of the stormwater management system(s) owners;
(2)
The names and addresses of party or parties responsible for operation
and maintenance, including how future property owners will be notified
of the presence of the stormwater management system and the requirement
for proper operation and maintenance;
(3)
Emergency contact information in the event a BMP is failing and creating
a safety hazard;
(4)
Provisions to notify the appropriate reviewing entity of a change
in responsible party;
(5)
The person(s) responsible for financing maintenance and emergency
repairs, including contact information (address, phone number, e-mail
address);
(6)
The routine and nonroutine maintenance tasks to be undertaken after
construction is complete and a schedule for implementing those tasks,
including what actions trigger a maintenance task to be performed
and the frequency with which the tasks will be performed;
(7)
BMP-specific O&M activities for each component of the stormwater
system;
(8)
Recordkeeping consistent with DEP requirements, including log forms
that will be submitted to the Committee as part of the long-term operation
and maintenance permit conditions;
(9)
A list of easements with the purpose and location of each;
(10)
A plan that is drawn to scale and shows the location of all
stormwater BMPs in each treatment train along with the discharge point;
(11)
Include for informational purposes description and delineation
of public safety features;
(12)
Estimated operations and maintenance budget for the routine and nonroutine maintenance tasks outlined under Subsection A(6) above; and
(13)
The signature(s) of the owner(s).
B.
The Stormwater Committee shall make the final decision of what maintenance
option is appropriate in a given situation. The Stormwater Committee
will consider natural features, proximity of site to water bodies
and wetlands, extent of impervious surfaces, size of the site, slopes,
the types of stormwater management structures, and potential need
for ongoing maintenance activities when making this decision. Once
approved by the Stormwater Committee, the operation and maintenance
plan shall be recorded at the South Middlesex Registry of Deeds by
the applicant and shall remain on file with the Stormwater Committee
and shall be an ongoing requirement as part of the stormwater management
permit.
Driveways pursuant to Chapter 235 of the Code of the Town of Groton shall comply with the following:
A.
The stormwater runoff from all impervious areas shall not discharge
into the public right-of-way. The peak rate of runoff at the street
shall not increase from the predeveloped rate during the two-, ten-,
twenty-five- or one-hundred-year storm event, unless an increase is
authorized by the Board of Selectmen or its designee. Stormwater runoff
may discharge directly into rights-of-way of new subdivision roads
if the overall stormwater management plan for the street is designed
to accommodate the runoff and a catch basin is located within 10 feet
of the downslope side of the driveway. The purpose is to prevent ice
jams in the winter.
A.
Proposed residential, commercial, or industrial subdivisions shall
apply these stormwater management criteria to the land development
as a whole. Individual lots in new subdivisions shall not be considered
separate land development projects but rather the entire subdivision
shall be considered a single land development project. Hydrologic
parameters shall reflect the ultimate land development and shall be
used in all engineering calculations.
B.
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 Committee, 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.
C.
Design basis. Facility design shall accommodate the following:
(1)
Closed drainage system (storm sewers) and swales: twenty-five-year
Rational Formula storm.
(2)
Detention basins: one-hundred-year TR-20 or TR-55 Type III twenty-four-hour
storm event.
(3)
Culverts, other stream crossings shall be designed in accordance
with the Massachusetts Stream Crossings Handbook and Massachusetts
River and Stream Crossing Standards.
(4)
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 feet and 10 feet per second and not more than five feet per second on unpaved surfaces.
D.
Closed drainage system (storm sewers).
(1)
All drain pipes shall be at least 12 inches inside diameter and made
of reinforced concrete conforming to Massachusetts Department of Transportation
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 Transportation
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.
E.
(Reserved)
F.
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 forty-eight-inch or greater sump below pipe invert, and shall be constructed of precast concrete units. Manholes used shall be at least four feet in 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 Transportation Standard Specifications for Highways and Bridges 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 Chapter 381, Part 2, of the Subdivision Rules and Regulations,[1] Construction Appendix. Granite curb inlets shall be provided
at all catch basins located within the roadway.
G.
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 of 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)
|
H.
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.
I.
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.
J.
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:
(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.
The Conservation Commission shall enforce the following general
performance standards:
A.
Minimize pre-project to post-project changes in site hydrology.
B.
Pre-project and post-project hydrology should remain fundamentally
the same as it pertains to protecting wetlands functions and values.
Of course, some minor degree of change in hydrology is inevitable
in any engineering/construction project, and within reasonable limits,
the Commission shall permit such variation when, in its judgment,
such changes will not produce a significant impact on wetlands functions
and values. The use of aboveground vegetated swales, infiltration
methods, and other low-impact drainage designs will be given preference
over subsurface drainage structures.
C.
Erosion control may require limiting stormwater discharge volumes
and velocities. Therefore, the Commission may require the construction
of such stormwater control structures, and specify particular engineering
and design details, as it deems necessary to protect wetland resources,
values, and functions.
D.
Minimize change in runoff water quality. The Commission shall impose
conditions that, in its judgment, reduce undesirable water quality
changes to levels that will not harm wetland functions or values,
immediately or cumulatively. The Commission may require the construction
of specific structures to improve stormwater runoff quality, such
as wet detention basins for pollutant removal and broad riprap swales
for aeration.
E.
Requirements for hydraulic calculations. In accordance with the above,
the Conservation Commission shall require, as part of the application
for permit, complete hydrological calculations for the one-, two-,
five-, ten-, twenty-five-, and one-hundred-year storm events. Such
calculations shall include:
(1)
Runoff from all impervious surfaces associated with the project,
including individual lot construction; and
(2)
Both pre- and post-project calculations for discharge volumes, concentration
times, discharge velocities, and other quantities that the Commission
may require for complete information.
F.
Requirements for turtle and amphibian migration shall include:
A.
Bioretention design guidelines. Bioretention systems offer excellent
pollutant removal, while providing some retention of stormwater runoff.
Bioretention systems rely on the soil and planting media to remove
pollutants and provide absorption of stormwater runoff. The following
design standards should be followed to achieve the maximum benefit:
(1)
Construction components. Bioretention systems are constructed in
an excavated hole and consist of, from bottom up:
(a)
Optional stone reservoir to provide additional holding capacity.
(b)
A minimum three-inch-thick transition layer of 3/8-inch pea
gravel.
(c)
A layer of lightly compacted soil media at least 18 inches thick.
(d)
Vegetation consisting of dense plantings of woody shrubs, herbaceous
perennials and, if the soil media is deep enough for the roots, small
trees. The plants should be native if possible and must be both drought
and flood tolerant.
(e)
A cover of three inches of aged, fibrous bark mulch.
(f)
A maximum six-inch depression for surface stormwater storage.
(3)
Filter fabric. Filter fabric should not be used between the soil
media and native soils or soil media and transition stone/stone reservoir.
The use of fabric between these layers will cause system failure due
to clogging from fines located within the soil media. If an underdrain
is to be used to drain the bioretention area, then filter fabric can
be used between the underdrain bedding and the native soils to prevent
the migration of native soils into the underdrain bedding.
B.
Naturalized basin design guidelines.
(1)
Naturalized basins have several advantages over traditional basins,
including:
(a)
The deeper root systems of the native plant materials encourage
infiltration, recharging groundwater tables and increasing base flows.
(b)
The plants trap pollutants, increasing the water quality of
the discharge.
(c)
The vegetation serves to cool water temperatures and slow stormwater
velocities.
(d)
They are visually more attractive and can help beautify a neighborhood,
increasing property values.
(e)
They require less maintenance. Generally, annual mowing and
periodic trimming of trees and plants is sufficient.
(2)
The following design standards shall be followed:
(a)
The basin shall be easily accessible for maintenance.
(b)
Construct the basin with a sediment forebay at the inlet, sized
to hold a minimum of one year's worth of sediment accumulation
if no other pretreatment is proposed.
(c)
Construct the basin to have a natural low-flow channel with
turf reinforcement material to remove pollutants and prevent erosion.
(d)
Incorporate a naturally landscaped area at the ground surface.
The ground surface around the basin shall be large enough to be in
scale with the overall landscaped area. The purpose is to filter and
soften views from residential areas.
(e)
Plant all areas of the naturalized basin, including basin floors,
side slopes, berms, impoundment structures, or other earth structures,
with suitable native vegetation such as naturalized meadow plantings
or lawn grass specifically suited for stormwater basins. Suggested
plants include:
[1]
Grasses: big bluestem, switchgrass and wildflower mixes. In
wet areas, plant sweetflag and soft rush for color and texture.
[2]
Shrubs: red chokeberry (Aronia arbutifolia); silky dogwood (Cornus
ammomum); arrowwood (Viburnum dentatum); cranberrybush (Viburnum trilobum);
service berry/shadbush (Amelanchier cavadensis or laevis); and elderberry
(sambucus).
[3]
Trees: red maple (Acer rubrum); river birch (Betula nigra);
various willows.
(f)
Trees may not be planted below the pool area of the basin. If
shrubs are used, they must be adapted to wet or moist soil conditions.
(g)
Mulch may be used in shrub beds located within the pool area
with a non-floating-type mulch.
(h)
Group trees or shrubs to avoid a spotty effect.
(i)
Provide access to the basin for maintenance. Blend access area
in with the surrounding landscape to the extent feasible.
(j)
The forebay/sediment trap shall be at least 10 feet long and
sized to hold at least the annual sediment loading and must meet the
sizing requirements of the DEP Stormwater Standards.
(k)
Maintenance access shall be planted with grass and at least
10 feet wide with a maximum slope of 15% and a maximum cross slope
of 3%.
(l)
Provide a means to prevent soil compaction on the floor of the
basin during construction.
(m)
Size treatment storage area to hold the water quality volume.
(n)
The perimeter of all 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. Basins shall follow natural landforms to the greatest extent
possible or be shaped to mimic a naturally formed depression.
(o)
Place inlets and outlets 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.
(p)
Minimum one foot of freeboard above the twenty-five-year storm
elevation.
(q)
The interior slopes of the basin within the pool area shall
not exceed a slope of four horizontal to one vertical.
(r)
A minimum of six inches of topsoil with at least six-percent
organic content shall be provided for all planting ground cover beds
or lawn areas.
(s)
Low-flow outlets shall be designed to prevent clogging.
(t)
For basins that cannot infiltrate the water quality volume,
use a soil filter conforming to the following:
[1]
Impoundment depth: Peak storage depth within the filter area
for water quality volume may not exceed 18 inches.
[2]
Pipe layout and spacing: Layout of the pipe underdrain system
must be sufficient to effectively drain the entire filter area. There
must be at least one line of underdrain pipe for every eight feet
of the filter area's width. The slope of the pipe must be 1%
or greater.
[3]
Pipe bedding: Minimum 12 inches over top of drainage pipe, six
inches thick at sides, and six inches below drainage pipe of clean,
well-graded gravel.
[4]
Filter bed: The soil must consist of loamy, coarse sand. The
soil filter must extend across the bottom of the entire filter area.
The soil must be at least 18 inches deep and underlain by a gravel
bedding.
[5]
Surface cover: The top of the underdrain system must be covered
with a four-inch layer of sandy loam and then covered with plantings
consisting of species tolerant of frequent inundation.
[6]
Underdrain outlet: Each system must discharge to an area capable
of withstanding concentrated flows and saturated conditions without
eroding.
[Added 3-21-2017]
B.
Any applicant must submit a written request to be granted such a
waiver. Such a waiver request shall be accompanied by an explanation
or documentation supporting the waiver request and demonstrating that
strict application of this chapter does not further the purposes or
objectives of this chapter.
C.
All waiver requests shall be discussed by the Committee and a decision
shall be made by the Committee within 30 days of receiving the waiver
request.
D.
If, in the Committee's opinion, additional time or information
is required for review of a waiver request, the Committee may continue
consideration of the waiver request to a date certain announced at
the meeting. In the event the applicant objects to a continuance,
or fails to provide the requested information, the waiver request
shall be denied.