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.
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.
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.
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Table 2
Approved CN Values for the SCS Methods (TR-20, TR-55)
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Hydrologic Soil Group
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Pre-Construction Runoff Curve Number (CN Values)
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A
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B
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C
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D
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Open space, such as lawns, parks, and cemeteries2
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68
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79
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86
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89
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Woods and forest3
|
30
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55
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70
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77
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Impervious areas, such as paved parking lots, driveways and
roofs
|
98
|
98
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98
|
98
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Gravel roads (processed, dense graded)
|
76
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85
|
89
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91
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Dirt roads
|
72
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82
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87
|
89
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Newly graded pervious areas (no vegetation)
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77
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86
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91
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94
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Post-Construction Runoff-Curve Number (CN Value)
|
A
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B
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C
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D
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Open space, such as lawns, parks, and cemeteries2
|
68
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79
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86
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89
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Woods and forest that is selectively cleared3
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43
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65
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76
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82
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Impervious areas, such as paved parking lots, driveways and
roofs
|
98
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98
|
98
|
98
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Gravel roads (processed, dense graded)
|
76
|
85
|
89
|
91
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Dirt roads
|
72
|
82
|
87
|
89
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Newly graded pervious areas (no vegetation)
|
77
|
86
|
91
|
94
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Source: TR-55, 1986
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NOTES:
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1.
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The runoff curve numbers are for use in calculating runoff with
TR-55 or other approved models.
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2.
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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.
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3.
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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.
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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.
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.
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:
(1) Curbing incline no greater than 45° from the horizontal (with
the exception of "roundings" and catch basins).
(2) Hay bales are to be arranged in a staggered formation where appropriate.
This is to allow migration paths for amphibians.