Code of Maine Rules › 06 - DEPARTMENT OF ENVIRONMENTAL PROTECTION › 096 - DEPARTMENT OF ENVIRONMENTAL PROTECTION - GENERAL › Chapter 500 - STORMWATER MANAGEMENT › Appendix 096-500-F - Vegetated buffers

Current through 2024-29, July 17, 2024
This appendix applies to all projects using vegetated buffers
				  for stormwater control. A buffer is a natural vegetated, non-lawn area or areas
				  located down gradient from a project that serves to store and remove pollutants
				  from stormwater runoff flowing from a project. Buffers must not be interrupted
				  by intermittent or perennial stream channels or other drainageways and must
				  have a relatively uniform slope so that stormwater does not concentrate in
				  channels. This appendix describes the design and sizing requirements for
				  vegetated buffers designed to meet the general standards. Requirements are
				  described for four different types of buffers, each of which is appropriate for
				  specific situations.
1. Types of vegetated buffers.
The applicability
				  of each of the four types of vegetated buffers is as follows.
(a) Vegetated buffer with stone bermed level lip spreaders. A
				  vegetated buffer with stone bermed level lip spreaders must be used when
				  treating stormwater runoff from any of the following:
(i) An impervious area greater than one acre;
(ii) Impervious areas where the flow path across the
				  impervious area exceeds 150 feet; or
(iii) Developed areas, including lawns and impervious
				  surfaces, where runoff is concentrated, intentionally or unintentionally, so
				  that it does not run off in well-distributed sheet flow when it enters the
				  upper end of a buffer, except that road ditch runoff may be treated using a
				  ditch turn-out buffer.
(b) Buffer adjacent to the downhill side of a road. A buffer
				  located along the downhill side of a road may only be used when the runoff from
				  the road surface and shoulder sheets immediately into a buffer. In no instance
				  may runoff from areas other than the adjacent road surface and shoulder be
				  directed to these buffers.
(c) Ditch turn-out buffer. A ditch turn-out buffer may only
				  be used when runoff from a road ditch is diverted to a 20-foot stone bermed
				  level lip spreader that distributes runoff into a buffer. No areas other than
				  the road surface, road shoulder and road ditch may be directed into a buffer.
				  No more than 400 feet of road and ditch may be treated in any ditch turn-out
				  buffer, and no more than 250 feet may be treated if more than one travel lane
				  is draining to the ditch.
(d) Buffer adjacent to residential, largely pervious or small
				  impervious areas. A buffer adjacent to a residential, largely pervious or small
				  impervious area that does not require that runoff be distributed by means of a
				  level spreader may only be used when:
(i) A buffer is located immediately downhill of the developed
				  area; and
(ii) Runoff from the developed area is not concentrated and
				  enters a buffer in well distributed sheet flow.
Only runoff from the following areas may be treated using
				  this type buffer:
(iii) A single family residential lot;
(iv) A developed area that is less than 10% impervious where
				  the flow path over the portion of the developed area for which treatment is
				  being credited does not exceed 150 feet; or
(v) An impervious area of less than one acre, where the flow
				  path across the impervious area does not exceed 100 feet.
2. Design requirements for all buffer types.
The
				  following design requirements apply to all types of buffers.
(a) Topography
.
The topography of a buffer area
				  must be such that stormwater runoff will not concentrate as it flows across a
				  buffer, but will remain well-distributed. Flow paths of runoff through a buffer
				  must not converge, but must be essentially parallel or diverging.
(b) Vegetative cover. The vegetative cover type of a buffer
				  must be either forest or meadow. In most instances the sizing of a buffer
				  varies depending on vegetative cover type.
(i) Forest buffer. A forest buffer must have a well
				  distributed stand of trees with essentially complete canopy cover, and must be
				  maintained as such. A forested buffer must also have an undisturbed layer of
				  duff covering the mineral soil. Activities that may result in disturbance of
				  the duff layer are prohibited in a buffer.
(ii) Meadow buffer. A meadow buffer must have a dense cover
				  of grasses, or a combination of grasses and shrubs or trees. A buffer must be
				  maintained as a meadow with a generally tall stand of grass, not as a lawn. It
				  must not be mown more than twice per calendar year. If a buffer is not located
				  on natural soils, but is constructed on fill or reshaped slopes, a buffer
				  surface must either be isolated from stormwater discharge until a dense sod is
				  established, or must be protected by a three inch layer of erosion control mix
				  or other woodwaste material approved by the Department before stormwater is
				  directed to it, with vegetation must be established using an appropriate seed
				  mix.
(iii) Mixed meadow and forest buffer. If a buffer is part
				  meadow and part forest, the required sizing of a buffer must be determined as a
				  weighted average, based on the percent of a buffer in meadow and the percent in
				  forest, of the required sizing for meadow and forest buffers.
(c) Deed restrictions and covenants. Areas designated as
				  vegetated buffers must be clearly identified on site plans and protected from
				  disturbance by deed restrictions and covenants.
3. Design specifications and sizing tables for a
				  vegetated buffer with stone bermed level lip spreaders.
Stormwater
				  runoff must be delivered to a vegetated buffer with stone bermed level lip
				  spreaders in either sheet or concentrated flow. These design specifications
				  direct runoff behind a stone berm constructed along the contour at the upper
				  margin of a buffer area. As a result of restriction of flow through the berm,
				  the runoff then spreads out behind the berm so that it seeps through the entire
				  length of the berm and is evenly distributed across the top of a buffer. The
				  stone must be coarse enough that it will not clog with sediment. The berm must
				  be well-graded and contain some small stone and gravel so that flow through the
				  berm will be restricted enough to cause it to spread out behind the
				  berm.
(a) Stone berm specifications. The stone berm must be at
				  least 1.5 feet high and 2.0 feet across the top with 2:1 side slopes
				  constructed along the contour and closed at the ends. Unless otherwise approved
				  by the Department, the design must include a shallow, 6-inch deep trapezoidal
				  trough with a minimum bottom width of three feet, and with a level downhill
				  edge excavated along the contour on the uphill edge of the stone berm. Stone
				  for stone bermed level lip spreaders must consist of sound durable rock that
				  will not disintegrate by exposure to water or weather. Fieldstone, rough
				  quarried stone, blasted ledge rock or tailings may be used. The rock must be
				  well-graded within the limits of Table 7, or as otherwise approved by the
				  Department.
Table 7
Percent of Rock Required to Pass Through Square Mesh
				  Sieves
Sieve Designation (Metric)
Sieve Designation (US Customary)
Percent by Weight passing Square Mesh
						  Sieves
300 mm
12 in
100
150 mm
6 in
84-100
75 mm
3 in
68-83
25.4 mm
1 in
42-55
4.75 mm
No. 4
8-12
(b) Buffer sizing. The required size of a buffer area below
				  the stone-bermed level lip spreader varies with the size and imperviousness of
				  the developed area draining to a buffer, the type of soil in a buffer area, the
				  slope of a buffer, and the vegetative cover type. The following table indicates
				  the required berm length per acre of impervious area and lawn draining to a
				  buffer for a given length of flow path through a buffer. Required berm length
				  varies by the Hydrologic Soil Group of the soils in a buffer and by the length
				  of flow path through a buffer. If more than one soil type is found in a buffer,
				  the required sizing of a buffer must be determined as weighted average, based
				  on the percent of a buffer in each soil type, of the required sizing for each
				  soil type buffer. Alternative sizing may be allowed if it is determined by a
				  site-specific hydrologic buffer design model approved by the Department. A
				  buffer meeting this standard is not allowed on Hydrologic Soil Group D soils
				  that are identified as wetland soils, unless measures are taken to improve
				  infiltrative capacity, as approved by the Department. A buffer meeting this
				  standard is not allowed on natural slopes in excess of 15% unless a buffer has
				  been evaluated using a site- specific hydrologic buffer design model approved
				  by the Department, and measures have been included to ensure that runoff
				  remains well-distributed as it passes through a buffer.
Table 8 below applies to a buffer with slopes ranging from 0
				  to 8%. For a buffer with slopes between 9% and 15%, the indicated berm length
				  must be increased by 20%.
NOTE
: The following tables were developed using
				  a 1.25 inch, 24 hour storm of type III distribution, giving a maximum unit flow
				  rate of less than 0.009 cfs per foot.
Table 8
Sizing Requirements for buffer with 0-8% slope and
				  stone bermed level lip spreader
Hydrologic Soil Group
Length of flow path through buffer
						  (feet)
Berm length for a
forested buffer (feet)
Berm length for a
meadow
						  buffer (feet)
Per acre of impervious area
Per acre
of lawn
Per acre of impervious area
Per acre
of lawn
Soil Group A
75
75
25
125
35
100
65
20
75
25
150
50
15
60
20
Soil Group B
75
100
30
150
45
100
80
25
100
30
150
65
20
75
25
Soil Group C
sandy loam
						  or loamy sand
75
125
35
150
45
100
100
30
125
35
150
75
25
100
30
Soil Group C
silt loam,
						  clay loam or silty clay loam
100
150
45
200
60
150
100
30
150
45
Soil Group D non-wetland
150
150
45
200
60
4. Design specifications and sizing tables for a buffer
				  adjacent to the downhill side of a road.
A buffer adjacent to a
				  disturbed area of less than one acre that continues to bea road may only be
				  used when a buffer is located such that the runoff from the road surface and
				  shoulder sheets immediately into a buffer. Required buffer design and sizing
				  for this type of buffer does not vary with soil type or slope, except that a
				  buffer meeting this standard is not allowed on soils identified as wetland
				  soils or on natural slopes in excess of 20%, unless the Department finds that
				  the buffer will have adequate adsorptive capacity, and measures have been
				  included to ensure that runoff remains well-distributed as it passes through
				  the buffer. Sizing depends on the vegetative cover type of a buffer and the
				  number of travel lanes draining to a buffer as indicated in Table 9.
Table 9
Required Flow Path Length of a Buffer Receiving Road
				  Runoff
Length of flow path
for a forested
						  buffer (feet)
Length of flow path
for a meadow
						  buffer (feet)
One travel lane draining to buffer
35
50
Two travel lanes draining to buffer
55
80
The inslope of the road bed may be included as part of a
				  meadow buffer only if it is designed and constructed to allow infiltration.
				  Design and construction to allow infiltration includes, but is not limited to,
				  the inslope fill material having slopes no steeper than 3:1; loaming and
				  seeding to meadow grasses; and maintaining a buffer area as a meadow
				  buffer.
5. Design specifications and sizing tables for a ditch
				  turn-out buffer.
A ditch turn-out buffer may only be used when runoff
				  from a road ditch is diverted to a 20-foot stone bermed level lip spreader that
				  distributes runoff into a buffer. No areas other than the road surface, road
				  shoulder, road ditch, and ditch back slopes may be directed to the stone bermed
				  level lip spreader.
(a) Stone berm specifications. The stone berm to which the
				  ditch turn-out delivers the runoff must be at least 20 feet in length and must
				  be constructed along the contour. It must be at least one- foot high and two
				  feet across the top with 2:1 side slopes. Stone for the berm must consist of
				  sound durable rock that will not disintegrate by exposure to water or weather.
				  Fieldstone, rough quarried stone, blasted ledge rock or tailings may be used.
				  The rock must be well-graded with a median size of approximately 3 inches and a
				  maximum size of 6 inches.
(b) Buffer sizing. The required size of a buffer area below
				  the stone bermed level lip spreader varies with the type of soil in a buffer
				  area, the slope of a buffer, the length of road ditch draining to a buffer and
				  the vegetative cover type within a buffer. A buffer meeting this standard is
				  not allowed on Hydrologic Soil Group D soils, unless measures are taken to
				  improve infiltrative capacity, as approved by the Department. A buffer meeting
				  this standard is not allowedon slopes in excess of 15%, unless the Department
				  finds that the buffer will have adequate adsorptive capacity, and measures have
				  been included to ensure that runoff remains well-distributed as it passes
				  through the buffer. Table 10 indicates the required length of the flow path
				  through a buffer for various vegetative covers and ditch lengths. The tables
				  below apply to a buffer with slopes ranging from 0 to 8%. For a buffer with
				  slopes between 9% and 15%, the indicated length of flow path should be
				  increased by 20%. If two travel lanes drain to the ditch, as in the case of a
				  super elevated road, the length of flow path indicated for 400 feet of road
				  must be used, but no more than 250 feet of ditch may drain to each
				  turn-out.
Table 10
Sizing Requirements for Buffer Based on Length of Road
				  or Ditch
Hydrologic soil group of
soil in
						  buffer
Length of road or ditch draining to a buffer
						  (feet)
Length of flow path for a forested buffer
						  (feet)
Length of flow path
for a meadow
						  buffer (feet)
A
200
50
70
300
50
85
400
60
100
B
200
50
70
300
50
85
400
60
100
C
Loamy Sand or Sandy
						  Loam
200
60
100
300
75
120
400
100
Not applicable
C
Silt Loam, Clay Loam,
						  or Silty Clay Loam
200
75
120
300
100
Not applicable
D
Non-wetland
200
100
150
6. Design specifications and sizing tables for a buffer
				  adjacent to a residential lot; developed area that is less than 10% impervious,
				  where the flow path over the portion of the developed area for which treatment
				  is being credited does not exceed 150 feet; or an impervious area where the
				  flow path across the impervious area does not exceed 100 feet.
The
				  design specifications and sizing tables below may only be used when a buffer is
				  located immediately adjacent to the downhill side of a developed area, and
				  where the topography and buildings and other facilities within the developed
				  area do not cause any significant concentration of runoff.
This design is appropriate for residential lots and other
				  mostly pervious areas with relatively uniform topography and for small
				  impervious areas. This design is not appropriate for treating large impervious
				  areas because, even if pavement is graded evenly, it is likely that some
				  concentration of runoff will occur as the stormwater travels across large areas
				  of pavement. For large areas of pavement where the average path of flow across
				  the pavement exceeds 100 feet, or where runoff will not be evenly distributed
				  across the downhill edge of the pavement, a stone bermed level lip spreader
				  must be used and the berm and buffer must be sized according to the
				  specifications in Section
				  3 above.
Table 11 below indicates the required minimum length of the
				  flow path through a buffer for various soil types and vegetative cover types.
				  Unless otherwise approved by the Department, the following apply:
(a) Length of flow paths defined in this table apply to
				  buffers with slopes between 0 and 8%.
(b) For buffers with slopes between 9% and 15%, the indicated
				  length of flow path must be increased by 20%.
(c) A buffer meeting this standard is not allowed on slopes
				  in excess of 15% or Hydrologic Soil Group D soils except that a forested buffer
				  is allowed if the D soils in a buffer are not wetland soils.
(d) Buffers described by this section must be located
				  downhill of the entire developed area for which it is providing stormwater
				  treatment, such that all runoff from the entire developed area has a flow path
				  through a buffer at least as long as the required length of flow path.
Table 11
Buffer Size Requirements Based on Soil and Vegetative
				  Cover Types
Hydrologic soil group of
soil in
						  buffer
Length of flow path
for a forested
						  buffer (feet)
Length of flow path
for a meadow
						  buffer (feet)
A
45
75
B
60
85
C
Loamy Sand or Sandy Loam
75
100
C
Silt Loam, Clay Loam, or Silty Clay
						  Loam
100
150
D
Non-wetland
150
Not applicable
7. Alternative buffer design for residential
				  subdivision lots.
This buffer design may be used as a Department approved
				  alternative treatment measure to meet either the general standards at Section
				  4(B) or the
				  phosphorus standard using compensation as provided in 06-096 CMR 501, but only
				  under the conditions described below. This design applies only to buffers
				  adjacent to subdivision projects in which all lots are for single or two-family
				  detached, residential housing, common areas or open space where: 1) the buffer
				  is located immediately downhill of the developed area; 2) runoff enters the
				  buffer as sheet flow without a level spreader; and 3) the flow path over the
				  portion of the developed area being treated by the buffer does not exceed 150
				  feet.
(a) Slope: To meet this alternative design, a buffer is not
				  allowed on natural slopes in excess of 15%.
(b) Soil restrictions: Such a buffer is allowed on Hydrologic
				  Soil Group D soils only if it is forested and non-wetland unless measures are
				  taken to improve infiltrative capacity, as approved by the Department.
(c) Buffer sizing: Table 12 below indicates the required
				  buffer flow path length based on soil types and vegetative cover types. Buffers
				  must be located downhill of the entire developed area for which they are
				  providing stormwater treatment; and with no converging contour, such that all
				  runoff from the developed area passes in sheet flow through the buffer for a
				  distance at least as long as the required length of flow path.
(d) Minimum sizing for phosphorus standard. If this buffer
				  standard is being used in a mitigation project to meet the phosphorus standard
				  and its size is being adjusted to provide a specific treatment factor, the
				  minimum sizing for this type of buffer is a flow path of 35 feet.
Table 12
Alternative Buffer Size Requirements for Residential
				  Subdivisions
Required minimum length of flow path
						  through the buffer
Based on a slope no greater than 15%
Hydrologic soil
For a forested buffer
(feet)
For a meadow buffer
(feet)
A
35
50
B
45
60
C Loamy sand or sandy loam
50
70
C Silt loam, clay loam or Silty clay loam
70
100
D
100
Not Applicable