Appendix F

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APPENDIX F. Vegetated buffers

This appendix applies to all projects using vegetated buffers for stormwater control. A buffer is a 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 down hill side of a road. A buffer located along the down hill 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 ft 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 with less than 10% imperviousness 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 following limits, or as otherwise approved by the department.

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. 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.

The table 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.

Required berm and flow length of buffer with 0–8% slope and a 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 down hill side of a road. A buffer adjacent to the down hill side of a 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%. Sizing depends on the vegetative cover type of a buffer and the number of travel lanes draining to a buffer as indicated in the following table.

Length of flow path

for a forested buffer (feet)

Length of flow path

for a meadow buffer (feet)

One travel lane draining to buffer

Two travel lanes draining to buffer

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 or on slopes in excess of 15%. The following table 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.

Required length of flow path per length of road or ditch draining to a buffer.

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 with less than 10% imperviousness, 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 structures 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.

The table below indicates the required minimum length of the flow path through a buffer for various soil types and vegetative cover types. Length of flow paths defined in this table apply to buffers with slopes between 0 and 8%. For buffers with slopes between 9% and 15%, the indicated length of flow path must be increased by 20%. 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. 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.

Required minimum length of the flow path through a buffer for various soil types 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)

Loamy Sand or Sandy Loam

100

Silt Loam, Clay Loam, or Silty Clay Loam

100

150

Non-wetland

150

Not applicable

Ch. 500 appendices

Appendix F