Current through Register No. 13, March 27, 2025
Vegetated buffers, including but not limited to
residential or small pervious area buffers, developed area buffers, roadway
buffers, and ditch turn-out buffers, shall be used only as follows:
(a) Buffers shall not be located in an area
of RSA 482-A jurisdiction;
(b) The
buffer shall be directly adjacent to the area being treated;
(c) The runoff shall enter the buffer as
sheet flow;
(d) The buffer shall
not be interrupted by any intermittent or perennial stream channel or other
drainage way;
(e) Only the
continuous flow path length shall be counted toward the buffer
length;
(f) The vegetative cover
type shall be forest or meadow or a combination of forest and meadow, and if a
combination, the required sizing of the buffer shall be determined as a
weighted average based on the percent of buffer in forest and the percent of
buffer in meadow;
(g) The
hydrologic soil group shall be identified, and if the buffer contains more than
one soil group, the required sizing of the buffer shall be determined as a
weighted average based on the percent of the buffer in each soil
type;
(h) The buffer area shall be
identified on the plans and protected by deed restrictions or covenants, or
both, so that it remains in an unaltered state;
(i) If the practice is for a residential or
small impervious area, the following requirements also shall be met:
(1) Runoff shall be received only from one or
more of the following:
a. A single family or
duplex residential lot;
b. A
developed area with less than 10% imperviousness where the flow path over the
developed area does not exceed 150 feet; or
c. An impervious area not greater than one
acre where the flow path across the impervious area does not exceed 100
feet;
(2) Runoff shall
enter the buffer as sheet flow without the aid of a level spreader;
(3) The buffer slope shall be uniform and not
exceed 15%; and
(4) The minimum
flow path through the buffer shall be the sum of the following distances or 45
feet, whichever is greater:
a. The base flow
path through the buffer shall be:
1. For
hydrologic group A soils, 25 feet;
2. For hydrologic group B soils, 45
feet;
3. For hydrologic group C
soils, 60 feet;
4. For hydrologic
group D soils, 142 feet; and
5. For
buffers containing more than one hydrologic soil group, a weighted average of
the distances in 1. through 4., above, based on the hydrologic group(s) of the
soils in the buffer;
b.
For every 1% slope of the buffer, 2 feet shall be added to the base flow path
length through the buffer; and
c.
For every 10% of the buffer area that is meadow, 3 feet shall be added to the
base flow path length through the buffer;
(j) If the practice is for a developed area
using a buffer with a stone berm level spreader, the following requirements
also shall be met:
(1) Runoff shall be
received from a developed area where the runoff is concentrated;
(2) A stone berm level spreader that meets
the requirements of Env-Wq 1508.19 and is no less than 20 feet and no greater
than 50 feet in length shall be provided to distribute the flow to the
buffer;
(3) The minimum flow path
length through the buffer shall be 50 feet;
(4) The buffer slope shall be uniform and not
exceed 15%;
(5) Subject to (10),
below, the total buffer area required shall be determined by multiplying the
total area draining to the buffer, in acres, by the sum of the following:
a. The base buffer area per acre, determined
pursuant to (6), below;
b. The
impervious area adjustment, determined pursuant to (7), below;
c. The slope adjustment, determined pursuant
to (8), below; and
d. The meadow
adjustment, determined pursuant to (9), below;
(6) The base buffer area per acre shall be
determined based on the hydrologic group of the soils and the area draining to
the buffer, as follows:
a. For hydrologic
group A soils, 400 square feet per acre draining to the buffer;
b. For hydrologic group B soils, 1,000 square
feet per acre draining to the buffer;
c. For hydrologic group C soils, 1,500 square
feet per acre draining to the buffer;
d. For hydrologic group D soils, 2,200 square
feet per acre draining to the buffer; and
e. For buffers containing more than one
hydrologic soil group, a weighted average of the areas in a. through d., above,
based on the hydrologic group(s) of the soils in the buffer;
(7) For every 1% of the area
draining to the buffer that is impervious, the following impervious area
adjustment shall be added to the base buffer area per acre:
a. For hydrologic group A soils, 45 square
feet per acre draining to the buffer;
b. For hydrologic group B soils, 66 square
feet per acre draining to the buffer;
c. For hydrologic group C soils, 83 square
feet per acre draining to the buffer; and
d. For hydrologic group D soils, 132 square
feet per acre draining to the buffer; and
e. For buffers containing more than one
hydrologic soil group, a weighted average of the areas in a. through d., above,
based on the hydrologic group(s) of the soils in the buffer;
(8) For every 1% slope of the
buffer, the slope adjustment shall be an additional 100 square feet per acre
draining to the buffer;
(9) For
every 1% of the buffer area that is meadow, the meadow adjustment shall be an
additional 24 square feet per acre draining to the buffer; and
(10) If a detention structure is used
upstream of the level spreader, the drainage area to the buffer shall be deemed
to be 1.0 acre of impervious area for every 1.0 cfs of peak 2-year, 24-hour
outflow from the detention structure;
(k) If the practice is a roadway buffer, the
following requirements also shall be met:
(1)
Runoff shall be received from the road surface and shoulder and sheet directly
into the buffer;
(2) No areas other
than the adjacent road surface and shoulder shall be directed to the
buffer;
(3) The road shall be
parallel to the contour of the buffer slope;
(4) Except as provided in (5) below, the
man-made buffer slope shall be uniform and not exceed 15%;
(5) A maximum of 20 feet of vegetated roadway
embankment slope of 3:1 or flatter shall count toward the required buffer
length as required in (8) below;
(6) The natural buffer slope shall be uniform
and not exceed 20%;
(7) The buffer
shall be vegetated; and
(8) The
buffer flow path shall be at least 50 feet for one travel lane draining to the
buffer and at least 30 additional feet for each additional travel lane draining
to the buffer; and
(l) If
the practice is a ditch turn-out buffer, the following requirements also shall
be met:
(1) No areas other than road surface,
road shoulder, and road ditch shall be directed to the buffer;
(2) No more than 6,000 square feet of
pavement shall be directed to a level spreader;
(3) A stone berm level spreader that meets
the requirements of Env-Wq 1508.19 and is no less than 20 feet and no greater
than 50 feet in length shall be provided at the end of the ditch to distribute
runoff to the buffer;
(4) The
buffer slope shall be uniform and not exceed 15%; and
(5) For every 1,000 square feet of area
draining to the buffer, the minimum flow path length through the buffer shall
be the sum of the following distances or 50 feet, whichever is greater:
a. The base flow path through the buffer
shall be:
1. For hydrologic group A soils, 7
feet;
2. For hydrologic group B
soils, 8 feet;
3. For hydrologic
group C soils, 15 feet;
4. For
hydrologic group D soils, 30 feet; and
5. For buffers containing more than one
hydrologic soil group, a weighted average of the distances in 1. through 4.,
above, based on the hydrologic group(s) of the soils in the buffer;
b. For every 1% slope of the
buffer, 2 feet shall be added to the base flow path length through the buffer;
and
c. For every 10% of the buffer
that is meadow, 3 feet shall be added to the base flow path length through the
buffer.
(See Revision Note #1 at chapter heading for Env-Wq 1500)
#9343, eff 1-1-09
