N.H. Code Admin. R. Env-Wq 1508.09

Current through Register No. 25, June 20, 2024

Section Env-Wq 1508.09 - Stormwater Treatment Practices: Vegetated Buffers

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 1508.18 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 1508.18 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.

N.H. Code Admin. R. Env-Wq 1508.09

(See Revision Note #1 at chapter heading for Env-Wq 1500) #9343, eff 1-1-09; (See Revision Note #2 at chapter heading for Env-Wq 1500) ss by #9817-A, eff 12-1-10

Amended by Volume XXXVII Number 32, Filed August 10, 2017, Proposed by #12342, Effective 8/15/2017, Expires 8/15/2027.