Current through Register No. 13, March 27, 2025
Filtering practices, including but not limited to surface
sand filters, underground sand filters, tree box filters, bioretention systems,
pervious asphalt, and pervious concrete, shall comply with the
following:
(a) Filtering practices
shall not be located in:
(1) An area where a
filtering practice sidewall is adjacent to hydrologic soil group A soils,
unless an impermeable liner is installed along the side wall so as to separate
the practice from the group A soils; or
(2) The areas identified in Env-Wq
1507.02(c)(1)-(3) unless the practice has an impermeable liner;
(b) Pretreatment as described in
Env-Wq 1508.12 through Env-Wq 1508.16 shall be provided unless:
(1) The practice is pervious pavement or a
tree box filter; or
(2) Roof runoff
is the only stormwater discharged to the practice;
(c) If a sediment forebay is used to meet (b)
above, it shall be designed to contain 25% of the WQV and otherwise meet the
criteria specified in Env-Wq 1508.12;
(d) The practice shall completely drain the
WQV within 72 hours or less;
(e) If
the practice has an impermeable liner, (f) through (i), below, shall not
apply;
(f) The design infiltration
rate of the underlying native soil or fill soil shall be determined in
accordance with Env-Wq 1504.14;
(g)
If the infiltration rate, prior to adding a factor of safety, of the underlying
soil or proposed fill, as determined pursuant to (f), above, is less than 0.50
inches per hour, an underdrain system shall be provided as follows:
(1) The underdrain shall consist of a 6-inch
diameter perforated pipe set in 3/4-inch to 2-inch diameter stone or gravel
washed free of fines and organic material;
(2) The stone or gravel layer shall extend at
least 4 inches above the drainage pipes and 4 inches below the drainage
pipes;
(3) The stone or gravel
layer shall be separated from the filter media with a 3-inch layer of 3/8-inch
pea gravel; and
(4) Underdrains
shall be provided at a maximum spacing of 25 feet center-to-center;
(h) If the practice is not located
within a groundwater protection area or water supply intake protection area,
the SHWT and bedrock may be reduced to not less than one foot below the bottom
of the filter course material, provided, however, that underdrains may be
installed to achieve the minimum one-foot separation between the SHWT and the
bottom of the filter course material;
(i) If the practice is located within a
groundwater protection area or water supply intake protection area, the
practice shall have a filter course depth of 24 inches and have at least one
foot of separation between:
(1) The bottom of
the filter course material and bedrock; and
(2) The bottom of the filter course material
and the SHWT, provided, however, that underdrains may be installed to achieve
the minimum one-foot separation between the SHWT and the bottom of the filter
course material;
(j) An
overflow structure that complies with Env-Wq 1507.02(d) shall be
included;
(k) If the practice
includes a surface filter, the following requirements also shall be met:
(1) The filter, including the storage area
above the filter, the filter media voids, and the pretreatment area, shall
store at least 75% of the WQV;
(2)
The contributing drainage area shall be less than 10 acres;
(3) The filter media shall be a minimum 18
inches deep;
(4) The filter media
shall consist of one of the following mixtures volume:
a. 50% to 55% by volume sand that is
certified by its producer as meeting the requirements for ASTM C-33 concrete
sand, 20% to 30% by volume of loamy sand topsoil with 15% to 25% fines passing
the number 200 sieve, and 20% to 30% by volume moderately fine shredded bark or
wood fiber mulch with less than 5% passing the number 200 sieve;
b. 20% to 30% by volume of moderately fine
shredded bark or wood fiber mulch that has no more than 5% fines passing the
number 200 sieve, with 70 to 80% by volume loamy coarse sand used in the
mixture meeting the following sieve analysis specification:
1. From 85 to 100 percent by weight shall
pass the number 10 sieve;
2. From
70 to 100 percent by weight shall pass the number 20 sieve;
3. From 15 to 40 percent by weight shall pass
the number 60 sieve; and
4. From 8
to 15 percent by weight shall pass the number 200 sieve;
(5) The filter shall have an
access grate; and
(6) The filter
shall not be covered with grass;
(l) If the practice includes an underground
filter, the following requirements also shall be met:
(1) The filter, including the filter media
voids and the pretreatment chamber, shall store at least 75% of the
WQV;
(2) The contributing drainage
area shall be less than 10 acres;
(3) The filter media shall be a minimum 18
inches deep;
(4) The filter media
shall consist of one of the mixtures specified in (l)(4) above; and
(5) The filter shall have an access
grate;
(m) If the
practice includes a bioretention system, the following requirements also shall
be met:
(1) The ponding area, including the
storage area above the filter and the filter media voids, shall store 100% or
more of the WQV;
(2) The
contributing drainage area shall be less than 5 acres;
(3) The filter media shall be a minimum 18
inches deep;
(4) The filter media
shall consist of one of the mixtures specified in (l)(4), above;
(5) Side slopes shall not exceed
3:1;
(6) The surface shall be
covered with a minimum of 3-inches of organic material; and
(7) The surface area shall be covered with
grasses or have the following planting design plan:
a. Only native, non-invasive species shall be
used;
b. Plant layout shall be
random and natural;
c. Woody
vegetation shall not be used near inflow locations;
d. Vegetation directly over the filter media
shall be limited to facultative wetland species as specified in National List
of Plant Species that Occur in Wetlands: Northeast (Region 1), May 1988,
published by U.S. Fish and Wildlife Services, available at
https://digitalmedia.fws.gov/cdm/ref/collection/document/id/1348,
or other species that can withstand periodic inundation, as determined by a
certified wetland scientist;
e.
Trees or large shrubs shall be planted along the perimeter; and
f. The plan shall establish a perimeter tree
canopy with an understory of shrubs and herbaceous plants;
(n) If the practice includes
pervious asphalt, the following requirements also shall be met:
(1) The practice shall be designed and
installed in accordance with UNHSC Design Specifications for Porous Asphalt
Pavement and Infiltration Beds, February 2014, revised September 2016,
published by University of New Hampshire Stormwater Center, available as noted
in Appendix B;
(2) The thickness of
the filter course shall be at least 12 inches; and
(3) The ratio of the total contributing area
to the area of the pervious surface shall be no more than 5:1;
(o) If the practice includes
pervious concrete, the following requirements also shall be met:
(1) The filter course material shall consist
of NHDOT Standard Specification for Road and Bridge Construction, 2016, Table
304-1, item number 304.1, available as noted in Appendix B, modified to have
0-6% passing the number #200 sieve;
(2) The practice shall be designed and
installed in accordance with American Concrete Institute, 522.1-13,
Specification for Pervious Concrete Pavement, 2013, available as noted in
Appendix B;
(3) The practice shall
be installed by a contractor certified in pervious concrete installation by the
National Ready Mix Concrete Association (NRMCA);
(4) The thickness of the filter course shall
be at least 12 inches; and
(5) The
ratio of the total contributing area to the area of the pervious surface shall
be no more than 5:1; and
(p) If the practice includes a tree box
filter, then:
(1) The tree box filter shall be
not less than 6 feet in diameter and not less than 4 feet deep; and
(2) The contributing drainage area shall be
less than 0.1 acres.
(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)
amd by #9817-A, eff 12-1-10
