HomeMy WebLinkAboutWPO202200038 Calculations 2022-09-14 (22)VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
VIRGINIA DEQ STORMWATER
DESIGN SPECIFICATION No. 2
SHEET FLOW TO A VEGETATED
FILTER STRIP OR
CONSERVED OPEN SPACE
VERSION 1.9
March 1, 2011
SECTION 1. DESCRIPTION
Filter strips are vegetated areas that treat sheet flow delivered from adjacent impervious and
managed turf areas by slowing runoff velocities and allowing sediment and attached pollutants to
settle and/or be filtered by the vegetation. The two design variants of filter strips are (1)
Conserved Open Space and (2) designed Vegetated Filter Strips. The design, installation, and
management of these design variants are quite different, as outlined in this specification.
In both instances, stormwater must enter the filter strip or conserved open space as sheet flow. If
the inflow is from a pipe or channel, an engineered level spreader must be designed in
accordance with the criteria contained herein to convert the concentrated flow to sheet flow.
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With proper design and maintenance, these practices can provide relatively high runoff reduction
as shown in Table 2.1.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
Table 2. 1: Summary of Stormwater Functions Provided by Filter Strips t
Conservation Area
Vegetated Filter Strip
HSG Soils
HSG Soils HSG Soils
HSG Soils
B 4, C and
Stormwater Function A and B C and D
A
D
3
No CA
2
With CA
Assume no CA in
Conservation Area
Annual Runoff Vol. Reduction RR
75% 50%
50%
50%
Total Phosphorus (TP) EMC
Reduction by BMP Treatment
0
0
Process
Total Phosphorus (TP) Mass Load
75%
50%
50%
50%
Removal
Total Nitrogen (TN) EMC Reduction
0
0
b BMP Treatment Process
Total Nitrogen (TN) Mass Load
75%
50%
50%
500:/.
Removal
Partial. Designers can use the RRM spreadsheet to
Channel Protection and
adjust curve number for each design storm for the
Flood Mitigation
contributing drainage area; and
designers can account for a lengthened Time -of -
Concentration flow path in computing eak discharge.
CWP and CSN (2008); CWP (2007)
2 CA = Compost Amended Soils (see Design Specification No. 4)
3 Compost amendments are generally not applicable for undisturbed A soils, although it may be
advisable to incorporate them on mass -graded A or B soils and/or filter strips on B soils, in order
to maintain runoff reduction rates.
4 The plan approving authority may waive the requirement for compost amended soils for filter
strips on B soils under certain conditions (see Section 6.2 below)
5 There is insufficient monitoring data to assign a nutrient removal rate for filter strips at this time.
SECTION 3. DESIGN TABLE
Conserved Open Space and Vegetated Filter Strips do not have two levels of design. Instead,
each must meet the appropriate minimum criteria outlined in Table 2.2 (next page) and Section
6 (below) to qualify for the indicated level of runoff reduction. In addition, designers must
conduct a site reconnaissance prior to design to confirm topography and soil conditions.
SECTION 4. TYPICAL DETAILS
Figure 1 shows a typical approach for sheetflow to a Conserved Open Space (Cappiella et al.,
2006). Figures 2 and 3 provide standard details for an engineered level spreader developed by
North Carolina State University (Hathaway and Hunt, 2006). An alternative design for an
"energy dissipater" can be found in Henrico County's Environmental Program Manual (Chapter
9, Minimum Design Standard 9.01) at: htti)://www.co.henrico.va.us/works/eesd/
Version 1.9, March 1, 2011 Page 2 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
Table 2.2. Filter Strip Design Criteria
Design issue I Conserved Open Space Vegetated Filter Strip
Cover Undisturbed soils and native
(Sections 6.1 and vegetation
0.5% to 3% Slope — Minimum 35 ft
Overall Slope and
width
Width (perpendicular
3% to 6% Slope — Minimum 50 ft
to the flow)
width
(Section 5)
The first 10 ft. of filter must be 2% or
less in all cases 2
cover or landscaped with
herbaceous cover, shrubs, and
trees
1 % 'to 4% Slope — Minimum 35
ft. width
4% to 6% Slope — Minimum 50 ft.
width
6% to 8% Slope — Minimum 65 ft.
width
The first 10 ft. of filter must be 2%
or less in all cases
Sheet Flow Maximum flow length of 150 ft. from adjacent pervious areas;
(Section 5) 1 Maximum flow lenath of 75 ft. from adiacent impervious areas
Concentrated Flow
(Section 6.3)
Construction Stage
(Section 8)
Typical Applications
(Section 5)
Amendments
(Section 6.1)
Boundary Spreader
Length of ELS " Lip = 13 lin. ft. per
each 1 cfs of inflow if area has 90%
Cover
Length = 40 lin. ft. per 1 cfs for
forested or re -forested Areas 4
(ELS 6 length = 13 lin.ft. min; 130
lin.ft. max.)
Located outside the limits of
disturbance and protected by ESC
Adjacent to stream or wetland buffer
or forest conservation area
No
GD 6 at top of filter
Length of ELS 6 Lip = 13 lin.ft. per
each 1 cfs of inflow (13 lin.ft. min;
1301ni.ft. max.)
Prevent soil compaction by heavy
equipment
Treat small areas of IC (e.g.,
5,000 so and/or turf -intensive land
uses (sports fields, golf courses)
Yes (B, C, and D soils)5
GD " at top of filter
PB 6 at toe of filter
'A minimum of 1 % is recommended to ensure positive drainage.
2 For Conservation Areas with a varying slope, a pro -rated length may be computed only if the
first 10 ft. is 2% or less.
3 Vegetative Cover is described in Section 6.2.
4 Where the Conserved Open Space is a mixture of native grasses, herbaceous cover and forest
(or re -forested area), the length of the ELS 6 Lip can be established by computing a weighted
average of the lengths required for each vegetation type. Refer to Section 6.3 for design criteria
5 The plan approving authority may waive the requirement for compost amended soils for filter
strips on B soils under certain conditions (see Section 6.1).
6 ELS = Engineered Level Spreader; GD = Gravel Diaphragm; PB = Permeable Berm.
Version 1.9, March 1, 2011 Page 3 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2
SHEET FLOW TO FILTER OR OPEN SPACE
Gravel diaphragm
(12 by 24 inches) for
pretreatment
0
k
hes
g
Ponding Gravel
Zone Berm
;l1f10 ii
Forest Zone
Figure 2.1. Typical Sheetflow to Conserved Open Space
Pipe From
Drvers�on
InRvent To Level
_ 1 5torrnwater 59reader
1ft
Figure 2.2. Level Spreader Forebay (Hathaway and Hunt 2006)
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
Frc
Filler Strip Could be a riparian buffer, vegetated
Flow Bypass , filter strip or restored npanan buffer_ '
Length = 13'to 130'
(parallel to contour lines)
Level 5preader Lip
Row PatF� �
Channel
Underdram - 51zed /
For Site Sod -
Conditions - Dram Appropnatey 5aed
Into Bypass 5wale Concrete Footer
eceiwng
beam
Reinforced Swale
(necessary if level
spreader is designed for
the 1 mfir storm)
3' Layer of
#57 Stone
Riparian Buller
or I'll Strip
3 R Wide
5tnp of
Filter Fabnc
Figure 2.3: Plan and Cross Section of Engineered Level Spreader (ELS)
(Hathaway 2006)
Version 1.9, March 1, 2011 Page 5 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
,..
Vol
RECEIVING
TREATMENT
11%►!:!:
MEASURE
�' �' •�
CLEAN WASHED GRAVEL OR
1
OTHER LINING AS
DESIGNED/SIZED FOR STABILITY
CLEAN WASHED GAAVEL OR
OTHER LINING AS
DESIGNEDYS¢ED FOR STABILITY
FILTER
DISCHARGETO
RECEIVING
TREATMENT
MEASURE
`6'A& TREATED TIMBERS OR CONCRETE LIP
PLAN VIEW
VARIABLE (MIN. 7 FT)
TIMBERS OR CONCRETE UP
05 REBAR TO SECURE
TIMBERS
61N. MIN
�I III III I III I III I III I III I III I III I I I�
MINN 5 FT.
PROFILE
Figure 2.4A: Section - Level Spreader with Rigid Lip
Version 1.9, March 1, 2011 Page 6 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2
SHEET FLOW TO FILTER OR OPEN SPACE
VEGETATED UP OF
u-
►++s+sw�wo++
DISCHARGE TO,
RECEIVING
MEASURE-
+♦+T�+ram+r1+,r►♦
.-
♦A/►<++++++fig
;►<�,�����+���++��++,,,ram<�
JUTE, IXCELSbFi, OR
PLAN VIEW
EQUIVALENT STAPLED IN PLACE
NOTE: CONSULT SPECIFICATIONS
FOR APPROPRIATE USE OF
JUTE, EXCELSIOR, OR VEGETATED LIP VERSUS RIGID LIP
EQUIVALENT STAPLED IN PLACEI
DISCHARGE TO
RECEIVING BURIED 6- MIN,
TREATMENT 1 .
MEASURE
& MIN.
61N_ MIN
LEVEL LIP OF-""TiT'=1If=`
SPREADER = 1i 1=1I-III-I GI L� III=TCI �F' 2:1 OR
MIN 6 FT.
PROFILE
Figure 2.4B: Section - Alternative Level Spreader with Vegetated Lip
Version 1.9, March 1, 2011 Page 7 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
SHEET FLOW FROM
IMPERVIOUS OR TURF
SHEET FLOW
PAVEMENT S_ ECTIO
PAVEMENT SUBGRADE
LINING AS SPECIFIED BY
DESIGN ENGINEER TO
- 4' MIN —
2' 2'
MIN MIN
2%4" DROP
Ir5:1 MAX II
�F�II I —II( —
FILTER FABRIC
WASHED
PREVENT SUBGRADE
IUN-11�1' MIN WITH 1:1 -1I -
SATURATION - SIDE SLOPES� IT�
=T1ILE. Ulll I _
Figure 2.5 — Gravel Diaphragm —Sheet Flow Pre-treatment
RIPRAP PLUNGE POOL
RIPRAP APRON
1-3 IN. COURSE AGGREGATE
OR OTHER LINING AS
DESIGNED FOR STABILITY
DIMENSION BASED ON
DESIGN DISCHARGE
3' MIN
10' MIN
-2
}MIN. DEPTH BASED ON
REQUIRED STONE SIZE
KEY LEVEL SPREADER
INTO EXISTING GRADE
r
- 6' X�6' TREATED 1
a TIMBERS OR
%Oti RETE LIP
LENGTH OF LEVEL SPREADER
LIP TO BE BASED ON DESIGN FLOW
AND ALLOWABLE VELOCITY
TOP OF LEVEL SPREADER(
SHALL -BE FLAT
PLAN VIEW
UNUIS IUHHG F bI ING 51 A
5%(20:1) OR LESS. IF VEGETATED
FILTER STRIP OR CONSERVED
OPEN SPAOE: FIRST 10 FEET LESS
THAN 2%
BY,35IIDI
PROFILE
Figure 2.6: Level Spreader. Pipe or Channel Flow to Filter Strip or Preserved Open Space
Version 1.9, March 1, 2011 Page 8 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2
SHEET FLOW TO FILTER OR OPEN SPACE
PROPOSED
SPACE VEGE )
PLAN VIEW
CONSERVED OPEN SPACE OR
VEGETATED FILTER STR:P
SHEET FLOW
SECTION A -A'
SIMPLE
DISCONNECTION
TO FILTER STRIP
OR CONSERVED
OPEN SPACE
PROPOSED LEVEL SPREADER
OR GRAVEL DIAPHRAGM
(IF NECESSARY)
Figure 2.7. Simple Disconnection to downstream Preserved Open Space
or Vegetated Filter Strip
Version 1.9, March 1, 2011 Page 9 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
SECTION 5. PHYSICAL FEASIBILITY & DESIGN APPLICATIONS
5.1 Conserved Open Space
The most common design applications of Conserved Open Space are on sites that are
hydrologically connected to a protected stream buffer, wetland buffer, floodplain, forest
conservation area, or other protected lands. Conserved open space is an ideal component of the
"outer zone" of a stream buffer, such as a Resource Protection Area (as is required in some parts
of the state), which normally receives runoff as sheetflow. Care should be taken to locate all
energy dissipaters or flow spreading devices outside of the protected area.
Designers may apply a runoff reduction credit to any impervious or managed turf cover that is
hydrologically connected and effectively treated by a protected Conserved Open Space that
meets the following eligibility criteria:
• The goal of establishing Conserved Open Space is to protect a vegetated area contiguous to a
receiving system, such as a stream or natural channel, for treating stormwater runoff.
Establishing isolated Conserved Open Space pockets on a development site may not achieve
this goal unless they effectively serve to connect the surface runoff to the receiving system.
Therefore, a locality may choose to establish goals for minimum acreage to be conserved (in
terms of total acreage or percentage of the total project site), and the physical location
(adjacent to a stream, or other criteria) in order for the cumulative conserved open space to
qualify for the RRM credit.
• No major disturbance may occur within the conserved open space during or after
construction (i.e., no clearing or grading is allowed except temporary disturbances associated
with incidental utility construction, restoration operations, or management of nuisance
vegetation). The Conserved Open Space area shall not be stripped of topsoil. Some light
grading may be needed at the boundary using tracked vehicles to prevent compaction.
• The limits of disturbance should be clearly shown on all construction drawings and protected
by acceptable signage and erosion control measures.
• A long term vegetation management plan must be prepared to maintain the Conserved Open
Space in a natural vegetative condition. Generally, Conserved Open Space management
plans do not encourage or even allow any active management. However, a specific plan
should be developed to manage the unintended consequences of passive recreation, control
invasive species, provide for tree and understory maintenance, etc. Managed turf is not
considered an acceptable form of vegetative management, and only the passive recreation
areas of dedicated parkland are eligible for the practice (e.g., the actively used portions of
ball fields and golf courses are not eligible), although conservation areas can be ideal
treatment practices at the edges of turf -intensive land uses.
• The Conserved Open Space must be protected by a perpetual easement or deed restriction
that assigns the responsible party to ensure that no future development, disturbance, or
clearing may occur within the area.
• The practice does not apply to jurisdictional wetlands that are sensitive to increased inputs of
stormwater runoff (e.g., bogs and fens).
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
5.2 Vegetated Filter Strips
Vegetated Filter Strips are best suited to treat runoff from small segments of impervious cover
(usually less than 5,000 sq. ft. ) adjacent to road shoulders, small parking lots and rooftops.
Vegetated Filter Strips may also be used as pretreatment for another stormwater practice such as
a dry swale, bioretention, or infiltration areas. If sufficient pervious area is available at the site,
larger areas of impervious cover can be treated by vegetated filter strips, using an engineered
level spreader to recreate sheet flow. Vegetated Filter Strips are also well suited to treat runoff
from turf -intensive land uses, such as the managed turf areas of sports fields, golf courses, and
parkland.
Conserved Open Space and Vegetated Filter Strips can be used in a variety of situations;
however there are several constraints to their use:
Filter Slopes and Widths. Maximum slopes for Conserved Open Space and Vegetated Filter
Strips are 6% and 8% respectively, in order to maintain sheet flow through the practice. In
addition, the overall contributing drainage area must likewise be relatively flat to ensure
sheet flow draining into the filter. Where this is not possible, alternative measures, such as an
engineered level spreader, can be used. Minimum widths (flow path) for Conserved Open
Space and Vegetated Filter Strips are dependent on slope, as specified in Table 2.2 above.
Soils. Vegetated Filter Strips are appropriate for all soil types, except fill soils. The runoff
reduction rate, however, is dependent on the underlying Hydrologic Soil Groups (see Table
2.1 above) and whether soils receive compost amendments.
Contributing Flow Path to Filter. Vegetated Filter Strips are used to treat very small
drainage areas of a few acres or less. The limiting design factor is the length of flow directed
to the filter. As a rule, flow tends to concentrate after 75 feet of flow length for impervious
surfaces, and 150 feet for pervious surfaces (Claytor, 1996). When flow concentrates, it
moves too rapidly to be effectively treated by a Vegetated Filter Strip, unless an engineered
level spreader is used. When the existing flow at a site is concentrated, a vegetated swale
should be used instead of a Vegetated Filter Strip (Lantin and Barrett, 2005).
Hotspot Land Uses. Vegetated Filter Strips should not receive hotspot runoff, since the
infiltrated runoff could cause groundwater contamination.
Turf -Intensive Land Uses. Both Conserved Open Space and Vegetated Filter Strips are
appropriate to treat managed turf and the actively -used areas of sports fields, golf courses,
parkland, and other turf -intensive land uses.
Proximity of Underground Utilities. Underground pipes and conduits that cross the
Vegetated Filter Strip are acceptable.
Version 1.9, March 1, 2011 Page 11 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
SECTION 6. DESIGN CRITERIA
6.1. Compost Soil Amendments
Compost soil amendments will enhance the runoff reduction capability of a vegetated filter strip
when located on hydrologic soil groups B, C, and D, subject to the following design
requirements:
• The compost amendments should extend over the full length and width of the filter strip.
• The amount of approved compost material and the depth to which it must be incorporated is
outlined in Stormwater Design Specification No. 4.
• The amended area will be raked to achieve the most level slope possible without using heavy
construction equipment, and it will be stabilized rapidly with perennial grass and/or
herbaceous species.
• If slopes exceed 3%, a protective biodegradable fabric or matting (e.g., EC-2) should be
installed to stabilize the site prior to runoff discharge.
• Compost amendments should not be incorporated until the gravel diaphragm and/or
engineered level spreader are installed (see Section 6.3).
• The local plan approval authority may waive the requirement for compost amendments on
HSG-B soils in order to receive credit as a filter strip if (1) the designer can provide
verification of the adequacy of the on -site soil type, texture, and profile to function as a filter
strip, and (2) the area designated for the filter strip will not be disturbed during construction.
6.2. Planting and Vegetation Management
Conserved Open Space . No grading or clearing of native vegetation is allowed within the
Conserved Open Space. An invasive species management plan should be developed and
approved by the local plan approval authority.
Reforested Conserved Open Space. At some sites, the proposed stream buffer or Conserved
Open Space may be in turf or meadow cover, or overrun with invasive plants and vines. In these
situations, a landscape architect or horticulturalist should prepare a reforestation or restoration
plan for the Conserved Open Space. The entire area can be planted with native trees and shrubs
or planted to achieve a gradual transition from turf to meadow to shrub and forest. Trees and
shrubs with deep rooting capabilities are recommended for planting to maximize soil infiltration
capacity (PWD, 2007). Over -plant with seedlings for fast establishment and to account for
mortality. Plant larger stock at desired spacing intervals (25 to 40 feet for large trees) using
random spacing (Cappiella et al., 2006). Plant ground cover or a herbaceous layer to ensure rapid
vegetative cover of the surface area.
Vegetated Filter Strips. Vegetated Filter Strips should be planted at such a density to achieve a
90% grass/herbaceous cover after the second growing season. Filter strips should be seeded, not
sodded. Seeding establishes deeper roots, and sod may have muck soil that is not conducive to
infiltration (Wisconsin DNR, 2007). The filter strip vegetation may consist of turf grasses,
meadow grasses, other herbaceous plants, shrubs, and trees, as long as the primary goal of at
least 90% coverage with grasses and/or other herbaceous plants is achieved. Designers should
Version 1.9, March 1, 2011 Page 12 of 20
VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
choose vegetation that stabilizes the soil and is salt tolerant. Vegetation at the toe of the filter,
where temporary ponding may occur behind the permeable berm, should be able to withstand
both wet and dry periods. The planting areas can be divided into zones to account for differences
in inundation and slope.
6.3. Diaphragms, Berms and Level Spreaders
Gravel Diaphragms: A pea gravel diaphragm at the top of the slope is required for both
Conserved Open Space and Vegetated Filter Strips that receive sheetflow. The pea gravel
diaphragm is created by excavating a 2-foot wide and 1-foot deep trench that runs on the same
contour at the top of the filter strip. The diaphragm serves two purposes. First, it acts as a
pretreatment device, settling out sediment particles before they reach the practice. Second, it acts
as a level spreader, maintaining sheet flow as runoff flows over the Filter Strip. Refer to Figure
2.5.
• The flow should travel over the impervious area and to the practice as sheet flow and then
drop at least 3 inches onto the gravel diaphragm. The drop helps to prevent runoff from
running laterally along the pavement edge, where grit and debris tend to build up (thus
allowing by-pass of the Filter Strip).
• A layer of filter fabric should be placed between the gravel and the underlying soil trench.
• If the contributing drainage area is steep (6% slope or greater), then larger stone (clean bank -
run gravel that meets VDOT #57 grade) should be used in the diaphragm.
• If the contributing drainage area is solely turf (e.g., sports field), then the gravel diaphragm
may be eliminated.
Permeable Berm: Vegetated Filter Strips should be designed with a permeable berm at the toe of
the Filter Strip to create a shallow ponding area. Runoff ponds behind the berm and gradually
flows through outlet pipes in the berm or through a gravel lens in the berm with a perforated
pipe. During larger storms, runoff may overtop the berm (Cappiella et al., 2006). The permeable
berm should have the following properties:
• A wide and shallow trench, 6 to 12 inches deep, should be excavated at the upstream toe of
the berm, parallel with the contours.
• Media for the berm should consist of 40% excavated soil, 40% sand, and 20% pea gravel.
• The berm 6 to 12 inches high should be located downgradient of the excavated depression
and should have gentle side slopes to promote easy mowing (Cappiella et al., 2006).
• Stone may be needed to armor the top of berm to handle extreme storm events.
• A permeable berm is not needed when vegetated filter strips are used as pretreatment to
another stormwater practice.
Engineered Level Spreaders. The design of engineered level spreaders should conform to the
following design criteria based on recommendations of Hathaway and Hunt (2006), or a locally
approved standard that meets the intent of these criteria, in order to ensure non -erosive sheet
flow into the vegetated buffer area. Figure 2.3 above represents a configuration that includes a
bypass structure that diverts the design storm to the level spreader, and bypasses the larger storm
events around the Conserved Open Space or Vegetated Filter Strip through an improved channel.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
An alternative approach is that utilized by Henrico County, where pipe or channels discharge at
the landward edge of a floodplain or stream (Resource Protection Area or RPA) buffer. The
entire flow is directed through a stilling basin energy dissipater and then a level spreader such
that the entire design storm for the conveyance system (typically a 10-year frequency storm) is
discharged as sheet flow through the buffer. (Refer to Henrico County's Environmental Program
Manual; Chapter 9, Minimum Design Standard 9.01 "Energy Dissipator":
httD://www.co.henrico.va.us/works/eesd/ .)
Key design elements of the engineered level spreader, as provided in Figures 2 and 3, include
the following:
• High Flow Bypass provides safe passage for larger design storms through the filter strip. The
bypass channel should accommodate all peak flows greater than the water quality design
flow.
• A Forebay should have a maximum depth of 3 feet and gradually transition to a depth of 1
foot at the level spreader lip (Figure 2). The forebay is sized such that the surface area is
0.2% of the contributing impervious area. (A forebay is not necessary if the concentrated
flow is from the outlet of an extended detention basin or similar practice).
• The length of the level spreader should be determined by the type of filter area and the design
flow:
0 13 feet of level spreader length per every 1 cubic foot per second (cfs) of inflow for
discharges to a Vegetated Filter Strip or Conserved Open Space consisting of native
grasses or thick ground cover;
0 40 feet of level spreader length per every 1 cfs of inflow when the spreader discharges to
a Conserved Open Space consisting of forested or reforested buffer (Hathaway and Hunt,
2006).
o Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish
the level spreader length by computing a weighted average of the lengths required for
each vegetation type.
o The minimum level spreader length is 13 feet and the maximum is 130 feet.
o For the purposes of determining the Level Spreader length, the peak discharge shall be
determined using the Rational Equation with an intensity of 1-inch/hour.
• The level spreader lip should be concrete, wood or pre -fabricated metal, with a well -
anchored footer, or other accepted rigid, non -erodible material.
• The ends of the level spreader section should be tied back into the slope to avoid scouring
around the ends of the level spreader; otherwise, short-circuiting of the facility could create
erosion.
• The width of the level spreader channel on the up -stream side of the level lip should be three
times the diameter of the inflow pipe, and the depth should be 9 inches or one-half the culvert
diameter, whichever is greater.
• The level spreader should be placed 3 to 6 inches above the downstream natural grade
elevation to avoid turf buildup. In order to prevent grade drops that re -concentrate the flows,
a 3-foot long section of VDOT # 3 stone, underlain by filter fabric, should be installed just
below the spreader to transition from the level spreader to natural grade.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
Vegetated receiving areas down -gradient from the level spreader must be able to withstand
the force of the flow coming over the lip of the device. It may be necessary to stabilize this
area with temporary (VDOT EC-2) or permanent (VDOT EC-3) materials in accordance with
the calculated velocity (on-line system peak, or diverted off-line peak) and material
specifications, along with seeding and stabilization in conformance with the Virginia Erosion
and Sediment Control Handbook.
6.4. Filter Design Material Specifications
Table 2.3 describes materials specifications for the primary treatment within filter strips.
Table 2.3. Vegetated Filter Strip Materials Specifications
Material
Specification
Quantity
Pea Gravel (#8 or ASTM equivalent) or
Diaphragm should be 2 feet wide, 1
Gravel
where steep (6% +) use clean bank -run
foot deep, and at least 3 inches
Diaphragm
VDOT #57 or ASTM equivalent (1-inch
below the edge of pavement.
maximum).
Permeable
40% excavated soil, 40% sand, and 20% pea gravel to serve as the media for the
Berm
berm.
Needled, non -woven, polypropylene geotextile meeting the following
specifications:
Grab Tensile Strength (ASTM D4632): > 120 lbs.
Geotextile
Mullen Burst Strength (ASTM D3786): > 225 lbs./sq. in.
Flow Rate (ASTM D4491): > 125 gpm/sq. ft.
Apparent Opening Size (ASTM D4751): US #70 or #80 sieve
Engineered
Level Spreader lip should be concrete, metal, timber, or other rigid material;
Level
Reinforced channel on upstream of lip: VDOT EC-2 (or EC-3 if velocities require
Spreader
permanent reinforcing).
See Hathaway and Hunt 2006 or Henrico County Program Manual.
Erosion
Control
Where flow velocities dictate, use woven biodegradable erosion control fabric or
Fabric or
mats that are durable enough to last at least 2 growing seasons. (e.g., VDOT
Matting
Erosion Control matting EC-2).
If existing topsoil is inadequate to support dense turf growth, imported top soil
(loamy sand or sandy loam texture), with less than 5% clay content, corrected pH
Topsoil
at 6 to 7, a soluble salt content not exceeding 500 ppm, and an organic matter
content of at least 2% shall be used. Topsoil shall be uniformly distributed and
lightly compacted to a minimum depth of 6 to 8 inches
Compost shall be derived from plant material and provided by a member of the
Compost
U.S. Composting Seal of Testing Assurance (STA) program, as outlined in
Stormwater Design Specification No. 4.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
SECTION 7: REGIONAL & SPECIAL CASE DESIGN ADAPTATIONS
7.1. Karst Terrain
Conserved Open Space areas are highly recommended in karst terrain, particularly when storm
flow discharges to the outer boundary of a karst protection area (see CSN, 2009).
Vegetated Filter Strips can also be used to treat runoff from small areas of impervious cover
(e.g., less than 5,000 square feet). Some communities use wide grass filter strips to treat runoff
from the roadway shoulder.
In no case should the use of a Conserved Open Space or Vegetated Filter Strip be considered as a
replacement for an adequate receiving system for developed -condition stormwater discharges,
unless the adequacy of the design has been demonstrated consistent with the the Virginia
Stormwater Management Handbook.
7.2. Coastal Plain
The use of Conserved Open Space areas and Vegetated Filter Strips are highly recommended in
the coastal plain, particularly when sheetflow (or concentrated flow with an appropriately -sized
level spreader) discharges to the outer boundary of a shoreline, stream or wetland buffer.
Vegetated Filter Strips can also be used to treat runoff from small areas of impervious cover
(e.g., less than 5,000 square feet). In both cases, however, the designer must consider the depth
to the water table. In general, shallow water tables may inhibit the function of Vegetated Filter
Strips.
7.3. Linear Highway Sites
Vegetated Filter Strips are highly recommended to treat highway runoff if the median and/or
road shoulder is wide enough to provide an adequate flow path.
SECTION 8: CONSTRUCTION
8.1. Construction Sequence for Conserved Open Space Areas
The Conserved Open Space must be fully protected during the construction stage of development
and kept outside the limits of disturbance on the Erosion and Sediment (E&S) Control Plan.
• No clearing, grading or heavy equipment access is allowed except temporary disturbances
associated with incidental utility construction, restoration operations or management of
nuisance vegetation.
• The perimeter of the Conserved Open Space shall be protected by super silt fence, chain link
fence, orange safety fence, or other measures to prevent sediment discharge.
• The limits of disturbance should be clearly shown on all construction drawings and identified
and protected in the field by acceptable signage, silt fence, snow fence or other protective
barrier.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
Construction of the gravel diaphragm or engineered level spreader shall not commence until
the contributing drainage area has been stabilized and perimeter E&S controls have been
removed and cleaned out.
Some light grading may be needed at the Filter Strip boundary; this should be done with
tracked vehicles to prevent compaction.
Stormwater should not be diverted into the Vegetated Filter Strip until the gravel diaphragm
and/or level spreader are installed and stabilized.
8.2. Construction Sequence for Vegetated Filter Strips
Vegetated Filter Strips can be within the limits of disturbance during construction. The following
procedures should be followed during construction:
• Before site work begins, Vegetated Filter Strip boundaries should be clearly marked.
• Only vehicular traffic used for Filter Strip construction should be allowed within 10 feet of
the Filter Strip boundary (City of Portland, 2004).
• If existing topsoil is stripped during grading, it shall be stockpiled for later use.
• Construction runoff should be directed away from the proposed Filter Strip site, using
perimeter silt fence, or, preferably, a diversion dike.
• Construction of the gravel diaphragm or engineered level spreader shall not commence until
the contributing drainage area has been stabilized and perimeter E&S controls have been
removed and cleaned out.
• Vegetated Filter Strips require light grading to achieve desired elevations and slopes. This
should be done with tracked vehicles to prevent compaction. Topsoil and or compost
amendments should be incorporated evenly across the filter strip area, stabilized with seed,
and protected by biodegradable erosion control matting or blankets.
• Stormwater should not be diverted into the Filter Strip until the turf cover is dense and well
established.
8.3. Construction Inspection
Construction inspection is critical to obtain adequate spot elevations, to ensure the gravel
diaphragm or ELS is completely level, on the same contour, and constructed to the correct design
elevation. As -built surveys should be required to ensure compliance with design standards.
Inspectors should evaluate the performance of the Filter Strip after the first big storm to look for
evidence of gullies, outflanking, undercutting or sparse vegetative cover. Spot repairs should be
made, as needed.
An example construction phase inspection checklist for Sheet Flow to a Filter Strip or Conserved
Open Space can be found on the Center for Watershed Protection website at:
http://www.cwp.org/Resource Library/Controlling Runoff and Discharges/sm.htm
(scroll to Tool6: Plan Review, BMP Construction, and Maintenance Checklists)
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
SECTION 9. MAINTENANCE
9.1. Maintenance Agreements
Section 4 VAC 50-60-124 of the regulations specifies the circumstances under which a
maintenance agreement must be executed between the owner and the local program. This section
sets forth inspection requirements, compliance procedures if maintenance is neglected,
notification of the local program upon transfer of ownership, and right -of -entry for local program
personnel.
All Vegetated Filter Strips must be covered by a drainage easement to allow inspection and
maintenance. If the filter area is a natural Conserved Open Space, it must be protected by a
perpetual easement or deed restriction that assigns the responsible party to ensure that no future
development, disturbance or clearing may occur within the area, except as stipulated in the
vegetation maintenance plan.
If the Vegetated Filter Strip is located on a residential private lot, the existence and purpose of
the Filter Strip shall be noted on the deed of record. Homeowners will need to be provided a
simple document that explains the purpose of the Filter Strip and routine maintenance needs. A
deed restriction or other mechanism enforceable by the qualifying local program must be in place
to help ensure that Filter Strips are maintained and Conserved Open Space Areas are not
converted or disturbed. The mechanism should, if possible, grant authority for local agencies to
access the property for inspection or corrective action.
9.2. Maintenance Inspections
Annual inspections are used to trigger maintenance operations such as sediment removal, spot re -
vegetation and level spreader repair. Ideally, inspections should be conducted in the non -growing
season when it easier to see the flow path. Example maintenance inspection checklists for Sheet
Flow to a Filter Strip or Conserved Open Space areas can be accessed in Appendix C of Chapter
9 of the Virginia Stormwater Management Handbook or at the Center for Watershed Protection's
website at:
htto://www.cwi).org/Resource Library/Controlling Runoff and Discharges/sm.htm
(scroll to Tool6: Plan Review, BMP Construction, and Maintenance Checklists)
Inspectors should check to ensure that:
• Flows through the Filter Strip do not short-circuit the overflow control section;
• Debris and sediment does not build up at the top of the Filter Strip;
• Foot or vehicular traffic does not compromise the gravel diaphragm;
• Scour and erosion do not occur within the Filter Strip;
• Sediments are cleaned out of Level Spreader forebays and flow splitters; and
• Vegetative density exceeds a 90% cover in the boundary zone or grass filter.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
9.3. Ongoing Maintenance
Once established, Vegetated Filter Strips have minimal maintenance needs outside of the spring
clean up, regular mowing, repair of check dams and other measures to maintain the hydraulic
efficiency of the strip and a dense, healthy grass cover. Vegetated Filter Strips that consist of
grass/turf cover should be mowed at least twice a year to prevent woody growth.
SECTION 10. REFERENCES
Cappiella, K., T. Schueler, and T. Wright. 2006. Urban Watershed Forestry Manual, Part 2.
Conserving and Planting Trees at Development Sites. Center for Watershed Protection. Prepared
for United States Department of Agriculture, Forest Service.
City of Portland, Environmental Services. 2004. Portland Stormwater Management Manual.
Portland, OR. Available online at: hfD://www.l)ortlandonline.com/bes/index.cfm?c=dfbbh
Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for
Watershed Protection. Ellicott City, MD.
CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for
Watershed Protection, Ellicott City, MD.
Hathaway, J. and B. Hunt. 2006. Level Spreaders: Overview, Design, and Maintenance.
Department of Biological and Agricultural Engineering. NC State University. Raliegh, NC.
htto://www. bae. ncsu.ed u/stormwater/Publication Files/Level Si)readers2006. odf.
Henrico County, Virginia. Henrico County Environmental Program Manual. Available online at:
hftp://www.co.henrico.va.us/works/eesd/
North Carolina State University. Level Spreader Design Worksheet. Available online at:
North Carolina Department of Environment and Natural Resources, Division of Water
Quality. "Level Spreader Design Guidelines." January 2007. Available online at:
htto://h2o.enr.state.nc.us/su/Manuals Factsheets.htm
Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the
Northern Virginia BMP Handbook. Fairfax, Virginia.
Philadelphia Stormwater Management Guidance Manual. Available online at:
htti)://www. Phillyriverinfo.oro/Proa rams/S u boroct ram Mai n.asox?ld=StormwaterManual
Schueler, T., D. Hirschman, M. Novotny and J. Zielinski. 2007. Urban Stormwater Retrofit
Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for
Watershed Protection, Ellicott City, MD.
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VA DEQ STORMWATER DESIGN SPECIFICATION NO. 2 SHEET FLOW TO FILTER OR OPEN SPACE
Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake
Stormwater Network. Baltimore, MD. www.chesapeakestormwater.net
Virginia Stormwater Management Handbook. 1999. Volumes I and 2. Richmond, VA.
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