VIRGINIA DCR STORMWATER
DESIGN SPECIFICATION No. 8

INFILTRATION PRACTICES

VERSION 1.6
September 30, 2009

SECTION 1: DESCRIPTION

Infiltration practices utilize temporary surface or underground storage to allow incoming stormwater runoff to exfiltrate into underlying soils. Runoff first passes through multiple pretreatment mechanisms to trap sediment and organic matter before it reaches the practice. As the stormwater penetrates the underlying soil, chemical and physical adsorption processes remove pollutants. Infiltration practices have the greatest runoff reduction capability of any stormwater practice and are suitable for use in residential and other urban areas where measured soil permeability rates exceed 0.5 inch per hour. Infiltration should not be utilized at sites designated as stormwater hotspots to prevent possible groundwater contamination. When used appropriately, infiltration has a very high runoff reduction capability, as shown in Table 1.

Table 1: Summary of Stormwater Functions Provided by Infiltration
Stormwater Function	Level 1 Design	Level 2 Design
Annual Runoff Reduction	50%	90%
Total Phosphorus Removal 1	25%	25%
Total Nitrogen Removal 1	15%	15%

Channel and Flood Protection

Use RRM spreadsheet to calculate Curve Number Adjustment;

Design extra storage (optional; as needed) on the surface or in the subsurface storage to accommodate larger storm volumes, and use NRCS TR-55 Runoff Equations2 to compute CN Adjustment.

1 Change in event mean concentration (EMC) through the practice. Actual nutrient mass load removed is the product of the removal rate and the runoff reduction rate.
2 NRCS TR-55 Runoff Equations 2-1 thru 2-5 and Figure 2-1 can be used to compute a curve number adjustment for larger storm events based on the retention storage provided by the practice(s).
Sources: CWP and CSN (2008) and CWP (2007).

SECTION 2: LEVEL 1 AND LEVEL 2 DESIGN TABLE

The major design goal for Infiltration is to maximize nutrient removal and runoff reduction. To this end, designers may choose to go with the baseline design (Level 1) or choose an enhanced Level 2 that maximizes nutrient and runoff reduction. To qualify for Level 2, the infiltration practice must meet all design criteria shown in the right hand column of Table 2.

Table 2: Level 1 and Level 2 Infiltration Design Guidelines
Level 1 Design (RR:50; TP:25; TN:15)	Level 2 Design (RR:90; TP:25; TN:15)
Tv = [(Rv)(A)/12] – volume reduced by upstream BMP	Tv = [1.1(Rv)(A)/12] – volume reduced by upstream BMP
At least two forms of pretreatment (Table 6)	At least three forms of pretreatment (Table 6)
Soil infiltration rate of 0.5 to 1.0 in/hr (Section 5.1 & Appendix A); number of tests depends on scale (Table 3)	Soil infiltration rates of 1.0 to 4.0 in/hr (Section 5.1 & Appendix A); number of tests depends on scale (Table 3)
Minimum of 2’ between bottom of infiltration practice and seasonal high water or bedrock (Section 4)
Tv infiltrates within 36 to 48 hours (Section 5.6)
Building Setbacks – See Table 3
All Designs: are subject to hotspot runoff restrictions/prohibitions

SECTION 4: PHYSICAL FEASIBILITY & DESIGN APPLICATIONS

Since infiltration practices have a very high runoff reduction capability, they should always be initially considered when evaluating a site. Designers should evaluate the range of soil properties during initial site layout, and seek to configure the site to conserve and protect the soils with the greatest recharge and infiltration rates. In particular, areas of Hydrologic Soil Group A or B soils shown on NRCS soil surveys should be considered as primary locations for infiltration practices. At this point, designers should carefully analyze and define constraints on infiltration, as follows:

Contributing Drainage Area: The maximum contributing drainage area (CDA) to an individual infiltration practice should be less than 2 acres and as close to 100% impervious as possible. This specification covers three scales of infiltration practices (1) Micro-infiltration (250 to 2,500 sq. ft of CDA), (2) small scale infiltration (2,500 to 20,000 sq. ft. of CDA) and (3) conventional infiltration (20,000 to 100,000 sq ft of CDA). The design, pretreatment and maintenance requirements differ depending on the scale at which infiltration is applied (see Table 3 for a summary).

Site Topography: Unless slope stability calculations demonstrate otherwise, infiltration practices should be located a minimum horizontal distance of 200 feet from down gradient slopes greater than 20%. The average slope of the contributing drainage areas should be less than 15%.
Practice Slope: The bottom of an infiltration practice should be flat to enable even distribution and infiltration of stormwater (i.e., 0% longitudinal slope) although a maximum longitudinal slope of 1% is permissible if an underdrain is employed. Lateral slopes should be 0%.

Minimum Head: The elevation difference needed to operate a micro-scale infiltration practice is nominal, although two or more feet of head may be needed to drive small-scale and conventional infiltration practices.
Minimum Depth to Water Table: A minimum vertical distance of 2 feet shall be provided between the bottom of the infiltration practice and the seasonal high water table or bedrock layer.

Soils: Native soils in proposed infiltration areas must have a minimum infiltration rate of 0.5 inches per hour (typically Hydrologic Soil Group (HSG) A and B soils meet these criteria). Initially, projected soil infiltration rates can be estimated from NRCS soil data, but they must be confirmed by an on-site infiltration evaluation. Native soils must have silt/clay content less than 40% and clay content less than 20%.
Use on Urban Soils/Redevelopment Sites: Sites that have been previously graded or disturbed do not retain their original soil permeability due to compaction. In addition, infiltration practices should never be situated above fill soils.

Dry Weather Flows: Infiltration practices should not be used on sites receiving regular dry weather flows from sump pumps, irrigation nuisance water and similar kinds of flows.
Setbacks: Infiltration practices should not be hydraulically connected to structure foundations or pavement in order to avoid harmful seepage. Setbacks to structures and roads vary based on the scale of infiltration (see Table 1). At a minimum, conventional and small-scale infiltration practices should be located a minimum horizontal distance of 100 feet from any water supply well, and 50 feet from septic systems, and at least 5 feet down-gradient from dry or wet utility lines.

High Loading Situations: Infiltration practices are not intended to treat sites with high sediment or trash/debris loads, as they will cause the practice to clog and fail.
Groundwater Protection: Section 9 of this specification presents a list of potential stormwater hotspots that pose a risk of groundwater contamination. Infiltration of runoff from designated hotspots is highly restricted or prohibited

Site Specific Considerations: Infiltration practices can be applied to most land uses that have measured soil infiltration rates that exceed 0.5 inches per hour. However, there is no single infiltration application that fits every development situation. The nature of the actual design application depends on four key design factors, described below:
- The first factor is the Design Scale at which infiltration will be applied:

Micro-infiltration is intended for residential rooftop disconnection, rooftop rainwater harvesting systems, or other small scale (250 to 2,500 ft2 of impervious area treated) application;
Small scale infiltration is intended for residential and/or small commercial applications that meet the feasibility criteria noted above; and
Conventional infiltration can be considered for most typical development and re-development applications and as such has more rigorous site selection and feasibility criteria.

Table 3 compares the different design approaches and requirements associated with each infiltration scale.

The second key design factor relates to the mode of temporarily storing runoff prior to infiltration – either on the surface or in an underground trench. When storing runoff on the surface (e.g., an infiltration basin), the maximum depth should be no greater than one foot, unless pretreatment cells are used, in which case, a maximum depth of two feet is permissible. In the underground mode, runoff is stored in the voids of the stones, and infiltrates into the underlying soil matrix. Perforated corrugated metal pipe, plastic or concrete arch pipe, or comparable materials can be used in conjunction with stone to increase the available temporary underground storage. In some instances, a combination of filtration and infiltration cells can be installed in the floor of a detention pond or a dry extended detention (ED) pond.
The third design factor relates to the degree of confidence that exfiltration can be maintained over time, given the measured infiltration rate for the subsoils at the practice location and the anticipated land uses. This determines whether an underdrain is needed, or whether an alternative practice, such as bioretention, is needed at the site (see Table 4).

Table 3: The Three Design Scales for Infiltration Practices
Design Factor	Micro Infiltration	Small-Scale Infiltration	Conventional Infiltration
Impervious Area Treated	250 to 2,500 sq. ft.	2,500 to 20,000 sq. ft.	20,000 to 100,000 sf
Typical Practices	Dry Well French Drain Paving Blocks	Infiltration Trench Permeable Paving*	Infiltration Trench Infiltration Basin
Min.Infiltration Rate	0.5 inches/hour	0.5 inches/hour	0.5 inches per hour
Design Infil Rate	50% of measured rate
Observation Well	No	Yes	Yes
Type of Pretreatment (Refer to Table 6)	External (leaf screens, grass filter strip, etc)	Vegetated filter strip or grass channel, forebay, etc.	Pretreatment Cell
Depth Dimensions	Max 3 Foot Depth	Max 5 Foot Depth	Max 6 Foot Depth,
UIC Permit Needed	No	No	Only if surface width is less than max depth
Head Required	Nominal 1 to 3 feet	Moderate 1 to 5 feet	Moderate 2 to 6 feet
Underdrain Requirements?	Elevated underdrain only on marginal soils	None Required	Back up underdrain
Required Soil Tests	One per practice	One per 1,000 sq. ft of surface area or Two per practice	One per 1,000 sq. ft of surface area
Building Setbacks	5 feet down-gradient1 25 feet up-gradient	10 feet down-gradient 50 feet up-gradient	25 feet down-gradient 100 feet up-gradient
* Although permeable pavement is an infiltration practice, a more detailed specification is provided in Design Specification No. 7 1 Note that the building setback of 5 feet is intended for simple foundations. The use of a dry well or french drain adjacent to an in-ground basement or finished floor area should be carefully designed and coordinated with the design of the structure’s water-proofing system (foundation drains, etc.), or avoided altogether.

The final factor is whether the infiltration practice will be designed as an on-line or off-line facility, as this determines the nature of conveyance and overflows needed. Off-line practices are sized to only accept some portion of the treatment volume, and employ a flow splitter to safely bypass large storms. On-line infiltration practices may be connected to underground perforated pipes to detain the peak storm event, or have suitable overflows to pass the storms without erosion.

Table 4: Design Choices Based on Measured Infiltration Rate
	Measured Infiltration Rate (inches/hour)
Recommended Design Solution	Less than 0.5	0.5 to 1.0	1.0 to 4.0	More than 4
Recommended Design Solution	Use Bioretention or Dry Swale w/ underdrain	Use Infiltration w/o underdrain, or 12” stone reservoir below underdrain invert. Alternately, use Bioretention w/elevated underdrain	Use Infiltration Bioretention, or Dry Swale w/o underdrain	Use Infiltration, Bioretention, or Dry Swale w/o underdrain

SECTION 5: DESIGN CRITERIA

5.1. Defining the Infiltration Rate

Soil permeability is the single biggest factor when evaluating infiltration practices. A minimum field verified infiltration rate of at least 0.5 inches/hour is needed to make the practice work.

Projected infiltration rate: For planning purposes, the projected infiltration rate for the site can be estimated using the SCS soil textural triangle for the prevailing soil types indicated on the local NRCS Soil Survey. This data is solely used to locate portions of the site where infiltration may be feasible and to pinpoint where actual on-site infiltration tests will be taken to confirm feasibility.

Measured Infiltration rate: On-site infiltration investigations should always be conducted to establish the actual infiltration capacity of underlying soils using the methods presented in Appendix A.
Design Infiltration rate: Several studies have shown that ultimate infiltration rates decline by as much as 50% from initial rates, so designers should be very conservative and not force infiltration on questionable soils. Therefore, the infiltration rate used in design shall be 50% of the measured rate.

5.2. Sizing of Infiltration Facilities

Several equations are needed to size infiltration practices. The first equations establish the maximum depth of the infiltration practice, depending on whether it is a surface (equation 1)or underground reservoir (equation 2).

(1) Basin

(2) Underground Reservoir

where:
dmax      = maximum depth of the infiltration practice (feet)
f           = measured infiltration rate (ft/day)
td          = maximum drawn down time (normally 1.5 to 2 days) (day)
Vr        = void ratio of the stone reservoir (0.4)

Designers should compare these results to the maximum allowable depths in Table 5, and use whichever value is less for subsequent design.

Table 5. Maximum Depth for Infiltration Practices (feet)
Mode of Entry	Scale of Infiltration
Mode of Entry	Micro Infiltration	Small Scale Infiltration	Conventional Infiltration
Basin	1.0	1.5	2.0
Reservoir	3.0	5.0	varies

Once the maximum depth is known, then calculate the surface area needed for an infiltration practice using equation 2:

(3) Basin SA = TV / (d + ½ f*tf )

(4) Underground Reservoir SA = TV / (Vr*d + ½ f*tf )

Where:
SA        = surface area (sf)
TV       = design volume (e.g., portion of treatment volume (ft3)
Vr         = Void Ratio (assume 0.4)
d          = infiltration depth (maximum depends on scale of infiltration and results of equation 1 (ft)
f           = measured infiltration rate (ft/day)
tf          = Time to fill infiltration facility (typically 2 hours, or 0.083 days) (day)

If designers choose to infiltrate less than the full Treatment volume (e.g., use of micro-infiltration or small scale infiltration) the runoff reduction rates shown in Table 6 shall be directly prorated in the runoff reduction spreadsheet. To qualify for Level 2 runoff reduction rates, designers must provide 110% of the site-adjusted water quality volume.

5.3. Soil Infiltration Rate Testing

The acceptable methods for on-site soil infiltration rate testing procedures are outlined in Appendix A. Since soil infiltration properties can vary, the following testing is recommended for each scale of infiltration.

Micro-infiltration: one test per facility
Small Scale Infiltration: One per 1,000 sq. ft of surface area or two tests per facility
Conventional Infiltration: one test per 1,000 sq. ft. of proposed infiltration bed

5.4. Pretreatment Features

Every infiltration practice must include multiple pretreatment techniques, although the nature of pretreatment practices depends on the scale at which infiltration is applied. The number, volume and type of acceptable pretreatment techniques needed for the three scales of infiltration are provided in Table 6.

Table 6: Required Pretreatment Elements for Infiltration Practices
	Scale of Infiltration
Pretreatment 1	Micro Infiltration	Small Scale Infiltration	Conventional Infiltration
Number and Volume of Pretreatment	2 external techniques, no minimum pretreatment volume required	3 techniques, 15% minimum pretreatment volume required (inclusive)	3 techniques, 25% minimum pretreatment volume required (inclusive) At least one separate pre-treatment cell
Acceptable Pretreatment Techniques	Leaf gutter screens Grass buffer Upper sand layer Washed bank run gravel	Grass filter strip Grass channel Plunge pool Pea gravel diaphragm	Sediment trap cell Sand filter cell Sump pit Grass channel or filter strip
1 A minimum of 50% of the runoff reduction volume must be pre-treated by a filtering or bioretention practice prior to infiltration if the site is a restricted stormwater hotspot

Note that conventional infiltration practices require pretreatment of at least 25% of the treatment volume, including a surface pretreatment cell capable of keeping sediment and vegetation out of the infiltration cell. All pretreatment practices should be designed such that exit velocities are non-erosive for the two year design storm and evenly distribute flows across the width of the practice (e.g., using a level spreader).

5.5. Conveyance and Overflow

The nature of the conveyance and overflow to an infiltration practice depends on the scale of infiltration and whether the facility is on-line or off-line (Table 7). Where possible, conventional infiltration practices should be designed offline to avoid damage from the erosive velocities of larger design storms. Micro-scale and small scale infiltration practices shall be designed to maintain non-erosive conditions for overland flows generated by the 2-year design storm (typically 3.5 to 5.0 feet per second).

Table 7: Conveyance and Overflow Choices Based on Infiltration Scale
	Scale of Infiltration
Conveyance and Overflow	Micro Infiltration	Small Scale Infiltration	Conventional Infiltration
Online Design	Non-erosive pervious overland flow path to street or storm drain system for two year design storm	An overflow mechanism such as a elevated drop inlet or flow splitter should be used to redirect flows to an overflow channel or stabilized water course. Non-erosive down slope overflows for ten year design event
Off-line Design	Not Recommended	A flow splitter or overflow structure can be used for this purpose using design guidance in Claytor and Schueler(1996) and ARC (2001).

5.6. Internal Geometry and Drawdowns

Runoff Reduction Volume Sizing:. The design approach for infiltration is to not force a large amount of infiltration into a small area. Therefore, individual infiltration practices that are limited in size due to soils permeability and available space need not be sized to achieve the full water quality volume of the contributing drainage area, as long as other runoff reduction practices are applied at the site to meet the remainder. The total runoff reduction volume shall be documented using the VA DCR runoff reduction spreadsheet or another locally approved methodology that achieves equivalent results. The minimum amount of runoff from a given drainage area that can be treated in individual practices is noted in Table 2.
Infiltration Basin Restrictions: The maximum vertical depth to which runoff may be ponded over an infiltration area is 24 inches (i.e., infiltration basin). The side-slopes should be 4:1 or gentler, and if the basin serves a CDA greater than 20,000 sf, a surface pretreatment cell must be provided (can be sand filter or dry sediment basin.
Rapid Drawdown: When possible, infiltration practices should be sized so that the target runoff reduction volume infiltrates within 36 hours to 48 hours to provide a factor of safety that prevents nuisance ponding conditions.
Conservative Infiltration Rates: Designers should always use the design infiltration rate rather than the measured infiltration rate to approximate long term infiltration rates (see Section 5.1).
Void Ratio: A porosity value of 0.40 shall be used in the design of stone reservoirs, although a larger value may be used if perforated corrugated metal pipe, plastic or concrete arch pipe, or comparable materials are provided in the reservoir.

5.7. Landscaping and Safety

Infiltration trenches can be effectively integrated into the site planning process, and aesthetically designed with adjacent native landscaping or turf cover, subject to the following additional design considerations:

Infiltration practices should NEVER be installed until all up-gradient construction is completed AND pervious areas are stabilized with dense and healthy vegetation.
Vegetation associated with the infiltration buffers should be regularly mowed and maintained to keep organic matter out and maintain enough native vegetation to prevent soil erosion from occurring
Infiltration practices do not pose any major safety hazards after construction. However, if an infiltration practices will be excavated to a depth greater than 5 feet OSHA health and safety guidelines need to be consulted for safe construction practices.
Fencing of infiltration trenches is neither necessary nor desirable.

Designers should always evaluate the nature of future operations to determine if the proposed site will be designated as a potential stormwater hotspot (see Section 9.1), and comply with the appropriate restriction or prohibition on infiltration.

5.8. Maintenance Reduction Features

Maintenance is a crucial element to ensuring the long-term performance of infiltration practices. The most frequently cited maintenance problem for infiltration practices is clogging caused by organic matter and sediment. The following design features can minimize the risk of clogging:

Observation Well: Small scale and conventional infiltration practices should include an observation well consisting of an anchored six-inch diameter perforated PVC pipe fitted with a lockable cap installed flush with the ground surface to facilitate periodic inspection and maintenance.

No Filter Fabric on Bottom: The use of geotextile filter fabric along the bottom of infiltration practices should be avoided. Experience has shown that filter fabric is prone to clogging, and that a layer of coarse washed stone (choker stone) is a more effective substitute. Permeable filter fabric shall be installed on the trench sides to prevent soil piping.

Direct Maintenance Access: Access shall be provided to allow crews and heavy equipment to perform non-routine maintenance tasks, such as practice reconstruction or rehabilitation. While a turf cover is permissible for micro and small-scale infiltration, the surface shall never be covered by an impermeable material, such as asphalt or concrete

5.9. Infiltration Material Specifications

The basic material specifications for infiltration practices are outlined in Table 8.

Table 8: Infiltration Material Specifications
Material	Specification	Notes
Stone	Clean, aggregate with maximum diameter of 3.5 inches and minimum diameter of 1.5 inches (VDOT No. 1 Open Graded Coarse Aggregate) or equivalent
Observation Well	Vertical 6-inch Schedule 40 PVC perforated pipe, with lockable cap and anchor plate.	One per 50 feet of length of infiltration practice
Trench Bottom	6 to 8 inch sand layer	VDOT Fine Aggregate Grade A or B
Trench Surface Cover	3 inch layer of river stone or pea gravel. Turf is acceptable when inflow is subsurface (i.e., roof leader)	Attractive surface cover that can suppress weed growth
Buffer Vegetation	Keep adjacent vegetation from forming an overhead canopy above practices, in order to keep vegetation, fruits and other material from clogging trench
Filter Fabric (sides only	Non-woven polyprene geotextile with a flow rate of > 110 gallons/minutes/square foot (e.g., Geotex 351 or equivalent)
Choking Layer	2 to 4 inch layer of choker stone (typically #8 or # 89 washed gravel) over the underdrain stone
Underdrain (where needed)	6-inch rigid schedule 40 PVC pipe, with 3/8” perforations at 6-inchs on center, each underdrain to use perforated pipe on 1% slope for length of infiltration practice	Non-perforated pipe as needed from downspout to a point 15 feet from structure; one or more caps. T’s as needed for underdrain configuration.
Stone Jacket for Underdrain	Stone should be double-washed and clean and free of all soil and fines.	6 to 8 inches of # 57 stone in underdrain

SECTION 6: REGIONAL DESIGN ADAPTATIONS

6.1. Karst Terrain

Conventional infiltration practices should not be used in active karst regions due to concerns about sinkhole formation and groundwater contamination. Micro or small scale infiltration areas are permissible IF geotechnical studies indicate at least 4 feet of vertical separation exist between the bottom of these facilities and the underlying active karst layer AND an impermeable liner and underdrain are used. In many cases, bioretention is a preferred stormwater alternative to infiltration in karst areas.

6.2. Coastal Plain

The flat terrain, low head and high water table of many coastal plain sites can constrain the application of conventional infiltration practices. However, such sites are still suited for micro-infiltration and small-scale infiltration practices. Designers should maximize the surface area of the infiltration practice, and keep the depth of infiltration to less than 24 inches. Where soils are extremely permeable (more than 4.0 inches per hour) shallow bioretention is a preferred alternative. Where soils are more impermeable (i.e., marine clays with less than 0.5 inches/hour), designers may prefer to use a constructed wetland practice.

6.3. Steep Terrain

Forcing conventional infiltration practices in steep terrain can be problematic with respect to slope stability, excessive hydraulic gradients and sediment delivery. It is generally recommended that, unless slope stability calculations demonstrate otherwise, infiltration practices should be located a minimum horizontal distance of 200 feet from down gradient slopes greater than 20%. Micro-infiltration and small-scale infiltration can work well as long as their smaller up-gradient and down-gradient building setbacks are satisfied.

6.4. Winter Performance:

Infiltration practices can be designed to withstand more moderate winter conditions. The main problem is caused by ice forming in the voids or the subsoils below the practice which may briefly result in nuisance flooding when spring melt occurs. The following design adjustments are recommended for infiltration practices installed in higher elevations of the Bay watershed, such as New York, Pennsylvania, and West Virginia:

The bottom of the practice should extend below the frost line.
Infiltration practices are not recommended at roadside locations that are heavily sanded and/or salted in the winter months (to prevent movement of chlorides into groundwater and prevent clogging by road sand).
Pretreatment measures can be oversized to account for the additional sediment load caused by road sanding (up to 40% of the Treatment Volume).
Infiltration practices must be setback at least 25 feet from roadways to prevent potential frost heaving of road pavements

6.5. Linear Highway Sites

Infiltration practices can work well at linear highway projects, where soils are suitable and can be protected from heavy disturbance and compaction during road construction operations.

SECTION 7: INFILTRATION CONSTRUCTION SEQUENCE AND INSPECTION

7.1. Construction Sequence

The following is a typical construction sequence to properly install infiltration practices. The sequence may need to be modified to reflect the scale of infiltration, expected site conditions and whether or not an underdrain needs to be installed.

Infiltration practices are particularly vulnerable to failure during the construction phase for two reasons. First, if the construction sequence is not followed correctly, construction sediment can clog the practice. In addition, heavy construction can result in compaction of the soil, which can then reduce the soil’s infiltration rate. For this reason, a careful construction sequence needs to be followed.

During site construction, the following steps are absolutely critical:

Avoid excessive compaction by preventing construction equipment and vehicles from traveling over the proposed location of the infiltration practice.

Keep the infiltration practice “off-line” until construction is complete. Sediment shall be prevented from entering the infiltration site by using super silt fence, diversion berms or other means. The erosion and sediment control plan shall indicate the earliest point at which storm runoff may be directed to a conventional infiltration basin, and the specific means by which the delay in runoff shall be accomplished

Infiltration practices should never serve as a sediment control device during site construction

Upland drainage areas need to be completely stabilized with a thick layer of vegetation prior to commencing excavation of an infiltration practice, as verified by the local erosion and sediment control inspector/program.

The actual installation of an infiltration practice is done using the following steps:

1. Excavate the infiltration practice to design dimensions from the side using a backhoe or excavator. The floor of the pit should be completely level.

2. Correctly install filter fabric on trench sides- Large tree roots should be trimmed flush with the sides of infiltration trenches to prevent puncturing or tearing of the filter fabric during subsequent installation procedures. When laying out the geotextile, the width should include sufficient material to compensate for perimeter irregularities in the trench and for a 6-inch minimum overlap at the top of the trench. The filter fabric itself should be tucked under the sand layer on the bottom of the infiltration trench, and stones or other anchoring objects should be placed on the fabric at the trench sides to keep the trench open during windy periods. Voids may occur between the fabric and the excavated sides of a trench. Natural soils should be placed in all voids to ensure the fabric conforms to the sides of excavation.

3. Scarify the bottom of the infiltration practice, and spread six inches of sand for the bottom filter layer.

4. Install the underdrain, if one is needed.

5. Anchor the observation well and add stone to the practice in 1-foot lifts.

6. Use sod to establish dense turf cover 10 feet on each side of the infiltration practice to reduce erosion and sloughing using sod. If seeding instead, native grasses are recommended primarily due to their adaptability to local climates and soil conditions.

7.2. Construction Inspection

Inspections during construction are needed to ensure that the infiltration practice is built in accordance with the approved design and standards and specifications. Qualified individuals should use detailed inspection checklists to include sign-offs at critical stages of construction to ensure that the contractor’s interpretation of the plan is acceptable to the designer. An example construction phase inspection checklist for infiltration practices can be accessed at the CWP website at http://www.cwp.org/Resource_Library/Center_Docs/SW/pcguidance/Tool6.pdf

SECTION 8: INFILTRATION MAINTENANCE

8.1. Maintenance Agreements

Section 4VAC 50-60-124 of the regulations specifies a maintenance agreement to be executed between the owner and the local program. The section requires a schedule of inspections, compliance procedures if maintenance is neglected, notification of the local program upon transfer of ownership, and right-of-entry for local program personnel.

When micro or small-scale infiltration practices are applied on private residential lots, homeowners will need to be educated on their routine maintenance needs, understand the long-term maintenance plan, and be subject to a deed restriction or other mechanism enforceable by the qualifying local program to ensure that infiltrating 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. In addition, the GPS coordinates for all infiltration practices shall be provided upon facility acceptance to ensure long term tracking.

8.2. Maintenance Inspections

Annual site inspections are critical to the performance and longevity of infiltration practices, particularly for small-scale and conventional infiltration practices. Maintenance of infiltration practices is driven by annual inspections that evaluate the condition and performance of the practice (see Table 9). Based on inspection results, specific maintenance tasks will be triggered. An annual maintenance inspection form for infiltration practices can be accessed at the CWP website at www.cwp.org/postconstruction (Tool #6).

8.3. Ongoing Maintenance

Effective long-term operation of infiltration practices requires a dedicated and routine maintenance inspection schedule with clear guidelines and schedules, as shown in Table 10. Where possible, facility maintenance should be integrated into routine landscaping maintenance tasks.

Table 9: Suggested Annual Maintenance Inspection Points for Infiltration Practices
Activity
The drawdown rate should be measured at the observation well for three days following a storm event in excess of 0.5 inch in depth. If standing water is still observed in the well after three days, this is a clear sign that that clogging is a problem
Check inlets, pretreatment cells and any flow diversion structures for sediment buildup and structural damage. Note if any sediment needs to be removed
Inspect the condition of the observation well and make sure it is still capped
Check that no vegetation forms an overhead canopy that may drop leaf litter, fruits and other materials that could clog the device
Evaluate the vegetative quality of the adjacent grass buffer and do spot reseeding if cover is less than 90%
Generally inspect the upland CDA for any controllable sources of sediment or erosion
Look for weedy growth on rock surface that might indicate sediment deposition or clogging
Inspect maintenance access to ensure it is free of woody vegetation and check to see whether valves, manholes or locks can be opened and operated.
Inspect internal and external infiltration side slopes for evidence of sparse vegetative cover, erosion or slumping, and repaired immediately

	Table 10: Typical Maintenance Activities for Infiltration Practices
	Activity	Schedule
Replace pea gravel/topsoil and top surface filter fabric (when clogged) Mow grass filter strips as necessary and remove clippings.		As needed
Ensure that contributing area, practice and inlets are clear of debris. Ensure that the contributing area is stabilized. Remove sediment and oil/grease from pretreatment devices, as well as overflow structures. Repair undercut and eroded areas at inflow and outflow structures		Quarterly
Check observation wells following 3 days of dry weather. Failure to percolate within this time period indicates clogging. Inspect pretreatment devices and diversion structures for sediment build-up and structural damage. Remove trees that start to grow in the vicinity of the trench.		Semi-annual Inspection
Clean out accumulated sediments from the pretreatment cell		Annually

SECTION 9: COMMUNITY AND ENVIRONMENTAL CONCERNS

9.1. Designation of Stormwater Hotspots

Stormwater hotspots are operations or activities that are known to produce higher concentrations of stormwater pollutants and/or have a greater risk for spills, leaks or illicit discharges. Table 11 presents a list of potential land uses or operations that may be designated as a stormwater hotspot. It should be noted that the actual hotspot generating area may only occupy a portion of the entire proposed use, and that some “clean” areas (such as rooftops) can be diverted away to another infiltration or runoff reduction practice. Communities should carefully review development proposals to determine if future operations, in all or part of the site, will be designated as a potential stormwater hotspot. Based on this designation, one or more design responses are required, as shown below:

1.   Stormwater Pollution Prevention Plan (SWPPP). The SWPPP required as part of a VPDES industrial activity or a municipal stormwater permit outlines pollution prevention and treatment practices that will be implemented to minimize polluted discharges from the on-going operations of the facility. (This is different from the SWPPP required as part of regulated construction activities.) Other facilities or operations that are not classified as industrial activities (SIC Codes) are not required to have an Industrial VPDES permit, but may still be designated as potential stormwater hotspots by the local review authority, as part of their local stormwater ordinance (these are shown in the shaded areas of Table 4). It is recommended that these facilities include an addendum to their stormwater plan that details the pollution prevention practices and employee training measures that will be used to reduce contact of pollutants with rainfall or snowmelt.
2.   Restricted Infiltration. A minimum of 50% of the total treatment volume must be treated by a filtering or bioretention practice prior to any infiltration. Portions of the site that are not associated with the hotspot generating area should be diverted away and treated by another acceptable stormwater practice.
3.   Infiltration Prohibition. The risk of groundwater contamination from spills, leaks or discharges is so great at these sites that infiltration of stormwater or snowmelt is prohibited.

Table 11: Potential Stormwater Hotspot and Site Design Responses
Potential Stormwater Hotspot Operation	SWPP Required?	Restricted Infiltration	No Infiltration
Facilities w/NPDES Industrial permits	Yes	■	■
Public works yard	Yes		ü
Ports, shipyards and repair facilities	Yes		ü
Railroads/ equipment storage	Yes		ü
Auto and metal recyclers/scrapyards	Yes		ü
Petroleum storage facilities	Yes		ü
Highway maintenance facilities	Yes		ü
Wastewater, solid waste and composting facilities	Yes		ü
Industrial machinery and equipment	Yes	ü
Trucks and trailers	Yes	ü
Airfields	Yes	ü
Aircraft maintenance areas	Yes		ü
Fleet storage areas	Yes		ü
Parking lots (40 or more parking spaces)	No	ü
Gas stations	No		ü
Highways (2500 ADT)	No	ü
Construction business (paving, heavy equipment storage and maintenance	No	ü
Retail/wholesale vehicle/ equipment dealers	No	ü
Convenience stores/fast food restaurants	No	ü
Vehicle maintenance facilities	No		ü
Car washes	No		ü
Nurseries and garden centers	No	ü
Golf courses	No	ü
Note: For a full list of potential stormwater hotspots. please consult Schueler et al (2004)
Key: ■ depends on facility

9.2. Other Environmental and Community Issues

Several community and environmental concerns may also arise when infiltration practices are proposed:

Nuisance Conditions: Poorly designed infiltration practices can create potential nuisance problems such as basement flooding, poor yard drainage and standing water. In most cases, these problems can be minimized through proper adherence to setback, soil testing and pretreatment requirements outlined in this specification.

Mosquito Risk: Infiltration practices have some potential to create conditions favorable to mosquito breeding if they clog and have standing water for extended periods.

Groundwater Injection Permits: Groundwater injection permits are required if the infiltration practice is deeper than the longest surface area dimension of the practice (EPA, 2008). Designers should investigate whether or not a proposed infiltration practice is subject to a state or local groundwater injection permit.

SECTION 10: DESIGN REFERENCES

The following references and resources were used to develop this master specification.

Center for Watershed Protection (CWP). 2007. Urban Stormwater Retrofit Practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Ellicott City, MD.

Center for Watershed Protection (CWP). 2003. New York State Stormwater Management Design Manual. Prepared for the New York State Department of Environmental Conservation.

Delaware Urban Runoff Management Approach
http://www.dnrec.state.de.us/DNREC2000/Divisions/Soil/Stormwater/New/GT_Stds%20&%20Specs_06-05.pdf

Maryland Department of Environment (MDE). 2000. Maryland Stormwater Design Manual. Baltimore, MD.

New Jersey Stormwater Best Management Practices Manual
http://www.nj.gov/dep/watershedmgt/bmpmanualfeb2004.htm

North Shore City 2007. Infiltration Design Guidelines. Sinclair, Knight and Merz. Auckland, New Zealand

Pennsylvania Draft Stormwater Best Management Practices Manual
http://www.dep.state.pa.us/dep/subject/advcoun/Stormwater/stormwatercomm.htm

Schueler, T., C. Swann, T. Wright and S. Sprinkle. 2004. Pollution source control practices. Manual No. 8 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection. Ellicott City, MD.

Virginia Department of Conservation and Recreation (VA DCR). 1999. Virginia Stormwater Management Handbook. Volumes 1 and 2. Division of Soil and Water Conservation. Richmond, VA.

APPENDIX A: INFILTRATION TESTING PROCEDURES

On-site testing should be conducted to establish the infiltration capacity of the native soils and determine the feasibility of infiltration. This appendix presents a basic infiltration testing procedure that can be used to determine soil infiltration rates at a development site.

I. Test Pit/Boring Procedures

1 test pit or standard soil boring should be provided for every 1,000 square feet of the proposed infiltration area.
The location of each test pit or standard soil boring should correspond to the location of the proposed infiltration area.
Excavate each test pit or dig each standard soil boring to a depth at least 2 feet below the bottom of the proposed infiltration area.
If the groundwater table is located within 2 feet of the bottom of the proposed facility, determine the depth to the groundwater table immediately upon excavation and again 24 hours after excavation.
Conduct Standard Penetration Testing (SPT) every 2 feet to a depth that is 2 feet below the bottom of the proposed infiltration area.
Determine the USDA or Unified Soil Classification system textures at the bottom of the proposed infiltration area and at a depth that is 2 feet below the bottom. All soil horizons should be classified and described.
If bedrock is located within 2 feet of the bottom of the proposed area, determine the depth to the bedrock layer.
Test pit/soil boring stakes should be left in the field to identify where soil investigations were performed.

II. Infiltration Testing Procedures

One infiltration test should be provided for every 1,000 square feet of surface area for the infiltration area.
The location of each infiltration test should correspond to the location of the proposed infiltration area.
Install a test casing (e.g., rigid, 4 to 6 inch diameter pipe) to a depth 2 feet below the bottom of the proposed infiltration area.
Remove all loose material from sides of the test casing and any smeared soil surfaces from the bottom of the test casing to provide a natural soil interface into which water may percolate. If desired, a 2-inch layer of coarse sand or fine gravel may be placed at the bottom of the test casing to prevent clogging and scouring of the underlying soils. Fill the test casing with clean water to a depth of two feet and allow the underlying soils to pre-soak for 24 hours.
After 24 hours, refill the test casing with another 2 feet of clean water and measure the drop in water level within the test casing after 1 hour. Repeat the procedure three additional times by filling the test casing with clean water and measuring the drop in water level after one hour. A total of four observations must be completed. The infiltration rate of the underlying soils may either be reported as the average of all four observations or the value of the last observation. The infiltration rate should be reported in terms of inches per hour.
Infiltration testing may be performed within an open test pit or a standard soil boring.
After infiltration testing is completed, the test casing should be removed and the test pit or soil boring should be backfilled and restored.

VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 8