DRAFT VA DCR STORMWATER DESIGN SPECIFICATIONS No. 8: INFILTRATION
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 8
INFILTRATION PRACTICES
VERSION 1.5
Note to Reviewers of the Stormwater Design Specifications
The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of non-proprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These updated standards and specifications were developed with significant assistance from Tom Schueler of the Chesapeake Stormwater Network (CSN), staff of the Center for Watershed Protection (CWP), the Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards are based on both the more traditional BMPs and the newer LID practices. The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR.
Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:Status as of 9/24/2008
# / Practice / Notes / Status 1
1 / Rooftop Disconnection / Includes front-yard bioretention / 2
2 / Filter Strips / Includes grass and conservation filter strips / 2
3 / Grass Channels / 2
4 / Soil Compost Amendments / 3
5 / Green Roofs / 1
6 / Rain Tanks / Includes cisterns / 2
7 / Permeable Pavement / 1
8 / Infiltration / Includes micro- small scale and conventional infiltration techniques / 2
9 / Bioretention / Includes urban bioretention / 3
10 / Dry Swales / 2
11 / OPEN
12 / Filtering Practices / 2
13 / Constructed Wetlands / Includes wet swales / 2
14 / Wet Ponds / 2
15 / ED Ponds / 2
1 Codes: 1= partial draft of design spec; 2 = complete draft of design spec;
3 = Design specification has undergone one round of external peer review as of 9/24/08
Reviewers should be aware that these draft standards and specifications are just the beginning of the process. Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided.
Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP Clearinghouse Committee:
The CSN will be soliciting input and comment on each standard and specification until the end of 2008 from the research, design and plan review community. This feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region. The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR.
The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009. The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations become final.
The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff.
As with any draft, there are several key caveats, as outlined below:
· Many of the proposed design standards and specifications lack graphics. Graphics will be produced in the coming months, and some of graphics will be imported from the DCR 1999 Virginia Stormwater Management (SWM) Handbook. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach.
· There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified in subsequent versions.
· While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity.
· Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure. These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility.
These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake Stormwater Network’s web site at http://www.chesapeakestormwater.net, with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR requests that you provide an email copy of your comments, etc., to Scott Crafton (). The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived.
Thank you for your help in producing the best stormwater design specifications for the Commonwealth.
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 8
INFILTRATION PRACTICES
VERSION 1.5
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 is not recommended at sites designated as stormwater hotspots, to prevent possible groundwater contamination. When used in the appropriate setting, infiltration has a very high runoff reduction capability, as shown in Table 1.
SECTION 2: PERFORMANCE CRITERIA
Table 1: Summary of Stormwater Functions Provided by InfiltrationStormwater 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 Protection / Moderate
RRv can be subtracted from CPv
Flood Mitigation / Partial
Reduced Curve Numbers and Time of Concentration
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 and will be higher than these percentages, as calculated using the Runoff Reduction Spreadsheet Methodology.
Sources: CWP and CSN (2008) and CWP (2007).
SECTION 3: PRACTICE APPLICATIONS AND FEASIBILITY
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 (HSG) A or B soils shown on NRCS soil surveys should be considered as primary locations for infiltration practices. Designers should carefully analyze and define constraints on infiltration, as follows:
Maximum Drainage Area: The 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 2500 sq. ft. of CDA); (2) small-scale infiltration (2500 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 (summary in Table 3).
Site Slopes: 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 2 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 some 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. The silt/clay content of native soils must be less than 40%, and the clay content must be 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, or other 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 3). At a minimum, conventional and small-scale infiltration practices should be located a minimum horizontal distance of 100 feet from any water supply well, 50 feet from septic systems, and at least 5 feet downgradient from dry or wet utility lines.
High Loading Situations: Infiltration practices are not intended to treat sites with high sediment or trash/debris loads, since they will cause the practice to clog and fail.
Groundwater Protection: Section 4 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.
SECTION 4: ENVIRONMENTAL AND COMMUNITY ISSUES
4.1: 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 2 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 need to be designated as a stormwater hotspot. Based on this designation, one or more design responses are required, as shown below:
1. Stormwater Pollution Prevention Plan (SWPPP). This plan is required as part of an industrial or municipal stormwater permit. It outlines pollution prevention and treatment practices that will be implemented to minimize polluted discharges from the site. Other facilities or operations are not technically required to have NPDES permits, but may be designated as potential stormwater hotspots by the local review authority as part of the local stormwater ordinance (these are shown in the shaded areas of Table 2). It is recommended that these facilities include an addendum to their stormwater plans 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 (Tv) 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 2: Potential Stormwater Hotspot and Site Design ResponsesPotential Stormwater Hotspot Operation / SWPP Required? / Restricted Infiltration / No
Infiltration
Facilities w/NPDES Industrial permits / Yes / ■ / ■
Public works yard / Yes / ●
Ports, shipyards and repair facilities / Yes / ●
Railroads/ equipment storage / Yes / ●
Auto and metal recyclers/scrapyards / Yes / ●
Petroleum storage facilities / Yes / ●
Highway maintenance facilities / Yes / ●
Wastewater, solid waste, and composting facilities / Yes / ●
Industrial machinery and equipment / Yes / ●
Trucks and trailers / Yes / ●
Airfields and aircraft maintenance areas / Yes / ●
Fleet storage areas / Yes / ●
Parking lots (40 or more parking spaces) / No / ●
Gas stations / No / ●
Highways (2500 ADT) / No / ●
Construction business (paving, heavy equipment storage, and maintenance / No / ●
Retail/wholesale vehicle/equipment dealers / No / ●
Convenience stores/fast food restaurants / No / ●
Vehicle maintenance facilities / No / ●
Car washes / No / ●
Nurseries and garden centers / No / ●
Golf courses / No / ●
Notes:1. Gray-shaded rows indicate uses the locality may optionally designate as hotspots.
2. For a full list of potential stormwater hotspots, please consult Schueler et al (2004).
Key: ■ depends on facility ● Yes
4.2: Other Environmental and Community Issues
The following community and environmental concerns may 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 the proper adherence to setback, soil testing, and pretreatment requirements outlined in this specification.