ROOFTOP DISCONNECTION VERSION 1.5

Transcription

1 DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1 ROOFTOP DISCONNECTION VERSION 1.5 Note to Reviewers of the Stormwater Design Standards and Specifications The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of nonproprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These standards and specifications were developed with assistance from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards and specifications are based on both the traditional BMPs and Low Impact Development (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 5 Green Roofs 1 6 Rain Tanks Includes cisterns 2 7 Permeable Pavement 1 8 Infiltration Includes micro- small scale and conventional infiltration techniques 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 Status: 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. 3 2 Rooftop Disconnection 1 of 10 9/24/08

2 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 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 with instructions regarding how to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR requests that you provide an copy of your comments, etc., to Scott Crafton (scott.crafton@dcr.virginia.gov). 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. Rooftop Disconnection 2 of 10 9/24/08

3 DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1 ROOFTOP DISCONNECTION VERSION 1.5 SECTION 1: DESCRIPTION OF PRACTICE This strategy involves treating runoff close to its source by intercepting rooftop runoff and infiltrating, filtering, treating, or reusing it before it moves from the roof into the storm drain system. Two kinds of practices are allowed. The first is for simple rooftop disconnection, whereas the second involves disconnection combined with supplementary runoff treatment, including the following: Compost amended soils in the filter path Installation of dry wells or french drains Installation of rain gardens or front yard bioretention Storage and reuse in a rain tank or cistern Storage and release in a foundation planter With proper design and maintenance, each of the disconnection options can provide relatively high runoff reduction rates, as shown in Table 1. With the exception of dry wells and rain gardens, most disconnection options have little or no capability to remove nutrients (Table 2). Any runoff reduction achieved by rooftop disconnections can be subtracted from the overall Rooftop Disconnection 3 of 10 9/24/08

4 channel protection volume for the site, and may also increase the time of concentration used to model larger design storms to control flooding. Table 1: Runoff Reduction Capabilities For Rooftop Disconnection Annual Runoff Reduction Rate HSG Soils A and B HSG Soils C and D Simple Disconnection Compost-Amended Filter Path Dry Wells or French Drains Rain Gardens/Bioretention Rain Tank or Cistern actual storage volume x 0.75 Foundation Planter 40 1 CA= Compost Amended Soils, see Design Specification No. 4 Sources: CWP and CSN (2008), CWP, 2007 Rooftop Disconnection 4 of 10 9/24/08

5 SECTION 2: PERFORMANCE CRITERIA Table 2: Pollutant Removal Capabilities For Rooftop Disconnection EMC Reduction Rate Total Phosphorus Total Nitrogen Simple Disconnection 0 0 Compost-amended Filter Path 0 0 Dry Wells or French Drains Rain Gardens/Bioretention Rain Tank or Cistern 0 0 Foundation Planter 0 0 Sources: CWP and CSN (2008), CWP (2007) SECTION 3: DESIGN APPLICATIONS AND VARIATIONS The flow chart in Figure 1 below provides guidance for choosing the best disconnection option for an individual rooftop: Figure 1: This simple flow chart helps homeowners decide whether simple disconnection, rain barrels, french drains or rain gardens are most appropriate for their lot. Rooftop Disconnection 5 of 10 9/24/08

6 3.1: Simple Rooftop Disconnection SECTION 4: DESIGN CRITERIA Simple disconnection is only allowed for residential lots greater than 6000 sq. ft. For smaller sites, disconnection with supplementary runoff treatment may be considered. The contributing flow path from impervious areas should not exceed 75 feet. The disconnection length must exceed the contributing flow path. A compensatory mechanism is needed if the disconnection length is less than 40 feet and/or the post construction Hydrologic Soil Group is in the C or D Category. Pervious areas used for disconnection should be graded to have a slope in the 1 to 2% range, and should never exceed 5%. The total impervious area contributing to any single discharge point shall not exceed 1000 square feet and shall drain continuously through a pervious filter until reaching a property line or drainage swale. The disconnection shall not cause basement seepage. Normally, this involves extending downspouts at least 10 feet from the building if the ground does not slope away from the building. Maintenance of disconnected downspouts is essentially maintaining the lawn or landscaped areas in the path of the water from the downspout. 3.2: Compost-amended filter path The design should conform to Stormwater Design Specification No. 4 (Soil Compost Amendments), and include the following elements: Flow from the downspout should be spread over a 10-foot wide strip extending downgradient from the building to the street or conveyance system. A pea gravel or riverstone diaphragm should be installed at the downspout outlet to distribute flows across the filter path. Existing soils in the strip will be scarified or tilled to a depth of 12 to 18 inches and amended with well-aged compost to achieve an organic matter content in the range of 8 to 13%. The depth of compost amendment is based on the relationship of the contributing rooftop area (RA) to the area of the soil amendment strip (SA), using the following general guidance: o RA/SA = 1, use 4 inches compost. Rooftop Disconnection 6 of 10 9/24/08

7 o RA/SA = 2, use 8 inches compost. o RA/SA = 3, use 12 inches of compost and till to a depth of inches. 3.3: Dry Wells and French Drains Depending on soil properties, roof runoff may be infiltrated into a shallow dry well or French drain. The design for this option should meet the requirements of micro-infiltration, as described in Design Specification No. 8 (Infiltration), and summarized in Table 3. Table 3: Design Requirements for Micro-Infiltration Design Factor Micro Infiltration Design Impervious Area Treated 250 to 2500 sq. ft. Typical Practices Dry Well and French Drain Runoff Reduction Sizing Minimum 0.1 inches over CDA (the RA) Minimum Soil Infiltration Rate 0.5 inches/hour Observation Well No Type of Pretreatment External (leaf screens, etc) Depth Dimensions Max. 3 foot depth UIC Permit Needed No Head Required Nominal, 1 to 3 feet Underdrain Requirements? Only on marginal soils Required Soil Test One per practice Building Setbacks 5 feet downgradient, 25 feet upgradient In general, the size of Micro-Infiltration Facilities needs to be10-15% of the contributing roof area. An on-site soil test is needed to determiner if soils are suitable for infiltration. The microinfiltration facility should be located in an expanded right of way or stormwater easement so that it can be accessed for maintenance. 3.4: Rain Gardens and Front Yard Bioretention Depending on soil properties, roof runoff may be filtered through a shallow bioretention area. The design for this option should meet the requirements of micro-bioretention, as described in Design Specification No. 7 (Bioretention), and summarized in Table 4. Rooftop Disconnection 7 of 10 9/24/08

8 Table 4 Design Requirements for Micro-Bioretention Design Factor Micro-Bioretention aka Rain Garden Impervious Area Treated 250 to 2500 sq. ft. Type of Inflow Sheetflow or roof leader Runoff Reduction Sizing Minimum 0.1 inches over CDA Minimum Soil Infiltration Rate 0.5 inches/hour (or use underdrain) Observation Well/ Cleanout Pipes No Type of Pretreatment External (leaf screens, etc) Minimum Filter Media Depth 24 inches Media Source Mixed on site Head Required Nominal, 1 to 3 feet Required Soil Borings One, only when an underdrain is not used Building Setbacks 5 feet downgradient, 25 feet upgradient For high density sites, front yard bioretention may be an attractive option. This form of bioretention captures roof overflow and lawn and driveway runoff from low to medium density residential lots in a slight depressed area between the home and the street. The bottom of the bioretention area then connects by an underdrain to the main storm drain pipe located underneath the street The concept is to take advantage of the drop from the roof leader to the street storm drain pipe, by creating a 10 foot wide bioretention corridor from roof to the street. The minimum effective length of the bioretention corridor is 20 feet long. The bioretention corridor is subtly graded to create a shallow 6-12 inch deep ponding area between the roof leader and the edge of a sidewalk or road. The ponding area may have a turf or landscape cover, depending on homeowner preference. The bioretention media is approximately 3 feet deep, and is located over a inch deep stone reservoir. A perforated underdrain is located above the stone reservoir, to promote storage and recharge, even on poorly draining soils. In highly urban settings, the underdrain is directly connected into the major storm drain pipe running underneath the street or in the street right of way. A trench needs to be excavated during construction to connect the underdrain to the street storm drain system. Construction of the remainder of the front yard bioretention system is deferred until after the lot has been stabilized. The front yard design should reduce the risk of homeowner conversion because it allows them to choose whether they want turf or landscaping. Front yard bioretention requires regular mowing and/or landscape maintenance to perform effectively and should be located in an expanded right of way of stormwater easement so that it can be accessed in the event that it fails to drain properly. 3.5: Rain Tanks and Cisterns This form of disconnection must conform to the design requirements outlined in Design Specification No. 6 (Rain Tanks and Cisterns). The runoff reduction rates for rain tanks and cisterns depend on their storage capacity and ability to drawdown water in between storms for reuse as potable water, greywater, or irrigation use. Rooftop Disconnection 8 of 10 9/24/08

9 Designers will need to estimate the water reuse volume, based on the method of distribution, frequency of use, and seasonally adjusted indoor and/or outdoor water demands for the building. Based on the prevailing climate for the region, a conservative runoff reduction estimate of 40% is recommended for initial design. Pretreatment measures may need to be employed to keep leaves, bird droppings, and other pollutants from entering the tank or cistern. All devices should have a suitable overflow area to route extreme flows into the next treatment practice or stormwater conveyance system. 3.6: Foundation Planter This form of disconnection must conform to the design requirements for foundation planters as outlined in Design Specification No. 9 (Bioretention). Foundation planters are another option to disconnect and treat rooftop runoff. They consist of confined planters that store and/or infiltrate runoff through a soil bed to reduce runoff volumes and pollutant loads. Stormwater planters combine an aesthetic landscaping feature with a functional form of stormwater treatment. Stormwater planters generally receive runoff from adjacent rooftop downspouts and are landscaped with plants that are tolerant to both periods of drought and inundation. The two basic design variations for stormwater planters are the infiltration planter and the filter planter. An infiltration planter filters rooftop runoff through soils in the planter followed by infiltration into soils below the planter. The recommended minimum width is 30 inches; length and shape can be decided by architectural considerations. The planter should be sized to temporarily store at least 1/2-inch of runoff from the contributing rooftop area in a reservoir above the planter bed. Infiltration planters should be placed at least 10 feet away from a building to prevent possible flooding or basement seepage damage. A filter planter has an impervious liner on the bottom. The minimum planter width is 18 inches with the shape and length governed by architectural considerations. Runoff is temporarily stored in a reservoir located above the planter bed. Overflow pipes are installed to discharge runoff when maximum ponding depths are exceeded to avoid water spilling over the side of the planter. Since a filter planter is self-contained and does not infiltrate into the ground, it can be installed right next to a building. All planters should be placed at grade level or above ground, and sized to allow captured runoff to drain out within four hours after a storm event. Plant materials should be capable of withstanding moist and seasonally dry conditions. Planting media should have an infiltration rate of at least 2 inches per hour. The sand and gravel on the bottom of the planter should have a minimum infiltration rate of 5 inches per hour. The planter can be constructed of stone, Rooftop Disconnection 9 of 10 9/24/08

10 concrete, brick, wood, or other durable material. If treated wood is used, care should be taken so that trace metals and creosote do not leach out of the planter. SECTION 4: OPERATIONS AND MAINTENANCE The rooftop disconnection and supplementary treatment device must be covered by a drainage easement to allow inspection and maintenance. When the disconnection occurs on a private residential lot, its existence and purpose shall be noted on the deed of record. Homeowners must be provided a simple document that explains their purpose and routine maintenance needs. A legally binding maintenance agreement must be in place to ensure that downspouts remain disconnected, treatment units are maintained and filtering/infiltrating areas are not converted or disturbed. The agreements should grant authority for local agencies to access the property for inspection or corrective action. SECTION 5: REFERENCES City of Portland, Environmental Services Portland Stormwater Management Manual. Portland, OR. CWP National Pollutant Removal Performance Database Version 3.0. Center for Watershed Protection, Ellicott City, MD. Northern Virginia Regional Commission Low Impact Development Supplement to the Northern Virginia BMP Handbook. Fairfax, Virginia Philadelphia Stormwater Management Guidance Manual Schueler, T., D. Hirschman, M. Novotney and J. Zielinski Urban stormwater retrofit practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for Watershed Protection, Ellicott City, MD Schueler, T Technical Support for the Baywide Runoff Reduction Method. Chesapeake Stormwater Network. Baltimore, MD Rooftop Disconnection 10 of 10 9/24/08