Natural Drainage Practice Guidelines for Single Family Residential Development Pervious Pavement

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1 Applies to permits issued through December 31, 2011 Revised June 23, 2011 These guidelines are designed to help single family homeowners determine whether pervious pavement is appropriate for their project and, if so, provide guidance on the design, sizing, construction, inspection, and maintenance of pervious pavement to meet the City of Bellevue s requirements for on-site stormwater management. On-site stormwater management is required when a project triggers Minimum Requirement #5 (MR #5) as described in Bellevue s current Storm and Surface Water Engineering Standards (Storm Engineering Standards). For single family residential projects that are subject to additional requirements beyond MR #5, a licensed engineer must perform the design in accordance with the Storm Engineering Standards. This handout provides the following information: What is pervious pavement? When an engineer is required for pervious pavement design Options for pervious pavement wearing course Sizing pervious pavement to meet stormwater requirements Essential components of pervious pavement Installing pervious paver systems Inspections required for pervious pavement Scheduling inspections Maintaining pervious pavement Finding qualified material providers and installers Resources and contacts What is? Pervious pavement is a paving system which allows rainfall to percolate through the system into the underlying soil or layer of rocks. Stormwater either soaks into the underlying soils, flows through an overflow pipe to an approved discharge point, or both. In residential settings, homeowners can use pervious pavement in driveways, walkways, patios, or other hardscaped surfaces (see section D6-03.2(B) of the Storm Engineering Standards). Using pervious pavement instead of conventional concrete or asphalt prevents floods, minimizes stream erosion, reduces water pollution, reduces the amount of water flowing into streams during storms, and keeps water temperatures cool in streams and lakes. It helps protect streams, lakes and wetlands, and the fish and other animals that live in them. Questions? Utilities Permit Center utilityreview@bellevuewa.gov These Guidelines for storm drainage are strictly for use on single family residential projects where Minimum Requirements 1-5 only apply. Minimum Requirements are usually determined by the amount of impervious area (such as driveways or roofs) that will be new and/or replaced. Contact Bellevue s Utilities Permit Center to determine which Minimum Requirements apply to your project.

2 When an engineer is required for pervious pavement design If the surface material will be pervious asphalt or pervious cement concrete, an engineer must design the facilities for all projects with no exceptions. If the surface material will be a pervious paver system such as concrete or gravel pavers, an engineer is required if the area will have vehicle traffic. Other applications of pervious pavers, such as walkways or patios, do not require an engineer for design. Options for pervious pavement wearing course You can select one of five main types of pervious pavement surface material, also known as the wearing course: 1. Pervious asphalt: Pervious asphalt is an open-graded pavement with reduced fines and stable air pockets encased within the asphalt. This design allows water to drain to the base below. It is produced and mixed at an asphalt plant. Special care is needed to avoid overconsolidation of the asphalt and must be installed by an experienced pervious asphalt installer. Slopes must be less than 5% for pervious asphalt. 2. Pervious cement concrete: Pervious cement concrete is an opengraded pavement with reduced fines and stable air pockets encased within the concrete. This design allows water to drain to the base below. It has a rougher appearance than traditional cement concrete. It must be produced AND mixed at a concrete plant. Special care is needed for successful performance. Pervious cement concrete must be installed by a certified pervious cement concrete installer. Slopes must be less than 6% for pervious concrete. 3. Pervious Paver Systems: Pervious paver systems typically consist of manufactured interlocking pavers composed of concrete. Infiltration occurs through the gaps between the pavers that allow stormwater to penetrate quickly into the subsurface soil. The gaps are filled with a pervious material, usually small stone, unless the pavers themselves are pervious. Pavers must be installed per the manufacturer s specifications to qualify as a pervious pavement facility. Manufacturers typically recommend these systems only in low traffic volume areas. Slopes must be less than 10% for pervious paver systems. 2

3 4. Open-celled paving grid with vegetation: Open-celled paving grids consist of a rigid grid made of concrete or a durable plastic that is filled with a mix of sand, gravel and topsoil for planting vegetation. The cells can be planted with a variety of grasses or low-growing groundcovers. The support base and the grid walls prevent soil compaction and reduce rutting and erosion by dispersing the weight of traffic. Vegetation in the grid openings provides habitat for beneficial microbes, nutrient cycling, pollutant removal through root uptake, and stormwater volume reduction though evaporation and transpiration. These systems are only recommended for use in low traffic volume areas. Slopes must be less than 10% for pervious paver systems. 5. Open-celled paving grid with gravel: This structure is similar to the open-celled grid with vegetation, but the openings are filled with a gravel mix to provide greater load bearing support for driveways or areas with longer parking durations. Clean gravel fill must meet the manufacturer s specifications. These systems are only recommended for use in low traffic volume areas. Slopes must be less than 10% for pervious paver systems. Sizing pervious pavement to meet stormwater requirements The first step in sizing pervious pavement is determining the native soil infiltration rate using the simplified infiltration test. A description of the simplified infiltration test can be found in Section D of the 2011 Engineering Standards or in the Rain Garden Handbook for Western Washington Homeowners If the measured infiltration rate is less than 0.1 inches per hour, the site is not suitable for pervious pavement. If no water will be directed to the pervious pavement from another hard surface (such as a roof or patio awning), no sizing calculations are needed it can be built as large or small as needed. If water from a roof, driveway, or other hard surface needs to be managed on-site, use the sizing factors provided to either: 1) calculate the required size of pervious pavement needed to manage a certain amount of contributing impervious area, or 2) see how much of the contributing area can be directed to the pervious pavement if the size available for the pervious pavement is limited. Use the following table and refer to the examples below. Additional sizing factors can be found in Section D of the Storm Engineering Standards. 3

4 Sizing Factors for Pervious Pavers, Paving Grid with Vegetation or Gravel, or 3-inch Wearing Course, and a 6-inch Reservoir Course: Sizing Factors (Size of Area Design Native Soil Infiltration Rate as Percent of Contributing Area) 0.1 inches/hour less than 0.25 inches/hour 41.2%.25 inches/hour or more 33.3% Equations for sizing pervious pavement to receive runoff from another area (no calculations necessary if it will only manage the rain that falls on it directly): 1- Facility Area = Contributing impervious area to be mitigated x sizing factor Contributing Impervious Area Mitigated = Pervious pavement facility sizing factor x 100 Note: The above sizing factors can only be used for pervious pavement with no underdrain and no impermeable liner. Option 1: Determine the square footage of a proposed pervious pavement facility. Then use the sizing chart to calculate how much contributing impervious area is allowed to drain onto it (based on Minimum Requirement #5). Example: The total proposed area of the driveway and parking pad is 710 square feet. The homeowner determined a design native soil infiltration rate of 1.2 inches per hour using the simplified infiltration test. Select the appropriate sizing factor from the above table using this design infiltration rate. The associated sizing factor equals 33.3%. Answer: 710 square feet of proposed pervious pavement 33.3 x 100 = 2,132 square feet of total impervious surface mitigated. Therefore, an additional 1,422 square feet (2,132 square feet 710 square feet) of roof or other impervious surface area can be routed to this facility. Option 2: Measure the square footage of the contributing area that will drain to the pervious pavement facility. Then use the sizing chart to determine the required amount of pervious paver area needed to manage the incoming water (pervious paver area needs to have less than 10% slope). Example: The roof area added to a house measures 480 square feet, and the runoff needs to be managed on-site. The design native soil infiltration rate in the location of the pervious pavement area is 0.1 inches per hour. Based on the design native soil infiltration rate, use the chart above to select the sizing factor (equals 41.2%) Answer: 480 x = 198 square feet of pervious interlocking concrete paver facility required to manage 480 square feet of contributing roof area. 4

5 Essential components of pervious pavement Wearing Course: The wearing course is the surface layer of pervious pavement that allows water to percolate into the underlying rock layer and/or into the native soil. The wearing course may be pervious concrete, pervious asphalt, open celled paving grids, or pervious pavers. Leveling Course: This layer of bedding material is used to protect drain pipes and provides a uniform surface for paver systems. It must be included when required by the designer or recommended by the manufacturer. It generally consists of fine gravel. Underdrain (optional): The underdrain is a slotted drain pipe installed just above the base of the facility. It conveys excess flows to the approved discharge point. Grass or Gravel Paving Reservoir course: This base layer is a layer of crushed rock that Pervious Concrete Block or Paver System provides storage space for stormwater as it gradually infiltrates into the soil below. A minimum 6-inch depth is required for pervious pavement, 4-inches for pervious paver systems, or 22-inches if an underdrain is used. Native soil/subgrade: This is the native soil directly below the pervious pavement facility. It receives the infiltrating stormwater and provides support for the permeable pavement. The infiltration rate must be at least 0.1 inches per hour to use pervious pavement systems. The infiltration rate can be determined as described above under Sizing pervious pavement to meet stormwater requirements. Observation Port (optional): An observation port installed in the furthest downslope area helps the owner see the water level and determine whether the water is soaking in as intended. This is optional under Minimum Requirement #5. See the Storm Engineering Standards (NDP-16 & 17) for more detail. Filter Fabric (optional): Filter fabric, also called nonwoven geotextile fabric, should be installed if the native soil is loose and finer than coarse sand to prevent the migration of fine soil particles from the native soil into the reservoir course, which can cause clogging. If used, geotextile fabric must be placed between the reservoir course and the subgrade. It must wrap underneath and along the sides of the reservoir course, and pass water at a rate greater than the infiltration rate of the existing subgrade. 5

6 Installing pervious paver systems An experienced or certified pervious pavement installer is required whenever pervious asphalt or concrete are used. See below for how to find an installer. When pervious paver systems are used in non-traffic areas, a certified designer and/or installer is not required. Always follow the selected manufacturer s listed installation methods as they vary from manufacturer to manufacturer. In general, the installation process involves: Step 1: Delineate the area planned for pervious pavement using spray paint (non-toxic), string, or similar methods. Step 2: Prepare the subgrade by excavating the area where the pavers will be installed down to an appropriate depth for the reservoir course, leveling course (if required by the manufacturer), and paver units. Keep heavy equipment out of the planned pervious pavement area to prevent soil compaction. Place filter fabric if specified by the engineer. Step 3: Place and compact reservoir course according to manufacturer recommendations. Check for porosity of the subgrade with a hose turn the water on all the way and leave in one place for at least 10 minutes. All of the water should flow into the base and not pond at the surface. Step 4: Apply leveling course if necessary to level out the surface or if required by the manufacturer. Step 5: Install pervious pavers. Step 6: Fill pavers or the spaces between them with sandy topsoil or gravel, depending on the specified paver system. Spread out with rake or broom. Step 7: If using a grass or vegetated paver system, lay turf or hydroseed. Step 8: Roll with a heavy roller to eliminate air pockets and to make sure roots are in contact with sandy topsoil (if using a grass or vegetated paver system). Level Pavers & Compact 6

7 Inspections required for pervious pavement First Inspection Clearing & Grading Precon/Jobcon: The first ground disturbance inspection is required of all permitted projects that disturb the ground by digging, clearing vegetation, or otherwise moving soil on the site. This inspection will occur after the project has been permitted and approved, but before any soil or vegetation is disturbed. The steps below are described in more detail in the Single Family Clearing and Grading Permit requirements at During this meeting, the Clearing & Grading Site Inspector will review the specifications of all permitted Natural Drainage Practice (NDP) systems, including pervious pavement, and discuss them with the homeowner or construction personnel. To prevent sediment and other pollution from the site during construction, the applicant will have prepared a Construction Stormwater Pollution Prevention Plan (CWSPPP), which will include techniques (known as Best Management Practices, or BMPs) for Erosion and Sediment Control (ESC). See the Clearing and Grading Permit requirements for instructions. The table below will help prepare for this inspection. DO NOT BEGIN EARTH DISTURBANCE PRIOR TO THIS INSPECTION Builder Requirements: Inspector Requirements: Note: Have the approved plan set on site. Review approved plan set. If the approved set of plans is not on-site, the inspection cannot be conducted and must be rescheduled. Have knowledge of the impervious material, details, areas, and quantities. Review allowed impervious surface per plans. Discuss flow entrance locations, underdrains and overflow details. Changes to the approved square footage of impervious will likely require re-sizing the NDP facilities. Plan revision submittals may be required. Carefully read the Construction Stormwater Pollution Prevention Plan (CSWPPP) narrative and review the site plan and the Erosion and Sediment Control (ESC) Plan. Confirm setback requirements for the NDP facilities. Have a construction sequencing, staging and NDP protection plan prepared. Install approved erosion and sediment controls and tree protection prior to start of work. Have a construction sequencing plan specifically for pervious pavement area. Have the approved manufacturer s specifications and details. Review the CSWPPP narrative, the site plan, and ESC Plan with the Owner or Owner s Representative. Discuss setback requirements. Discuss NDP protection requirement. Review erosion and sediment control requirements and expectations. Review NDP protection and discuss construction access. Verify pervious pavement is either protected from compaction or mitigated during subgrade preparation. Verify pervious pavement area will be protected from all construction runoff. Review manufacturer s details for paver and grid systems, if applicable. Contractor must have non-disturbed areas fenced off. If construction activity has recently impacted the site, the NDP design may need to be revised. Setbacks for pervious pavement are only needed if there will be water directed to it from other impervious areas. NDPs must be protected from traffic, compaction and silt during all phases of construction. No tracking/dirt/debris allowed in right-of-way. Minimal traffic and compaction on the construction access is vital. Use small, lightweight excavation, grading, and compaction equipment to minimize the potential for subbase over-compaction. Sub-grade scarification must be completed during dry weather on the same day the base layer or reservoir course is placed. 7

8 Subgrade Inspection: The subgrade (native soil under the pervious pavement) inspection and verification must be scheduled after all excavation work has been completed, and before the installation of the reservoir course rock base layer. This inspection will be completed by the Utilities Inspector. Use your UB permit number to schedule this inspection. Builder Requirements: Inspector Requirements: Note: Have the approved plan set on site. Know the impervious square footage. Know the NDP protection requirements and have NDP protection in place. Monitor the CSWPPP BMPs for effectiveness. Review allowed impervious surface per plan. Confirm protection of the NDP areas. This inspection cannot be conducted without the approved plan set. The excavated subgrade for the NDP shall be no greater than 5% slope for pervious concrete, 6% for pervious asphalt, or 10% for pervious pavers. Dig the permeable pavement areas to the depth per plan. Scarify a minimum four inches of the approved subgrade material. Prepare overflow route as site conditions allow. Schedule additional inspections as needed. Confirm excavated subgrade slope and check dam spacing and elevation for slopes greater than 2%. Confirm the depth and scarification of the existing subgrade. Review the overflow and underdrain system if required. Review NDP protection and discuss Assess the need for additional inspections. When the excavated subgrade is greater than 2%, the subbase must be designed to provide subsurface ponding (check dams) or an interceptor infiltration trench. Subgrade scarification shall be completed during dry weather on the same day as the aggregate base of the reservoir is placed. No tracking/dirt/debris allowed in right-of-way (ROW). Minimal traffic and compaction in the NDP areas. Scheduling inspections To schedule an inspection, call and press 1. Schedule the first inspection with the Clearing & Grading Site Inspector by entering the Clearing & Grading Permit number and inspection code 100. You can schedule an inspection up until 6:00 a.m. on the day of the inspection. Inspectors will arrive between 8:00 am and 3:30 pm, or on busy days, inspections may be rescheduled for the next day. To find out your estimated inspection time, contact the inspector at between 7:00 am and 8:00 am on the day of the inspection. The remaining inspections ( Subgrade Inspection, Base and Drainage Inspection and Pavement Final Inspection), and any additional inspections required to correct problems will be done by the Utilities Inspector. Schedule these inspections by calling (425) and entering the UB permit number and inspection code 635. You can schedule inspections until 4:30 pm on the day before the inspection. The Utilities inspector will arrive at the scheduled appointment time. Charges for these and any other additional inspections will apply. 8

9 Base and Drainage Inspection: The base and drainage inspections occur during and after placement of the reservoir course aggregate, leveling course, flow entrance and underdrain pipes. Discuss and agree on the timing of these inspections during the previous subgrade base inspection. This inspection will be completed by the Utilities Inspector. Use your UB permit number to schedule this inspection. Builder Requirements: Inspector Requirements: Note: Have the approved plan set on site. Monitor the BMPs for effectiveness. This inspection cannot be conducted without the approved plan set. Establish temporary or permanent erosion control needed to protect the NDP areas at all times. Install the underdrain pipe in the reservoir course (base) of the facility. Have approved reservoir course material placed and ready for verification. Install the flow entrances per plan, if applicable. Have leveling course material placed (if needed). Provide manufacturer details for paver and grid systems, including reservoir material and site preparation details. Schedule additional inspections as needed. Confirm ongoing protection around the NDP areas. Verify the overflow system and underdrain system, if required. Verify reservoir course aggregate material (rock). Verify the flow entrances properly route water to the pavement Certify contributing areas match approved plans. Verify leveling course material. Review manufacturer details for paver and grid systems. Assess the need for additional inspection. A separate under drain overflow inspection may be necessary. If an underdrain is required, the reservoir course must be at least 22 inches, with at least 6 inches on top of the underdrain to protect the pipe. No flow entrances needed if rainwater is the only source of water to the pervious pavement. This layer can protect the underdrain pipe from the permeable pavement, or may help level the reservoir course for placing pavers. A separate underdrain/overflow inspection may be scheduled. During the Clearing and Grading Permit process, it will be determined whether a Geotechnical Special Inspector is required. If so, the City of Bellevue Site Final Inspection does not replace any Geotechnical Special Inspection requirements. 9

10 Pavement Final Inspection: The pavement final site inspection can be done at the same time as other types of inspections associated with NDPs, such as plantings for rain gardens, amended soils, final erosion control, and tree installation. The table below will help you prepare for the Pavement Final Inspection. This inspection will be completed by the Utilities Inspector. Use your UB permit number to schedule this inspection. Builder Requirements: Inspector Requirements: Note: Have the approved plan set onsite and have permanent erosion control installed. Know the impervious square footage per plan. If the project is over 5,000 square feet, submit a test panel to the inspector prior to installation. Permanently stabilize all areas surrounding the pervious pavement. Establish permanent erosion control around the pervious pavement. Have a water source and a clean 5 gallon bucket onsite to test the flow entrance. Have a water source and a clean 5 gallon bucket onsite to perform the water infiltration test. Provide as-built drawings for the pervious pavement, drainage, and storm stub connection installations. Schedule additional inspections as needed. Verify permanent erosion control. Measure actual impervious surface and pervious pavement receiving run-on. Confirm the design infiltration rate and pavement design (e.g., pavement thickness). Confirm stabilization of the pervious pavement areas. Confirm permanent erosion control is established around the pervious pavement. Verify flow entrance performance. Verify the pervious pavement performance by water test. Review as-built drawings. Assess the need for additional inspections. This inspection cannot be conducted without the approved plan set. Test panel only applies for pervious cement concrete systems. Not needed if rain will be the only source of water to the pervious pavement. Water source can be a hose attached to a water supply. As-built drawings document what was actually built, rather than what was originally planned. If a Geotechnical Special Inspector is involved in the project, the City of Bellevue Site Final Inspection does not replace any Geotechnical Special Inspection requirements. Maintaining pervious pavement Maintenance of pervious pavement systems is required routinely (at regular intervals), and when certain conditions trigger a need for maintenance. Conditions triggering the need for maintenance include (but are not limited to) cracking or damage in the pavement, or if materials are spilled onto the pavement (soil, sand, oil, concrete, etc). For more information on maintenance of pervious pavement systems, refer to the Storm and Surface Water Maintenance Standards, City of Bellevue Utilities Department February

11 Finding qualified material providers and installers Incorporating pervious pavement into a project is different from typical construction practices. While pervious pavement materials are not new products, the materials, production methods, and installation standards continue to evolve. Given the rapid pace of this evolution, Bellevue strongly recommends that applicants work with experienced material providers. Pervious concrete must be installed by a Certified Pervious Concrete Installer (see list available through the National Ready Mixed Concrete Association (NRMCA) at The table below provides a quick reference for materials specifications and preapproved products for the various types and components of pervious pavement. Type/Component: Pervious Asphalt Pervious Cement Concrete Pervious Paver Systems Reservoir Course Filter Fabric Specifications: Pervious asphalt must meet the specifications listed in the Low Impact Development: Technical Guidance Manual for Puget Sound (Puget Sound Partnership, 2005 or current version) or must meet the specifications prepared by the designer or manufacturer. Pervious concrete providers should meet the specifications of ACI Specifications for Pervious Concrete Pavement published the American Concrete Institute, Farmington Hills, Michigan. For projects larger than 5,000 square feet, a test panel must be submitted to the inspector prior to installation. Pervious pavers must meet the specifications listed in the Low Impact Development: Technical Guidance Manual for Puget Sound (Puget Sound Partnership, 2005 or current version) or must meet the specifications prepared by the designer or manufacturer. The reservoir course must be constructed of 2.5 inch to 0.5 inch uniformly graded crushed (angular) thoroughly washed stone (AASHTO No. 3), or thoroughly washed Permeable Ballast meeting the requirements of Standard Specification (2) in Standard Specifications for Road, Bridge, and Municipal Constructions (Washington State Department of Transportation 2010). Filter fabric (geotextile) is optional. If specified by the engineer, use nonwoven geotextile for separation (Standard Specifications, (1) Table 3, separation, nonwoven in Standard Specifications for Road, Bridge, and Municipal Constructions - Washington State Department of Transportation 2010). Qualified Products:. A list of approved pavers is provided in Section D of the 2011 Engineering Standards. Paver and grid suppliers must provide manufacturer specifications for the installer. A list of approved filter fabrics is provided in Section D of the 2011 Engineering Standards. Local material providers may recommend installers working in the local area. Installers can provide references and completed projects for evaluation. This is the most effective way to ensure a competent installation. Installers can also provide references for material providers they have worked with. A list of local suppliers can be found at the Seattle Public Utilities RainWise website: 11

12 Resources and contacts Contact the permit desk at City Hall with questions to make the permit process easier. City of Bellevue Storm and Surface Water Engineering Standards Resources Web Link rengstds_2011.pdf City of Bellevue Storm and Surface Water Utility Code llevue24/bellevue24.html City of Bellevue Clearing and Grading Development Standards City of Bellevue Clearing and Grading Code llevue23/bellevue2376.html#23.76 City of Bellevue Storm and Surface Water Maintenance Standards _Storm_Maintenance_Standards_Feb_2010.pdf Key contacts: Utilities Permit Center (425) Inspection Requests (425) Clearing & Grading Inspector (425)