12.1 BMP L633 - Permeable Pavements
Permeable paving surfaces are an important integrated management practice within the LID approach and can be designed to accommodate pedestrian, bicycle and auto traffic while allowing infiltration, treatment and storage of stormwater.
The general categories of permeable paving systems include:
Porous hot or warm-mix asphalt pavement is a flexible pavement similar to standard asphalt that uses a bituminous binder to adhere aggregate together. However, the fine material (sand and finer) is reduced or eliminated and, as a result, voids form between the aggregate in the pavement surface and allow water to infiltrate.
Pervious Portland cement concrete is a rigid pavement similar to conventional concrete that uses a cementitious material to bind aggregate together. However, the fine aggregate (sand) component is reduced or eliminated in the gradation and, as a result, voids form between the aggregate in the pavement surface and allow water to infiltrate.
Permeable interlocking concrete pavements (PICP) and aggregate pavers. PICPs are solid, precast, manufactured modular units. The solid pavers are (impervious) high-strength Portland cement concrete manufactured with specialized production equipment. Pavements constructed with these units create joints that are filled with permeable aggregates and installed on an open-graded aggregate bedding course. Aggregate pavers (sometime called pervious pavers) are a different class of pavers from PICP. These include modular precast paving units made with similar sized aggregates bound together with Portland cement concrete with high-strength epoxy or other adhesives. Like PICP, the joints or openings in the units are filled with open-graded aggregate and placed on an open-graded aggregate bedding course. Aggregate pavers are intended for pedestrian use only.
Grid systems include those made of concrete or plastic. Concrete units are precast in a manufacturing facility, packaged and shipped to the site for installation. Plastic grids typically are delivered to the site in rolls or sections. The openings in both grid types are filled with topsoil and grass or permeable aggregate. Plastic grid sections connect together and are pinned into a dense-graded base, or are eventually held in place by the grass root structure. Both systems can be installed on an open-graded aggregate base as well as a dense-graded aggregate base.
The City of Tacoma has developed Standard Plans for permeable pavement sections that can be used for both public and private projects within the City of Tacoma. These plans are available at https://www.cityoftacoma.org/government/city_departments/public_works/engineering/standard_plans_and_g_i_s_typical_details.
Permeable paving systems can generally be used where traditional paving surfaces are used.
Limit run-on to permeable pavement surfaces to the maximum extent practicable. Run-on shall only be allowed from fully stabilized areas.
Unless the pavement, base course, and subgrade have been designed to accept runoff from adjacent impervious surfaces, slope impervious runoff away from the permeable pavement to the maximum extent practicable. Sheet flow from up-gradient impervious areas is not recommended, but permissible if the permeable pavement area is greater than the impervious pavement area.
This BMP can be utilized to ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management.
The BMP can be used solely to mitigate for other hard surfaces if utilizing the List Approach or may be used in combination with other BMPs to satisfy the LID Performance Standard.
This BMP may also be used to help ensure compliance with Minimum Requirement 6 - Stormwater Treatment, Minimum Requirement 7 - Flow Control and Minimum Requirement 8 - Wetlands Protection.
Where compliance with Minimum Requirements is not required, where feasible, this BMP is recommended to be used to help protect receiving waterbodies from the effects of stormwater.
The following infeasibility criteria describe conditions that make permeable pavement infeasible when applying The List Approach for compliance with Minimum Requirement 5 - Onsite Stormwater Management.
The infeasibility criteria shall also be used to determine the appropriateness of installing permeable pavement on a project site. Project proponents may be allowed to install permeable pavement even if considered infeasible, unless otherwise stated within the specific infeasibility criteria, provided documentation is included that substantiates the design. Documentation may be required by a Washington State Licensed Professional Engineer or other appropriate professional depending upon the project conditions.
These criteria also apply to impervious pavements that would employ stormwater collection from the surface of the impervious pavement with redistribution below the pavement.
Setback distances are measures from the edge of the pavement section.
A site characterization study must be completed in order to determine if the following infeasibility criteria apply.
The following infeasibility criteria require evaluation of site specific conditions and a written recommendation from an appropriate Washington State Licensed Professional (e.g., Professional Engineer, Professional Geologist, Professional Hydrogeologist)
Where professional geotechnical evaluation recommends infiltration not be used due to reasonable concerns about erosion, slope failure, or down gradient flooding.
Within an area whose groundwater flows into an erosion hazard or landslide hazard area.
Where infiltrating and ponded water below new permeable pavement would compromise existing adjacent impervious pavements.
Where infiltrating and ponded water below new permeable pavement would threaten existing below grade basements.
Where infiltrating water would threaten shoreline structures such as bulkheads.
Downslope of steep, erosion prone areas that are likely to erode sediment.
Where fill soils are used that can become unstable when saturated.
Excessively steep slopes where water within the aggregate base layer or at the subgrade surface cannot be controlled by detention structures and may cause erosion and structural failure, or where surface runoff velocities may preclude adequate infiltration at the pavement surface.
Where permeable pavements cannot provide sufficient strength to support the anticipated loads.
Where underlying soils are unsuitable for supporting traffic loads when saturated.
Where installation of permeable pavement would threaten the safety or reliability of preexisting underground utilities, preexisting underground storage tanks, preexisting structures, or preexisting road or parking lot surfaces.
The following infeasibility criteria are based upon subsurface characteristics and require a soils report to determine infeasibility. See Appendix B - Appendix B Soils Reports for Soils Report Requirements.
Where the minimum vertical separation between the elevation of the lowest layer designed to be part of the permeable pavement section and the seasonal high groundwater elevation, bedrock or other impermeable layer is 1 foot or less.
Where the native soils below a pollution-generating permeable pavement do not meet the soil suitability criteria for providing treatment. This applies only if the soils under the permeable pavement are being used for treatment. These include:
Cation exchange capacity (CEC) of the treatment soil must be ≥5 milliequivalents CEC/100g dry soil (USEPA Method 9081). Consider empirical testing of soil sorption capacity, if practicable. Ensure that soil CEC is sufficient for expected pollutant loadings, particularly heavy metals. CEC values of >5 meq/100g are expected in loamy sands, according to Buckman and Brady (1996).
Depth of soil used for infiltration treatment must be a minimum of 18 inches. This depth may be reduced to 12 inches if the permeable pavement does not accept runon.
Organic content shall be a minimum of 1%. Organic content shall be measured on a dry weight basis using ASTM D2974.
A measured (initial) saturated hydraulic conductivity of 12 inches/hour or less.
If the applicant wishes to use permeable pavement but the criteria above for treatment are not met, they can elect to use a 6” sand layer meeting the sand filter specification (Sand Filtration BMPs) to provide treatment.
Where the field testing indicates potential permeable pavement locations have a measured (initial) native soil saturated hydraulic conductivity less than 0.3 inches/hour.
Permeable pavement installed for the purposes of Minimum Requirement 5 - Onsite Stormwater Management shall only be constructed where infiltration is feasible.
If the measured soil infiltration rate is less than 0.30 inches/hour, permeable pavement cannot be used to meet The List Approach.
Where replacing existing impervious surfaces unless the existing surface is a non-pollution generating surface over an outwash soil with a saturated hydraulic conductivity of 4 inches/hour or greater.
The following infeasibility criteria are based on conditions such as topography and distances to predetermined boundaries. Citation of the following do not need site-specific written recommendations from a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist though some criteria may require professional services to determine if the infeasibility criteria apply.
Within an area designated as an erosion hazard or landslide hazard.
For properties with known soil or groundwater contamination:
Within 100 feet of an area known to have deep soil contamination;
Where groundwater modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the groundwater;
Wherever surface soils can be found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area;
Any area where these facilities are prohibited by an approved cleanup plan under the state Model Toxics Control Act or Federal Superfund Law, or an environmental covenant under Chapter 64.70 RCW.
Within 100 feet of a closed or active landfill.
Within 100 feet of a drinking water well, or a spring used for drinking water supply if the permeable pavement is (or has run-on from) a pollution-generating hard surface
Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and greywater reuse systems. For setbacks from a “large on-site sewage disposal system”, see WAC Chapter 246-272B.
Within 10 feet of an underground storage tank and connecting underground pipes, regardless of tank size. As used in this criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.
At multi-level parking garages, and over culverts and bridges.
Where the subgrade slope exceeds 6% after reasonable efforts to grade.
Where the permeable pavement wearing course slope exceeds 6% after reasonable efforts to design grade.
Where the road receives more than very low traffic volumes. Roads with a projected average daily traffic volume of 400 vehicles or less are very low volume roads (AASHTO, 2001), (USDOT, 2013). This infeasibility criterion does not extend to sidewalks and other non-traffic bearing surfaces.
Where the area receives more than very low truck traffic. Areas with very low truck traffic are roads or other areas not subject to through truck traffic but may receive up to weekly use by utility trucks (e.g., garbage, recycling), daily school bus use, and multiple daily use by pick-up trucks, mail/parcel delivery trucks, and maintenance vehicles. This infeasibility criterion does not extend to sidewalks and other non-traffic bearing surfaces.
Where replacing existing impervious surfaces unless the existing surface is a non-pollution generating surface over an outwash soil with a saturated hydraulic conductivity of 4 inches/hour or greater.
In areas that typically generate high concentrations of oil due to high traffic turnover or the frequent transfer of oil. These include:
Areas of a commercial or industrial site subject to an average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area, or 300 total trip ends per day.
Areas of commercial or industrial sites subject to petroleum storage and transfer in excess of 1,500 gallons per year, not including routinely delivered heating oil. This petroleum storage and transfer criterion is intended to address regular transfer operations such as gasoline service stations, not occasional filling of heating oil tanks.
Areas of a commercial or industrial site subject to parking, storage or maintenance of 25 or more vehicles that are over 10 tons gross weight (trucks, buses, trains, heavy equipment, etc.). In general, all-day parking areas are not included in this criteria.
Road intersections with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primary pedestrian or bicycle use improvements.
In areas with “industrial activity” as identified in 40 CFR 122.26(b)(14).
Where the risk of concentrated pollutant spills is more likely such as gas stations, truck stops, and industrial chemical storage sites.
Where routine, heavy applications of sand occur in frequent snow zones to maintain traction during weeks of snow and ice accumulation. Most lowland western Washington areas do not fit this criterion.
If there are any conflicts with any of the following competing needs criteria:
Requirements of the following federal or state laws, rules, and standards:
Historic Preservation Laws and Archaeology Laws as listed at: https://dahp.wa.gov/project-review/preservation-laws
Federal Superfund or Washington State Model Toxics Control Act
Federal Aviation Administration requirements for airports
American with Disabilities Act
When found to be in conflict with special zoning district design criteria adopted and being implemented pursuant to a community planning process.
Public health and safety standards
Transportation regulations to maintain the option for future expansion or multi-modal use of public rights-of-way.
City of Tacoma Critical Area Ordinances that provides protection of tree species or other critical areas.
Within 10 feet of a building structure.
Within 5 feet of any other structure or property line.
Within 50 feet from the top of any slope greater than 20% and from geologically hazardous areas. The permeable pavement may be sited within 50 feet of a slope greater than 20% and/or geologically hazardous area if a geotechnical analysis performed by a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist determines that there will be no negative impacts to the slopes and/or geologically hazardous areas caused by the permeable pavement.
Where the project site design cannot avoid putting pavement in areas likely to have long-term excessive sediment deposition after construction.
12.1.4 Subsurface Characterization
A soils report is required to design permeable pavement. A soils report is also required if citing that permeable pavement is not feasible due to subsurface characteristics. See Appendix B - Appendix B Soils Reports requirements.
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
At a minimum, comply with the design criteria in the sections below. The City of Tacoma Design Manual, Chapter 4 contains additional requirements for permeable pavement sections that are placed in the ROW. The Low Impact Development Technical Guidance Manual for Puget Sound is also a good guide for general guidance when designing permeable pavements.
Subgrade shall be a maximum 3% slope. If roadway surface exceeds 3%, subsurface detention structures per Section 12.1.5.5 may be required.
Compact the subgrade to the minimum necessary for structural stability and at a minimum be "firm and unyielding" and be 90-92% Standard Proctor. See the City of Tacoma Right of Way Design Manual for additional subgrade preparation for applications within the City ROW. The exposed subgrade shall maintain preconstruction infiltration rates. The subgrade shall be protected during construction including minimizing traffic to the subgrade.
To prevent compaction when installing the aggregate base, the following steps are recommended:
Dump aggregate onto subgrade from the edge of the installation.
Dump subsequent aggregate base from on top of the aggregate base.
12.1.5.2 Separation or Bottom Filter Layer
A layer of sand or crushed stone (0.5 inches or smaller) is recommended to promote infiltration across the surface, stabilize the base layer, protect underlying soils from compaction, and serve as a transition between the base course and the underlying geotextile material or subgrade.
12.1.5.3 Geotextile and GeoGrids (Optional)
Geotextiles between the subgrade and aggregate base are not required or necessary for many soil types. Geotextile is recommended on the side slopes of the open graded base perimeter next to the soil subgrade if concrete curbs or impermeable liners are not provided that extend the full depth of the base. Geotextile is required between the permeable ballast and sand layer used for treatment (if proposed). Geotextile shall conform to WSDOT Standard Specification 9-33.2(1) - Table 3 - Geotextile for Separation - Woven. Geotextile shall be installed per WSDOT Standard Specification 2-12.3(1).
12.1.5.4 Permeable Ballast Base Course
Refer to the City of Tacoma Standard Plan PD-01 for the minimum permeable ballast thickness necessary for structural integrity for permeable pavement roadways, accessways, and trails. The applicant shall submit modeling with an Ecology approved continuous simulation model, assuming a 15-minute timestep, showing the proposed ballast thickness is sufficient as a reservoir layer to manage stormwater based on the design criteria applicable to the project.
For private onsite permeable pavement surfaces, the minimum permeable ballast thickness shall be 6 inches for structural integrity and to function as a minimum reservoir layer. The applicant shall submit modeling results from an Ecology approved continuous simulation model, assuming a 15-minute timestep, showing the proposed ballast thickness is sufficient as a reservoir layer based on the design criteria applicable to the project. Projects required to comply with Minimum Requirements #6, #7, or #8 and Minimum Requirement #5: Onsite Stormwater Management - Low Impact Development (LID) Performance Standard shall submit modeling results from an Ecology approved continuous simulation model. The City of Tacoma may require other projects, such as installations that receive runon to submit calculations.
For pervious concrete sidewalks the minimum ballast thickness shall be 4 inches. Where runon is minimized to the pervious sidewalk sections, calculations are not necessary to determine if the reservoir layer is sufficient for stormwater management. If runon to the permeable sidewalk is allowed, additional modeling may be required by Environmental Services/Site Development Group per Joint Administrative Policy and Procedure Directive No. 2021-02-001.
Permeable ballast base course shall meet the requirements of WSDOT Standard Specification Section 9-03.9(2) - Permeable Ballast except as modified by this section. The permeable ballast base course shall be seated or compacted until no visible movement of aggregate is observed. Immediately following spreading and final shaping each layer of surfacing shall be lightly compacted in one lift to a firm and unyielding condition.
The above permeable ballast criteria are specific to pervious concrete and permeable asphalt. Other permeable pavements such as PICP may use different materials provided the materials are free draining and information is submitted that substantiates the design.
The above permeable ballast criteria are specific to pervious concrete, permeable asphalt, and ballasted sidewalk. Other permeable pavements such as PICP may use different materials provided they materials are free draining and information is submitted that substantiates the design.
Permeable ballast base course shall not be manufactured from recycled concrete. The materials shall be uniform in quality and substantially free from wood, roots, bark, and other extraneous material and shall meet the following quality test requirements:
The fracture requirement shall be at least two fractured faces and will apply to the combined aggregate retained on the No. 4 sieve in accordance with FOP for AASHTO T335. Permeable ballast base course shall meet the requirements for grading and quality when placed in hauling vehicles for delivery to the site, after placement in temporary location, when in stockpiles on site, during installation, and after installation and in place after compacted to project specifications.
12.1.5.5 Subsurface Detention Structures
For permeable pavements on slopes ≥3%, provide subsurface detention structures or terrace subgrade to increase infiltration, improve flow attenuation, and reduce structural issues with subgrade erosion on slopes. See the “Low Impact Technical Guidance Manual for Puget Sound” for examples and design recommendations.
The wearing course includes the actual driving surface and the asphalt treated permeable base layer.
For permeable pavement roadways and accessways refer to City of Tacoma Standard Plan PD-01 for minimum wearing layer thickness.
For private onsite permeable pavement surfaces, the minimum wearing layer thickness shall be 4” for porous asphalt and 6” for pervious concrete though actual thickness shall be based on projected traffic use.
The maximum slope for porous asphalt and permeable concrete shall be 6%.
A minimum initial infiltration rate of 20 inches per hour is required. High infiltration rates are recommended.
For porous asphalt, products shall have adequate void space, commonly 16-25%.
For pervious concrete, products shall have adequate void space, commonly, 15-35%.
For permeable interlocking concrete pavement and aggregate pavers, pavement joints should be filled with No. 8, 89, or 9 stone.
For grid/lattice systems filled with gravel, sand, or a soil of finer particles with or without grass, the fill material shall be at least a minimum of 2 inches of sand, gravel, or soil.
12.1.5.7 Permeable Pavement - Underdrain
Underdrains are not recommended. Underdrains placed at or near the bottom of the aggregate base should not be used and if used, the permeable pavement is not considered a low impact development technique and cannot be used to satisfy List Approach of Minimum Requirement 5 - Onsite Stormwater Management and cannot be used to satisfy flow control requirements. Elevated underdrains that are placed within the aggregate base course to protect the pavement wearing course from saturation can be used to satisfy the List Approach of Minimum Requirement 5 - Onsite Stormwater Management though an underdrain is not required and not recommended. The underdrain perforations or slots shall only be located on the lower half of the pipe.
Permeable roads should be designed to ensure safe driving conditions in the event of an intense storm event and to account for potential clogging.
For driveways or small permeable pavement sections (1000 square feet or less), test by pouring a bucket of water on the permeable surface. Additional testing prior to acceptance will be needed if there is runoff from the surface.
For permeable pavement sections greater than 1000 square feet, test the initial infiltration rate using ASTM C1701. Test to determine if the minimum infiltration rate of 20 inches per hour is met. The number of test locations is specified in ASTM C1701.
12.1.7 Permeable Pavement Modeling
Use the permeable pavement element of an Ecology approved continuous simulation model, assuming a 15-minute timestep.
The models allow for specifying pavement thickness and porosity, aggregate base material thickness and porosity, maximum allowed ponding depth, and native soil infiltration rate.
Use a value of zero for the pavement thickness and pavement porosity. The design shall not depend on water storage in the pore space of the permeable pavement section itself but should rely on the reservoir layer.
For grades less than 2% no adjustments to the below grade volumes are necessary.
For grades greater than 2% without internal dams within the base materials, the below ground storage volume must be adjusted as follows:
Permeable pavement surfaces that are below the surrounding grade and that are on a slope can be modeled as permeable pavement with an infiltration rate and a nominal depth.
The dimensions of the permeable pavement are:
◦ the length (parallel to and beneath the road) of the base materials that are below grade
◦ the width of the below grade base materials
◦ an Effective Total Depth of 1 inch.
◦ If the continuous simulation model required the permeable pavement to have an overflow riser for the model run, enter 0.04 ft (0.5") for the riser height and 1000 inches for the riser diameter. This will ensure no head build-up.
If an underdrain is elevated in the below grade base materials, the pipe should only have perforations or slots on the lower half (below the spring line) or near the invert. Pipe volume and trench volume above the pipe invert cannot be assumed as available storage space. If a underdrain is placed at the bottom of the base material, model the pavement as an impervious area.
For roads on a slope with internal dams within the base materials that are below grade, the below ground storage volume must be adjusted as follows:
Each stretch of permeable pavement (cell) that is separated by barriers can be modeled separately. For each cell, determine the average depth of water within the cell at which the barrier at the lower end will be overtopped.
Specify the dimensions of each cell of the below-grade base materials using the permeable pavement dimension fields for:
◦ the pavement length (length of the cell parallel to the road
◦ the width of the bottom of the base material
Each cell should have its own contributing area within the permeable pavement element that includes the road above it, any project site areas whose stormwater drains onto and through the road and any offsite areas. Represent each contributing area with a permeable pavement icon and a lateral flow basin icon (if there is runon).
In the runoff modeling, similar designs throughout the project can be summed and represented as one facility. For example, all walkways can be summed into one facility. In these instances, a weighted average of the design infiltration rate (only if they are within a factor of 2) can be used. The averages shall be weighted by the contributing area.
In WWHM, in the Permeable Pavement Element, the Use Wetted Surface Area should be set to No if the below grade base material trench has sidewalls steeper than 2H:1V - which is typical of roadway design.
Sweep non-planted surfaces with a high-efficiency sweeper twice per year, once in autumn and once in early spring. Sweeping frequency can be reduced if infiltration rate testing indicates that a rate of 10 inches/hour or greater is being maintained.
The design engineer should consider the turning radius of the sweeper when designing permeable pavement surfaces to ensure the facility can be properly maintained