Permeable Pavement

5.4.8 Permeable Pavement

5.4.8.1 BMP Description

Permeable pavement (BMP T5.15 in Volume V, Chapter 5 of the Ecology Manual) is a paving system that allows rainfall to infiltrate into an underlying aggregate storage reservoir, where stormwater is stored and infiltrated to the underlying subgrade or removed by an overflow drainage system.

A permeable pavement BMP consists of a pervious wearing course (e.g., porous asphalt, pervious concrete, etc.) and an underlying storage reservoir. The storage reservoir is designed to support expected loads and store stormwater to allow time for the water to infiltrate into the underlying soil.

While not explicitly addressed in this section, infiltration may be allowed under impermeable pavements, outside of public rights of way, in lieu of permeable pavement.

Pavement for vehicular and pedestrian travel occupies roughly twice the space of buildings. Stormwater from vehicular pavement can contain significant levels of solids, heavy metals, and hydrocarbon pollutants. Both pedestrian and vehicular pavements also contribute to increased peak flow durations and associated physical habitat degradation of streams and wetlands. Optimum management of stormwater quality and quantity from paved surfaces is, therefore, critical for improving fresh and marine water conditions in Puget Sound.

The general categories of permeable paving systems include:

Porous Hot or Warm-Mix Asphalt Pavement – 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 – 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.

5.4.8.2 Performance Mechanism

Flow control occurs through temporary storage of stormwater runoff in the voids of the aggregate material and subsequent infiltration of stormwater into the underlying soils. Pollutant removal mechanisms include sedimentation, infiltration, filtration, adsorption, and biodegradation.

5.4.8.3 Application and Limitations

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 runoff treatment and flow control of stormwater.

Permeable pavements are appropriate in many applications where traditionally impermeable pavements have been used. Typical applications for permeable paving include parking lots, sidewalks, pedestrian and bike trails, driveways, residential access roads, and emergency and facility maintenance roads.

Limitations to permeable pavements include:

No run-on from pervious surfaces is preferred. If runoff comes from minor or incidental pervious areas, those areas shall be fully stabilized to reduce or prevent erosion.
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.
Soils shall not be tracked onto the wear layer or the base course during construction.

As shown in Table II-5.12, permeable pavement may be used to meet on-site stormwater management, runoff treatment and flow control requirements. Regarding runoff treatment, Ecology recognizes permeable pavement as a basic treatment BMP if it meets either of the following criteria:

The native soils below the permeable pavement meet the criteria for Runoff Treatment per Volume V, Section 5.6 (Site Suitability Criteria) of the Ecology Manual; OR
The permeable pavement design includes a 6‑inch layer of sand that meets the size gradation (by weight) given in Sand Medium Specification table in Volume V, Chapter 6 of the Ecology Manual.

See the Permeable Pavement as a Runoff Treatment section in BMP T5.15 in Volume V, Chapter 5 of the Ecology Manual for additional information.

Table II-5.12. Permeable Pavement Applicability.^a
BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
BMP	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus	MR #7: Flow Control
Permeable Pavement	X	X	X^b				X
Notes: Site suitability criteria apply (see Volume V, Section 5.6 of the Ecology Manual). Ecology recognizes permeable pavement as a basic treatment BMP if the native soils below the permeable pavement meet the criteria for Runoff Treatment per Volume V, Section 5.6 in the Ecology Manual; or the permeable pavement design includes a 6‑inch layer of sand that meets the size gradation (by weight) given in the Sand Medium Specification table in Volume V, Chapter 6 of the Ecology Manual.

5.4.8.4 Site Considerations

Unlike many BMPs that require dedicated space on a site, permeable pavement BMPs are part of the usable lot area and can replace conventional pavements, including:

Sidewalks and pedestrian plazas
Pedestrian and bike trails
Driveways
Most parking lots
Low volume roads, alleys, and access drives

Site considerations for the applicability of permeable pavement BMPs include:

Setbacks and Restrictions – Permeable pavement BMPs shall meet the siting and infiltration rate requirements for infiltration BMPs presented in Vol II–5.3.2 Determine Infiltration Feasibility.
Site Topography – The recommended maximum surface (wearing course) slope for permeable pavement BMPs is 6% to allow efficient storage of water within the subbase. For vehicular traction, the maximum surface slope varies by wearing course type (see industry guidelines). Minimum wearing course slope shall be 1% unless provision is made for positive drainage in event of surface clogging.
Slope – The recommended maximum subgrade slope for permeable pavement applications is 6%. Subgrades that are sloped require subsurface check dams to promote storage in the subgrade. At steeper subgrades slopes, design and construction become more complex and the construction cost increases.
Land Use – Because permeable pavement can clog with sediment, permeable pavement BMPs are not recommended where sediment and pollutant loading are unavoidable, including the following conditions:
- Excessive sediment contamination is likely on the pavement surface (e.g., construction areas, landscaping material yards).
- It is infeasible to prevent stormwater run-on to the permeable pavement from unstabilized erodible areas without pre-settling.
- Regular, heavy application of sand is anticipated for maintaining traction during winter, or the BMP is in close proximity to areas that will be sanded. A minimum 7‑foot clearance is required between a permeable pavement BMP and the travel lane of sanded arterial roads.
- Sites where the risk of concentrated pollutant spills are more likely (e.g., gas stations, truck stops, car washes, vehicle maintenance areas, industrial chemical storage sites, etc.).
Accessibility – As for standard pavement design, Americans with Disabilities Act (ADA) accessibility issues shall be addressed when designing a permeable pavement BMP, particularly when using pavers.

5.4.8.5 Design Information

See BMP T5.15 in Volume V, Chapter 5 of the Ecology Manual for design requirements and Section 6.3 in the LID Technical Guidance Manual (Hinman and Wulkan 2012) for additional guidance. BMP T5.15 in Volume V, Chapter 5 of the Ecology Manual provides guidance on Runoff Model Representation.

5.4.8.6 Minimum Construction Requirements

Proper construction methods and pre-planning are essential for the successful application of any permeable pavement BMP. Over-compaction of the underlying soil or fine sediment contamination onto the existing subgrade and pavement section during construction will significantly degrade or effectively eliminate the infiltration capability of the BMP.

Minimum requirements associated with construction of a permeable pavement BMP include the following:

Conduct field infiltration and compaction testing of the water quality treatment course (if included) prior to placement of overlying courses.
Prevent intermixing of the various base course materials with fines and sediment. Remove and replace all contaminated material.
Complete final subgrade excavation during dry weather on the same day bottom aggregate course is placed, when practicable.
Use traffic control measures to protect permeable pavement subgrade areas from heavy equipment operation or truck/vehicular traffic.
Select excavation, grading, and compaction equipment to minimize the potential for over-compaction.
Isolate the permeable pavement site from sedimentation during construction, either by use of effective erosion and sediment control measures upstream. Alternatively, delay the excavation of the lowest 1 foot of material above the final subgrade elevation for the entire pavement area until after all sediment-producing construction activities have been completed and upstream areas have been permanently stabilized. Once the site is stabilized, the lowest 1 foot of material may be removed. For more information on site stabilization, see Vol I–4.2.2 Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP).
Conduct infiltration test in accordance with the Infiltration Test for Permeable Pavement Surface subsection in BMP T5.15 in Volume V, Chapter 5 of the Ecology Manual.

5.4.8.7 Operations and Maintenance

See the Maintenance section in BMP T5.15 in Volume V, Chapter 5 and Volume V, Appendix V‑A of the Ecology Manual or further detail on specific maintenance activities and schedules for permeable pavement systems.