Low Impact Development (LID) is coming to Washington State in the newest round of municipal stormwater permits, called
National Pollution Discharge Elimination System (NPDES) Permits. These permits are issued by the Washington State
Department of Ecology and cover more than 90 Cities and Counties in Western Washington and 24 cities and counties in Eastern
Washington. By June 2015, Seattle, Tacoma, and unincorporated King, Snohomish, Pierce and Clark Counties will require the use
of low impact development (LID) on both new and redevelopment projects. Most other Western Washington NPDES permittees
will need to adopt these requirements by the end of 2016. The Eastern Washington NPDES permittees are required to update
their codes, if needed, to allow the use of LID by the end of 2017.
Permeable pavement is a good addition to Low Impact Development (LID) landscapes because it allows water to seep through designed voids in its structure. This allows the water to infiltrate into native soil, reducing and filtering stormwater before it enters nearby water bodies. But soil conditions, correct components, and base layers are critical to proper function. Start with the soil Testing the soil on site to identify soil texture, organic matter content, ability to treat pollutants, compaction and infiltration rate will determine whether the soil will be adequate for absorbing stormwater and removing pollutants. Soils that don’t drain well may not be suitable for installation of permeable pavements unless they include an overflow system or under-drains. Soils that do not allow for treatment of pollutants require an engineered sub layer to treat stormwater and receive credit on new or redeveloped LID sites. Does permeable pavement work on any site? Just like any other LID Best Management Practice (BMP), a thorough site assessment will determine whether permeable pavement is appropriate. The site assessment should include an inventory of existing structures and function such as hardscaping, utilities, contributing impervious area, vegetation, soil quality including texture (clay, silt, and sand ratio) and compaction, natural drainage, infiltration rates, slope, and exposure. Site conditions that are not appropriate for permeable pavements include areas with excessive sediment deposits; locations that could experience concentrated pollutant spills like gas stations or industrial chemical storage sites; seasonally high groundwater within one foot of the bottom of the aggregate storage course; steep slopes where water may cause erosion and structural failure; water seeps and; known flooding issues. When working on large or complex sites it is always recommended that you consult with an engineer to determine the suitability of permeable pavement for your site prior to getting started. What are the common components of permeable pavement? Each type of permeable pavement has different installation specifications to allow for infiltration. Research and assessment of each project is important. Specifications will depend on the type of pavement product as well as the site requirements and local (municipal and county) specifications. Zsofia Pasztor emphasizes the importance of making sure the specifications being used are up to date. Each site typically includes these components: Subgrade –the exposed native sub-soil under the storage or reservoir course (layer). Storage or reservoir course (layer) –clean washed gravel, or aggregate with no fines, large enough to create adequate voids for water storage, and with angular surfaces that lock together to provide structural support. Bedding or “Choker” course – clean washed aggregate with no fines, smaller than for storage or reservoir course. This layer provides a level, stable under-surface and is not necessary for all products. Surface or Wearing course – the porous concrete, asphalt, pavers, or other permeable materials used at the surface of the installation. Geotextiles are sometimes used between the sub grade and reservoir layers to keep sediments out of the subgrade. Drains or Overflow- are used depending on the amount of rain water the site receives, the height of the water table, and the infiltration rate of the subgrade. Do permeable pavements require a lot of maintenance? Rick Crooks of Mutual Materials describes, “While permeable pavements catch everything that falls on or flows to them, routine pavement maintenance will allow these systems to function properly for many years. Sites receiving large amounts of silt or with a large amount of falling organic material may clog more readily and will require more maintenance.” In addition to regular maintenance, Crooks recommends monitoring adjacent property activity for possible sediment sources. For example, debris from a construction project could spill over and fill the voids in permeable pavement. Provide clear maintenance plans when installing permeable pavement projects that include: • Periodic(at least once per year) evaluation of infiltration rate • Periodic (at least once per year) evaluation of drains and overflow systems • Cleaning of surfaces once or twice annually using suction, vacuum, or high pressure wash equipment (shop vacs are appropriate for smaller installations). • Weed removal – weed flamers are effective for this purpose; avoid using herbicides • Periodic replenishment of aggregate in joints, if required • Drain and overflow system cleaning, as needed
Permeable pavement is a good addition to Low Impact Development (LID) landscapes because it allows water to seep through designed voids in its structure. This allows the water to infiltrate into native soil, reducing and filtering stormwater before it enters nearby water bodies. But soil conditions, correct components, and base layers are critical to proper function. Start with the soil Testing the soil on site to identify soil texture, organic matter content, ability to treat pollutants, compaction and infiltration rate will determine whether the soil will be adequate for absorbing stormwater and removing pollutants. Soils that don’t drain well may not be suitable for installation of permeable pavements unless they include an overflow system or under-drains. Soils that do not allow for treatment of pollutants require an engineered sub layer to treat stormwater and receive credit on new or redeveloped LID sites. Does permeable pavement work on any site? Just like any other LID Best Management Practice (BMP), a thorough site assessment will determine whether permeable pavement is appropriate. The site assessment should include an inventory of existing structures and function such as hardscaping, utilities, contributing impervious area, vegetation, soil quality including texture (clay, silt, and sand ratio) and compaction, natural drainage, infiltration rates, slope, and exposure. Site conditions that are not appropriate for permeable pavements include areas with excessive sediment deposits; locations that could experience concentrated pollutant spills like gas stations or industrial chemical storage sites; seasonally high groundwater within one foot of the bottom of the aggregate storage course; steep slopes where water may cause erosion and structural failure; water seeps and; known flooding issues. When working on large or complex sites it is always recommended that you consult with an engineer to determine the suitability of permeable pavement for your site prior to getting started. What are the common components of permeable pavement? Each type of permeable pavement has different installation specifications to allow for infiltration. Research and assessment of each project is important. Specifications will depend on the type of pavement product as well as the site requirements and local (municipal and county) specifications. Zsofia Pasztor emphasizes the importance of making sure the specifications being used are up to date. Each site typically includes these components: Subgrade –the exposed native sub-soil under the storage or reservoir course (layer). Storage or reservoir course (layer) –clean washed gravel, or aggregate with no fines, large enough to create adequate voids for water storage, and with angular surfaces that lock together to provide structural support. Bedding or “Choker” course – clean washed aggregate with no fines, smaller than for storage or reservoir course. This layer provides a level, stable under-surface and is not necessary for all products. Surface or Wearing course – the porous concrete, asphalt, pavers, or other permeable materials used at the surface of the installation. Geotextiles are sometimes used between the sub grade and reservoir layers to keep sediments out of the subgrade. Drains or Overflow- are used depending on the amount of rain water the site receives, the height of the water table, and the infiltration rate of the subgrade. Do permeable pavements require a lot of maintenance? Rick Crooks of Mutual Materials describes, “While permeable pavements catch everything that falls on or flows to them, routine pavement maintenance will allow these systems to function properly for many years. Sites receiving large amounts of silt or with a large amount of falling organic material may clog more readily and will require more maintenance.” In addition to regular maintenance, Crooks recommends monitoring adjacent property activity for possible sediment sources. For example, debris from a construction project could spill over and fill the voids in permeable pavement. Provide clear maintenance plans when installing permeable pavement projects that include: • Periodic(at least once per year) evaluation of infiltration rate • Periodic (at least once per year) evaluation of drains and overflow systems • Cleaning of surfaces once or twice annually using suction, vacuum, or high pressure wash equipment (shop vacs are appropriate for smaller installations). • Weed removal – weed flamers are effective for this purpose; avoid using herbicides • Periodic replenishment of aggregate in joints, if required • Drain and overflow system cleaning, as needed
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