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All pavement designs require an overflow outlet connected to a storm sewer with capacity to convey larger storms. One option is to set storm drain inlets slightly above the surface elevation of the pavement, which allows for temporary shallow ponding above the surface. If the surface is overloaded or clogged, then flows that are too large to be treated by the system can be bypassed through the storm drain inlets. Another design option intended as a backup water removal mechanism is an overflow edge (Figure 4.7.5). An overflow edge is a gravel trench along the downgradient edge of the pavement surface that drains to the stone reservoir below. If the pavement surface is overloaded or clogs, stormwater will flow over the surface and into the overflow edge and underlying stone reservoir, where infiltration and treatment can still occur. On smaller sites, overflow can simply sheet flow onto the traditional paving and drain into the storm sewer system.
All pavement designs require an overflow outlet connected to a storm sewer with capacity to convey larger storms. One option is to set storm drain inlets slightly above the surface elevation of the pavement, which allows for temporary shallow ponding above the surface. If the surface is overloaded or clogged, then flows that are too large to be treated by the system can be bypassed through the storm drain inlets. Another design option intended as a backup water removal mechanism is an overflow edge (Figure 4.7.5). An overflow edge is a gravel trench along the downgradient edge of the pavement surface that drains to the stone reservoir below. If the pavement surface is overloaded or clogs, stormwater will flow over the surface and into the overflow edge and underlying stone reservoir, where infiltration and treatment can still occur. On smaller sites, overflow can simply sheet flow onto the traditional paving and drain into the storm sewer system.
Pavements designed for full infiltration, where native soil infiltration rate is 15 mm/hr or greater, do not require incorporation of a perforated pipe underdrain. Pavements designed for partial infiltration, where native soil infiltration rate is less than 15 mm/hr (i.e., hydraulic conductivity less than 1x10-6 cm/s) should incorporate a perforated pipe underdrain placed near the top of the granular stone reservoir. Partial infiltration designs can also include a flow restrictor assembly on the underdrain to optimize infiltration with desired drawdown time between storm events (Figure ).
Pavements designed for full infiltration, where native soil infiltration rate is 15 mm/hr or greater, do not require incorporation of a perforated pipe underdrain. Pavements designed for partial infiltration, where native soil infiltration rate is less than 15 mm/hr (i.e., hydraulic conductivity less than 1x10-6 cm/s) should incorporate a perforated pipe underdrain placed near the top of the granular stone reservoir. Partial infiltration designs can also include a flow restrictor assembly on the underdrain to optimize infiltration with desired drawdown time between storm events (Figure ).
<h4>Monitoring Wells</h4>
A capped vertical standpipe consisting of an anchored 100 to 150 millimetre diameter perforated pipe with a lockable cap installed to the bottom of the facility is recommended for monitoring the length of time required to fully drain the facility between storms.
<h4>Stone Reservoir</h4>
The stone reservoir must be designed to meet both runoff storage and structural support requirements. Clean washed stone is recommended as any fines in the aggregate material will migrate to the bottom and may prematurely clog the native soil (Smith, 2006). The bottom of the reservoir should be flat so that runoff will be able to infiltrate evenly through the entire surface. If the system is not designed for infiltration, the bottom should be sloped at 1 to 5% toward the underdrain. A hybrid permeable pavement/soakaway design can feature connection of a roof downspout directly to the stone reservoir of the permeable pavement system, which is sized to store runoff from both the pavement surface and the roof drainage area.
<h4>Geotextile</h4>
A non-woven needle punched, or woven monofilament geotextile fabric should be installed between the stone reservoir and native soil. Woven slit film and non-woven heat bonded fabrics should not be used as they are prone to clogging. The primary function of the geotextile is separation between two dissimilar soils. When a finer grained soil or aggregate bedding layer overlies a coarser grained soil or aggregate layer (e.g., stone reservoir), the geotextile prevents clogging of the void spaces from downward migration of soil particles. When a coarser grained aggregate layer (e.g., stone reservoir) overlies a finer grained native soil, the geotextile prevents slumping from downward migration of the aggregate into the underlying soil. Geotextile may also enhance the capacity of the facility to reduce petroleum hydrocarbons in runoff, as microbial communities responsible for their decomposition tend to concentrate in geotextile fabrics (Newman et al., 2006a). Specification of geotextile fabrics in permeable pavement systems should consider the apparent opening size (AOS) for non-woven fabrics, or percent open area (POA) for woven fabrics, which affect the long term ability to maintain water flow. Other factors that need consideration include maximum forces to be exerted on the fabric, the load bearing ratio and permeability of the underlying native soil, and the texture (i.e., grain size distribution) of the overlying pavement bedding material. Table 4.7.5 provides further detail regarding geotextile specifications.
<h4>Pavement</h4>
The costs and benefits vary for each of the permeable pavement types. Review the design specifications in Table 4.7.5 and consult the other design resources to determine which pavement type is appropriate for your application.
<h4>Edge Restraints </h4>
The provision of suitable edge restraints is critical to the satisfactory performance of permeable pavements. Pavers must abut tightly against the restraints to prevent rotation under load and any consequent spreading of joints. The restraints must be sufficiently stable that, in addition to providing suitable edge support for the paver units, they are able to withstand the impact of temperature changes, vehicular traffic and/or snow removal equipment. Metal or plastic stripping is acceptable in some cases, but concrete edges are preferred (Figure 4.7.6). Edge restraints should be used for pervious concrete and porous asphalt to prevent pavement unravelling at the edges. Curbs, gutters, or curbed gutter, constructed to the dimensions of municipal standards (these standards generally refer to cast-in-place concrete sections), are considered to be acceptable edge restraints for heavy duty installations. Where extremely heavy industrial equipment is involved such as container handling equipment, the flexural strength of the edge restraint should be carefully reviewed, particularly if a section that is flush with the surface is used and may be subjected to high point loading. Concrete edge restraints should be supported on a minimum base of 150 mm of aggregate.
<h4>Landscaping</h4>
Landscaping plans should reflect the permeable pavement application. Landscaping areas should drain away from permeable pavement to prevent sediments from running onto the surface. Urban trees also benefit from being surrounded by permeable pavement rather than impervious cover, because their roots receive more air and water. Permeable pavers used around the base of a tree can be removed as the tree grows.