Volume 2-05, Chapter 3 - City of Wichita
Volume 2-05, Chapter 3 - City of Wichita Volume 2-05, Chapter 3 - City of Wichita
Section 3.4.4 - Modular Porous Paver Systems 3.4.4.1 General Description Modular porous pavers are structural units, such as concrete blocks, bricks, or reinforced plastic mats, with regularly interspersed void areas used to create a load-bearing pavement surface. The void areas are filled with pervious materials (gravel, sand, or grass turf) to create a system that allows for the infiltration of stormwater runoff. The use of porous paver systems results in a reduction of the effective impervious area on a site (see Section 3.4.4.3), and the additional infiltration may be used to infiltrate all or part of the difference between the pre- and post-development 1-year, 24-hour CP V . There are many different types of modular porous pavers available from different manufacturers, including both pre-cast and mold in-place concrete blocks, concrete grids, interlocking bricks, and plastic mats with hollow rings or hexagonal cells (see Figure 3-59). Modular porous pavers are typically placed on a gravel (stone aggregate) base course. Runoff infiltrates through the porous paver surface into the gravel base course, which acts as a storage reservoir as it infiltrates to the underlying soil. The infiltration rate of the soils in the subgrade must be adequate to support drawdown of the entire runoff capture volume within 24 to 48 hours. Special care must be taken during construction to avoid undue compaction of the underlying soils, which could affect the soils’ infiltration capability. Modular porous paver systems are typically used in low-traffic areas such as the following types of applications: • Parking pads in parking lots; • Overflow parking areas; • Residential driveways; • Residential street parking lanes; • Recreational trails; • Golf cart and pedestrian paths; • Emergency vehicle and fire access lanes. Slopes should be flat or gentle to facilitate infiltration versus runoff. Bedrock should be a minimum of two feet below the bottom of the gravel layer, and historically high groundwater must be at least 5 feet below the bottom of the gravel bed. Porous paver systems have the positive characteristics of volume reduction due to infiltration, groundwater recharge, and an ability to blend into the normal urban landscape relatively unnoticed. It also may result in a reduction in the need for (and cost of) other stormwater infrastructure which may somewhat offset the placement cost. A major drawback is the cost and complexity of modular porous paver systems compared to conventional pavements. Porous paver systems require a very high level of construction Page 3 - 202 Volume 2, Technical Guidance
Section 3.4.4 - Modular Porous Paver Systems workmanship to ensure that they function as designed and do not settle unevenly. In addition, there is the difficulty and cost of rehabilitating the surfaces should they become clogged. Therefore, consideration of porous paver systems should include the construction and maintenance requirements and costs. Like other infiltration controls, porous paver systems should not be used in areas that experience high rates of wind deposited particulates, or receive inflow from areas with highly erosive (unstable) soils. Also it cannot be used in traffic areas where sanding or salting is used during winter weather. 3.4.4.2 Pollutant Removal Capabilities The following deisgn pollutant removal rates are average polluntant reduction percentages for design purposes derived from sampling data, modeling, and professional judgment. • Total Suspnded Solids – must not be used to remove this pollutant • Total Phosphorus – 80% • Total Nitrogen – 80% • Metals – 90% • Fecal Coliform – must not be used to remove this pollutant 3.4.4.3 Design Criteria and Specifications • Porous paver systems can be used where the underlying in-situ subsoils have an infiltration rate of between 0.5 and 3.0 inches per hour. Therefore, porous paver systems are not suitable on sites with hydrologic group D or most group C soils, or soils with a high (>30%) clay content without an under drain system. During construction and preparation of the subgrade, special care must be taken to avoid compaction of the soils. • Porous paver systems should ideally be used in applications where the pavement receives tributary runoff only from impervious areas. The ratio of the contributing impervious area to the porous paver surface area should be no greater than 3:1. • If runoff is coming from adjacent pervious areas, it is important that those areas be fully stabilized to prevent sediment loads and clogging of the porous paver surface. Pretreatment using filter strips or vegetated swales for coarse sediment removal is recommended. • Porous paver systems are not recommended on sites with a slope greater than 2%. • A minimum of 2 feet of clearance is required between the bottom of the gravel base course and underlying bedrock, or 5 feet above the historically high groundwater table. Inflow from untreated hotspots is not permitted. • Porous paver systems should be sited at least 10 feet down gradient from buildings and a safe distance from drinking water wells. Volume 2, Technical Guidance Page 3 - 203
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Section 3.4.4 - Modular Porous Paver Systems<br />
workmanship to ensure that they function as designed and do not settle unevenly. In addition,<br />
there is the difficulty and cost <strong>of</strong> rehabilitating the surfaces should they become clogged.<br />
Therefore, consideration <strong>of</strong> porous paver systems should include the construction and<br />
maintenance requirements and costs.<br />
Like other infiltration controls, porous paver systems should not be used in areas that<br />
experience high rates <strong>of</strong> wind deposited particulates, or receive inflow from areas with highly<br />
erosive (unstable) soils. Also it cannot be used in traffic areas where sanding or salting is<br />
used during winter weather.<br />
3.4.4.2 Pollutant Removal Capabilities<br />
The following deisgn pollutant removal rates are average polluntant reduction percentages for<br />
design purposes derived from sampling data, modeling, and pr<strong>of</strong>essional judgment.<br />
• Total Suspnded Solids – must not be used to remove this pollutant<br />
• Total Phosphorus – 80%<br />
• Total Nitrogen – 80%<br />
• Metals – 90%<br />
• Fecal Coliform – must not be used to remove this pollutant<br />
3.4.4.3 Design Criteria and Specifications<br />
• Porous paver systems can be used where the underlying in-situ subsoils have an<br />
infiltration rate <strong>of</strong> between 0.5 and 3.0 inches per hour. Therefore, porous paver systems<br />
are not suitable on sites with hydrologic group D or most group C soils, or soils with a high<br />
(>30%) clay content without an under drain system. During construction and preparation<br />
<strong>of</strong> the subgrade, special care must be taken to avoid compaction <strong>of</strong> the soils.<br />
• Porous paver systems should ideally be used in applications where the pavement receives<br />
tributary run<strong>of</strong>f only from impervious areas. The ratio <strong>of</strong> the contributing impervious area<br />
to the porous paver surface area should be no greater than 3:1.<br />
• If run<strong>of</strong>f is coming from adjacent pervious areas, it is important that those areas be fully<br />
stabilized to prevent sediment loads and clogging <strong>of</strong> the porous paver surface.<br />
Pretreatment using filter strips or vegetated swales for coarse sediment removal is<br />
recommended.<br />
• Porous paver systems are not recommended on sites with a slope greater than 2%.<br />
• A minimum <strong>of</strong> 2 feet <strong>of</strong> clearance is required between the bottom <strong>of</strong> the gravel base<br />
course and underlying bedrock, or 5 feet above the historically high groundwater table.<br />
Inflow from untreated hotspots is not permitted.<br />
• Porous paver systems should be sited at least 10 feet down gradient from buildings and a<br />
safe distance from drinking water wells.<br />
<strong>Volume</strong> 2, Technical Guidance Page 3 - 203
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