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Landscaped areas ''must'' be [[grading|graded]] drain away from permeable paving to prevent sediment from running onto the surface. <br>Urban [[trees]] benefit from being surrounded by permeable pavement rather than impervious cover, because their roots receive more air and water. Block pavers around the base of a tree can be removed as the tree grows.
==Performance==Permeable pavers can be classified into two categories according to the infiltration rate of the underlying subsoil:*Full Infiltration: Full infiltration designs are more effective, because little if any of the pollutants generated on the impermeable surfaces leave the site as surface runoff*Partial Infiltration: Partial infiltration designs with underdrains generate more runoffStudies in North Carolina have shown the average curve number of permeable pavements to range from a low of 45 to a high of 89. <ref>Bean, E.Z., Hunt, W, F., Bidelspach, D.A. 2007a. Evaluation of Four Permeable Pavement Sites in Eastern North Carolina for Runoff Reduction and Water Quality Impacts. Journal of Irrigation and Drainage Engineering. Vol. 133. No. 6. pp. 583-592.</ref><br>Partial infiltration designs with underdrains generate more runoff, and as a result, are often used in studies investigating the water quality impact of permeable pavements on surface waters. These studies show load reductions above 50% for total suspended solids, most metals and hydrocarbons <ref>Legret, M and V. Colandani. 1999. Effects of a porous pavement structure with a reservoir structure on runoff water: water quality and fate of metals. Water Science and Technology. 39(2): 111-117</ref> <ref>Pratt, C.J., Mantle, J.D.G., Schofield, P.A. 1995. UK research into the performance of permeable pavement reservoir structures in controlling stormwater discharge quantity and quality. Water Science Technology. Vol. 32. No. 1. pp. 63-69.</ref> <br>As with all stormwater infiltration practices, risk of groundwater contamination from infiltration of runoff laden with road de-icing [[salt]] constituents (typically sodium and chloride) is a significant concern. Chloride ions are extremely mobile in the soil and are readily transported by percolating water to aquifers.
Permeable pavements require regular inspection and maintenance to ensure proper functioning. The limiting factor for permeable pavers is clogging within the aggregate layers, filler, or underdrain. Ideally, signs should be posted on the site identifying permeable paver and porous pavement areas. This can also serve as a public awareness and education opportunity. See: [[Permeable paving: Maintenance]]
==Life cycle costs==Initial construction costs for permeable pavements are typically higher than conventional asphalt pavement surfaces, largely due to thicker aggregate base needed for stormwater storage. However, the cost difference is reduced or eliminated when total life-cycle costs, or the total cost to construct and maintain the pavement over its lifespan, are considered. Other potential savings and benefits include reduced need for storm sewer pipes and other stormwater practices, less developable land consumed for stormwater treatment, and ancillary benefits (improved aesthetics and reduced urban heat island effect). These systems are especially cost effective in existing urban development where parking lot expansion is needed, but there is not sufficient space for other types of BMPs. They combine parking, stormwater infiltration, retention, and detention into one facility.See also: [[Cost analysis resources]]
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==Landscaping and grading==
===Water Balance===
Research on the volumetric runoff reduction performance of permeable pavements have been conducted on pavements with and without an underdrain in the base. Volumetric performance improves when:
* Native soils have high infiltration capacity.
* Impervious surface draining onto the permeable pavement is limited or absent.
* Underdrain is elevated above the native soil and/or a flow restrictor is installed on the underdrain.
All permeable pavements have very high surface infiltration rates when appropriately maintained. Therefore, the surface course type (i.e. PICP, pervious concrete) is not a key factor in determining volumetric runoff reduction performance.
==Inspection and Maintenance==
{|class="wikitable"
|+Volumetric runoff reduction from permeable pavements
|-
!'''LID Practice'''
!'''Location'''
!'''Runoff Reduction<sup>1</sup>'''
!'''Reference'''
|-
|rowspan="6" style="text-align: center;" | Permeable pavement without underdrain
|Guelph, Ontario
|style="text-align: center;" |90%
|style="text-align: center;" |James(2002)
|-
|Pennsylvania
|style="text-align: center;" |90%
|style="text-align: center;" |Kwiatkowski et al. (2007)
|-
|France
|style="text-align: center;" |97%
|style="text-align: center;" |Legret and Colandini (1999)
|-
|Washington
|style="text-align: center;" |97 to 100%
|style="text-align: center;" |Brattebo and Booth (2003)
|-
|Connecticut
|style="text-align: center;" |72%<sup>2</sup>
|style="text-align: center;" |Gilbert and Clausen (2006)
|-
|King City, Ontario
|style="text-align: center;" |99%<sup>4</sup>
|style="text-align: center;" |TRCA (2008b)
|-
|rowspan="6" style="text-align: center;" | Permeable pavement with underdrain
|-
|Vaughan, Ontario
|style="text-align: center;" |45%<sup>2</sup>
|style="text-align: center;" |Van Seters and Drake (2015)
|-
|North Carolina
|style="text-align: center;" |98 to 99%
|style="text-align: center;" |Collins et al. (2008)
|-
|United Kingdom
|style="text-align: center;" |50%
|style="text-align: center;" |Jefferies (2004)
|-
|United Kingdom
|style="text-align: center;" |53 to 66%
|style="text-align: center;" |Pratt ''et al.'' (1995)
|-
|Maryland
|style="text-align: center;" |45% to 60%
|style="text-align: center;" |Schueler ''et al.'' (1987)
|-
|Mississauga
|style="text-align: center;" |61 to 99%
|style="text-align: center;" |CVC (2018)
|-
| colspan="2" style="text-align: center;" |'''Runoff Reduction Estimate<sup>3</sup>'''
|colspan="2" style="text-align: center;" |'''85% without underdrain;
45% with underdrain'''
|-
|colspan="4"| Notes:
1. Runoff reduction estimates are based on differences between runoff volume from the practice and total precipitation over the period of monitoring unless otherwise.
2. Runoff reduction estimates are based on differences in runoff volume between the practice and a conventional impervious surface over the period of monitoring.
3. This estimate is provided only for the purpose of initial screening of LID practices suitable for achieving stormwater management objectives and targets. Performance of individual facilities will vary depending on site specific contexts and facility design parameters and should be estimated as part of the design process and submitted with other documentation for review by the approval authority.
4. In this study, there was no underdrain in the pavement base, but an underdrain was located 1 m below the native soils to allow for sampling of infiltrated water. Temporary water storage fluctuations in the base were similar to those expected in a no underdrain design.
|}
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