205
edits
Changes
Jump to navigation
Jump to search
Line 47:
Line 47:
no edit summary
The bottom of the reservoir should be level so that water infiltrates evenly. If the system is not designed for infiltration, the bottom should slope at 1 - 5% toward the underdrain.
The bottom of the reservoir should be level so that water infiltrates evenly. If the system is not designed for infiltration, the bottom should slope at 1 - 5% toward the underdrain.
{{:Gravel}}
{{:Gravel}}
<h5>Sizing Stone Reservoirs</h5>
The following calculation is used to size the stone storage bed (reservoir) used as a base course for designs without underdrains. It is assumed that the footprint of the stone bed will be equal to the footprint of the pavement. The following equations are taken from the ICPI Manual (Smith, 2006).
The equation for the depth of the stone bed is as follows:
db= [Qc * R + P – i * T ] / Vr
Where:
db = Stone bed depth (m)
Qc = Depth of runoff from contributing drainage area, not including permeable paving surface(m)
R = Ac/Ap = Ratio of contributing drainage area (Ac) to permeable paving area (Ap)
P = Rainfall depth (m)
i = Infiltration rate for native soils (m/day)
T = Time to fill stone bed (typically 2 hr)
Vr = Void ratio for stone bed (typically 0.4 for 50 mm dia. stone)
Note that the contributing drainage area (Ac) should not contain pervious areas.
For designs that include an underdrain, the maximum depth of the stone reservoir below the invert of the underdrain pipe can be calculated as follows:
dr max = i * ts / Vr
Where:
dr max = Maximum stone reservoir depth (m)
i = Infiltration rate for native soils (m/hr)
Vr = Void space ratio for aggregate used (typically 0.4 for 50 mm clear stone)
ts = Time to drain (design for 48 hour time to drain is recommended)
The value for native soil infiltration rate (i) used in the above equations should be the design infiltration rate that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation
On highly permeable soils (e.g., infiltration rate of 45 mm/hr or greater), a maximum stone reservoir depth of 2 metres is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.
If trying to size the area of permeable paving based on the contributing drainage area, the following equation may be used:
Ap = Qc * Ac / [Vr * dp – P + i * T]
<h4>Geotextile</h4>
<h4>Geotextile</h4>
{{:Geotextiles}}
{{:Geotextiles}}
==Performance==
Permeable pavers can be classified according to the infiltration rate of the underlying subsoil into two categories:
*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 runoff.
==Construction Considerations==
Construction of permeable pavement is a specialized project and should involve experienced contractors. The following general recommendations apply:
*Sediment Control: The treatment area should be fully protected during construction so that no sediment reaches the permeable pavement system and proper erosion and sediment controls must be maintained on site.
*Weather: Porous asphalt and pervious concrete will not properly pour and set in extremely high and low temperatures (City of Portland, 2004; U.S. EPA, 1999). One benefit to using permeable pavers is that their installation is not weather dependent.
*Pavement placement: Properly installed permeable pavement requires trained and experienced producers and construction contractors.
==Inspection and Maintenance==
Like all other stormwater practices, permeable pavement requires regular inspection and maintenance to ensure that it functions properly.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.
==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 savings and benefits may also be realized, including reduced need for storm sewer pipes and other stormwater practices, less developable land consumed for stormwater treatment, and ancillary benefits such as 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.
==Proprietary Links==
==Proprietary Links==