Tennessee Valley Authority Economic Development
Sustainable Development Guide
Design Principles for Parking Lots
Parking lots
Minimize parking ratios and stall dimensions
Use pervious materials in spillover parking
Treat storm water at the edges of parking lots
Best practices for parking lots
Minimize parking ratios and stall dimensions
Local municipalities often require parking for a particular land use. The number of spaces is usually determined by the building square footage or number of seats. Demand studies are a valuable tool for determining parking requirements for planned activities at an industrial site. They determine parking requirements better than parking ratios.
Land Use |
Parking Requirement | Actual Average Parking Demand |
|
Parking Ratio |
Typical Range |
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Single family homes |
2 spaces per dwelling unit |
1.5 – 2.5 |
1.11 spaces per dwelling unit |
| Shopping Center | 5 spaces per 1000 ft2 GFA | 4.0 – 6.5 | 3.97 per 1000 ft2 GFA |
| Convenience store | 3.3 spaces per 1000 ft2 GFA | 2.0 – 10.0 | – |
| Industrial | 1 space per 1000 ft2 GFA | 0.5 – 2.0 | 1.48 per 1000 ft2 GFA |
| Medical/Dental office | 5.7 spaces per 1000 ft2 GFA | 4.5 – 10.0 | 4.11 per 1000 ft2 GFA |
| GFA = Gross floor area of a building without storage or utility spaces | |||
Shared parking and parking structures can reduce the number of parking spaces, resulting in less imperviousness. Compact stalls create up to 35 percent less impervious cover than larger stalls. (8)
Refer to the following document for more information about this best practice:
Environmental Protection Agency Post-Construction Storm Water Management, Green Parking
Use pervious materials in spillover parking
Alternative surfaces such as porous pavers or concrete effectively reduce the runoff generated by parking lots. Select pervious pavers suited to the vehicles that will use the area. Porous pavers, as shown in Figure 4, can replace conventional asphalt or concrete in both new developments and redevelopment projects. They are an excellent choice for overflow parking areas because they capture and treat runoff from other site areas. (9)
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| Grass Paver Surface Used for Parking |
There are four things to consider in deciding about alternative pavement:
- Accessibility: Generally, use conventional pavement in handicap parking areas and sidewalks. However, the city of Olympia, Washington, found that Uni-Eco-Stone complies with ADA requirements. (10)
- Site conditions: Paver performance depends on site conditions. Soils must allow 0.5 inches per hour of percolation. Extremely pervious soils, like sandy soil, may not provide adequate treatment (depending on groundwater depth). Augment soil that is too sandy with a peat liner (11).
- Performance: Achieving expected storm-water treatment performance requires proper installation and maintenance. Problems arise if subsurface soils are compacted during construction, or if the paving system is clogged during installation or because it is not maintained.
- Economics: Construction costs for pervious pavement are generally greater than those for conventional pavements. However, reduced use of curb and gutter and reduced storm-water system requirements may offset these extra costs. Similarly, lower storm-water system maintenance costs may offset higher maintenance costs for pervious pavement. (12)
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Examples of Modular Porous Pavers |
Alternative pavement systems were compared at an office parking lot in Olympia, Washington. The project set out to demonstrate permeable pavement systems’ potential to restore soil infiltration functions in urban areas. Two adjacent parking stalls were constructed using four types of permeable pavement systems:
- 90 percent impervious block with gravel
- 60 percent impervious concrete block with grass
- Reinforced grass pavement
- Reinforced gravel and grass pavement.
The installations were monitored and the following conclusions were drawn:
- The systems dramatically reduce surface runoff volume and attenuate peak discharge.
- Although the systems were structurally different, the hydrologic benefits were consistent.
- Storm characteristics and weather conditions influenced the systems’ hydrologic responses.
- Permeable pavement systems vary widely in cost and are more expensive than typical asphalt pavements. Although cost comparisons between permeable pavement and conventional systems are limited, eliminating conventional systems reduces life-cycle costs.
- Permeable pavement can significantly reduce the impervious area directly connected to drainage systems. It can control runoff timing, reduce runoff, and benefit water quality.(13)
The table below summarizes the relative cost and effectiveness of paving systems (14):
Materials |
Initial Cost |
Maintenance Cost |
Water Quality Effectiveness |
Conventional Asphalt / Concrete |
Medium |
Low |
Low |
Pervious Concrete |
High |
High |
High |
Porous Asphalt |
High |
High |
High |
Turf Block |
Medium |
High |
High |
Brick |
High |
Medium |
Medium |
Natural Stone |
High |
Medium |
Medium |
Concrete Unit Pavers |
Medium |
Medium |
Medium |
Gravel |
Low |
Medium |
High |
Wood Mulch |
Low |
Medium |
High |
Cobbles |
Low |
Medium |
Medium |
Refer to the following documents for more information about these best practices:
Federal Highway Administration Stormwater Best Management Practices
http://www.fhwa.dot.gov/environment/ultraurb/uubmp3p6.htm
http://www.fhwa.dot.gov/environment/ultraurb/3fs15.htm
Georgia Stormwater Management Manual Details
http://www.georgiastormwater.com/vol2/3-3-7.pdf
http://www.georgiastormwater.com/vol2/3-3-8.pdf
Environmental Protection Agency Post-Construction Storm Water Management
Treat storm water at the edges of parking lots
Structural drainage systems and storm sewers are designed to efficiently remove storm water. However, in doing so these systems tend to increase peak runoff discharges, flow velocities, and the delivery of pollutants to downstream waters. Where possible, provide storm-water treatment for parking lot runoff using dry swales, vegetative channels, bioretention areas, filter strips, and other practices that can be integrated into landscaping areas. Natural open channels store more storm water onsite, lower storm-water peak flows, reduce erosive runoff velocities, infiltrate a portion of the runoff volume, and capture and treat storm-water pollutants. Used upstream from natural drainage ways, they reduce post-development flows and prevent erosion and degradation. (15)
Bioretention, dry swales, perimeter sand filters, and filter strips in parking lot landscaping are all options for managing storm water. (16)
Runoff can also be directed toward riparian buffers and other undisturbed natural areas delineated in the initial stages of site planning. They can infiltrate runoff, reduce runoff velocity, and remove pollutants. Natural depressions can temporarily store and infiltrate water. Natural areas can intercept and infiltrate runoff before it becomes substantially concentrated. A level spreader can distribute this flow evenly to a buffer or natural area. (17). Carefully constructed berms around natural depressions and below undisturbed vegetated areas with porous soils provide additional runoff storage and infiltration of flows. (18)
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From King County Surface Water Design Manual: Level Spreader |
Best practices for parking lots
The following are best practices for water quality and quantity treatment in and around parking lots:
Sand filters, or infiltration basins
Infiltration trenches, or dry wells
Manufactured oil and grit separators
Enhanced swales (also referred to as vegetated open channels or water quality swales) are conveyance channels engineered to capture and treat water quality volume for a drainage area. Unlike a normal drainage channel or swale, they remove storm-water pollutants more effectively. Enhanced swales have limited longitudinal slopes to force a slow and shallow flow. This allows particulate to settle and limits erosion. Berms and check dams installed perpendicular to the flow path promote settling and infiltration.
 The dry swale is a vegetated conveyance channel that includes a filter bed of prepared soil over an under-drain system. It is sized to filter the entire water quality volume through its bottom. |
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A wet swale (or wetland channel) is a vegetated channel designed to retain water or marshy conditions that support wetland vegetation. A high water table or poorly drained soil is necessary to retain water. The wet swale acts as a linear, shallow wetland treatment system that retains the water quality volume.Refer to the following documents for more information about these best practices:
Federal Highway Administration Stormwater Best Management Practices
Georgia Stormwater Management Manual Details (PDF file)
Environmental Protection Agency Post-Construction Storm Water Management
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| From Georgia Stormwater Management Manual, Volume 2 |
Bioretention areas convey runoff as sheet flow to the "treatment area" consisting of a grass buffer strip, ponding area, organic or mulch layer, planting soil, and vegetation. An optional sand bed can further aerate and drain the planting soil. Filtered runoff is collected and returned to a conveyance system. (19) It can be exfiltrated into surrounding porous soil.
Refer to the following documents for more information about this best practice:
Federal Highway Administration Stormwater Best Management Practices
Georgia Stormwater Management Manual Details
Bioretention Areas (PDF)
Filter Strips (PDF)
Environmental Protection Agency Post-Construction Storm Water Management
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| Sand filter |
Sand filters, or infiltration basins, capture and temporarily store storm-water runoff and filter it through a bed of sand. Most sand filter systems have two chambers. The first chamber is a sediment forebay that removes floating pollutants and heavy sediments. The second is the filtration chamber, which removes additional pollutants by filtering runoff through a sand bed. The filtered runoff is typically collected and returned to the conveyance system. It can be exfiltrated into surrounding porous soil. (20)
Refer to the following documents for more information about this best practice:
Federal Highway Administration Stormwater Best Management Practices
Infiltration Basin
Georgia Stormwater Management Manual Details
Sand Filter (PDF)
Underground Sand Filter (PDF)
Environmental Protection Agency Post-Construction Storm Water Management
Infiltration Basin
Infiltration trenches, or dry wells, primarily remove storm-water pollutants. However, they can also control runoff quantity from smaller storms. An infiltration trench is not sufficient to provide over-bank and extreme flood protection.(21)
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| Infiltration Trench Example |
Refer to the following documents for more information about this best practice:
Federal Highway Administration Stormwater Best Management Practices (PDF)
Georgia Stormwater Management Manual Details
Environmental Protection Agency Post-Construction Storm Water Management
Constructed wetlands are mostly covered with wetland vegetation. They remove significant amounts of sediment, nutrients, heavy metals, toxic materials, floatable materials, oxygen-demanding substances, and oil and grease, as well as some bacteria and viruses. (22)
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| Constructed wetland. From Franklin Stormwater Manual, Volume 2. |
Storm-water ponds (also called retention ponds, wet ponds, or wet extended detention ponds) have a permanent pool of water throughout the year. They can be created by excavating an already existing natural depression or by constructing embankments. Multiple storm-water ponds can be placed in series or parallel to increase performance or meet site design constraints. There are several types of storm-water pond designs:
- A wet pond is constructed with a permanent pool of water equal to the water quality volume. Runoff displaces water already present in the pool. Temporary storage can be provided above the permanent pool elevation for larger flows.
- A wet extended detention pond splits water quality volume evenly between the permanent pool and the extended detention storage provided above the permanent pool. During storm events, water is detained above the permanent pool and released over 24 hours. This design removes pollutants as effectively as a traditional wet pond, but requires less space.
- The micropool extended detention pond maintains a small micropool at the outlet to the pond. The outlet structure detains the water for 24 hours. The micropool prevents resuspension of previously settled sediments and prevents clogging of the low flow orifice.
- Multiple pond systems provide water quality and quantity volume storage in two or more cells. The additional cells create longer pollutant-removal pathways and improved downstream protection. (23)
Refer to the following documents for more information about these best practices:
Federal Highway Administration Stormwater Best Management Practices
Wetlands and Shallow Marsh Systems
Georgia Stormwater Management Manual Details
Dry Detention and Extended Detention Ponds (PDF)
Multi-Purpose Detention Areas (PDF)
Stormwater Wetlands (PDF)
Submerged Gravel Wetlands (PDF)
Environmental Protection Agency Post-Construction Storm Water Management
Manufactured oil and grit separators improve the pollution-removal performance of catch basins, which capture floatable pollutants and settle some solids. Inserts improve the catch basin’s ability to remove oil and grease, trash, debris, and sediment. Some inserts drop directly into an existing catch basin, while others require extensive retrofit construction. To maintain their pollution-removal capability, catch basins must be cleaned once or twice annually.
Refer to the following documents for more information about these best practices:
Federal Highway Administration Stormwater Best Management Practices
Georgia Stormwater Management Manual Details
Gravity (Oil-Grit) Separator (PDF)
Environmental Protection Agency Post-Construction Storm Water Management