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Low Impact Developement
Most of the 'paving over' in developed areas is due to common roads and parking lots, which play a major role in transporting increased stormwater runoff and contaminant loads to receiving waters. Alternative paving materials can be used to locally infiltrate rainwater and reduce the runoff leaving a site. This can help to decrease downstream flooding, the frequency of combined sewer overflow (CSO) events, and the thermal pollution of sensitive waters. Use of these materials can also eliminate problems with standing water, provide for groundwater recharge, control erosion of streambeds and riverbanks, facilitate pollutant removal, and provide for a more aesthetically pleasing site. The effective imperviousness of any given project is reduced while land use is maximized. Alternative pavers can even eliminate the requirement for underground sewer pipes and conventional stormwater retention / detention systems. The drainage of paved areas and traffic surfaces by means of permeable systems is an important building block within an overall Low Impact Development scheme that seeks to achieve a stormwater management system close to natural conditions.
...with permeable pavers*
Some current studies on the effectiveness of permeable pavers for reducing Total Suspended Solid (TSS), nutrient, metal and thermal loadings are being conducted in Florida, Toronto, and Washington State.
The parking lot of the Florida Aquarium in Tampa, which serves 700,000 visitors annually, has been innovatively designed as a research and demonstration project for the use of permeable pavers as part of a treatment train approach, comparing three paving surfaces in conjunction with swales.1 First-year results found that load removal efficiencies for metals (copper, iron, lead, manganese and zinc) ranged from 23 to 59% for asphalt pavement with a swale; 62 to 84% for cement pavement with a swale; and 75 to 92% for porous concrete with a swale. In general, metals were measured at much higher concentrations in the basins paved with asphalt than those paved with cement products. The porous system with a swale also achieved 91% removal efficiency for total suspended solids, higher than the other two paving systems.
Studies at the University of Guelph in Canada have also observed greater pollutant loads from asphalt surfaces than from concrete or permeable pavers. There, a research team led by Professor William James has been performing field and laboratory tests since 1993 on the influence of permeable pavers on runoff pollutant levels and thermal characteristics. They have found that a permeable paver made up of interlocking concrete blocks can significantly reduce the surface runoff loads of such contaminants as nitrite, nitrate, phosphate, phosphorus, metals, BOD, and ammonium.2 In addition, during a lab simulation, the permeable pavers were found to reduce surface runoff temperatures by 2 to 4 degrees Celsius compared to the runoff from asphalt paving. Since the permeable pavers also increase infiltration, the total heat content of runoff leaving a site is reduced substantially.3
Finally, surface and subsurface runoff samples are being collected by the Center for Urban Water Resources Management in Washington State from a test parking area, which contains five different surface materials.4 Constructed in 1996, the King County employee parking lot contains nine stalls, of which one is traditional asphalt, and the others are four pairs of alternative permeable pavement surfaces: gravel-filled interlocking concrete blocks, soil and grass-filled interlocking concrete blocks, gravel-filled plastic cell networks, and soil and grass-filled plastic cell networks.
The project's primary goal is to determine the long-term water quality benefits of these systems under real world usage. A system of pipes, gutters and gauges collect and enable the measurement of the volume and chemistry of both the surface runoff and the subsurface infiltrate. A comprehensive water quality analysis is being conducted over the winter of 2001/2002. Preliminary results indicate that the subsurface runoff is consistently cleaner than the surface runoff; statistical analyses and reports will be produced in future months (Derek Booth, Feb. 2002, personal communication).
For more specialized users, continuing research at Coventry University in England has been looking at applying nutrients to permeable pavers in order to support a microbial population that can serve as an in-situ bioreactor for oil degradation in highway and parking lot runoff.5 Studies have demonstrated the potential to maintain microbial activity for over 12 months from one application of a slow-release fertilizer, with warnings given about ensuring the effective use of the nutrients so that high effluent levels will not cause eutrophication in receiving waters.
Most of the above studies have also examined the influence of permeable pavers on runoff volume, tending to show a marked reduction in the surface runoff that leaves a permeable paver site due to increased infiltration. In the University of Guelph experiments, field sites with permeable interlocking concrete pavers demonstrated a 90% reduction in runoff volume.3 The treatment train studies at the Florida Aquarium showed that, in general, the use of swales reduced runoff volume but that paving type also played a major role in runoff reduction, with permeable pavers being the most effective. The figure below demonstrates this fact as well as the caveat that the use of swales and permeable pavers has the most influence on runoff during small storms.6 For high intensity rainfalls or when soil conditions are saturated, runoff is not reduced as substantially. Note the different scales on the two graphs; the first is for a rain event that produced just over 0.5 inch of rain in about 75 minutes, while the second is for an event producing almost 2.5 inches in under 2 hours and occurring less than 24 hours after four preceding days with rain.
The studies by the Center for Urban Water Resources Management in Washington State have looked for similar differences in the hydrologic response of pavers based on storm intensity or if the storm followed a long dry period versus a period of abundant rain. To date, however, results show a general absence of surface runoff from the permeable pavers regardless of conditions: "it all just infiltrates, all the time" (Derek Booth, personal communication). The figure below, representing a typical observation during the study's first year, compares surface runoff produced from traditional (asphalt) and permeable (Turfstone) pavements.7 The Turfstone permeable paver is a 60% impervious surface made up of soil and grass-filled interlocking concrete blocks. The measured surface runoff from the Turfstone is less than 1 percent of the total rainfall and is probably a result of observed leaks in the covering over the collection system. All other permeable pavement systems showed equivalent results. The asphalt paving, however, responds quickly to the rainfall, with most of the rain that hits the surface running off.
It is likely that results are different from those in Florida due to differences in the two regions' rainfall regimes. The Washington rain event had a maximum rainfall intensity that was under 0.2 in/hr; this was typical of the storms recorded. In comparison, the heavier rain event presented in the Florida graph had a maximum rainfall intensity of 1.5 in/hr. Rain events in Washington State are generally of a lower intensity and longer duration than those measured in Florida, where the rainfall, particularly in the summer, is dominated by short and more intense convective events.
1 Rushton, B.T., 2001: Low-impact parking lot design reduces runoff and pollutant loads. Journal of Water Resources Planning and Management, (May/June), 172-179.
2 James, W., ed., 1997: Advances in Modeling the Management of Stormwater Impacts Volume 5. Proceedings of the Stormwater and Water Quality Management Modeling Conference, Toronto, Ontario, February 22-23, 1996, 520 pp.
3 James, W., 2002: Green roads: Research into Permeable Pavers. Stormwater, (March/April), 48-50.
4 Booth, D.B., J. Leavitt and K. Peterson, 1996: The University of Washington Permeable Pavement Demonstration Project. Background and First-Year Results, available online at http://dept.washington.edu/cuwrm/ under Research.
5 Pratt, C.J., A.P. Newman and P.C. Bond, 1999: Mineral oil bio-degradation within a permeable pavement: long term observations. Wat. Sci. Tech., 39 (2), 103-109.
6 Southwest Florida Water Management District, 2001: Florida Aquarium Parking Lot - A Treatment Train Approach to Stormwater Management. Final Report for FDEP Contract No. WM 662, Brooksville, Florida, 220 pp.
7 Booth, D.B. and J. Leavitt, 1999: Field evaluation of permeable paver systems for improved stormwater management. Journal of the American Planning Association, 65(3), 314-325.