FIELD TESTING THE EFFECTIVENESS OF PERVIOUS PAVEMENTS AS A WATER SENSITIVE URBAN DESIGN INITIATIVE

Transcription

1 FIELD TESTING THE EFFECTIVENESS OF PERVIOUS PAVEMENTS AS A WATER SENSITIVE URBAN DESIGN INITIATIVE N. Kadurupokune 1, N. Jayasuriya 2 1 Postgraduate student, School of Civil Environmental and Chemical Engineering, RMIT University, Melbourne, nilmini.kadurupokune@rmit.edu.au 2 Senior Lecturer, School of Civil Environmental and Chemical Engineering, RMIT University, Melbourne, nira.jayasuriya@rmit.edu.au ABSTRACT Pervious pavements in car parks and driveways reduce peak discharge and the volume of runoff flowing in to urban drains and improve the water quality by trapping the sediments in the infiltrated water. This reduces the risk of pollutants such as suspended solids and particle bound chemicals such as phosphorous, nitrogen, heavy metals and oils and hydrocarbons entering receiving waters. The key objectives of the study are to establish relationships between rainfall and pervious pavement runoff and quantify improvements to infiltrated stormwater quality through the pervious pavement. This paper focuses on presenting results from field tests carried out in Melbourne to evaluate improvements to water quality from stormwater infiltrating through the pervious pavement surface. INTRODUCTION Water Sensitive Urban Design (WSUD) is a practice that is increasingly emerging throughout the world as a cutting edge initiative adopted to establish sustainable urban communities. WSUD is a measure or structural control that is used for a given set of conditions to manage the quantity and improve the quality of stormwater runoff in the most cost-effective manner. Pervious pavements have been identified as a successful element of the WSUD concept. A pervious pavement is a structure that allows water to permeate through its structure while bearing loads. It overlies a reservoir storage layer. The water holding capacity of the sub-base is of vital importance to reduce peak runoff. The main difference between a pervious and a conventional pavement is the permeability of the surface and the water holding capacity of the sub-base. The water infiltrates through the pavement to a sub-base reservoir, from where it infiltrates slowly to the sub-grade soil and/or to drains. By reducing peak flow rates and volumes in downstream receiving waters, pervious pavements decrease overland flows and recharge groundwater. Pervious pavements have an ability to reduce pollutants in stormwater runoff at source before it is transported to receiving waters. Pervious pavements can be categorized as porous pavements or permeable pavements. Even if the benefits of both are the same, they vary significantly in the way they operate and in their appearance. Porous pavements are cast monolithically with open-graded aggregate, bound together by binders such as asphalt or Portland cement into a coherent mass, with sufficient interconnected voids to provide high rate of permeability to water through the entire surface. Permeable pavements are constructed from pre cast blocks that are impervious, laid such a way that there are some gaps (voids) between interconnected blocks. The block paved surface allows the passage of water through voids between the paving blocks to the underlying reservoir structure. The main focus of this research is to investigate the use of pervious pavements for managing stormwater quantity and quality. The infiltration characteristics of the ROCLA Ecotrihex have been studied in the laboratory by Zhang et al (26). The current study examines two types of pavement surfaces (permeable surfaces), namely ROCLA Ecotrihex pavers and Atlantis turf

2 cells. The structural and hydraulic properties of these pavers have been studied extensively and reported in literature (Shackel et al., 1996; The current research is based on a field scale experimental setup located at the Centre for Education and Research in Environmental Strategies (CERES) in Melbourne, Australia. The flow data and water quality data collected from ROCLA Ecotrihex pavers, Atlantis turf cells and a conventional asphalt pavement (control surface) were analysed in the laboratory to investigate improvements to water quality parameters and the reduction in surface runoff. Stormwater management using permeable pavements in Australia Recent major Australian receiving water quality management initiatives such as the Port Phillip Bay Study, Perth Coastal Water Study, South East Queensland Regional Water Quality Management Strategy and (Sydney) Clean Waterways program have identified a need for greater attention to be paid to the way we deal with, and manage, urban stormwater quantity and quality (Wong, 26). Although the use of pervious pavements is rapidly emerging as a popular urban stormwater management practice in Australia, there is limited data to support efficient and effective design. Therefore more laboratory and field tests are needed before researchers, practicing engineers, regulators and Councils accept pervious pavements in Local Government design. Based on literature, pervious pavements initially increase the infiltration rate and improve stormwater quality whilst with time, unless properly maintained, the pervious pavement will clog with entrapped sediments and reduce its efficiency. Over the last six years a significant number of projects in Australia have successfully utilized this new construction concept. The Olympic Boulevard at the Homebush Olympic site in Sydney, constructed in 1999 is a major example. Furthermore, there are some roads that have been installed since Examples include roads in Kiama and Smith Street in, Manly Sydney. Another car park was constructed in July 1999 by the City of Charles Sturt, using BORAL Formpave units at Kirkaldy Avenue, Adelaide. The car park located adjacent to the North Haven Football Clubrooms was constructed by the City of Port Adelaide Enfield late 1999, using BORAL Formpave units. A grasspave installation was done in 1997 at the premises of St. Elizabeth Anglican Church in Adelaide. The authors believe the use of the pervious pavements would further increase if their stormwater improvement qualities are quantified and design guidelines are developed to assist WSUD. Research on pervious pavements Zhang (26) reported that permeable surfaces are more suitable in car parks and driveways than porous pavements. The voids between the paver materials in permeable surfaces are more widely open and can infiltrate higher rainfall intensities than porous pavements. Shackel and Pearson (24) indicated that infiltration capacities of porous pavements are not sufficiently high for Australian rainfall conditions and can easily clog within a short period. According to above authors, permeable pavements are more suitable for Australian hydrological conditions. Urban Water Resources Center at the University of South Australia reported that the hydraulic performance of three types of permeable pavements namely BORAL, ROCLA and grass subject to certain sediment loads through in-situ and laboratory tests (Urban Water Resources Center, University of South Australia, 22). Four test beds were set up in the laboratory and

3 measured sediment loads were introduced with the input water before starting the tests. The results revealed that with 35 years simulated sediment loads, the hydraulic conductivity reduced by 59%, 68% and 75% from BORAL, ROCLA and Grasspave respectively. However, the sediment retention rate of different paver surfaces did not decrease significantly during the long term simulation. BORAL, ROCLA and Grasspave can retain up to 94%, 89% and 97% of sediment loads contributing to significant improvements in water quality. The findings from the field tests revealed similar decreases in hydraulic conductivity when compared to simulated laboratory tests. Based on a number of previous studies, Melbourne Water (23) reported that if pervious pavements are correctly designed and maintained, they can retain up to 8 % of sediments, 6 % of phosphorus, 8 % of nitrogen, 7 % of heavy metals, and 98 % of oils and greases in stormwater. EXPERIMENTAL SITE A car park was built with two types of pervious (permeable) surfaces namely ROCLA Ecotrihex pavers and Atlantis Turf cells and impervious asphalt surface as a control (4.8m*1.5m each). Details of the pavement design are reported in Jayasuriya et al (26). The structure of the ROCLA Ecotrihex and Atlantis pavements are given below. ROCLA Ecotrihex The design of the ROCLA Ecotrihex structure was based on recommendations in Shackel et al (23) and verified using LOCKPAVE Version (Concrete and Masonry Association of Australia, 23) The pavement configuration, thickness of the bedding and subbase, and the aggregate sizes are given below. - 8 mm ROCLA Ecotrihex paver - 3 mm bedding layer consists of 2 to 5 mm coarse aggregate - Permeable geo-textile fabric - 25 mm subbase consists of 5 to 2 mm open graded aggregate - Geo-textile - Subgrade Atlantis Turf Cell The design of the Atlantis Turf pavement was based on the information provided on and personnel communication with the Atlantis Turf Cell provider, Wayne Alexander and verified using LOCKPAVE Version The pavement configuration and design details of the pavement structure are given below mm Atlantis turf cell filled with sand and fertiliser mix - Permeable geo-textile fabric - 38 mm sub-base consists of 5 to 2 mm open graded aggregate. - Geo-textile - Subgrade The turf cell was placed on a permeable geo-textile and filled with clean fine sharp sand (washed concrete sand) mixed with a starter fertilizer. Buffalo grass was used as this has high resistance to traffic wear.

4 Agricultural (aggi) pipes were placed around the catchment to prevent stormwater from the surrounding areas entering the experimental site and the three surfaces are under control conditions. Three on-line flow meters were installed to measure the surface flow from the control surface and infiltrated water from the two pervious surfaces. The flow meters were calibrated to activate when the depth of water in the channel was 1 mm. Three water quality auto samplers were also installed in special pits in the field to collect event based water quality samples from the three types of pavements. The quality of the infiltrated water will be benchmarked against the surface water quality flowing through the conventional asphalt car park. As it is not possible to manually activate the auto-samplers during a storm event, they were connected to the flow meters by a special cable to ensure the activation of the autosampler and the flow meter simultaneously. Rainfall data were collected from a Tipping Bucket rain gauge installed at the site. These rainfall values were compared with the nearby continuous rain gauges operated by Melbourne Water (within a 4 km radius) to determine the rainfall pattern and the intensity of the rain event at the experimental site. The lack of rainfall events in Melbourne in 26 and beginning of 27 have been a major factor impacting the progress of the project resulting in, a decision taken to install sprinklers to simulate rainfall. The sprinklers were selected and placed so that the water was uniformly sprayed over each surface. As mentioned earlier, Aggi pipes were placed around the experimental field to prevent surface water flowing from adjacent areas to the experimental site corrupting runoff data. Hence there will be no difference between the stormwater generated from a real storm and a sprinkler simulated storm. The flow rate from the sprinklers was adjusted to simulate different rainfall intensities between 14 mm/hr to 21 mm/hr. The storm durations were varied between 25 mins to 55 mins to cover typical rainfall events in Melbourne. The stormwater collected from the auto sampler were analysed for the following water quality parameters: total Suspended Solids (TSS); ph; total Nitrogen (TN); total Phosphorous (TP); Oil and Greases; Zinc (Zn); Copper (Cu); Cadmium (Cd) and Lead (Pb). RESULTS AND DISCUSSION Reductions in surface runoff The installation of all the flow meters and water quality samplers were completed in September 26. Figure 1 gives the magnitude of the total rainfall and the runoff generated from the surfaces from actual storms until April 27. The maximum daily rainfall recorded at the catchment was 14mm. The Asphalt surface produced runoff with a minimum total rainfall of 3 mm. So far the ROCLA Ecotrihex surface produced runoff only from the 13 mm and 14 mm actual storm events on 2 nd of November, 26 and 24 th of March, 27 respectively. The Atlantis Turf Cell pavement did not produce any runoff from actual events with all the precipitation infiltrated. As shown in Figure 1, there was no runoff generated from the grass pavement for these storm events as well. The rainfall intensities were higher and the storm durations longer in simulated storms than in actual events recorded. The rainfall runoff relationships for the simulated storms are depicted in Figure 2.

5 noff volume (L) Ru noff volume (L) Ru Figure 1: The total rainfall and runoff relationship for natural storm events Figure 2: The total rainfall and runoff relationship for simulated storm events A daily water balance was carried out for the temporary storage in the pavement with actual and simulated storms. Potential evaporation for the day was obtained from the web site. It was assumed that the evaporation is at the potential rate as the surface was saturated after simulating the rainfall event. Furthermore, it was assumed that there was no groundwater percolation from the pavement as there was an impermeable geotexlite layer laid underneath the pavement. Although there was an impermeable geotextile laid, the permeability of the subgrade soil pavement was silty clay with a swell of ½% (Jayasuriya et al.; 26). The geotextile layer was laid especially to prevent any percolation from the pavement adversely affecting the water balance calculations. Figures 3 and 4 give the relationship between rainfall (actual and simulated), runoff and water retained in ROCLA Ecotrihex and Atlantis Turf pavements respectively. The latter retains more water in the pavement reducing the runoff to the drains. This is confirmed by the data obtained from the actual storm events, where there was no runoff generated from the turf surface. The average ratios of total runoff to total rainfall from the two surfaces are.5 and.6 respectively. The reasons for higher ratio in the ROCLA Ecotrihex surface would be due to high hydraulic conductivity of the 2-5 mm coarse aggregate compared to the sand used in the Atlantis turf cells. In addition, the sub-base layer was 58 mm deep in the turf cell pavement thus holding more water in the storage Total rainfall volume(l) Asphalt Rocla Total rainfall volume(l) Asphalt Rocla Grass Volume (L)L Day Rainfall Runoff Storage Figure 3: The rainfall, runoff and water storage relationships from the ROCLA Ecotrihex pavement

6 12 1 Volume(L) Day Rainfall Runoff Storage Figure 4: The rainfall, runoff and water storage relationships from the Atlantis Turf pavement Figures 5 and 6 show the hourly rainfall pattern for the 13mm and 14mm events and the runoff hydrographs produced from the above two storms. Above two figures clearly depict that the lag time to the peak from the two storms are 1 hour and 45 mins respectively from the ROCLA Ecotrihex pavement when compared with runoff characteristics of the Asphalt surface. The peak discharge also has dropped by 5% and 54% respectively from the ROCLA Ecotrihex pavement compared to the Asphalt surface. The above values are summarised in Table 1 Similar results were obtained from the simulated storms. As stated previously intensities were varied between 14mm/hr and 21mm/hr. The high intensities were selected to ensure surface runoff from all three pavements. On average, the peak flow reduced by 5% and 6% from ROCLA Ecotrihex and Atlantis Turf pavements respectively. Figure 5: Runoff hydrographs and rainfall intensities produced by the 13mm storm (2/11/26) for the ROCLA Ecotrihex surface Figure 6: Runoff hydrographs and rainfall intensities produced by the 14 mm storm (24/3/7) the Atlantis turf cells

7 Table 1: Flow data for the natural rainfall events on 2/11/26 and 24/3/27 2/11/26 24/3/27 Asphalt ROCLA Asphalt ROCLA Lag in commencement of runoff (min) N/A 3 N/A 135 Time to peak (min:sec) 3: 36: 15: 18: Lag time to the peak (min) N/A 6 N/A 3 Peak discharge Qp (L/s) Difference in peak Qp compared to Asphalt N/A.12 N/A.14 (L/s) Difference in peak Qp compared to Asphalt (%) N/A 5.2 N/A 53.5 Improvements to Water Quality The auto samplers were set to collect water every 15 minutes from the start of the event. As mentioned before, there was no runoff collected from the Atlantis turf surface for actual storm events. As a result it was decided to graphically present the results obtained from a simulated storm. Figure 7 depicts the event mean concentrations calculated from the rain simulated on the 2 nd of April 27. The removal efficiencies of all pollutants are in the same range from both pavements except for Cu, TP and TN. Approximately 9% of Zinc (Zn), TSS and oil in the surface runoff were removed by passing through the pervious surfaces. The removal of Cu is much higher from the grass surface (54%) than from the ROCLA Ecotrihex pavement (33%). Initially the TN and TP concentrations in the runoff from both pervious pavements were higher than from the Asphalt surface. This could be due to the fertilizer mixed with the sand in the bedding layer of the turf pavement and the nutrients that were in the aggregates used in the subbase for both pavements. However, it should be noted that with time the removal efficiencies have improved and concentrations in the runoff from the pervious pavements have reduced. The reduction efficiency from the ROCLA Ecotrihex pavement is around 5 % and 6% for TP and TN respectively. The removal rate of both TP and TN from the Atlantis Turf pavement is around 4%. The Cd and Pb values were below detectable levels and as such, the values are not reported herein. Event Mean Concentrat (mg/l) Zn Cu TN TP Pollutant Asphalt Rocla Atlantis Event Mean Concentrat (mg/l) Oil TSS Pollutant Asphalt Rocla Atlantis nd Figure 7: Event Mean Concentrations (EMC) obtained from the 2 April 27 simulated rainfall

8 6. CONCLUSIONS Engineers and environmental practitioners are being challenged to address water related environmental impacts resulting from increased impermeable areas due to rapid urbanisation. Therefore the impacts of runoff, infiltration and stormwater quality have to be considered when designing urban pavements and surface areas such as car parks, driveways and pedestrian pavements. The two pavements tested ROCLA Ecotrihex and Atlantis Turf Cell reduced runoff volume by 5%-6%, the peak flow by 3% to 55% and improved the TSS, Zn and oil capture by 9%. Both types of commercially available pervious pavements operated efficiently to improve stormwater quality and infiltrated precipitation reducing surface runoff significantly. Pervious pavements could play an important role in Water Sensitive Urban Design as it helps to reduce peak flows, improve stormwater quality and if properly designed, permits the infiltrated water to be harvested to be put to fit for purpose productive use. Preliminary pervious pavement studies conducted in the field yielded positive results. The outcomes from the study provide useful information to design environmentally friendly car parks, pedestrian paths, light traffic driveways and public areas in the future. AKNOWLEDGEMENT This project has been assisted with funding from the Victorian Government s Stormwater and Urban Water Conservation Fund. Assistance provided by Keith Jesse from the Centre for Education and Research in Environmental Strategies (CERES) for conducting this study is greatly appreciated. The senior management at ROCLA and Atlantis pavement manufactures are also acknowledged for providing the paving material. REFERENCES Jayasuriya J., Jarrar A. and Jesse K. (26), Stormwater quality reductions and quality improvements using pervious pavements, AWA Journal, Vol. 3, pp LOCKPAVE Version 15.2; Concrete and Masonry Association of Australia; www@cmaa.com.au Melbourne Water, Website: (14/11/23) (25), Sustainable Water Stratergy Central Region Action to 255. Shackel B. and Pearson A. (24), Eco paving can help control road run-off Erosion & Stormwater, pp Shackel B., Kaligis J.O., Muktiarto Y. and Pamudji (1996), Infiltration and structural tests of permeable Eco-paving. Proc.6th International Conference on Concrete Block Paving, Telaviv. Urban Water Resource Centre at University of Adelaide (22), Research into effective Life of permeable pavement source control instalations, Project no: Wong T.H.F. (26), Australian runoff quality A guide to water Sensitive Urban Design. Zhang J. (26), Laboratory scale study of infiltration from pervious pavements, School of Civil Environmental and Chemical Engineering, RMIT University, Thesis, 26 Zhang J., Jayasuriya N. and Setunge S. (26), Application of pervious pavements - a laboratory scale study, Proceedings of the 7th Urban Drainage Modelling and the 4th Water Sensitive Urban Design, Melbourne, Australia, April 2-7, pp Asphalt