Evaluation of the performance of permeable pavement at Lamb Drove FINAL

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1 Evaluation of the performance of permeable pavement at Lamb Drove FINAL T M Hess & C Ibe Department of Environmental Science and Technology October 2011

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3 Executive Summary The permeable paving at Lamb Drove, Cambourne, Cambs., was designed to collect, treat, store and slowly release runoff to the environment as part of the Lamb Drove sustainable drainage showcase project. Although biannual suction sweeping was recommended, it has not taken place. Therefore, the permeable pavement has been in operation since its completion in 2006 with no suction sweeping and its hydraulic performance is unknown. The aim of this study was to carry out an assessment of the hydrological performance of the permeable paving at Lamb Drove, by assessing the infiltration capacity of the pavement in its present condition in relation to design standards and extreme rainfall intensities for the region. Where possible, the effect of maintenance activities on the infiltration capacity was to be assessed and the hydrological impact modelled. Ten points were randomly selected on the permeable pavement and the infiltration rate was measured by means of a double-ring infiltrometer. The pavement at each point was cleaned by hand and the infiltration rate measured again. The measured infiltration rates were compared with extreme recorded rainfall events and estimated 1 in 100 and 1 in 50 year rainfall intensities for the Cambourne. These results suggest that, although the pavement has not been maintained in accordance with the recommendations for 6 years since installation, it is still functioning well and should be able to infiltrate all but the most extreme, short duration rainfall events. Surface runoff from the pavement is therefore unlikely to occur. Manual cleaning of the pavement during the experiment did not result in any significant change in measured infiltration rates. It would therefore appear that there is no requirement for additional maintenance of the pavement in the medium term. However, there is anecdotal evidence that the pavement was cleaned mechanically in June 2011 in error and the results observed may reflect the vigorous cleaning of the joints in the pavement.

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5 Contents 1. Introduction Lamb Drove sustainable drainage showcase project Lamb Drove permeable pavement Construction Maintenance Previous studies Objectives Methods Sample point selection Infiltration measurement Cumulative infiltration Infiltration rate Performance assessment Design storm estimation Observed rainfall Observed runoff Effect of maintenance Results Site condition Infiltration rate Observed rainfall intensities Discussion Pavement performance Impact of manual cleaning Conclusion References... 11

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8 1. Introduction 1.1. Lamb Drove sustainable drainage showcase project As part of their membership of the EU Floodplain Land use Optimising Workable Sustainability (FLOWS) Project, Cambridgeshire County Council chose to showcase sustainable water management technologies in the newly built Cambourne urban development scheme. The main objective for the project was to demonstrate measures to attenuate water flow within the development - slowing down the rate of runoff and storing water in high rainfall events. The operations at Cambourne were intended to incorporate new development strategies and amenities at an early design stage with additional forms incorporated throughout development. Lamb Drove had originally been designed with a traditional drainage system but was repurposed with Sustainable Urban Drainage Systems (SuDS) and flood resistant technologies to demonstrate benefits they provide. The Lamb Drove SuDS won a design award as a practical demonstration of the implementation/integration of SuDS into an urban setting (2006 Royal Town Planning Institute, East of England Planning Achievement Award). The Lamb Drove SuDS was designed in 2004, with construction completed in The site is approximately one hectare in size, comprising 35 residential buildings owned and managed by Cambridge Housing Society. The landscape slopes to the south-east, and the development is constructed largely on impermeable gault clay. This meant infiltration was not a viable option to remove surface water and the entire treatment train drains into the river Burle, the nearest watercourse. The system comprises two permeable pavement zones, an internal system of swales and detention basins, and a more substantial deep swale system draining the site (Figure 1). The system was designed to cope with a 1:100 year return period storm. Figure 1 Lamb Drove Sustainable Drainage Showcase Project, Cambourne Cambridgeshire. Evaluation of the performance of permeable pavement at Lamb Drove 1

9 1.2. Lamb Drove permeable pavement Construction The 4,200 m 2 Formpave permeable paving at Lamb Drove (Figure 2) was designed to collect, treat, store and slowly release runoff to the environment (Royal Haskoning, 2006). It allows rainwater to infiltrate to a voided stone storage before discharge to a surface basin via underground swales (Figure 1). An Inbitex geotextile layer also provides substrate for bacteria that degrade oils in the water. Figure 2 Construction of the Formpave permeable pavement at Lamb Drove, showing, A. Impermeable liner, B. Lower sub-base, C. Reinforcement geogrid, D. Upper sub-base, E. Geotextile, F. Bedding layer. (Source: Royal Haskoning, 2006) Maintenance The maintenance of the site is the responsibility of Cambridge Housing Society and is carried out by a sub-contractor, Fordham Landscapes. Major maintenance aspects cover grass cutting, herbicide application, and litter clearance. The permeable pavements have been cleaned and manually swept six times during March and November every year. Suction sweeping of permeable pavements is required to ensure that the voids between the blocks do not become clogged with silt and debris. Borgwardt (2006) estimated that entrainment of mineral and organic matter in the joints of permeable block paving can reduce the infiltration rate by an order of magnitude over 10 years. Conversely, Pratt et al. (1995) cited two examples where infiltration rates still exceeded 1000 mm/h after 9 years without maintenance. Although biannual suction sweeping was recommended (Royal Haskoning, 2006), it has not taken place (Royal Haskoning, 2010) as the maintenance contractor does not have the appropriate equipment. As yet, the County Council have not signed the Section 38 agreement with the developer on the roads within the Lamb Drove development and the preceding phases of the development around the Lamb Drove. The Section 38 agreement will secure ultimate adoption of the roads by the County Council and Evaluation of the performance of permeable pavement at Lamb Drove 2

10 allow maintenance of the permeable paving to begin. Therefore, the permeable pavement has been in operation for over 5 years with no suction sweeping and its hydraulic performance is unknown Previous studies Royal Haskoning UK (RHUK) have been monitoring the hydrological performance of the Lamb Drove sustainable drainage showcase project (Royal Haskoning, 2010) since A comparison of runoff hydrographs from the SuDS site and a control site nearby has shown that; The SuDS site shows significant attenuation in discharge following rainfall events. The SuDS features act to delay discharge of water from the site. In higher intensity rainfall events there is less delay in peak flows through the SuDS system but the reduction in flow volumes is still very pronounced. No infiltration measurements were made immediately after construction, however, Donoghue (2010) reported an average infiltration rate of 991 mm/h (±41 mm/h). This suggests that the permeable pavement was still in a very good condition and despite the lack of suction sweeping, hydraulic performance had not deteriorated as much as might have been expected. However, his measurements were taken over very short time periods (<1 minute) and therefore only reflected the infiltration capacity of the permeable pavement at the start of any rainfall event. The performance over longer storm durations is unknown Objectives Given the incomplete maintenance of the permeable pavement, the aim of this study was to carry out an assessment of the hydrological performance of the permeable paving at Lamb Drove, by assessing the infiltration capacity of the pavement in its present condition in relation to design standards and extreme rainfall intensities for the region. Where possible, the effect of maintenance activities on the infiltration capacity was to be assessed and the hydrological impact modelled. 2. Methods The assessment was based on in-situ infiltration tests and analysis of hydrographs using data supplied by RHUK Sample point selection A preliminary site visit was made on the afternoon of 6 th June, 2011 to inspect the condition of the permeable pavement and to design the sampling strategy. The pavement was divided into five sections and two points were randomly selected in each section (Figure 3) to ensure even distribution and allow analysis of spatial variability. A handheld GPS was used to locate and record the coordinates of the measurement sites in each of the sections. Evaluation of the performance of permeable pavement at Lamb Drove 3

11 A B E C D Figure 3 Sketch showing the Infiltration measurement points ( ) in each section (A E) of the Lamb Drove permeable pavement site Infiltration measurement Infiltration measurements were taken between 24 th June and 15 th July, 2011 at each point in the existing condition Cumulative infiltration Laboratory tests on paving material can be carried out using rainfall simulators (e.g. Borgwardt, 2006); however, this is difficult in-situ, especially where the site is not perfectly level. Instead, cumulative infiltration was measured using the double-ring infiltrometer method as used by Bean, et al. (2007) and that broadly follows the British Standard for the determination of infiltration rate on synthetic sports surfaces (BSI, 2003). The infiltrometer used consisted of two 16-gauge steel rings with an inner ring diameter of 107 mm (area m 2 ) and outer ring diameter of 203 mm (area m 2 ) (Figure 4). The inner ring was located on the pavement to ensure a representative area of blocks and joints was enclosed within the ring. When used on soil, infiltration rings are pushed into the ground, however, this is not possible when used on a pavement. Therefore plumber s putty was used to seal the rings to the surface of the permeable pavement to prevent water from leaking during measurement. Both rings of the infiltrometer were filled with water up to 50 mm to check for leaks, and then filled to a depth of approximately 150 mm. The initial water level in the inner ring was recorded at time 0 and measured at five minute intervals until the water intake rate was approximately constant (i.e. the drop in water level was similar over consecutive 5 minute periods) or all the water from the inner ring had infiltrated (Figure 5). Water was added to the outer ring whenever the water level was about 10 mm below that of the inner ring in Evaluation of the performance of permeable pavement at Lamb Drove 4

12 order to maintain the same hydraulic head. The role of the outer ring was to act as a buffer and prevent lateral movement of water from the inner ring. Figure 4 Schematic diagram of double-ring infiltrometer Infiltration rate For each measurement point, the cumulative infiltration over time was fitted to the Philip (1954) equation, Where F t is the cumulative infiltration at time t, mm t is time, h a and b are empirical constants This was differentiated to produce an equation of infiltration rate with time. Where I t is the infiltration rate at time t, mm/h t is time, h Using this equation, infiltration rates were estimated for each sample point (and a mean for the site) for durations up to 1 hour in 3 minute intervals Performance assessment Typical infiltration rates through the joints of new concrete block paving can be in the order of 4000 mm/h (CIRIA, 2007) and highway drainage in the UK is usually designed to cope with a rainfall intensity of only 50 mm/h. However the in situ performance of the permeable pavement will depend on the infiltration capacity of the pavement and the intensity and duration of extreme rainfall events. The general guidance is that the surface infiltration rate of the permeable pavement should be greater than the expected rainfall intensity to avoid ponding of water on the surface (CIRIA, 2007). The measured infiltration rates on the permeable pavement were compared with 1 in 100 and 1 in 50 year return period (0.01 and 0.02 annual exceedance probability) events of varying durations. In addition, peak rainfall intensities from observed storms at Cambourne were compared with the measured infiltration rates. Evaluation of the performance of permeable pavement at Lamb Drove 5

13 Design storm estimation The maximum rainfall for return periods of 1 in 100 years and 1 in 50 years was estimated using the Flood Estimation Handbook (FEH) method (Faulkner, 1999). Storm durations of 15 minutes to 1 hour were considered as the FEH methods are not recommended for durations of less than 15 minutes. Mean rainfall intensity, was estimated from Where R t is the mean rainfall intensity for a duration of t hours, mm/h D t is the maximum rainfall depth for a duration of t hours, mm The average infiltration rate was compared with average storm intensity for storm durations ranging from 5 minutes to 1hour to determine the capacity of the permeable pavement to cope with extreme rainfall Observed rainfall Data from tipping-bucket rain gauges at Great Cambourne and Lower Cambourne were provided for the period 1 July 2008 to 30 April A double-mass analysis of daily rainfall totals from the two stations revealed that for the majority of the period, the two stations agreed, however, there were periods when one or other of the stations appears to have not been recording. The Lower Cambourne gauge has the most complete record, so that station was used for the entire period, with the exception of the period 22 February August 2010 when data from the Lower Cambourne gauge was used. Significant storms were selected from the rainfall record to determine observed peak rainfall intensities Observed runoff Data were provided for runoff from the permeable pavement (Lamb Drove) and a control site (Friar Way) covering the period 1 August June The performance of the permeable pavement could be assessed by comparing runoff hydrographs from the two sites. However, there are two important caveats; The monitoring point at Lamb Drive monitors runoff from an area of 4,200 m 2, of which the permeable pavement covers only 9.5% (Royal Haskoning, 2010). The measurements are therefore dominated by runoff from areas other than the permeable pavement, such as roofs, gardens and traditional pavements. Changes in the hydrological response measured at the monitoring point may have resulted from other changes in the catchment area, such as blocking of gutters and drains or compaction of gardens. The control site at Friar Way monitors runoff from 8,000 m 2, roughly double the area of the catchment at the Lamb Drove monitoring site (Royal Haskoning, 2010). As a result, it is unreasonable to infer anything about the performance of the permeable pavement from the observed runoff data. Evaluation of the performance of permeable pavement at Lamb Drove 6

14 2.4. Effect of maintenance Bean, et al., (2004), Hunt and Bean (2006), and Interpave, (2010) have clearly shown that vacuum cleaning to remove debris, silts and other clogging materials improves the infiltration rate of permeable pavements. Whilst it would be interesting to have compared the infiltration rate of the permeable paving before, and after, suction sweeping, no mechanical sweeping was planned during the period of observation. Instead, each location was cleaned by hand to remove the clogging materials. Joints were swept with a stiff hand brush and larger particles were prised out using a screw driver. Infiltration measurements were repeated at the same places in each section (to represent the swept condition) and the same analysis performed. An analysis of variance (ANOVA) was carried out on the fitted parameters of the Philip equation to test for a change in infiltration in response to hand-cleaning. 3. Results 3.1. Site condition The initial site observation suggested that parts of the pavement had recently been cleaned mechanically, as the joints between the blocks were free of fine material (Figure 5, a) whilst the joints in the corners of the pavement contained organic and mineral material (Figure 5, b). It appears that mechanical cleaning may have been done in error as neither Cambridgeshire County Council nor Royal Haskoning 1 were aware of any scheduled cleaning. There was evidence that the neighbouring conventional road surfaces had recently been cleaned, and it is possible that the conventional road sweeper was driven over Lamb Drove in error. a) Cleaned b) Uncleaned Figure 5 Cleaned and uncleaned sections of permeable pavement. (Photos taken 6 June 2011) Infiltration rate Figure 6 shows the mean infiltration rate (and confidence interval) for the Lamb Drove site. The infiltration rate is high initially as water is drawn into the dry pavement joints by suction as well as gravity, but reaches a more or less constant rate of 200 ( 27) mm/h after 1 Matthew Hardwick, Pers. Com. 8/06/2011. Evaluation of the performance of permeable pavement at Lamb Drove 7

15 approximately 20 minutes. The results are of a similar order of magnitude to those reported by Borgwardt (2006) for a permeable pavement with sand filled joints (250 mm/h). The best fit average infiltration function, for the site is Where I t is the infiltration rate at time t, mm/h t is time, h Figure 6 Mean infiltration rate, mm/h (and confidence interval) for permeable pavement at Lamb Drove, compared to 1 in 100 and 1 in 50 year return period rainfall intensities. Table 1 Observed maximum rainfall intensities for selected storms from Lower Cambourne From To Rainfall mm Duration min Average Intensity mm/h 01/10/ :02 01/10/ : /07/ :00 11/07/ : /07/ :27 27/07/ : /09/ :37 04/09/ : /07/ :00 11/07/ : /07/ :29 07/07/ : /07/ :34 17/07/ : /06/ :41 26/06/ : /06/ :13 26/06/ : Evaluation of the performance of permeable pavement at Lamb Drove 8

16 3.3. Observed rainfall intensities Table 1 shows the calculated rainfall intensities for select storms (and parts of storms) from the Lower Cambourne rainfall record. Three intense short-duration (<5 minute) storm periods were observed with an average intensity of 50 mm/h. Longer duration events had lower rainfall intensities, with the exception of the 8 minute storm on 7 July 2009 which had an average intensity of 52 mm/h. 4. Discussion 4.1. Pavement performance The infiltration measurements show that, even after 1 hour of wetting, the average infiltration rate of the permeable pavement was 162 mm/h. This considerably exceeds the 50 mm/h rainfall intensity typically used for highway drainage design. Over the three-year monitoring period, the highest observed average rainfall intensity was 52.5 mm/h over a duration of 8 minutes (on 7 July 2009). The profile of this storm is shown in Figure 7. For two minutes, the rainfall intensity exceeded 130 mm/h. However, from the calibrated infiltration function, after 4 minutes of wetting, the infiltration rate of the pavement would be expected to be >300 mm/h, suggesting that the permeable pavement in its present condition could adequately cope with the rainfall intensity during that storm. Figure 7 Storm profile for storm on 7 July 2009 The 1 in 100 year rainfall intensity for a 15 minute event was estimated as 222 mm/h, which is close to the estimated mean infiltration rate at that time. Indeed, Table 2 suggests that the infiltration rate of the permeable pavement would be adequate to cope with many of the highest ever recorded rainfall intensities. Although there is considerable uncertainty over the estimation of rainfall intensity for such short duration storms, the results suggest Evaluation of the performance of permeable pavement at Lamb Drove 9

17 that the pavement may not be able to cope with very short extreme events, such as occurred in Preston, Lancs., in Table 2 Maximum recorded rainfall intensities in UK compared to estimated infiltration rate (after Met Office). Duration Location Date Average Rainfall Intensity, mm/h Estimated infiltration rate, mm/h 5 min Preston, 10 August Lancashire min Eskdalemuir, 26 June Scotland hour Maidenhead, Berkshire 12 July Beyond 25 minutes, the 1 in 100 year event is well below the estimated average infiltration capacity of the pavement 4.2. Impact of manual cleaning The infiltration measurements taken after manual cleaning of the permeable pavement showed no significant change in infiltration rate. This may be attributed to a) the improvised nature of manual cleaning and b) the possible mechanical cleaning that appeared to have been carried out before the tests took place. This does not suggest that cleaning is not worthwhile, as the results of Borgwardt (2006) and Bean et al., (2007) both show a significant improvement in infiltration following cleaning. 5. Conclusion These results suggest that, although the pavement has not been maintained in accordance with the recommendations for 6 years since installation, is still functioning well and should be able to infiltrate all but the most extreme, short duration rainfall events. Surface runoff from the pavement is therefore unlikely to occur. It would therefore appear that there is no requirement for additional maintenance of the pavement in the medium term. However, the extent to which this can be attributed to the apparent mechanical cleaning in June 2011 is unknown. The wide confidence intervals shown for very short durations in Figure 6 reflect the spatial variability in infiltration capacity across the Lamb Drove site. This suggests that although, on average, very extreme short duration rainfall events may exceed the infiltration capacity of the permeable pavement, local ponding and runoff may be infiltrated in other parts of the pavement, such that runoff from the pavement is unlikely. Evaluation of the performance of permeable pavement at Lamb Drove 10

18 6. References Bean, E.Z., Hunt, W.F., Bidelspach, D.A. (2007) Field survey of permeable pavement surface infiltration rates. Journal of Irrigation and Drainage Engineering-ASCE, 133: Borgwardt, S. (2006), Long-Term In-Situ Infiltration Performance of Permeable Concrete Block Pavement, In: Proceedings of the 8 th International Conference on Concrete Block Paving, Interlocking Concrete Pavement Institute, Washington, DC. BSI (2003) BS EN 12616:2003. Surfaces for sports areas. Determination of water infiltration rate. British Standards Institution. London. CIRIA (2007) The SUDS Manual. Construction Industry Research and Information Association. Donoghue, B. T. (2010) Performance of permeable pavements: A Case Study at Lamb Drove, Cambourne. MSc thesis. Cranfield University. Faulkner, D. (1999) Rainfall frequency estimation. In: Flood Estimation Handbook, Wallingford: Inst. of Hydrology. Hunt, W.F. and Bean, E. Z. (2006) NC State University permeable pavement research and changes to the state of NC runoff credit system. 8 th International Conference on Concrete Block Paving, November 6-8, 2006, San Francisco, California USA. Interpave (2011) e:pave; News from Interpave: the digital magazine from Interpave. Interpave: The Precast Concrete Kerb Association. Available at: (accessed: 23 May, 2011). Philip. J.R. (1954). An infiltration equation with physical significance. Soil Science 77: Pratt, C. J., Mantle, J. D. G. and Schofield, P. A. (1995) UK research into the performance of permeable pavement, reservoir structures in controlling stormwater discharge quantity and quality. Water Science and Technology. 32(1): Royal Haskoning (2006) Sustainable Drainage Systems for New Homes Best Practice Guidance. FLOWS report WP3Cvii-1. Cambridgeshire County Council. June Royal Haskoning (2010) Lamb Drove SUDS Showcase Project, Cambourne. Interim Monitoring Report. January Evaluation of the performance of permeable pavement at Lamb Drove 11

19 Appendix 1 Infiltration data Point: A1 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Point: A2 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Evaluation of the performance of permeable pavement at Lamb Drove 12

20 Point: B1 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Point: B2 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Evaluation of the performance of permeable pavement at Lamb Drove 13

21 Point: C1 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Point: C2 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Evaluation of the performance of permeable pavement at Lamb Drove 14

22 Point: D1 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Point: D2 NGR: TL Time(min) Time (h) Water level (cm) Water Intake (mm) Cumulative intake(mm) Infiltration rate (mm/h Evaluation of the performance of permeable pavement at Lamb Drove 15