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P me .M.A veme ly bet .4%� a e per most ance�, event tchme 67 for of an correl sture c ed tha ven st care m D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. CE Database subject headings: Runoff; Urban areas; Porous media; Pavements; Permeability; Stormwater management; Nonpoint pollution; Maintenance; Best management practice; Sustainable development. Author keywords: Porous pavements; Permeable pavement; Permeability; Storm-water management; Low impact development; Non- point pollution; Maintenance; Best management practice. Introduction Permeable pavement is one of many tools available to achieve a low impact development �LID� scheme on a lot-level scale. The term LID was coined by Prince George’s County, Maryland De- partment of Environmental Resources to describe a new paradigm for design of storm-water management systems �Dept. of Envi- ronmental Resources Programs and Planning Div. 1999b�. To mitigate or prevent receiving water degradation, all aspects of the urban water cycle, including timing, rates, and volumes of storm- water runoff must be incorporated into the basis of design for storm-water management. Main drivers for this goal originated from observation that peak flow-only type controls do not ad- equately prevent stream erosion or physical habitat degradation. It is proposed that if the overall hydrologic regime of the catchment is kept to predevelopment levels, then the receiving stream should be able to maintain its physical structure with an urbanized con- dition, contributing to preservation of its ecological integrity. From a technical standpoint, this hypothesis forms the basis of design for LID hydrology. Permeable pavement replaces conventional paving surfaces with an at-source control to prevent or significantly delay runoff generation. In most applications, permeable pavement “treats” or manages only the surface over which it covers. Precipitation fall- ing over the surface of the permeable pavement infiltrates into an aggregate-filled storage reservoir �the basecourse� below the per- meable surface �Fig. 1�. Designed as a retention system where site conditions permit, water flows through the basecourse porous media to exfiltrate into the surrounding subsoil. Otherwise, per- meable pavement designed as a detention system �e.g., installed over poorly draining subgrade or with an impervious liner� dis- charges via an underdrain to reticulation or to the receiving water. In some cases, permeable pavement may also manage runon �flow� from adjacent impervious surfaces. “Outflow” from a per- meable pavement area usually refers to discharge of treated, man- aged flow from the underdrain. In very intense storm events, on steep slopes, or during periods of frequent rainfall, surface runoff may also occur from the permeable paving surface. The hydraulic characteristics of a permeable pavement system contribute to four areas of hydrologic control: peak flow, volume, hydrograph tim- ing, and duration. Studies of permeable or porous pavement retention systems generally show a significant potential to reduce runoff volume 1Senior Lecturer, Dept. of Civil and Environmental Engineering, Univ. of Auckland, Private Bag 92019, Auckland Mail Centre, Auckland 1142, New Zealand. E-mail: e.fassman@auckland.ac.nz 2Environmental Engineer, AECOM, 6th Fl, 1901 Rosser Ave., Burnaby, BC, Canada V5C 6S3. Note. This manuscript was submitted on October 12, 2009; approved on February 18, 2010; published online on March 12, 2010. Discussion period open until November 1, 2010; separate discussions must be sub- mitted for individual papers. This paper is part of the Journal of Hydro- logic Engineering, Vol. 15, No. 6, June 1, 2010. ©ASCE, ISSN 1084- 0699/2010/6-475–485/$25.00. JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 / 475 Urban Runoff Mitigation by a over Imper Elizabeth A. Fassman, Ph.D., A Abstract: The respective runoff from a 200-m2 permeable pa catchment in Auckland, New Zealand, was monitored concurrent meable subgrade soils, and on an atypically high slope �6.0–7 permeable pavement was exceptional. Measured discharge from th comparable to or below modeled predevelopment conditions for 24-h annual recurrence interval event. For large events �5% exceed conditions, but it was substantially less than asphalt runoff for all flow and volume were statistically different between the asphalt ca level of significance�. Runoff coefficients ranged from 0.29–0. 0.41–0.74 when permeable pavement comprises about one-half drograph duration were reminiscent of a vegetated area. Spearman on runoff parameters from both catchments, while antecedent moi infiltration measurement at four permeable pavement sites reveal clogging than does traffic load. Permeable pavements should be gi and can also mitigate conventional large design storm flows, but DOI: 10.1061/�ASCE�HE.1943-5584.0000238 J. Hydrol. Eng., 2010, ermeable Pavement System able Soils SCE1; and Samuel Blackbourn2 nt test site and an adjacent 850-m2 conventional asphalt road ween 2006 and 2008. Despite installation over relatively imper- nd active roadway, the overall hydrologic performance of the meable pavement underdrain demonstrated peak flow �81 storms� storms, regardless of antecedent conditions, including a 10-year, underdrain discharge volume was comparable to predevelopment s up to approximately 70% exceedance. The distributions of peak nt runoff and the permeable pavement underdrain discharge �0.05 underdrain discharge �10th–90th percentile events�, and from otherwise impervious catchment. Underdrain lag time and hy- ation indicated influences of rainfall depth, intensity, and duration ondition was correlated to underdrain discharge lag time. Surface t surrounding land uses likely have more influence on pavement rong consideration as an low impact development source control, ust be taken during installation to ensure proper function. 15(6): 475-485 francis Realce francis Realce to do with clogging potential than direct vehicular loadings �Bean et al. 2007�. Pezzaniti et al. �2009� measured a 59–75% decline in D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. compared to that from asphalt surfaces �Gilbert and Clausen 2006; James and Thompson 1997; Pratt et al. 1995; Rushton 2001� or rainfall volume �Gilbert and Clausen 2006; James and Thompson 1997; Pratt et al. 1995�. Gilbert and Clausen �2006� compiled data from three studies which reported permeable pave- ment system discharge to range from 37 to 61% of rainfall vol- ume, whereas asphalt runoff volume was approximately equal to rainfall volume. As most of these studies were performed on sys- tems designed for exfiltration, peak flow effects were not readily reported, except in a few instances of major storm events. Several studies of permeable pavements installed over clayey subgrade soils show meaningful stormwater retention. Dreelin et al. �2006� found that for storms less than 20 mm, 93% of total rainfall exfiltrated through grassed plastic grid pavement over clay soils. Collins et al. �2008� compared runoff from an asphalt parking space with underdrain discharge from four parking spaces of different permeable pavements installedover sandy loam to sandy clay loam soils. The study determined median underdrain discharge volume was 36–67% less than rainfall volume for storms less than 50 mm across the different types of permeable pavements. Median peak flow differences of approximately 63– 83% were reported between asphalt runoff and permeable pave- ment underdrain discharge. Tyner et al. �2009� measured exfiltration from pervious concrete through a clay subgrade at 8 mm/d, which could be enhanced by preparing subgrade with ei- ther boreholes �46 mm/d�, ripping �100 mm/d�, or trenching �258 mm/d�. Regardless of outflow design, permeable pavement systems can serve to increase times of concentration, thereby decreasing hydraulic efficiency of the catchment and peak flow. Wetting, pore space storage in the aggregate basecourse, and the tortuous flow path through the basecourse all contribute to reports of 45– 145 min lag times, depending on storm size �Brattebo and Booth 2003; Gilbert and Clausen 2006; Schluter et al. 2002�. Antecedent moisture conditions were also found to affect lag time and exfil- tration rates to the subgrade �Brattebo and Booth 2003; Schluter et al. 2002�. Collins et al. �2008� identified negative statistical correlation of time to peak with rainfall intensity, rather than an- tecedent moisture. Surface clogging is the primary failure mechanism of perme- able pavement in terms of stormwater management. The per- ceived propensity for clogging and the associated maintenance burden can deter application. Bean et al. �2007� reported the re- sults of several studies as well as their own field investigation of 40 permeable pavement sites along the mid-Atlantic U.S. coast. The writers observed that unstable catchments and catchments with fine-graded soils exacerbate the potential for clogging. It appears that catchment characteristics as a whole may have more Fig. 1. Typical permeable pavement longitudinal section 476 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 J. Hydrol. Eng., 2010, hydraulic conductivity in laboratory studies simulating 35 years of sediment delivery on three different types of permeable pave- ments. Field tests showed rapid decline in hydraulic conductivity at three sites that received runon from adjacent bitumen asphalt catchments. Clogging was attributed to coarse sediments trapped in joints, as opposed to the fine sediments which were retained by the geotextile layer �Pezzaniti et al. 2009�. The different conclu- sions by Pezzaniti et al. �2009� and Bean et al. �2007� regarding the clogging potential and sediment size may be due to the types of permeable pavements tested. The former tested primarily inter- locking block pavers and grass pavers, where the latter also tested pervious concrete. Many LID manuals emphasize the importance of creating op- portunities for infiltration within the developed landscape �Dept. of Environmental Resources Programs and Planning Div. 1999a; Hinman 2005�. This objective is challenging in the Auckland Re- gion of New Zealand, where soils are largely characterized by weathered mudstone and sandstone �Waitemata and Onerahi se- ries clays� �Beca Carter Hollings & Ferner Ltd. 1999�, with hy- drologic soil group C �poor infiltration capacity�, and very frequent rainfall. Mean annual rainfall for the Auckland region is 1,240 mm with 137 wet days ��1.0-mm rainfall� spread rela- tively evenly through the year �NIWA Science 2000�. The viability of permeable pavement over impermeable soils to substantially mitigate hydrologic effects of urban runoff ac- cording to LID design objectives subject to the Auckland climate is questionable. To consider it, this study investigates the hydro- logic performance of a 200-m2 permeable pavement system in- stalled over clay subsoils with an estimated permeability of 0.01 mm/d. The test site was located on an active roadway �Birkdale Rd., Auckland, New Zealand� as it was hypothesized that in- creased loading from frequent traffic would accelerate failure, and hence yield a better understanding of long-term performance. Current local �Auckland� regulations may require peak flow reduction for the 2-year, 10-year, and 100-year, 24-h annual re- currence interval �ARI� �based on rainfall depth� design storm events to predevelopment conditions, and extended detention re- lease of the first 34.5 mm of rainfall over a 24-h period, depend- ing on site specific conditions. The aim of LID is to maintain runoff at the predevelopment condition for frequently occurring “every day” events �i.e., less than a 2-year return period� with respect to all hydrograph characteristics �i.e., flow rate, volume, and timing�. Data from the permeable pavement and an adjacent conven- tional asphalt roadway section were collected concurrently be- tween 2006 and 2008. In addition, surface infiltration rates were measured at the Birkdale Rd. site and three other permeable pave- ment installations in Auckland to assess maintenance require- ments as indicated by clogging. Site data are used herein to evaluate the permeable pavement’s ability to achieve both regu- latory goals and LID goals, despite installation over relatively impermeable subgrade soils. Site Description Constructed in February 2006, the trial site �Fig. 2� comprises a standard asphalt section and a permeable pavement section that was purposely built for concurrent monitoring. Surface coverage per catchment is described in Table 1. The entire 850 m2 asphalt catchment includes pavement, sidewalk, and grass and drains to a 15(6): 475-485 francis Realce francis Realce D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. single catchpit. The asphalt surface has a 8.9–10% longitudinal slope and 3.6–5.9% cross fall. The catchpit discharges to a storm sewer along Birkdale Rd. via a 225-mm-diameter concrete pipe. The total permeable pavement catchment is 395 m2, including 200 m2 of permeable pavement and 195 m2 of sidewalk, drive- way, and grass. The permeable pavement includes a live traffic lane and a parking lane with a bus stop. Longitudinal surface slope was measured at 6.0–7.4% over the permeable pavement, with 3.0–3.7% cross fall toward a catchpit which also collects overland flow from the sidewalk, driveway, and grass. The �35-m-long, �6-m-wide permeable pavement test sec- tion was constructed from impermeable paver blocks, with en- larged joint spaces �Fig. 3� filled with 2–5 mm aggregate chip to allow flow into a two-layer aggregate basecourse. Joint spacing was calculated to allow infiltration of the peak flow generated by a 2-year, 24-h ARI storm from an equivalent entirely impervious area, and allow for deterioration over a 10-year pavement life, resulting in a design surface infiltration rate of 1,200 mm/h and a ratio of joint space to impervious paver area of 0.19. The same Fig. 2. Birkdale Rd. test site. Asphalt catchment �white border� =850 m2 including asphalt, sidewalk, and grass. Permeable pave- ment catchment �black border�=395 m2 including 200 m2 perme- able pavement, plus sidewalk, driveway, and grass. Table 1. Catchment Characteristics Catchment �total area� Fraction of area as surface Asphalt Other imperviousa Grass verge Permeable pavement Asphalt �850 m2� 0.58 0.19 0.23 0 Permeable pavement �395 m2� 0.09 0.23 0.11 0.50 aConcrete sidewalk, curb, and driveway. Fig. 3. Birkdale Rd. permeable pavement surface detail JOU J. Hydrol. Eng., 2010, 2–5 mm chip was used as bedding material for the pavers. A geotextile was placed between the bedding layer andthe base- course. An overall two-layer 480-mm basecourse thickness was re- quired to provide structural stability to support the daily 458 equivalent standard axle load along Birkdale Rd. �including 5% heavy traffic�. Structurally, the top 150-mm-deep layer of a 12- mm-diameter aggregate would only be adequate for lighter loads �e.g., cars in parking lots�; the bottom 230-mm-deep layer of a 40-mm-diameter aggregate was required because of the truck traf- fic and loading frequency in the live roadway. A secondary moti- vation for the two layers is that the 40-mm aggregate costs less than the 12-mm aggregate, resulting in a greater specified depth of the former, but the latter is still necessary to prevent migration of the 2–5 mm joint/bedding chip. A 2007 investigation into un- derdrain construction revealed that the actual basecourse depth varied from 590–660 mm in some areas near the down-hill end of the test section. The extent of the thicker basecourse over the entire test area is not known; investigation of approximately 10-m upslope of the down-hill end revealed installation that conformed to the 480-mm basecourse design depth. In situ testing performed by the installation team �of which the writers were not part� characterized the subgrade soils as silty clay and clayey silt. The writers were informed that soil perme- ability testing equipment broke during field measurements but that permeability was later estimated at 0.01 mm/d via “perc” testing. Information provided does not detail the specific testing method, other than to assert a falling head methodology. The re- sulting permeability falls into the range of fissured clay �Scott 1980�, but is also deemed “impermeable” �Bear 1972�. Experi- ence of the writers in attempting to install a rain gauge on-site anecdotally confirms dense clay soils �a gas-powered auger was lodged into the soil, requiring significant creativity to extract it�. As exfiltration was not expected to be significant compared to storm volume, a single underdrain at the down-hill end of the test section was installed. The underdrain discharges by a 150-mm- diameter PVC pipe to a storm sewer along Birkdale Rd. A subsurface perforated drain runs along the length of Birkdale Rd. underneath the sidewalk for conventional pavement basecourse drainage. To promote and isolate all permeable pave- ment discharge through the underdrain �for measurement�, a poly- thene lining was placed between the basecourse edge and the existing road basecourse drain along the length of the test section, which extended for approximately 1.5 m under the basecourse �measured perpendicularly to the sidewalk�. Supplemental to the Birkdale Rd. permeable pavement site, three additional sites in the Auckland Region were tested for sur- face infiltration capacity every 3 months in 2006. The Kahika Rd. carpark is constructed from hexagonal shaped interlocking block pavers with enlarged joint spacing �Fig. 4�. The site serves a Fig. 4. Additional infiltration testing sites: Kahika Rd. car park �left�, Upland Rd. driveway �middle�, and North Shore Events Centre car park �right� RNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 / 477 15(6): 475-485 curve number of 74 �Beca Carter Hollings & Ferner Ltd. 1999� for each measured rainfall depth. According to local procedures, D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. wastewater pumping station, which is used infrequently to park trucks during station maintenance. Trees surrounding the site de- posit leaf litter onto the pavement. The Upland Rd. residential driveway site is the same type of interlocking concrete pavers with enlarged joint spacing �Fig. 4� as the Birkdale Rd. site. The North Shore Events Centre carpark is constructed from permeable blocks �Fig. 4�. It is occupied by buses at night and cars during the day. The heavy loading from the buses was not anticipated during the design of the pavement. There is a builder’s yard ad- jacent to the carpark which deposits fines onto the pavement from traffic and wind. Methodology Runoff Data Collection For each monitoring station, equipment was installed in the 150 mm and 225 mm pipes from the underdrain and catchpit, respec- tively, upstream of the discharge point to the storm sewer. Water depth in the pipes was recorded at 5-min intervals by a Sigma 900MAX �Hach Company, Loveland, Colo.� automatic sampler with pressure transducer rated for up to 1.75-m depth with �2 mm accuracy. The pressure transducers were installed upstream of a 120° v-notch weir installed in each pipe. A Sigma 2149 �Hach Company, Loveland, Colo.� tipping bucket rain gauge lo- cated near the end of the permeable pavement section recorded total rainfall depth in 5-min intervals based on 0.25-mm bucket tips. Runoff Data Processing Laboratory replicas were used to develop rating curves for each monitoring station, including all equipment that might cause flow turbulence. Each rating curve �Fig. 5� was determined from at least 10 flow depths and a weighing bucket apparatus. Linear interpolation between measured depth-flow data points was ap- plied to calculate flow from field measurements. Field results were normalized by the size of the contributing drainage area to allow direct comparison. Predevelopment hydrology was modeled using the TR-55/ NRCS curve number method as per local regulatory requirements, assuming a grass paddock with Hydrologic Soil Group C and a Fig. 5. Asphalt catchment and permeable pavement underdrain dis- charge rating curves 478 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 J. Hydrol. Eng., 2010, an initial abstraction of 5 mm is applied to all pervious areas �Beca Carter Hollings & Ferner Ltd. 1999�. While Pitt �1999� demonstrated significant errors with applying the TR-55 method in urban areas to individual events, it nonetheless remains the method of choice for many regulatory authorities. In lieu of direct measurement, asphalt catchment runoff can be used to estimate the surface runoff from the 195 m2 of sidewalk, driveway, and grass verge parts of the permeable pavement catch- ment. Based on sustained high infiltration rates through the per- meable pavement surface �refer to the discussion under “Surface Infiltration”�, it was assumed that surface runoff rarely occurred from the permeable pavement, which is also supported by Collins et al. �2008�. Conversely, the grass verge �over clay soils� in either catchment likely provides minimal infiltration especially when wet �Pitt 1999�. Total permeable pavement catchment dis- charge volume �surface runoff+underdrain discharge� was deter- mined by Eq. �1� Vppc = 195 395 Vasph + 200 395 Vperm �1� where Vppc, asph, perm=discharge volume �millimeters �mm� per unit area� from the permeable pavement catchment, asphalt catch- ment, and permeable pavement underdrain, respectively. The ra- tios represent the weighted contributions of each surface to the total catchment discharge. SPSS �PASW Statistics 18� was used to compare asphalt catchment runoff, permeable pavement underdrain discharge, per- meable pavement catchment discharge �underdrain+surface run- off�, and modeled predevelopment conditions. The Mann- Whitney U test for independent samples was used to test equality of peak flow or volume distributions between the various sce- narios. Correlation between measured climate parameters with as- phalt catchment and permeable pavement underdrain discharge characteristics were assessed by Spearman correlation. Nonpara- metric tests were required as appropriatetransformations were not easily identified. Surface Infiltration Testing Surface infiltration was measured using a modified double ring infiltrometer apparatus �ASTM 2003�. A two-stage ring was de- vised such that the water contact area with the pavement between the rings was consistent with standard design, but the reservoir volume was increased to slow the fall of water in the rings, al- lowing for more accurate timing at high infiltration rates �Fig. 6�. In addition, flush contact between the bottom of the infiltrometer and the permeable pavement for the outer ring was incorporated into apparatus design, as per Bean et al. �2007�. During testing, the inner and outer infiltrometer rings were sealed to the pavement using plumber’s putty; care was taken to push the putty into the cracks between the pavers �Fig. 6�. The seal was then tested by pouring water into the rings, which also saturated the pavement basecourse. The water in the rings was initially filled to a height of 10–25 cm above the pavement surface. The water depth in the inner ring was recorded at regular intervals. As tests proceeded, the outer ring water level was topped up so that it matched that of the inner ring, to ensure that vertical flow through the inner ring. Infiltration tests were conducted at various locations at each test site. Along Birkdale Rd., quarterly road closures allowed for 15(6): 475-485 ity of storms were less than 20 mm in depth, comprising 51% of an 2-m n of nfall h D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. periodic testing within the live roadway in 2006/2007. All other testing was in the parking lane. Wilcoxon rank-sum testing was performed to determine if surface infiltration in the live roadway was statistically different than in the parking lane, and if the rate changed because of underdrain maintenance activity and joint chip topup in April 2007. Two locations were consistently tested at the Kahika Rd. pumping station, one of which was underneath a tree canopy. The original testing location at the North Shore Events Centre was moved after the first round of testing to the end of the lot farthest away from the builder’s yard �further explained in “Results and Discussion”�. One location was consistently tested at the Upland Rd. driveway. Results and Discussion Runoff Control Field data collection occurred primarily over two periods, span- ning from winter to early summer in both cases. A total of 566.3 mm of rainfall was monitored in September 2006–January 2007, while 561.8 mm occurred during the July 2008–December 2008 monitoring period. Individual events considered for analysis had a minimum of 2 mm of rainfall. The distribution and characteristics of the storm events analyzed are presented in Table 2. The major- Fig. 6. Modified double ring infiltrometer Table 2. Hydrologic Characteristics from Complete Storms of Greater th Number of storms Storm depth �mm� Fractio total rai dept 36 �2–7 0.13 10 �7–10 0.07 24 �10–20 0.31 5 �20–30 0.10 3 �30–50 0.10 3 �50 0.28 Overall mean 14 Overall median 8 Maximum 152 Minimum 2 Standard deviation 21 Note: “Complete” indicates flow measured at all monitoring stations. aMaximum 5-min rainfall over storm duration. JOU J. Hydrol. Eng., 2010, the total rainfall, and 86% of the number of events. Distinct hydrographs were often difficult to isolate during back-to-back events, when catchment discharge from the first storm might still be occurring despite several hours’ gap in rain- fall. Rainfall and runoff volume could be resolved across both events, but hydrograph tails were somewhat arbitrarily cut off between events with the result that apparent measured runoff slightly exceeded rainfall for some individual events, and vice versa. Total volume determinations were also challenged in the asphalt catchment, where debris often washed through the catch- pit or bypass flow from upstream a catchpit seemingly increased measured catchment runoff volume. Conversely, extended dry pe- riods that caused significant evaporation from the catchpit caused questionably low discharge volumes. Where data from these events could not be resolved, the volumes were excluded from calculation of runoff coefficients. Full discharge hydrographs from the permeable pavement un- derdrain discharge for storms producing less than 7 mm of rainfall could not be measured accurately, as they resulted in only a trickle of water which was less than 2-mm depth over the weir. Any fluctuation in water level was below the monitoring instru- ments’ sensitivity. Hence these very small storms, which contrib- ute 13% of the overall volume of rainfall, but 44% of the number of storms monitored, can only be considered for limited analysis. Peak flows were estimated based on 2-mm depth over the weir and the rating curves for each monitoring station �Fig. 5�. For the underdrain discharge, a best estimate for 26 of the 36 storms that were less than 7 mm is that peak flow was less than 0.13–0.21 mm/h. The problem of low flow measurement applied to only 13 of the 36 storms in the asphalt runoff catchment; peak flows were estimated at 0.01 mm/h �0.004 L/s�. Total volume, lag time, and duration could not be satisfactorily estimated for these events. Event-Based Performance Representative hydrographs for small storms �25 mm�, and ap- proximately 2-year �63 mm�, 5-year �98 mm�, and 10-year �152.3 mm�, 24-h ARI storms successfully captured are shown in Figs. 7–10. In all cases, asphalt runoff usually mirrored spikes in rain- fall intensity, whereas discharge from the permeable pavement underdrain was relatively stable once the peak rate was reached. The hydrographs also demonstrate significantly reduced perme- m Rainfall Depth Rainfall intensity �mm/h� Median duration �h�Mean Peaka 2.5 11.6 3.1 1.7 17.9 6.8 2.1 15.9 8.4 1.9 31.7 14.7 3.6 35.2 13.8 3.8 44.8 27.1 2.3 17.0 7.9 1.4 14.4 6.1 27.6 79.2 40.8 0.3 3.6 0.1 3.3 12.9 6.7 RNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 / 479 15(6): 475-485 D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. able pavement underdrain discharge volume compared to the as- phalt catchment runoff. Table 3 compares the hydrographs in Figs. 8–10 plus one additional measured 5-year, 24-h ARI storm against conventional stormwater management goals, for which the system was not expressly designed. Assuming no surface flow was assumed to occur from the permeable pavement �refer to Fig. 7. Small storm hydrograph �25 mm� Fig. 8. Approximate 2-year storm hydrograph �63 mm� Fig. 9. Approximate 5-year storm hydrograph �98 mm� 480 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 J. Hydrol. Eng., 2010, Surface Infiltration section�, Table 3 demonstrates that the perme- able pavement is able to reduce peak discharge below predevel- opment conditions for conventionally regulated storm events. Table 4 summarizes median hydrologic characteristics of the monitored systems according to storm size, while Table 5 presents Spearman correlations with measured climate parameters. The as- phalt catchment peak flow unsurprisingly showed a steady in- crease in depth with increasing storm size; parameters were positivelycorrelated at the 0.05 level of significance, while rain- fall intensity was also positively correlated at the 0.01 level of significance. For the majority of events �storms up to 20 mm, Table 2�, median asphalt peak flow is approximately an order of magnitude greater than median modeled predevelopment peak flow, and is approximately four times greater for the remainder of events. While permeable pavement underdrain peak flow was also positively correlated to storm size and intensity �both at 0.01 level of significance�, the median peak flow for each storm size range �Table 4� were substantially less than the median asphalt catch- ment peak flow. The Mann Whitney U test for independent samples suggested statistically significant different peak flow dis- tributions �0.05 level of significance�. For events exceeding 20-mm rainfall, median permeable pavement underdrain peak flow was less than modeled predevelopment conditions, corrobo- rating results from Table 3 over a wider range of events. The difference in peak flow distributions was not statistically different �0.05 level of significance� between underdrain discharge and modeled predevelopment peak flow, when storms with greater than 2 mm over the weir were considered. Interestingly, positive correlation �0.01 level of significance� between peak rainfall in- tensity and peak underdrain discharge suggests that intense storms push flow through the basecourse, reducing flow control. Effective capacity for stormwater retention despite clayey sub- soils is demonstrated by substantially less median permeable pavement underdrain discharge volume compared to the median asphalt catchment discharge volume for each rainfall category �Table 4�. As might be expected, discharge volumes were posi- tively correlated to rainfall depth and average intensity for both systems �0.01 level of significance�. Underdrain discharge volume was also positively correlated to peak intensity �0.01 level of significance�, suggesting that rainfall bursts push water through Fig. 10. Approximate 10-year storm hydrograph �152.3 mm� 15(6): 475-485 Table 3. Peak Flow Control for Conventional Design Storms m Size Pre 0. 0. 1. 3. D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. the system. The Mann Whitney U test for independent samples rejected the null hypothesis of equal volume distributions be- tween the systems �0.05 level of significance�. Greater than expected rainfall retention measured in 2006/ 2007 prompted inspection of the permeable pavement underdrain. In April 2007, the underdrain was reinstalled and all pipe joint seals were checked to ensure positive drainage without leaks. Consequently, approximately 3 m of the permeable pavement sur- face and basecourse was replaced in the vicinity of the underd- rain. Joint chip along the entire length of the test section was also topped up. Subsequent data revealed that initial underdrain instal- lation was not problematic �i.e., there was no measureable differ- ence in retention�, and instead lead to the topic of the current paper. Losses in the permeable pavement can be attributed to a combination of: Significant storage in the subbase, followed by evaporation �wetting and drying of the pavement basecourse�; or Leakage under the sidewalk to the perforated road drain; or Exfiltration into subgrade soils. A presumed lack of capillary action viable in the generally large size basecourse aggregates suggests evaporation should not be significant. However, a few laboratory and field studies of permeable pavements indicate that evaporation does occur �Andersen et al. 1999; Gomez-Ullate et al. 2008; Karasawa et al. 2006�, and in one study, that it even contributes to surface cooling �Karasawa et al. 2006�. Collins et al. �2008� also suggested evaporation as a water loss mechanism between events, although it was not specifically measured. With respect to exfiltration, the Storm date Rainfall depth �mm� Storm duration �h� App ARI July 26, 2008 63.6 19.3 August 5, 2006 82.1 27.8 July 29, 2008 98.0 40.8 October 1, 2006 152.3 27.1 Note: n /a=not applicable. aSource: Beca Carter Hollings & Ferner Ltd. �1999�. Table 4. Median Discharge Hydrologic Characteristics according to Stor Storm depth �mm� Duration �h� Pred Aspha Perma �2–7 n/a 2.7 7.5 �7–10 0.1 11.4 14.1 �10–20 7.7 9.7 12.0 �20–30 13.3 14.8 14.2 �30–50 10.0 16.7 26.7 �50 24.2 24.3 28.5 Overall mean 11.5 10.4 15.8 Overall median 8.3 9.3 13.1 Overall maximum 44.7 29.6 43.3 Overall standard deviation 10.7 7.7 8.7 Note: Asph=Asphalt Catchment Runoff, Perm=Permeable Pavement U applicable. aFull hydrograph analysis is not available for the permeable pavement sys For 26 storms for the permeable pavement underdrain and 13 storms fo 0.13–0.21 mm/h and 0.01 mm/h, respectively. These events were exclude JOU J. Hydrol. Eng., 2010, subgrade may not be homogeneous. Cracks, fissures, or other het- erogeneity could promote exfiltration, as per Tyner et al. �2009�. Leakage to the perforated drain is less likely; in addition to the polythene liner, longitudinal slope toward the underdrain �6.0– 7.4%� is substantially greater than cross fall toward the road drain �3.0–3.7%�. Table 6 summarizes the overall fraction of rainfall that be- comes runoff, a.k.a., the runoff coefficient, C, which is a fre- quently endorsed approach by regulatory communities for storm- water management design. The coefficients show that even when permeable pavement is only about half of the total catchment area �Table 1�, the total catchment discharge �underdrain+surface run- off� is likely to be substantially less than from an asphalt catch- ment with similar nonasphalt fractions, particularly for larger events, presuming C increases with storm size �Pitt 1999�. For storms greater than 7.0 mm of rainfall, the distribution of asphalt catchment volume was statistically different from either the per- meable pavement underdrain volume or the permeable pavement catchment �underdrain+surface�. Storms less than 7.0 mm of rainfall could not be compared, as asphalt catchment volume and permeable pavement underdrain volume were not concurrently available in most cases. Flow Frequency Analysis Impacts of urban runoff arise not only from the magnitude of flows, but the frequency with which elevated flows occur. As described in the “Introduction,” flow control under a conventional te � Peak flow �mm/h� Predevelopment Asphalt surface Permeable pavement underdrain 7.7 16 2.9 7.4 n/a 5.5 12.4 46.0 6.5 24.5 48.6 14.9 Peak flow �mm/h� Volume �mm� d Aspha Perma Pred Aspha Perma 0 3.0 0.2 0.0 3.2 3.3 1 9.5 0.8 0.1 7.7 4.4 0 9.5 1.2 0.9 14.1 7.1 3 13.3 1.3 2.5 11.4 10.6 6 28.6 3.4 7.4 28.9 21.4 4 46.0 6.5 47.9 105.3 56.9 4 11.5 1.2 2.9 13.2 9.5 2 7.7 0.6 0.1 8.7 6.0 5 48.6 14.9 92.5 109.8 82.4 6 12.2 2.1 11.8 19.1 13.1 rain Discharge, and Pred=Modelled Predevelopment Runoff. n /a=not all storms of 2–7 mm rainfall due to low flow and instrument sensitivity. sphalt catchment in this storm depth range, peak flow was estimated at total volume and duration calculations. 6. 12. 1. 0. 24. 3. nderd tem for r the a d from roxima a �year 2 2 5 10 RNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 / 481 15(6): 475-485 Table 5. Spearman Rho Correlation between Climate and Runoff Char- acteristics D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cnol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. management scenario tends to match peak flow for isolated de- sign storm events, whereas the LID approach calls for matching flows and volumes over a range of conditions. Statistically significant Spearman correlations �Table 5� dem- onstrated that runoff control by permeable pavement is affected by antecedent moisture conditions and storm characteristics. A useful format to assess the reliability of a stormwater manage- ment system for flow mitigation under varying conditions without consideration of the sequence of events is with a flow duration curve �Smakhtin 2001�. Under the context of assessing storm- water management devices on an event basis, a flow frequency curve �FFC� is suggested as a more apropos name, and is adopted herein. The number of observations affects the shape of the FFC. Data included in the peak flow FFC �Fig. 11� include an equal number of records �81� for peak flows across the three scenarios of asphalt catchment runoff, permeable pavement underdrain discharge, and modeled predevelopment conditions. Fig. 11 demonstrates that regardless of conditions, measured underdrain discharge is ap- proximately equal to modeled predevelopment peak flow for ap- proximately 80% of occurrences. Excluding estimated permeable pavement underdrain peak flows when flow over the weir was approximately 2 mm, the modeled predevelopment peak flow dis- tribution is not statistically different from the distribution of un- Climate Asphalt runoff Permeable pavement underdrain Rainfall depth Volume �0.01� Volume �0.01� Peak flow �0.05� Peak flow �0.01� Duration �0.01� Duration �0.05� Rainfall duration Duration �0.01� Duration �0.05� Average rainfall intensity Volume �0.05� Volume �0.01� Peak flow �0.01� Peak flow �0.01� Peak rainfall intensitya Peak flow �0.01� Volume �0.01� Peak flow �0.01� Duration �0.01� Antecedent dry period Lag time �0.05� Note: Correlation tests exclude storms with estimated peak flow; all cor- relations between lag time and runoff duration performed only for storms with at least 7-mm rainfall; two-tailed level of statistical significance indicated in “��”. aMaximum 5-min rainfall over storm duration. Table 6. Estimated Runoff Coefficients Based on Storms �7-mm Rain- fall Percentile Asphalt catchment Permeable pavement underdrain Permeable pavement catchment 0.10 0.48 0.29 0.41 0.25 0.60 0.43 0.53 0.50 0.85 0.49 0.64 0.75 0.94 0.57 0.71 0.90 0.98 0.63 0.74 Standard deviation 0.20 0.15 0.13 Note: Runoff coefficient C=runoff volume/rainfall volume. 482 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 J. Hydrol. Eng., 2010, derdrain peak flow �0.05 level of significance�. Approximately 15% of all storms did not produce measureable peak flow from the observed or modeled scenarios. The volume FFC �Fig. 12� is compiled from 68, 55, and 61 records among the asphalt catchment runoff, permeable pavement underdrain discharge, and modeled predevelopment conditions, respectively. To promote comparability between modeled data and field observations, the number of modeled events was re- stricted to the average number of observed events between the two catchments for storm volume. Specific events excluded were those that did not produce modeled runoff, as low flows were the reason for exclusion of measured data. Permeable pavement total catchment data were not included as only 45 data points were available and did not contain any small storm ��7.0 mm� events. For large infrequent events �5% exceedance�, permeable pave- ment underdrain discharge again is shown to match predevelop- ment conditions. While predevelopment volume is not matched by the permeable pavement for frequently occurring events �at least 50% exceedance�, likely due to the clay subsoils, it is still substantially mitigated compared to the expected asphalt catch- ment volume regardless of antecedent conditions. Volume distri- butions were statistically different �0.05 level of significance� between asphalt catchment and permeable pavement underdrain discharge for storms greater than 7 mm of rainfall. Fig. 11. Peak FFCs Fig. 12. Volume frequency curves 15(6): 475-485 our pe D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. Together, results suggest that the permeable pavement can be used to meet LID objectives for matching peak flows for “fre- quently occurring” events, even when part of a larger impervious catchment. Volume control is slightly less satisfactory according to these objectives, but certainly the ability to self-mitigate even with poor subgrade soils, steep slope, and frequent rainfall should be acknowledged. Lag Time and Runoff Duration Antecedent dry period duration was positively correlated to lag time for the underdrain discharge �Table 5, 0.05 level of signifi- cance�, further supporting the concept of wetting and drying, and possible exfiltration. However, individual event data indicate that even when the permeable pavement is wet at the onset of rainfall �i.e., a short interevent time�, there is still a delay in the onset of runoff due to the tortuous flow path through the permeable pave- ment basecourse. Over 54 events, a median value of 1 h �2.2-h standard deviation� elapsed before discharge from the permeable pavement was measured, whereas only a median 12 min �48-min standard deviation� elapsed for the asphalt runoff over 61 events. The large standard deviation of asphalt lag time is due to a few storms where it is likely that the catchpit had to fill up before discharge through the monitoring point could occur. Data are not presented in Table 4, as lag time was not correlated to storm depth. Storage and flow through the aggregate basecourse can serve to extend the underdrain’s hydrograph duration. Asphalt catch- ment runoff and permeable pavement underdrain discharge dura- tions were positively correlated to rainfall depth �Table 5�. For storms with more than 2-mm depth over the weir, discharge from the permeable pavement underdrain typically lasted 3–6 h longer than from the asphalt catchment �Table 4�, reminiscent of a slow release from a vegetated site. Altogether, the permeable pavement can have an important effect on overall hydrograph timing, re- gardless of antecedent moisture conditions. Surface Infiltration Substantial variation was observed in the Birkdale Rd. surface infiltration rate �Fig. 13�. Removal of some paver blocks in April 2007 revealed that parts of the bedding layer had formed into Fig. 13. Surface infiltration rates at f JOU J. Hydrol. Eng., 2010, agglomerated chunks and a crust above the geotextile �Fig. 14�. However, since 70% of results are greater than the 1,200-mm/h design permeability and all results are greater than the 120-mm/h required permeability, the chunking and crusting does not appear to be a cause of concern for overall performance. Testing com- menced 2 months after initial construction, and continued for ap- proximately 1.5 years. The 2008 data �Fig. 13� is indicative of approximately 1.25–1.5 years of operation without maintenance, as joint chip was topped up in April 2007. When all data at Birkdale Rd. are considered, the Wilcoxon rank-sum testing is unable to detect statistically significant differ- ences in surface infiltration rate between pre- and postmainte- nance activity. Nordoes the surface infiltration rate in the live road appear to be statistically different from the surface infiltra- tion rate in the parking lane, but it is noted that roadway testing did not occur in 2008. When data from only premaintenance ac- tivity is tested, the surface infiltration on the roadway is statisti- cally greater than in the parking lane at the 0.025 level of significance. Birkdale Rd. results may be due to high site slope �6.0–7.4%�; over time, a large portion of the joint chip scoured out of cracks �causing the need for the topup in 2007, and recurrence was con- firmed in 2008�, effectively clearing any organic or other matter which might otherwise clog pore space. The loss of joint chip allowed paver blocks to shift under vehicle loads, deforming the road surface and joints, thus explaining the statistically significant rmeable pavement sites in Auckland Fig. 14. Agglomerated bedding sand �left� and crusting above geo- textile �right�, April 2007 RNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 / 483 15(6): 475-485 difference in surface infiltration rate between the live road and parking lane for the premaintenance period. In 2008, weeds and not statistically different. Conversely, asphalt catchment peak D ow nl oa de d fro m a sc el ib ra ry .o rg b y U TF PR - U ni ve rs id ad e Te cn ol og ic a Fe de ra l d o Pa ra na o n 10 /3 1/ 17 . C op yr ig ht A SC E. F or p er so na l u se o nl y; a ll rig ht s r es er ve d. leaf litter were also observed in joint space. Loss of joint chip might also suggest that surface flow did in fact occur from the permeable pavement. Flow measurement was attempted in the permeable pavement catchment catchpit, but re- sults were unreliable, despite checks on equipment calibration, and would have been confounded with overland flow from the driveway, sidewalk, and grass verge anyway. Sustained high sur- face infiltration rates �Fig. 13� suggest that overall, surface flow from the permeable pavement would have been a rare occurrence, would likely be minimal compared to the other catchment area contributions, and thus the underdrain discharge evaluation satis- factorily represents typical system performance. Trees overhanging one testing location at the Kahika Rd. site dropped leaf litter into pavement joints, resulting in low compara- tive permeability. Joints were pressure washed between the first and second rounds of testing. While leaf litter continued to accu- mulate throughout 2006, surface infiltration rate was well above 1,200 mm/h. Ultimately, pressure washing is not recommended as a maintenance method, unless the wash water is captured. Trapped pollutants are reentrained into a runoff system �the wash water� or simply disbursed around the site, where they may once again flow into the permeable pavement and cause clogging. The North Shore Events Centre surface infiltration rates were expected to be higher than the other sites due to its construction from permeable blocks and hence a larger surface area available for infiltration. However, the sediments tracked or wind blown from the builder’s yard resulted in a permeability of 523 mm/h when the end of the lot closer to the yard was tested. While it was considered important to demonstrate the detrimental effects of sediment deposition, alternative locations were chosen for future testing. Permeability measured at the opposite end of the lot was well above the design criteria despite the potential heavy loads from bus parking every night. The testing clearly demonstrates the importance of considering all land uses within a catchment when assessing the suitability of a permeable pavement installa- tion, as observed by Bean et al. �2007�. The Upland Rd. driveway demonstrated consistently high permeability measurements throughout the monitoring period, which is attributed to very low site traffic. Conclusions A 200-m2 interlocking block permeable pavement site was tested along an active roadway and parking lane with bus stop over impermeable soils. Data from the permeable pavement underdrain discharge and adjacent conventional asphalt section runoff were collected concurrently between 2006 and 2008, and over a total of 1,128.1 mm of rainfall. The permeable pavement compared favorably with modeled predevelopment conditions against several criteria for LID objec- tives: maintaining predevelopment conditions for runoff lag time, peak flow, and duration of flow. Peak flow from the permeable pavement underdrain was less flashy and tended to show less variation overall, compared to the asphalt runoff which usually mirrored spikes in rainfall intensity. Eighty-one complete storms contributing to a peak flow frequency assessment indicate rela- tively comparable peak flows between the permeable pavement underdrain discharge and modeled predevelopment flows, regard- less of antecedent conditions, and including large conventional design storms �up to 10-year, 24-h ARI�. Flow distributions were 484 / JOURNAL OF HYDROLOGIC ENGINEERING © ASCE / JUNE 2010 J. Hydrol. Eng., 2010, flow was substantially greater in all cases, and the flow distribu- tion was statistically different from the underdrain discharge flow distribution. The tortuous flow path through the permeable pavement basecourse delayed the onset of runoff compared to rainfall, with a median 1-h lag time, compared to median 12 min for the asphalt catchment. The permeable pavement underdrain extended the total runoff duration by 3–6 h over that for asphalt catchment during storms of 7–50 mm. Underdrain hydrograph timing was reminiscent of modeled predevelopment conditions in most cases. Relatively substantial volume control was measured, despite presumed impermeable subgrade soils, steep slope, and frequent rainfall occurrences, although predevelopment conditions were not matched. Losses in the permeable pavement are most likely attributed to significant storage in the subbase, followed by evaporation �wetting and drying of the pavement basecourse� and/or exfiltration into subgrade soils. Runoff coefficients for the permeable pavement underdrain discharge were measured at 0.29–0.67 �10th–90th percentile values� which increases to 0.41– 0.74 when permeable pavement comprises about one-half of the catchment area and the remainder is predominantly impervious area. For both scenarios, a statistically significant difference be- tween distributions was measured compared to the asphalt catch- ment. Assessment of maintenance requirements from the four tested permeable pavement sites is inconclusive. At Birkdale Rd., sur- face infiltration rate did not substantially deteriorate after 1 year of operation. Subsurface investigation in the midst of the project revealed crusting of the bedding layer below the pavers. One and a half years after joint chip replacement, at the end of the moni- toring period, surface infiltration rate was not significantly com- promised. High site slope �6.0–7.4%� likely helped maintain high surface infiltration rates, as joint chip was partially washed out in about 1 year �which also caused paver blocks to shift position�. At other testing sites, organic material and sediment significantly re- duced permeability, yet even reduced rates well exceeded design requirements of 120 mm/h to infiltrate the 2-year, 24-h ARI storm. Where pressure washing occurred at one site to clear the joints, surface permeability was restored by more than an order of magnitude. Pressure washing is not recommended as a mainte- nance procedure, however, as dislodged sediments may be washed into the reticulation system and ultimately to the receiv- ing water. Interpretation of results with respect to system design is com- plicated.Generally, greater system depth would positively con- tribute toward all of the hydrologic performance metrics considered herein. At Birkdale Rd., the basecourse depth was de- termined for structural requirements rather than a detention stor- age volume for a particular extent of flow control. Structural design will vary from site to site, but it likely will always exceed water quality design where vehicle loadings will be present �and assuming the source area is only the permeable pavement itself�. It is also likely that in practice, actual construction will always deviate in some way from design on paper, as it did at Birkdale Rd. �observed by the deeper subbase areas near the underdrain�. In this case, perhaps construction deviations contributed posi- tively toward performance, but it is also feasible that a different deviation�s� could detract from performance. Overall, the Birkdale Rd. system design could be considered typical; hence, the hydrologic characteristics observed herein would be expected elsewhere if the same design basis was adopted. Altogether, permeable pavement appears to be an effective 15(6): 475-485 tool for hydrologic mitigation of storms from “every day events” up to the 10-year, 24-h ARI, even on steep slopes located over impermeable soils and subject to frequent rainfall. Acknowledgments This study was funded by the North Shore City Council, the Auckland Regional Council through the Storm Water Action Plan, Maunsell, Ltd. �now AECOM�, and TechNZ. Viewpoints ex- pressed in this paper are those of the writers and do not reflect policy or otherwise of the funding agencies. 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