Highway Runoff in Ireland and Management with a French Drain System

Transcription

1 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 Highway Runoff in Ireland and Management with a French Drain System N.M.P. Higgins 1, P.M. Johnston 1, L.W. Gill 1, M Bruen 2 and M. Desta 2 1 Department of Civil and Environmental Engineering, Trinity College Dublin, Dublin 1, Ireland 2 School of Architecture, Landscape & Civil Engineering, University College Dublin, Dublin 4, Ireland N.M.P. Higgins higginsn@tcd.ie ABSTRACT A considerable body of research already exists on highway runoff and associated management strategies from various countries throughout Europe and beyond. However, very little research has been conducted in Ireland. The objective of this study was to characterise the runoff from major Irish roads under the current drainage structure and to evaluate the performance of a French Drain i.e. filter drain system. A number of highway sites within close proximity to the Dublin City were instrumented and monitored over a 2-year period. Annual average daily traffic numbers on these routes varied from 25, to 3, vehicles with precipitation ranging from 8 to 9mm. More than 1 storm events were monitored and the resultant runoff analysed for a number of parameters. Significant quantities of suspended solids and heavy metals were detected whereas any hydrocarbons detected were at low levels. The runoff quality results as measured are similar to other European studies particularly the UK for comparable traffic densities. The filter drain system performed as both an excellent attenuation and quality control mechanism. Average pollutant concentration removal efficiency was 89% for total suspended solids, 85% for total phosphate, 84% for total copper, 91% for total zinc, 44% for total cadmium and 64% for total lead respectively. KEYWORDS Highway runoff; filter drain; pollutant removal efficiency; heavy metals; solids; drainage system 1. INTRODUCTION The construction of motorway grade roads in Ireland has developed rapidly in recent years, along with the economy, and it has accelerated under the National Development Plan 2-26 and more recently the Transport 21 Plan At last report (NRA, 25), Ireland s network of public roads extends for 92, 3km. There are 26km of road per 1, population; approximately twice as much as in Belgium, France or Denmark, and over three times as much as in the Netherlands, Italy or Spain. Within the network, national primary routes (2,74km) are the major long distance through-routes of which 192 km are classified as motorway (7% of total). If dual carriageways are included, some 17% of the national primary routes might be classified as highway. The national route system represents 6% of the overall road network but carries 38% of total road traffic. Higgins et al. 1

2 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 Concomitant with the development of the road network has come an increase in traffic densities. With vehicle numbers approaching 2 million in Ireland (CSO, 26), traffic counts (AADT Annual Average Daily Traffic) on busy routes have reached over 93 per day on the M5 Dublin city ring road and around 3 on the N4 Dublin to Galway route (NRA, 25). While these densities are still not large compared to many other European locations, they do carry environmental implications. The combination of roads and high traffic densities represent an ongoing potential hazard in terms of emissions to the environment, exacerbated by prevailing meteorological conditions. The vehicle engine, as a combustion system burning fuels, emits exhaust gases and liquids (water, unburned hydrocarbons). Frictional resistance between the tyres and the road surface, combined with wind and rain, results in sediment/runoff from the road. A variety of detritus may fall from wear of the vehicles themselves and their engines in transit. The road infrastructure may also deteriorate with time (e.g. crash barriers) and the application of deicing salts in winter can result in significant emissions. Nevertheless, emissions are typically airborne (gases and aerosols) or water borne (via rainfall and runoff). Key indicator compounds for road and vehicle emissions include Poly Aromatic Hydrocarbons (PAHs), heavy metals (Cr, Pb, Cu, Cd and Zn), phosphorous and chloride. In this context, the Environmental Protection Agency (EPA) and the National Roads Authority (NRA) initiated a research project to investigate the nature of the water borne emissions (i.e. runoff) from highways in Ireland and their effect on principal receptors. By conventional design, drainage from highways in Ireland has concentrated on removal of excess water with a view to maintaining both safety for vehicle movement as well as the geotechnical integrity of the road infrastructure. Any improvements in the quality aspects of the drainage water have tended to be a beneficial side effect. However, under the increasing traffic densities on major new roads, the chemical quality of the runoff was seen to be a potential issue, especially as the intended discharge point for most highway drainage is a nearby surface water course. Measurement of road runoff and its quality at selected sites on new Irish highways was undertaken and the results compared with similar European studies. Specific attention was paid to the effects on the quality of receiving streams. In the light of the results of the analyses of runoff quantity and quality, the filter drain was evaluated as a possible method of road drainage management, conforming to Sustainable Urban Drainage System (SuDS) practice. Moreover, under the Water Framework Directive (EU, 2), a highway may constitute a possible contaminant source for both surface water and groundwater. The relevant pathways and their susceptibilities to migrating contamination needed to be assessed in an Irish context. 2. METHODS 2.1 Site Selection Runoff was monitored and sampled from four highway sites within close proximity to Dublin in the east of the country as presented Figure 1. The sites were selected on the basis of a range of physical factors, which included the type of drainage system; existence of a well-defined catchment area; proximity to receiving water body; climatic characteristics pavement type and condition and accessibility for operation and instrumentation (Caltrans, 1999). The two types of drainage systems encountered at these sites were a filter drain and kerb & gully system. Site details are presented in Table 1 including a list of the criteria. 2 Highway Runoff in Ireland and Management with a French Drain System

3 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 Figure 1. Detailed map of the sites in relation to Dublin City. CRITERIA SITE A SITE B SITE C SITE D Location Kildare E Monasterevin W Monasterevin Maynooth Land Type (rural/urban) Rural Rural Rural Rural AADT (vehs/day) 25,-3, 25,-3, 25,-3, 25,-3, Yearly precipitation (mm) Surface Area (m 2 ) 14,184 11,368 9,6 9,76 Regular Maintenance Yes Yes Yes Yes Drainage Surface Type Kerb & Gully Kerb & Gully Filter drain Filter drain Date Highway Opened Table 1. Summary table of the final sites selected for detail investigation. 2.2 Drainage System Design The standard kerb and gully drainage structure as presented in Figure 2 was constructed to guidelines in the Design Manual for Roads and Bridges (DMRB, 1998). The surface runoff collects at the kerb surface and then discharges to a pipe drainage system via on-line gully traps, installed at 2m intervals. The filter drain as presented in Figure 2 consists of an excavated trench, which is firstly lined with a geotextile material (woven or nonwoven) and then backfilled with CI 53 bedding material to a thickness < 75mm on to which perforated land drainpipes or butt jointed concrete pipes are laid horizontally. A layer of coarser CI 55 type stone, which is either natural, or mechanically crushed, is placed above. The finish is an overlap of the geotextile Higgins et al. 3

4 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 material at roughly 3 mm from the trench surface and additional CI 55 type stone is added to bring the filter level with the road surface. The geotextile material, which surrounds the filter drain provides extra strength to the filter drain structure but is also intended to stop filter material from being blocked by silt from the surrounding sub soils (NGSRW, 2). One of the aims of this research is to investigate the overall efficiency of this drainage system especially to investigate whether the overlap of the geotextile in the top section of the filter drain is affecting the pathway of the runoff from the highway surface through the filter material into the underlying carrier pipe, which is the design pathway. It may be that the designers of the filter drain have underestimated the sediment loading in the runoff from the highway and as a result the sediment has clogged the geotextile in the top section, which in turn prevents runoff migrating through the filter material into the carrier pipe. If the runoff cannot enter the drainage system it will either remain on the road surface resulting in a safety hazard or percolate into the subsurface of the road side in which case groundwater may become the receptor. Figure 2. Kerb & Gully System and System (DMRB, 26). 2.3 Monitoring Each of the sites were installed with an ISCO 674.1mm tipping bucket rain gauge, which recorded the rainfall on a one minute time series. The flow was measured with a low profile ISCO 77 area velocity flow module which was placed within the drainage pipe to measure the velocity (using the Doppler effect) and depth (hydrostatic pressure) of the flow in the pipe. Multi-function probes that measured the temperature, ph, dissolved oxygen and conductivity were also installed. All these devices led into a mother system an ISCO 6712 automatic sampler that acted as a storage cell but also took samples of the runoff via a suction tube depositing into a series of 3ml bottles. With installation complete, the sampler was programmed to activate when a certain criteria was met, such as the recording of a rainfall depth. The pacing at which the samples were taken was either regulated by the flow in the pipe or time step. This setup worked well in capturing the entirety of the main storm events. Following a storm event the samples of runoff captured were transported immediately back to the laboratory where they were analysed for a number of parameters commonly found in highway runoff. The water quality analysis was carried out in the laboratory in accordance with the Standard Methods (APHA, 1998). 4 Highway Runoff in Ireland and Management with a French Drain System

5 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, RESULTS AND DISCUSSION 3.1 Highway runoff quality In the course of the investigation, all specified contaminants (with the exception of MTBE) were detected during at least one monitored event. However, a number of these contaminants were only detected at a single location for a limited number of rainfall events. Table 3 summarises the results of the study for direct highway runoff event mean concentrations and compares them with results published in the UK Design Manual for Roads and Bridges (DMRB, 26). In general the pollutant concentrations detected in this study are comparable to those found in the UK under similar climatic and traffic conditions. THIS STUDY 27 UK DMRB 26 Parameter Events EMC Events EMC Detected UNITS MEAN MAX Detected UNITS MEAN MAX Total Suspended Solids 1 mg/l 35 2,34 1 mg/l 115 1,35 Total Phosphate 1 mg/l mg/l - - Total Organic Carbon 1 mg/l mg/l - - Total Chloride 1 mg/l mg/l 258 3,12 Heavy Metals Total Cd 1 ug/l ug/l Total Cu 1 ug/l ug/l Dissolved Cu 1 ug/l ug/l Total Pb 1 ug/l ug/l Dissolved Pb 1 ug/l ug/l - - Total Zn 1 ug/l 274 1,75 1 ug/l Dissolved Zn 1 ug/l ug/l Naphthalene 33 ug/l ug/l Acenaphthylene 33 ug/l ug/l.2.22 Acenaphthene 33 ug/l ug/l.2.31 Fluorene 33 ug/l ug/l.3.26 Phenanthrene 33 ug/l ug/l.8.8 Anthracene 33 ug/l ug/l.5.39 Fluoranthene 33 ug/l ug/l Pyrene 33 ug/l ug/l Benzo(a)anthracene 33 ug/l ug/l Chrysene 33 ug/l ug/l.12 1 Benzo(b)(k)fluoranthene 33 ug/l ug/l - - Benzo(a)pyrene 33 ug/l ug/l.15.7 Indeno(123cd)pyrene 33 ug/l ug/l.11.9 Dibenzo(ah)anthracene 22 ug/l ug/l.7.58 Benzo(ghi)perylene 33 ug/l ug/l.8.9 Table 3. Summary of event mean concentrations from this study and the DMRB 26. Analyses of the runoff waters showed that they contained significant levels of contaminants, which included suspended solids, total and dissolved heavy metals (cadmium, copper, lead and zinc), hydrocarbons including PAHs, total organic carbon, chlorides and phosphorous. The total heavy metal concentrations followed a pattern of Cd<Pb<Cu<Zn in magnitude with the dissolved phase following a similar trend. PAHs were not detected very often. Out of more than 2 events sampled only six of them had levels above the detection limit. The three dominant PAHs were found to be fluoranthene, phenanthrene and pyrene, which made up approximately 5% of the total PAH concentration. The highest concentrations of PAHs were Higgins et al. 5

6 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 detected for the two light 2-3 ring PAHs; naphthalene and phenanthrene, and three of the more toxic heavy 4-5 ring PAHs; fluoranthene, chrysene and pyrene. The lowest PAH concentrations detected were also a mixture of the light and heavy groups, acenaphthylene, fluorine, and indeno(123cd)pyrene respectively. The nature and concentrations of contaminants measured were broadly comparable with those reported from the UK study (DMRB, 26). In particular, there were similarities with a number of contaminant concentrations, mainly the heavy metals and PAHs. Those contaminant concentrations not similar were total suspended solids and total cadmium, which were detected at higher levels in the runoff from the highway sites in Ireland. The main reasons for these differences could be due to the location of the sites selected, which were on highways operating as major access routes to large construction sites. This would explain the high concentrations of suspended solids in the runoff that may have been associated with the construction vehicles accessing the site and subsequently washed-off in the storm event. Chloride concentrations were generally low at all sites and in comparison with the UK results. A plausible reason for this could be the low application rate of de-icers (NaCl) on Irish highways compared to a high application rate on similar highways in the UK 3.2 Analysis of the Hydraulics The filter drain system performed as an excellent attenuation mechanism and substantially reduced the peak flow compared to the kerb and gully system as displayed by the storm event hydrographs at each site in Figure 3. To investigate the overall hydraulic efficiency of each system the runoff coefficient, which is the ratio of precipitation falling onto the catchment and the resultant runoff, was calculated. The coefficient value may vary from site to site depending on a number of factors, which include the storm characteristics, vehicular splash off, the pre-storm history, the monitoring protocol, and the nature of the catchment, and the drainage system in place (Caltrans, 1999). Evaporation may also be a factor, but in such a temperate region as Ireland, this effect is likely to be minimal, particularly during the winter period. This is clear to see in the site results with the kerb and gully sites displaying similar runoff coefficients. The values are high at both sites with a mean value of.91 at site A and.78 at site B respectively. This was expected, as both sites had direct drainage with runoff entering the carrier pipe and discharging via the outfall directly to the surface water receptor. Runoff loss was minimal and may have been the result of depression storage on the highway surface and vehicular splash off. In contrast, the two sites with the filter drain system (C and D) had low runoff coefficients with a mean value of.48 at site D and.11 at site C respectively. The reason for this loss of water in the system (as evident with the low runoff coefficient) could be a catastrophic hydraulic failure of the filter drain system due to sediment transported in the runoff clogging the filter material, particularly where the liner is incorporated. These water balances (runoff coefficients) clearly suggest that the majority of the runoff is not getting to the designated surface water receptor via the carrier pipes. Therefore it can be concluded that a significant percentage of the highway runoff was lost in roadside percolation to the subsurface and into the underlying groundwater. As the groundwater is inadvertently the receptor of the highway runoff this raises the issue of groundwater contamination, as there would have been no assessment of any impact on the groundwater during the design stage of the highway. 6 Highway Runoff in Ireland and Management with a French Drain System

7 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 Site A (Kildare) May 21, 25 Rainfall Flow Flow (l/s) Rainfall (mm) 3 14:4 14:24 14:44 15:4 15:24 Time Site D (Maynooth) April 5,25 Rainfall Flow Flow (l/s) :3 23:1 :5 2:3 4:1 5:5 7:3 9:1 1:5 Time Rainfall (mm/5min) Figure 3. Hydrographs of two storm events sampled during Treatment Efficiency Comparisons were also made between the direct runoff at site A and the runoff via the filter drains at site C. Storm events, which occurred on the same date and had similar precipitation characteristics, were sampled and comparisons were made. Since the sites were in close proximity to each other the precipitation characteristics were similar in nature. Figure 4 presents bar charts of pollutant concentrations for each parameter at the two sites for the four storm events that occurred on the 21 st March, 17 th April, 3 rd May, and the 23 rd of July 25. It is clear from the bar charts that the concentrations of a number of the pollutants were significantly higher at site A with the kerb and gully drainage than at site C with the filter drain. The event mean concentrations of TSS were low at site C, with values ranging from 72 to 97% (ave.89%) less than that at site A. This would indicate that the filter drain is removing a significant load of suspended solids as the runoff passes through the filter material, particularly at the point were the liner is integrated into the design. The total phosphate values show a similar trend with values ranging from 7 to 9% (ave.85%) less than at site A. All the highest concentrations of total heavy metals were detected at site A. Values at the filter drain site C for cadmium ranged from 33 to 75% (ave.44%), copper from 79 to 93% (ave.84%), lead from 57 to 75% (ave.64%) and zinc from 84 to 95% (ave.91%) less than at site A. These results would indicate that the filter drain performed extremely well in pollutant removal efficiency for a number of parameters particularly for the suspended solids and heavy metals. Higgins et al. 7

8 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 TSS Concentrations: & Systems TP Concentrations: & Systems TSS EMC (mg/l) 12 8 TP EMC (mg/l) Mar.5 17.Apri.5 3.May.5 23.Jul.5 21.Mar.5 17.Apri.5 3.May.5 23.Jul.5 TZn Concentrations: & Systems TPb Concentrations: & Systems TZn EMC (mg/l) TPb EMC (mg/l) Mar.5 17.Apri.5 3.May.5 23.Jul.5 21.Mar.5 17.Apri.5 3.May.5 23.Jul.5 TCd Concentrations: & Systems TCu Concentrations: & Systems TCd EMC (mg/l).9.6 TCu EMC (mg/l) Mar.5 17.Apri.5 3.May.5 23.Jul.5 21.Mar.5 17.Apri.5 3.May.5 23.Jul.5 Figure 4. Event Mean Concentrations from the kerb & gully and filter drain systems. 4. CONCLUSIONS The results of this study show that pollutant concentrations in runoff from highways in Ireland are comparable to that observed in the United Kingdom for similar climatic and traffic conditions. Analysis of the current drainage practice indicated that the filter drain as a drainage/treatment system performed exceptionally well in pollutant removal efficiency for a number of parameters particularly suspended solids and heavy metals. The dominant process involved in the removal of pollutants appeared to be due to filtration of the solid fraction and associated pollutants. However, on sites studied with the filter drain, significant proportions 8 Highway Runoff in Ireland and Management with a French Drain System

9 11 th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 28 of the flow and the associated contaminant load from the road surface do not reach the intended receptor, surface water. Careful water balance calculations on each of the drainage systems investigated indicated that there are alternative, undocumented pathways taken by part of the runoff. Runoff coefficients particularly from the lined filter drain were as low as 11%. The implications of the difference in runoff coefficients are that a significant part of the highway runoff is being lost to roadside percolation to the subsurface and most likely into the underlying groundwater. This is an indication that the main receptor in the case of the filter drain system is groundwater and not the intended surface water body. As a number of the pollutant parameter concentrations in this investigation are above the drinking water standards, some form of risk analysis would be required to evaluate whether the system is suitable for a site. Future research should include further investigations into the unexplained losses of water from the filter drain system, with a view to determining if there are any implications for groundwater contamination. If the filter drain is to continue in use, a re-design is required to improve its efficiency in terms of drainage maintenance as well as treatment if it is specifically to assume that role. In particular, it is recommended that the role of the geotextile is re-examined. ACKNOWLEDGEMENTS This research was funded by the Irish Environmental Protection Agency (EPA) and the National Roads Authority (NRA) under the National Development Plan REFERENCES APHA (1998). Standard methods for the examination of water and waste water. Technical report, American Public Health Association, Washington DC, 2th. edition. Caltrans (1999). Storm Water Quality Handbook. Project Planning and Design Guide. Central Statistics Office (26). Measuring Ireland s Progress 26, Dublin, DMRB (1998). Design manual for roads and bridges: Hydraulic design of road edge surface water channels. Vol.4, section 2, part 4. Technical report, Highways Agency. DMRB (26). Design manual for roads and bridges: Water quality and drainage, volume 11, sec 3, part 1. Technical report, Highways Agency, UK. EU (2). Water Policy Framework Directive 2/6/EC, European Commission, December 2. NGSRW (25). Notes for Guidance on the Specification for Road Works. NRA (25). National Roads and Traffic Flow 25, National Roads Authority Preliminary Report, March 25. Higgins et al. 9