3.3 Research tools and methods
3.3.2 Wash-off sample collection
Pollutant wash-off primarily varies with the rainfall characteristics (Egodawatta, et al., 2007). Investigating the variability of wash-off using naturally occurring rainfall events is difficult due to uncertainties associated with occurrence and rainfall parameters (Egodawatta, 2007). The use of simulated rainfall events can overcome the practical issues relating to these uncertainties and enable effective control of the range of the variables influencing wash-off.
a. Rainfall simulator
For this study, the rainfall simulator designed by Herngren (2005) was used to generate rainfall events for predetermined rainfall intensities. Figure 3-3 shows the schematic diagram of the rainfall simulator. It consists of an A-frame structure made of 40 mm diameter aluminium tubing and stainless steel 32 mm diameter nozzle boom connected longitudinally at a height of 2.4 m. Three nozzles (Veejet 80100) spaced at 1m apart are embedded in the nozzle boom. Water input pressure is designed to be 41 kPa in order to maintain natural rainfall characteristics such as raindrop size distribution, impact velocity and kinetic energy through simulated rainfall. More details can be found in Herngren (2005).
Figure 3-3 Rainfall simulator (Adopted from Herngren (2005))
As continuous spray through the nozzles produce high rainfall intensities, the simulator is designed to generate intermittent rainfall by oscillating the nozzle boom using a small motor. The different rainfall intensities can be produced by controlling the cycle time of the nozzle boom by a control system, which can alter the speed and
delay time of oscillations. Different settings of speed and cycle time were used to simulate different rainfall intensities and the appropriate speed and delay settings for this study were obtained by undertaking a calibration procedure.
b. Calibration and uniformity testing of the rainfall simulator
The rainfall simulator was calibrated prior to the field experiments. This was to assign speed and cycle time settings for the control box for simulating the required rainfall intensities. The overall range of the control box was 3 to 20 seconds of cycle time under two speeds settings.
A procedure similar to that adopted by Herngren (2005) and Loch et al. (2001) was used for the calibration of the rainfall simulator. The procedure adopted in this study is outlined below.
Twelve containers were placed under the rainfall simulator as shown in Figure 3-4.
Rainfall simulator was operated for 5 min for each speed and delay settings of the control box. The complete sets of control box settings are available in Table A1, Appendix A.
The volume of water accumulated in each container during a 5 min simulation was measured.
Rainfall intensity for each control box setting was calculated using Eq. 3.1. Outcomes from the rainfall simulator calibration exercise are available in Table A1, Appendix A.
𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 (𝑚𝑚ℎ𝑟) =𝑉 (𝑚𝑚3)×60(
𝐴 (𝑚𝑚2)×5𝑚𝑖𝑛 Eq. 3.1
where V is the volume of water collected in the container and A is the surface area of the container
According to the test results given in Table A1, Appendix A, the maximum rainfall that can be produced by the rainfall simulator is 83 mm/hr.
Figure 3-4 Rainfall simulator calibration and uniformity of testing
Once the simulator was calibrated for its intensities, the spatial variability of rainfall intensity over the plot surface was also estimated. For this purpose, a uniformity coefficient was determined using the data obtained from the calibration as given in Eq. 3.2.
𝑈𝑛𝑖𝑓𝑜𝑟𝑚𝑖𝑡𝑦 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (%) = [1 − |∑ 𝑥|
𝑚 × 𝑛] × 100 Eq. 3.2
where 𝑥 is the deviation of an individual observation from mean, n is the number of observations and m is the mean intensity.
A higher uniformity coefficient indicates little spatial variability across the plot. According to the test results given in Table A1, Appendix A, uniformity coefficients for the complete range of simulated intensities were in the range of 77-85%. Egodawatta (2007) and Herngren (2005) also observed the similar range of uniformity coefficients for intensities that were simulated using the rainfall simulator. Accordingly, it can be concluded that a rainfall simulator has the capability to simulate predetermined intensities with little spatial variability over the sampling plot.
c. Rainfall event selection for pollutant wash-off sample collection
Four different rainfall intensities and their most frequent durations were selected for rainfall simulations. Events were selected based on a statistical analysis of measured rainfall events. The statistical analysis was conducted using rainfall data for the years, 1999, 2004 and 2005. These three years were selected after comparing the long-term trends in rainfall records. Based on the long-term rainfall records, three selected years represented, above average, average and below average total rainfall depths. For the analysis, all the events for the three representative years were separated and maximum 6, 12, 18, 24, 30, 36, 42, 60 min rainfall intensities (greater than 10mm/hrs.) and their frequency of occurrence were extracted. Frequency distribution is plotted in Figure 3-5. The selected intensities are marked in Figure 3- 5.
Figure 3-5 Frequency of occurrence of the rainfall events
The rainfall events selected for the wash-off study were also based on the capability of the rainfall simulator in re-producing rainfall intensities. The selected rainfall events are shown in Table 3-3.
0 5 10 15 20 25 30 0 25 50 75 100 125 150 175 200 225 250 Fre q u en cy o f Occu rre n ce Rainfall Intensity (mm/hr) max 6 max 12 max 18 max 24 max 30 max 36 max 42 max 60 Selected Intensities
Table 3-3 Selected rainfall intensities and durations
Rainfall Intensity (mm/hr)
Rainfall duration (min)
Event 1 Event 2 Event 3 Event 4 Event 5 Event 6
83 6 12 18 24 - -
64 6 12 18 24 - -
38 6 12 18 24 30 36
25 6 12 18 24 30 36
Design rainfall intensities for 6, 12, 18, 24, 30 and 36 min durations
1 year ARI (mm/hr) 104 80.3 67.1 58.1 51.1 47.6
d. Protocol for wash-off sample collection in the field
The rainfall simulator was required to be set up at the selected road site for wash-off sample collection as illustrated in Figure 3-6. Wash-off was collected from a 3 m2 surface area similar to that of build-up collection. For that, a 2.0 m x 1.5 m plot boundary was demarcated on the road surface using a plastic frame (Figure 3-6). It was sealed using gutter tape and silicon sealant to avoid water escaping through the boundary. The downstream end of the plot was kept open to fix the catch tray (collection trough) which has a capacity of 30 L, to temporarily capture runoff during rainfall simulations. The catch tray was also fixed to the road surface and sealed using silicon so the runoff generated within the plot boundary flows into it. Collected water in the tray was immediately transferred into 25 L polyethylene containers with the use of the vacuum cleaner as shown in Figure 3-7.
Selected four rainfall intensities were simulated in four different plots at each selected road site. For particular rainfall intensity, the simulator was operated continuously and samples were collected at discrete intervals. For example, at 83 mm/hr intensity, the first wash-off sample was collected after a duration of 6 min, the second wash-off sample was collected at 6-12 min and so on. The collected samples were labelled and transported to the laboratory on the same day of collection.
Figure 3-6 Rainfall simulator set-up for wash-off sample collection
Figure 3-7 Wash-off collection into the polyethylene container from catch tray