NOISE BARRIERS CERC 1. INTRODUCTION

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1 Augut 217 P33/1B/17 NOISE BARRIERS CERC In thi document ADMS refer to ADMS-Road 4.1, ADMS-Urban 4.1 and ADMS-Airport 4.1. Where information refer to a ubet of the lited model, the model name i given in full. 1. INTRODUCTION The effect of noie barrier on air quality cloe to road i of interet, particularly in mainland Europe. Invetigation uing wind tunnel and diperion model how that ground level concentration are reduced immediately downtream of a noie barrier, due to the fact that emiion are effectively elevated by the barrier. In ADMS, thi reduction in ground level concentration ha been repreented by modelling a road with a noie barrier a an elevated road ource. The model i able to take into account noie barrier on either or both ide of a road, parallel to the road. Noie barrier are alo modelled in the TNO traffic model ued in the Netherland, and reult from the TNO model (hown for example in [1]), are conitent with thoe from ADMS. The relationhip between the height of the noie barrier and the increae in elevation of the modelled road ource i given in Section 2. Detail of model retriction and aumption are alo given in thi ection. The Appendix detail the methodology ued to derive the relationhip between the height of the noie barrier and the increae in road elevation. Page 1 of 6

2 2. CALCULATION OF THE ELEVATED ROAD SOURCE HEIGHT The relationhip between the noie barrier height, ource, ẑ, i where road z ˆ NB H NB, and the height of the elevated road A H z (1) _ road z _ i the height at which the line repreenting the road without a noie barrier i modelled, and A i a contant. Recall from the road ource Technical Specification (P31/1) that z _ i given by road z z, (2) _ road h where h 1 m i the initial mixing height and z i the road height a entered by the uer. 2.1 Region of influence When ADMS model a road ource, the ource i divided up into a number of wind-aligned egment. For each egment, a conervative region of influence i defined outide which the ource ha a negligible effect (for further detail pleae refer to the Technical Specification, P25/3). Source egment Region of influence of tripped ource egment WIND USE NON BARRIER CONCENTRATION USE BARRIER CONCENTRATION Road Barrier Figure 1 Diagram howing where the barrier and non-barrier concentration are ued downtream of a road ource egment Figure 1 how a diagram of a road ource, with the wind coming from the left of the figure. A noie barrier i located to the right of the road. The road ource i divided up into four wind-aligned ource egment, and the region of influence of one of the egment i hown. Between the road and the noie barrier, the unadjuted concentration value are ued, and P33/1B/17 Page 2 of 6

3 downwind of the barrier, the concentration due to the elevated road ource i ued, in order to repreent the effect of the barrier on concentration. 2.2 Aumption and retriction The main retriction of thi module i that adjutment to concentration are only made outide the noie barrier, when clearly the preence of the barrier will affect concentration within the road. To alo conider concentration within the road, where there are noie barrier of imilar height on both ide of the road, the road hould be treated a a baic treet canyon, refer to Technical pecification document P28/1. Alternatively, for a more advanced treatment, epecially in cae where the barrier height are aymmetric, the advanced treet canyon module can be ued in place of the noie barrier and baic canyon module. The advanced treet canyon module calculate the effect of noie barrier on concentration both inide and outide the canyon, pleae refer to Technical Specification document P28/2 for further detail on the advanced treet canyon module. Aumption made during the derivation of the algorithm given by equation (1) above include: Only neutral meteorological condition were conidered. The emiion wa conidered to be at ambient temperature. Barrier height of up to 1 m were invetigated. 3. REFERENCES [1] Weeling, J. and Vier, G. (23) Meauring and modelling of the A13 motorway near Overchie. Arena Magazine, no.8. P33/1B/17 Page 3 of 6

4 APPENDIX DERIVATION OF THE ELEVATED ROAD SOURCE HEIGHT ALGORITHM Thi Appendix decribe the method of invetigation ued to derive the algorithm given by (1), which relate a given noie barrier height to the height of the correponding elevated road that can be ued to repreent the decreae in concentration downwind of the barrier. The indutrial ource model ADMS 3 wa ued for thi invetigation in order to be able to model the effect of explicitly defined building on concentration. Note that: A road ource are not modelled in ADMS 3, thi invetigation wa conducted uing a line ource (for detail of the difference between line ource and road ource in ADMS pleae refer to P31/1). The relative difference between concentration due to line and road ource wa invetigated to enure that it wa correct to apply the derived methodology for road ource (ee Section A.2 below). Non-point ource cannot be modelled with building in ADMS 3. However, a line ource can be accurately repreented by a erie of point ource; if ufficient point ource are modelled, the difference between modelled concentration i negligible. The invetigation decribed are effectively two-dimenional i.e. a building relatively long in the crowind direction wa modelled and end effect were not conidered. The along-wind olution at the building centre ha been applied more generally in three dimenion to road, which are modelled a crowind ource egment. Barrier edge effect on concentration are therefore neglected. A.1 Repreenting the barrier in ADMS 3 Figure 2 how a vertical cro ection of the ADMS 3 model et up. The crowind width of the noie barrier i 5 m and the along wind width i.2 m. Barrier height of up to 1 m and road centreline to barrier ditance of up to 4 m are conidered, the latter accounting for road width of up to 8 m. Figure 3 how an example et of reult for a ource-barrier ditance of 1 m. Here, the thick black line give the concentration in the abence of a barrier, and reult for barrier height of 2 m, 3 m, 4 m, 6 m, 8 m and 1 m are alo hown. A would be expected, a the barrier height increae, the initial concentration downtream of the barrier decreae, due to the fact that the ource i effectively elevated. For the lower barrier height (2 m and 3 m), the downtream concentration are predicted to exceed the concentration in the abence of a barrier, wherea for barrier height 4-1 m, the concentration initially remain lower. Source height h = 1 m Source Barrier Road centreline to barrier ditance up to 4 m Noie barrier height up to 1 m P33/1B/17 Page 4 of 6

5 Concentration (µg/m³) Figure 2 Vertical cro ection of the ADMS 3 model et up When a building i modelled, the flow field i divided into different region (for further detail of the variou region, pleae refer to Technical Specification paper P16/1). One very apparent part of the flow field i the cavity region immediately downtream of the building. Due to thi region, there i a light dicontinuity in the concentration gradient downtream of a building, and thi ha been indicated in Figure 3 for each of the cae where the barrier ha been modelled. The length of the cavity region i related to the barrier height, with the cavity region being longer for higher barrier No barrier 2m barrier 3m barrier 4m barrier 6m barrier 8m barrier 1m barrier End of cavity region Ditance downtream of barrier (m) Figure 3 Concentration downwind of a barrier a modelled in ADMS 3, for ourcebarrier ditance 1 m and barrier height 2 m, 3 m, 4 m, 6 m, 8 m and 1 m; end of cavity region indicated by arrow; concentration in the abence of a barrier alo hown. A model attempting to repreent the behaviour of the concentration decay downtream of a noie barrier hould, in the limit of the noie barrier height tending to zero, tend to the profile given in the abence of a barrier. For thi reaon, it wa decided to ignore the behaviour predicted by the low level barrier at 2 m and 3 m, and the unphyical dicontinuity in concentration gradient at the edge of the cavity region. By conideration of the firt downwind maximum concentration hown in Figure 3, it i poible to approximate the concentration downwind of a noie barrier by an elevated line ource uing the expreion given by equation (1) above. Full detail are omitted in thi document, but Figure 4 compare the line ource approximation to the explicitly modelled barrier, for barrier height 4m, 6m, 8m and 1m and a road centerline to barrier ditance of 1m. Note that the vertical cale on thi plot ha been magnified to how more detail than in Figure 3. Concluion of the full invetigation include: P33/1B/17 Page 5 of 6

6 Concentration (µg/m³) the approximation how good agreement cloe to the barrier, in particular for the lower barrier height, and reult for horter road centreline to barrier ditance (le than 2 m ay) are better than for larger ditance (2-4 m, ay) No barrier 4m barrier 6m barrier 8m barrier 1m barrier Approx to 4m barrier Approx to 6m barrier Approx to 8m barrier Approx to 1m barrier Ditance downtream of barrier (m) Figure 4 Comparion of line ource approximation to concentration downtream of a barrier in ADMS 3, for ource-barrier ditance 1 m and barrier height 4 m, 6 m, 8 m and 1 m; concentration in the abence of a barrier alo hown. A.2 Extenion to from line ource in ADMS 3 to road ource A outlined in document P31/1, ADMS model road a line ource, with: a minimum height of h 1 m, and an increaed vertical plume pread. It i important to check if the ource height predicted by (1) give imilar concentration for the line and road ource, relative to the maximum concentration in the abence of a barrier for both ource type. Maximum concentration downtream of a road ource are both lower and cloer to the ource than for a line ource of the ame emiion and elevation. Thi i a expected due to the increaed vertical plume pread for road ource. However, maximum concentration for elevated ource relative to the maximum concentration of a ource at a height of h for road ource were found to be within 1% of the ame ratio for line ource, o the expreion relating barrier height and ource elevation derived uing ADMS 3 line ource i valid for modelling road ource in ADMS. P33/1B/17 Page 6 of 6