Dust Control Project: Sand Fence Array

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1 Dust Control Project: Sand Fence Array Sand Fences: Design Considerations 20 rows (not all continuous), approximately 800 feet long ( 15 acres), 4 foot high, composed of plastic mesh material of 50% porosity, spaced 10 fence heights apart. No theoretical or empirical relationships were known that could be used to design a priori a sand fence array to meet sand/dust flux reduction targets. Based on known aerodynamics of wind flow past single porous fences and some assumptions on the effect on sand transport it suggested that sand flux interior to the array would be 50% of that upwind and external to the array. Conservatively estimated that dust flux would be reduced by 50% from within the control area.

2 Sand Fence: Results A reduction of normalized sand flux (NSF=sand flux interior to the array/upwind sand flux exterior to the array) is observed with increasing distance into the array, which extends to approximately row 10. For NDD 92.5 (112 m), the measured mean sand flux appears to stabilize.

3 The sand flux after row 10 is, on average, 56% of that measured upwind and exterior to the sand fences (excluding days of very low transport and likely a sand trap collection efficiency issue) Mean Normalized Sand Flux, NSF for NDD Mean NSF for NDD 92.5 for the available sampling periods based on the BSNE trap measurements. Pink squares are days with low mass in the exterior and interior traps.

4 Sand Fence: Results For the available data when PM10 measured at the upwind position was greater than 100 µg/m 3, indicating the sand surface was actively emitting dust due to saltation, the mean difference in the PM10 between the upwind and downwind positions was 32% (±7%). This suggests that for the sand fence array, which reduced sand flux by 56%, across 50% of the surface, reduced the PM10 concentration measured at the reference height of the instrument (3 m a.g.l.) by a minimum of 32%. The actual control efficiency was likely greater than this as even no change in the PM10 concentration would indicate that the control measure was reducing the input of PM to the airstream.

5 Dust Control Project: Straw Bale Arrays Sand Fences: Design Considerations The straw bale control area had 189 bales per acre (spaced center-to-center and row-to-row at 4.8 m), which should provide a reduction in sand flux of 50% compared to the flux in the absence of that roughness (Gillies and Lancaster, 2013). The empirical relationship of Gillies and Lancaster (2013), it should be noted, was derived from measurements made on relatively flat topography, so the effect of the ODSVRA dune topography was unknown.

6 Straw Bale: Results The NSF for each BSNE trap, data are normalized to the trap furthest to the west and upwind of the first straw bale array, is shown below. Mean Normalized Sand Flux, NSF West B35 B36 B37 B38 B39 B40 B41 B42 B43 B44 B45 B46 B47 B48 B49 B50 B51 B52 B53 B54 B55 B56 B57 B58 B59 B60 B61 B62 B63 B64 BSNE Number East NSF at each BSNE sand trap along a west (left side) east (right side) gradient through the straw bales. Brown colored bars indicate the BSNEs at each leading edge of the five areas with straw bales. B35 B36 B37 B38 B39 B40 B41 B42 B43 B44 B45 B46 B47 B48 B49 B50 B51 B52 B53

7 Straw Bale: Results There are several patterns in the sand flux data that are noteworthy. Once inside the arrays no measured sand flux is greater than 57% of the flux upwind and exterior to the array. The mean NSF for all measurements inside the straw bale array is 0.11 (±0.10), which suggest a greater degree of sand flux reduction than expected. The pattern in sand flux reduction from west to east is unlike the pattern observed for similar measurements made on relatively flat or gently undulating surfaces.

8 Straw Bale: Results Sand appears to have been eroded from the lower portions of the upwind side of the dunes and deposited near the crests, buying the straw bales there.

9 Straw Bale: Results The mean percent change in the measured PM10 concentration for cases where the mean hourly wind speed is 4 m/s (a conservative value for the threshold wind speed for sand transport in the ODSVRA) was 60% less than the upwind concentration. PM10 range for this wind speed was µg/m 3. The mean percent change when mean wind speed is 5 m/s, was 15% (lower), suggesting a rapid loss in control efficiency for dust emissions.

10 2014 Dust Control Project: Conclusions The APCD and DRI have reviewed the e-bam PM10 data set for the upwind and downwind positions at the sand fence and straw bale arrays and agree that a demonstrable reduction in PM10 occurs from upwind to downwind over the length of the straw bale arrays. Sand fence: 32% less Straw Bales 60% less for low winds, 15% for moderate to higher winds.

11 2014 Dust Control Project: Conclusions It is difficult to reconcile whether there was an observable affect on the PM10 concentration measured at CDF. This could be due to: 1) the reduction of PM10 from the control areas being insufficient due to the sand transport not being reduced to an appropriate, 2) the size of the areas controlled was insufficient, and 3) the location of the areas being controlled was not optimized so the effectiveness of the reduction was not observed at CDF. The information gained from the 2014 study and the 2013 PM10 and Wind Speed Monitoring efforts are being used to guide the development of a dust control project for 2015, with the major goals being to: 1) increase the sand flux reduction in the areas designated for control, and 2) select the area for control to maximize the potential for reducing the contributions from areas in the ODSVRA, to CDF based on our current understanding of emissivity, wind speed and direction patterns, and the dispersion of PM10 from these areas to CDF.