The role of water content in wildfire ash mobilization by wind erosion under laboratory conditions

The Portuguese ASHMOB project (CENTRO-01-0145-FEDER-029351) is addressing the overall lack of knowledge on the mobilization of wildfire ash with time since fire. More specifically, ASHMOB is looking into the lateral transport of wildfire ash by water as well as wind erosion. ASHMOB involves a combined measurement-modeling approach comprising five phases: (i) wildfire ash collection and characterization; (ii) wind tunnel experiments of ash mobilization by wind erosion; (iii) hydraulic laboratory experiments of ash mobilization by rainfall splash, run-on and their combination; (iv) adjusting selected wind and water erosion models to accommodate erosion of wildfire ash; (v) validating the adjusted models by field measurements in a recently burnt area.  The current presentation concerns the second phase and, in particular, wind tunnel experiments aiming to assess the influence of water content on the post-fire ash mobilization. The experiments were performed in the wind tunnel of the Atmosphere Aerodynamics Laboratory of Aveiro University’s Department of Environment and Planning, which is an open-circuit wind tunnel with 13 m long and a test section of 6.5x1.5x1.0 m (LxWxH), often used for physical modelling of urban flows and air quality. For these experiments, wildfire ash was used from three different land cover types, i.e. Maritime Pine and Eucalypt forest plantations and Strawberry tree woodlands (or, more concretely, woodland patches). These ashes were collected as soon as possible after three wildfires that occurred during the summer of 2019 in central Portugal, typically within 2 weeks. The pine, strawberry tree and eucalypt ashes were collected following wildfires in july in Vila de Rei, august in Fátima and september in Albergaria-a-Velha, respectively. The experiments involved one specific ash load of 10 mm, based on findings of earlier experiment with varying ash loads, and ash-to-water ratio of 1:0, 1:0.25, 1:0.5, 1:0.75, 1:1, 1:1.5 and 1:2 (w_ash/w_water) at the start of the experiments. Ash mobilization with stepwise increasing wind speeds up till 9 m.s^-1 was measured continuously using a measurement scale linked to a computer and was also filmed from above using a Go-Pro video camera. For each combination of ash type and ash-to-water ratio, 5 replicate experiments were run. Preliminary analysis of the obtained results revealed a clear role of water content in the mobilization by wind of all three types of wildfire ash, with marked increased shear velocity with increasing ash-to-water ratio. At the same time, shear velocity also differed markedly between the woodland types, with consistently lower shear velocity for pine ash than for eucalypt as well as strawberry tree ash at the different ash-to-water ratios.