Measurement of water content as a control of particle entrainment by wind

Abstract

Of all controls on particle transport by wind, which include texture, crusting, vegetation cover and roughness, the role of water content is one of the most difficult to parameterize because of its high degree of spatial and temporal variability and its operation at a particlescale level directly at the surface. This study demonstrates that measurement of the distribution of brightness for all pixels in an image, now routinely employed in digital photography, is strongly correlated with gravimetric water content. Wind tunnel experiments further suggest that measurement of the distribution of β β β β β, as normalized against the brightness of the dry sand surface, is very useful in determining the order of magnitude of the mass transport rate (q). Finer resolution will likely never be achieved because of the heterogeneity of the particle transport phenomenon. Analysis of the variability in surface brightness does suggest that q is governed by the partitioning of momentum to particle motion that terminates in adhesion to surrounding areas of the surface that remain relatively wet. The proportion of surface particles that becomes dry appears to be of less importance. Preliminary work suggests that field application of digital photography in tracking spatial and temporal changes in the water content of beach deposits looks promising.

Field Study of Beach Water Content as a Guide to Wind Erosion Potential

“…Despite a high degree of control in complementary wind tunnel simulations, order-of-magnitude variations in q under constant flow conditions were similarly observed for damp surfaces in laboratory work by MCKENNA NEUMAN and LANGSTON (2006). The authors attribute this outcome to…”

“…Sediment transport by wind is highly intermittent under such circumstances, and the net amount also is most uncertain, similar to transport conditions I-IV identified for coastal environments by JACKSON and NORDSTROM (1997). BAUER and DAVIDSON-ARNOTT (2002), and MCKENNA NEUMAN and LANGSTON (2006) comment that researchers would do well to identify the correct order of magnitude of q for these damp conditions. An important boundary condition for this aeolian transport system is established when the water table drops below about 400-500 mm, as identified in this study and also in earlier work by YANG and DAVIDSON-ARNOTT (2005).…”

“…While there is good agreement on the spatial extent of this zone between all three isopleth maps, direct measurements of w indicate higher pore water contents (26% Ͻ w Ͻ 34%, Figure 7B) than those inferred from the image brightness values (26% Ͻ w Ͻ 28%, Figure 7A). From previous calibrations in both lab and field settings, MCKENNA NEUMAN and LANGSTON (2006) also have suggested that the surface brightness becomes less sensitive to water contents exceeding about 15%. Finally, Figures 7A and 7B display very similar generalized patterns for pore water content distribution on the back beach (30 m Ͻ y Ͻ 60 m), but they are not well matched with that of the groundwater table depth where 200 mm Ͻ ␦ Ͻ 350 mm ( Figure 7C).…”

Aeolian sediment transport on a beach: Surface moisture, wind fetch, and mean transport

“…The maximum rate of transport, however, depends on the mean wind speed, the sediment grain size, and the condition of the sand surface. Most notably on beaches, it is widely appreciated that moisture content can play an important role in controlling the dynamics of sediment transport by increasing the sediment entrainment threshold (via increased inter-particle cohesion) and thereby reducing the overall rate of transport (e.g., Logie, 1982;Hotta et al, 1984;McKenna Neuman and Nickling, 1989;Namikas and Sherman, 1995;Nordstrom, 1997, 1998;McKenna Neuman and Maljaars, 1998;Cornelis and Gabriels, 2003;Wiggs et al, 2004a,b;Davidson-Arnott et al, 2005;McKenna Neuman and Langston, 2006). The limited empirical evidence that is available also suggests that the critical fetch distance increases with increasing moisture content of the surface (Davidson-Arnott and Dawson, 2001), so the maximum rate of transport is less on a wet beach, and the Fig.…”

Sediment Mobilisation by the Wind

“…16 Critical shear velocity for sediment entrainment as a function of sand moisture content (fromSherman, 1990).present(McKenna-Neuman and Langston, 2006;Darke and McKenna-Neuman, 2008; Darke, Robin and Oller- head, 2009).…”