Trapping of Sand-Sized Particles Exterior and Interior to Large Porous Roughness Forms in the Atmospheric Surface Layer

Six same-sized, double-walled, porous cubes constructed of plastic mesh material of different porosity ε were deployed at a field site where they interacted with wind-driven saltation to evaluate their relative potential for sand sequestration (internal and external). The internal mass collected and externally deposited mass and lengths demonstrate that, for large three-dimensional porous forms with both well-defined geometric shape and dimensional properties of the permeable walls, sequestration of saltating sand is largely controlled by the characteristic three-dimensional permeability K ′ and the hydraulic diameter H d of the wall material and not simply the value of ε of the walls of the forms. These two properties collapse the relationships for the particle-sequestration effectiveness [i.e., the (internal) trapping efficiency, normalized (external) deposit length, and normalized (external) deposit mass] for the five forms with geometrically-similar square/rhomboid-shaped pores. The form with rounded-rectangular holes and very thin walls does not correspond to the same relationship, suggesting that pore geometry plays a key role in the magnitude of the amount of sand sequestered, as the data from the form with the differently-shaped pores are consistent outliers compared with the other five forms with similarly-shaped pores. This is due to the physical properties of the form, the pore shape, and the shape of the solid material around the pore, as the change in flow speed between the exterior and interior scales continually as a function of permeability K ′, with no apparent effect related to the pore geometry.