The effects of surface moisture on aeolian sediment transport threshold and mass flux on a beach

Abstract

This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta-T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two-dimensional sonic anemometer. Transport intensity was measured at a height of 2-4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10-20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1-2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance.

The application of terrestrial laser scanning to aeolian saltation cloud measurement and its response to changing surface moisture

“…Previous field experiments have captured some of the spatial and temporal characteristics of this transport mechanism using saltation impact sensors at single or multiple points (Baas and Sherman, 2005;Baas and Sherman, 2006;Baas, 2008), whilst wind tunnel work has elucidated characteristic surface controls on saltation dynamics (McKenna Neuman and Scott, 1998;Bauer et al, 2004;McKenna Neuman, 2004), but there is a need to concurrently determine both surface characteristics and saltation metrics over larger spatial scales through time. Surface moisture is a key controlling factor in transport dynamics as it increases the shear velocity threshold required to entrain sediment (Sarre, 1988;Namikas and Sherman, 1995;Wiggs et al, 2004b;Davidson-Arnott et al, 2005;Davidson-Arnott et al, 2008;Bauer et al, 2009;Davidson-Arnott and Bauer, 2009). However, moisture can also facilitate the formation of a hard surface, inducing elastic rebound of particles which results in an increase in saltation height and efficiency (McKenna Neuman and Scott, 1998).…”

Dune Growth Due to Aeolian Sediment Transport and the Role of the Beach and Intertidal Zone

“…Differences in erodibility (or sediment supply) in cross shore direction can however exist due to variability in surface moisture content (Davidson-Arnott et al, 2008) but also due to lag deposits (Carter, 1976). Since surface moisture, which can limit sediment supply, is large at the water line, it can be expected that most erosion of the beach will occur from the (dry) upper beach.…”

Backshore coarsening processes triggered by wave‐induced sand transport: the critical role of storm events

“…This transport depends on the sand grain size (Bagnold, 1941;Kawamura, 1951;Owen, 1964), sand moisture content (Belly, 1964;Hotta et al, 1984;van Dijk et al, 1996;Cornelis and Gabriels, 2003), beach slope (Iversen and Rasmussen, 1999;Lancaster et al, 2002;Lamb et al, 2008), and thickness of beach grass (Raupach, 1991;Lancaster and Baas, 1998;Udo and Takewaka, 2007). However, backshore processes are caused by both wave-and wind-induced sand transport (Short and Hesp, 1982;Haxel and Holman, 2004;Ruz and MeurFerec, 2004;Udo and Yamawaki, 2007;Davidson-Arnott et al, 2008;. Therefore, it follows that the change in shoreline position with swash and water level is a crucial factor for the description of backshore processes.…”