Steven L. Namikas

A Review of the Effects of Surface Moisture Content on Aeolian Sand Transport

Over the past several decades, a number of studies have shown that intergranular cohesion associated with the presence of moisture significantly increases the critical shear velocity required to initiate motion in sand grains, and decreases transport rates. This paper examines currently available models of moisture effects and compares model predictions for several hypothetical situations. Model predictions exhibit considerable disagreement regarding the magnitude of moisture effects. For 0.27-mm sands, predicted increases in threshold shear velocity associated with a 1% moisture content ranged from about 8% to 148% of the expected dry threshold velocity, and with 4% moisture increased to 47%–206% of the dry value. Based on the predicted threshold shear velocities, the expected transport rates at a 1% moisture content under a 0.50 m s−1 shear velocity range from no transport to more than 100 kg m−1 hr−1.

Design and field test of a continuously weighing, tipping‐bucket assembly for aeolian sand traps

A new tipping-bucket assembly (T-BASS) for aeolian sand traps has been designed and field tested with encouraging results. It facilitates high-frequency monitoring of sediment flux over extensive time periods, and therefore offers improved performance over other continuously weighing mechanisms that are generally limited to either a small total-load capacity or poor resolving ability. The T-BASS is a modified version of the tipping-bucket meteorological rain gauge mounted on a cantilever and pulley system linked to an electronic load cell. As trapped sand accumulates in one of the buckets, the increasing mass exerts a downward force on the cantilever arm, which translates into a slight deflection of the thin-beam element of the load cell. The resulting voltage output is proportional to the load, and the analogue signal may be monitored by a data-acquisition system. Eventually the bucket fills to capacity and tips, the sediment load is emptied into a reservoir container, the other bucket of the bucket pair is positioned beneath the funnel, and the system is automatically reset to zero load for continued measurement. In this way, a high-frequency record of sediment accumulation is obtained. Field testing of five prototypes demonstrated that the T-BASS can be configured to yield: (i) linear calibrations (for conversion of voltage to gram weight) with R2 values exceeding 0·99; (ii) weight resolution of 0·99; (ii) weight resolution of 0·1 g or better depending on load-cell specifications and bucket capacity; and (iii) detailed temporal information (order of 1 s) on sediment flux allowing investigation of its relation to attributes of the wind field. Suggested modifications may produce improved performance in future versions. Copyright

Predicting Aeolian Sand Transport: Revisiting the White Model

The derivation and history of the frequently cited aeolian transport model of White are considered in light of the continued replication of an error in the original expression. The error may have escaped notice because the expression is still dimensionally correct and it yields predictions that appear reasonable in comparison with both the predictions of other models with field data. The incorrect expression has come to be identified as a distinct model. However, the correct formulation of the ‘White model’ is, in fact, a rearrangement of the Kawamura model with a slightly smaller (c.6%) empirical coefficient.

Utility of the Delta-T Theta Probe for Obtaining Surface Moisture Measurements from Beaches

Abstract Recent studies have employed a new device to measure beach “surface” moisture content, the Delta-T Theta probe. A key weakness of the device for this application is that the sensor length (6.0 cm) exceeds the desirable depth for “surface” measurements in the context of coastal-aeolian processes. This study investigated the reliability of the Delta-T Theta probe when modified to restrict measurement depths to 0.5–1.5 cm. Field investigations were conducted at two beaches in Texas and North Carolina to allow assessment of the influence of different sediment sizes. Results demonstrated that sensor output becomes less sensitive as the sensor length is decreased. However, R2 values reveal very strong relationships between probe output and laboratory-measured moisture content, with virtually all sampling runs exceeding 0.90. Further, although the standard error approximately doubled (from ±1% to ±2%) for the modified versions of the probe, the error remained within the accuracy ranges reported in the literature and did not appear to consistently increase as the sensor length was shortened. Grain size was found to have no consistent influence on sensor performance. Comparisons of multiple runs and multiple probes indicated that it is possible to achieve high levels of repeatability both between runs and between probes, but several instances of significant departures were identified that suggest caution with this application and that other environmental parameters may influence results.

AEOLUS II : an interactive program for the simulation of aeolian sedimentation

Abstract A computer program has been developed to simulate two- or three-dimensional topographic change associated with aeolian sediment transport. Because predictions generated by available models of aeolian transport can vary by about an order of magnitude for a given set of environmental conditions, selection of an appropriate model is problematic. This program includes a number of models and provides for interactive selection of a desired model combination(s), to allow for comparison between model predictions. Topography, represented by a set of coordinates in two or three dimensions, is defined by ‘bins’ of sediment. Differential rates of transport between adjacent bins, resulting from variations in grain size, slope, and surface moisture content, are used to determine mass flux and to adjust the topography in an iterative fashion. Sub-routines allow the simulation of temporally varying levels of surface moisture resulting from evaporation, and periodic restoration of upwind boundary conditions. The wind field can be represented by a constant shear velocity, by a spatial (cross-shore) distribution of shear velocities, or by a time series of values representing multiple wind events. Output is in the form of a graphic display or tabulated results written to disk. Example results are presented which (qualitatively) suggest that simulated topographic changes are reasonable. Areas where further refinement is needed to improve correspondence with reality are identified and discussed.

Comparison of Surface Moisture Measurements with Depth-Integrated Moisture Measurements on a Fine-Grained Beach

ABSTRACT Edwards, B.L.; Schmutz, P.P., and Namikas, S.L., 2013. Comparison of surface moisture measurements with depth-integrated moisture measurements on a fine-grained beach. Surface moisture measurement on beaches is an important component of beach groundwater and aeolian transport studies. There are several approaches to measuring beach surface moisture, but each has significant limitations. Several recent studies have used techniques that integrate moisture content over shallow depths, and this study aims to assess the utility of these measurements for characterizing moisture conditions at the sediment surface, and also to briefly comment on the usefulness of a handheld spectroradiometer for measuring beach surface moisture. Depth-integrated moisture measurements of the top 1.5 and 6 cm of sediment obtained with a time domain reflectometry probe were compared with collocated surface moisture measurements from the spectroradiometer. Results show that depth-integrated measurements overestimated actual surface moisture by an average of 2.5 and 4.4% moisture content for the 1.5- and 6-cm sampling depths, respectively. The maximum difference between surface moisture and depth-integrated moisture content was about 12% for the 6-cm depth and about 9% for the 1.5-cm depth. These results suggest that moisture measurements integrated over even shallow depths may not depict conditions at the surface accurately enough for some applications and may potentially provide a misleading description of moisture conditions at the surface. The spectroradiometer proved to be a useful method of measuring beach surface moisture (field calibration with an R2  =  0.99 and standard error of ±1.5% moisture content), but from a logistical standpoint, may not be well suited to measuring and mapping surface moisture over large areas.