Aeolian sand strip mobility and protodune development on a drying beach: examining surface moisture and surface roughness patterns measured by terrestrial laser scanning

Abstract

Ephemeral aeolian sand strips are commonplace on beaches. Their formation during high energy sand transport events often precedes the development of protodunes and their dynamics present interesting feedback mechanisms with surface moisture patterns. However, due to their temporary nature, little is known of their formation, mobility or the specifics of their interaction with beach surface characteristics. Similarly surface moisture has an important influence on sediment availability and transport in aeolian beach systems, yet it is difficult to quantify accurately due to its inherent variability over both short spatial and temporal scales. Whilst soil moisture probes and remote sensing imagery techniques can quantify large changes well, their resolution over mainly dry sand, close to the aeolian transport threshold is not ideal, particularly where moisture gradients close to the surface are large. In this study we employed a terrestrial laser scanner to monitor beach surface moisture variability during a three and a half hour period after a rain event and investigated relationships between bedform development, surface roughness and surface moisture. Our results demonstrate that as the beach surface dries, sand transport increases, with sediment erosion occurring at the wet/dry surface boundary, and deposition further downwind. This dynamic structure, dependent upon changing surface moisture characteristics, results in the formation of a rippled sand strip and ultimately a protodune. Our findings highlight dynamic mobility relationships and confirm the need to consider transient bedforms and surface moisture across a variety of scales when measuring aeolian transport in beach settings. The terrestrial laser scanner provides a suitable apparatus with which to accomplish this.

Influence of meteorological conditions on aeolian processes along the Polish cliff coast

“…As the beach dries out, the distensions are dispersed by the wind. The same forms was observed by Nield (2011) and Rotnicka (2013b) on the beach of the Łeba Barrier, southern Baltic Sea coast, Poland.…”

The effect of short ground vegetation on terrestrial laser scans at a local scale

“…In the context of vegetated ground surfaces, the intensity may also be influenced by edge effects. Nield et al (2011) found that the moisture content of soils affected the intensity of the survey data acquired in a beach environment. Thus it is difficult to identify a general algorithm that can identify different types of materials based solely on the intensity recorded by the TLS.…”

Correction of Incidence Angle and Distance Effects on TLS Intensity Data Based on Reference Targets

“…Apart from discrete topography measurements, almost all current TLS instruments simultaneously measure the power of the backscattered laser signal of each point and record it as an intensity value [3]. The intensity recorded by current laser scanners, either in the form of the echo amplitude or full-waveform of the backscattered laser signal [4], can be utilized as complementary information along with the point cloud in a wide range of applications, especially in visualization, segmentation [5], classification [6][7][8][9][10], and multi-temporal analysis [11][12][13][14].…”

“…The intensity value is regarded as a significant source of spectral information associated with the surface properties, e.g., reflectance [1,[15][16][17][18], roughness [15,19], moisture [11,13,14,20,21], brightness [22][23][24][25] and grain size [26,27], of the scanned object. However, the original intensity data 1.…”

“…‚ Second, according to Equation (14) and Figure 4b, the intensity value of the 80% target at 1.54 m was interpolated between the intensity values at 1 m and 2 m. ‚ Finally, the intensity value of the 80% target at 7.6˝and 1.54 m according to Equation (20) was calculated as 1794; the measured intensity value of the 80% target at 7.6˝and 1.54 m was 1772. The measured and interpolated intensity values were approximately equivalent.…”