Abstract
Surface moisture plays a key role in limiting the aeolian transport on sandy beaches. However, the existing measurement techniques cannot adequately characterize the spatial and temporal distribution of the beach surface moisture. In this study, a mobile terrestrial LiDAR (MTL) is demonstrated as a promising method to detect the beach surface moisture using a phase-based Z&F/Leica HDS6100 laser scanner mounted on an all-terrain vehicle. Firstly, two sets of indoor calibration experiments were conducted so as to comprehensively investigate the effect of distance, incidence angle and sand moisture contents on the backscattered intensity by means of sand samples with an average grain diameter of 0.12 mm. A moisture estimation model was developed which eliminated the effects of the incidence angle and distance (it only relates to the target surface reflectance). The experimental results reveal both the distance and incidence angle influencing the backscattered intensity of the sand samples. The standard error of the moisture model amounts to 2.0% moisture, which is considerably lower than the results of the photographic method. Moreover, a field measurement was conducted using the MTL system on a sandy beach in Belgium. The accuracy and robustness of the beach surface moisture derived from the MTL data was evaluated. The results show that the MTL is a highly suitable technique to accurately and robustly measure the surface moisture variations on a sandy beach with an ultra-high spatial resolution (centimeter-level) in a short time span (12 × 200 m per minute).
Highlights
The measurement of surface moisture on a beach is a fundamental component of field studies that seek to model the aeolian transport from the beach [1,2,3,4,5,6,7,8,9], or investigate the distribution of the beach groundwater [10,11,12,13]
A mobile terrestrial LiDAR (MTL) is demonstrated as a promising method to detect the beach surface moisture using a phase-based Z&F/Leica HDS6100 laser scanner mounted on an all-terrain vehicle
The results show that the MTL is a highly suitable technique to accurately and robustly measure the surface moisture variations on a sandy beach with an ultra-high spatial resolution in a short time span (12 × 200 m per minute)
Summary
The measurement of surface moisture on a beach is a fundamental component of field studies that seek to model the aeolian transport from the beach (which contributes to dune growth and recovery after erosion from storm-wave processes) [1,2,3,4,5,6,7,8,9], or investigate the distribution of the beach groundwater [10,11,12,13]. The techniques which have been used for measuring the beach surface moisture could generally be classified into three approaches: (a) soil moisture probes, (b) sample gravimetric method, (c) the optical remote sensing methods. Instead of using destructive/disturbing and time-consuming methods, the optical remote sensing method holds great promise with a faster and repeatable detection of the real surface moisture [1] This approach is based on the principle that wet sand darkens upon wetting because of a reduced reflectance and there are at least two different theoretical hypotheses explaining this phenomenon. Several studies employed the optical remote sensing techniques so as to calculate the beach surface moisture by relating the beach surface brightness derived from digital cameras to the surface moisture contents [9,15,26,27,28,29]. We start with a theoretical introduction on the manner in which the distance, incidence angles and sand moisture contents’ impact on the backscattered intensity should be quantified, followed by the indoor calibration experiments and data processing
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