Detection of aeolian sand strips and their characteristics using terrestrial laser scanning

Abstract

Sea level rise causes more difficulties for coastal maintenance. Although hard structures were built in the past to prevent for flooding, nature based solutions are preferred more recently. For the application of nature based solutions, a better understanding of the coastal processes is desired. Dry sand is transported towards the dunes by aeolian processes, whose bedform development is understudied while knowledge on this is important. One of the most visually clear bedforms are organised dry sand over a moist beach, referred to as sand strips. In order to improve the knowledge on sand strips, this study focusses on sand strip properties and occurring environmental conditions during their presence at the Noordwijk beach, using data of a terrestrial laser scanning (TLS) device.

Sand strips are detected with the Fourier transform. Since surface moisture can be derived from the reflectance intensity of the TLS-data, and due to the different moisture content of the sand strips compared to the surrounding beach, the Fourier transform is applied on the reflectance intensity. Sand strips are detected based on the energy in the variance density spectrum for a wavenumber-range corresponding to sand strips. The detected sand strips were oriented alongshore to oblique-alongshore with a mean wavelength and height of 13.2 m and 4.0 cm respectively, which is in correspondence with similar sand strip-related studies.

According to sand samples of the beach the grain size varies in transverse direction of the sand strips, comparable with the grain size variation of aeolian sand ripples. The coarser grains were located at the crest and the finer grains at the lee-side. Additionally, the samples also showed a significant difference in gravimetric moisture content between the sand strips and surrounding beach, as expected due to the reflectance-based detection. At the sand strips, the maximum moisture content was 5.3%, while the minimum determined moisture content at the surrounding beach was 6.0%. The mean values were equal to 2.6% and 9.4% for the sand strips and surrounding beach respectively.

In addition, sand strips mainly occurred during (almost) alongshore wind events with a wind velocity in excess of 8 m/s. However, the threshold wind velocity for sand strip formation is determined at 10 m/s. Due to the significant height difference that can remain present during precipitation events, these events are not necessarily restrictive factors for sand strip development, although the reflectance intensity suggests different. Furthermore, sand strips mostly formed during falling tide and they were mostly destroyed during rising tide.

The one life cycle of the sand strips that is analysed showed dynamic sand strip behaviour. The migration rate of the sand strips varied over the width of the beach, causing more inland oriented sand strips. This dynamic behaviour cannot be related to weather conditions since they remained constant, however it could be related to topographic steering caused by the dune. Nevertheless, the results of the dynamic properties are only indicative and encourage further study of dynamic sand strip properties.