Mapping sub-annual beach growth using terrestrial laser scanning

Abstract

This thesis presents a case study on beach growth of a South Holland beach located slightly north of the Sand Motor over the course of six months, which was measured using a terrestrial laser scanner (TLS). This device was set up to continuously take hourly full coverage measurements of a one kilometre stretch of beach from a hotel rooftop.

As sea-levels rise, interest in the morphological processes that take place on beaches is growing, so that coastal safety can be continued to be guaranteed in the future. As a result, it becomes increasingly relevant to understand the transport of sediment towards the beach. Existing studies on the subject focus on timescales of years to decades, often making use of GPS measurements. However, no thorough research has been performed on sub-annual timescales in over a decade, leading to the following main research question for this thesis: How is beach volume growth distributed on sub-annual time scale, both in spatial and temporal dimensions?

To validate the data obtained by the TLS, an accuracy check was performed which proved the standard deviation of the measurements to be much smaller than the observed morphological change. A rotational instability of the scanning device was discovered and corrected, however a higher measurement accuracy could be obtained by developing a more detailed correction method. The applied correction method did however no longer allow for the study of smaller fluxes such as aeolian transport, as they are overruled by it. It was investigated how the raw 3D data obtained from the TLS should be processed to obtain a clean timeseries of cross-sections. A framework is presented that includes noise detection and removal, object filtering, interpolation and subsampling. Subsequently, timeseries of 132 cross-sections were extracted from the data by selecting a daily low tide scan for 132 days along 4 different transects.

The resulting timeseries clearly display morphological activity such as intertidal bar migration and storm erosion, and volumetric computations have displayed periods of beach growth. These periods generally occur between storms, during calm wind and wave conditions. The main driver for this growth is the onshore migration of intertidal bars. As bars enter the intertidal zone, they migrate onshore and grow, increasing the volume of the beach. A swash bar that formed high in the intertidal zone during neap tide was found to migrate at increased rate during the neap-spring tidal cycle and welded to the beach, as compared to a different bar which migrated during the springneap cycle. Following spring tide, the bar ceased onshore migration and an offshore expansion occurred. This offshore expansion had a great effect on the volumetric growth of the total beach profile and showcases the influence of tide in the migration of swash bars. However, due to the great number of factors that influence beach growth, only few significant correlations were found between beach volume changes and boundary conditions such as tide, and wave and wind forcing.

Over the entire research period, only limited growth of the beach has occurred (2.6 m3 over all transects). Periods of growth (up to 20 m3 in under a month) were followed by storms, which eroded the gained volume. No general linear trend in growth was observed, indicating the dominance of variability over trend on the regarded time scale. This result contradicts findings of studies that use monthly or yearly data. When regarding daily data for several months, the beach volume is very much influenced by bar migration and storm erosion which lead to much more variation in the volumetric signal.