University of Twente Student Theses


LiDAR measurements of swash-event bed dynamics : investigation of LiDAR application to monitor beach evolution at the inter-swash scale

Mūrnieks, A. (2022) LiDAR measurements of swash-event bed dynamics : investigation of LiDAR application to monitor beach evolution at the inter-swash scale.

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Abstract:The swash zone is the dynamic interface between the land and ocean that is alternately covered and exposed by onshore- and offshore-directed water movement. The challenge associated with the swash zone is the ability to obtain reliable measurements of its bed dynamics. As the result, the knowledge about the processes occurring in the swash zone is limited. To overcome the challenge, it is proposed to make use of LiDAR (light detection and ranging), which is an optical remote sensing technology that makes use of laser beams to detect the distance between LiDAR and an object or surface. The benefit of LiDAR use is its ability to replace point measurement systems and make spatially and temporally dense measurements without interfering with the observed environment. The objective of this research is to make use of available raw LiDAR data gathered in large-scale wave flume experiments and to examine if it is possible to quantify the inter-swash morphodynamical contribution of individual swash events to the longer-term, event-averaged morphological evolution of the sandy beach. In the validation part of this research, the LiDAR measurements along the horizontal and vertical axis were compared to other measurement devices. It was found that LiDAR error along the horizontal axis for the dry bed is below ~5mm margin, while if there are objects present on the slopes, the error can increase by one order of magnitude. Along the y-axis, not all the vertical positions of the obtained beach profile measurements were within the allowed error margin. The main cause for differences along the y-axis could be caused by measuring two separate locations or the inherent error of the mechanical measurement devices. The LiDAR error along the vertical axis of the static beach slope did not exceed ~5mm. Subsequently, the LiDAR data on the water level was separated from the bed level of the beach. In this case, three different methods were compared: varying variance, decreased variance and relative signal strength index. Both, the variance and decreased variance methods provided similar results. The extracted water lines were capable of following the water column that is higher than ~20mm. The method with relative strength index was capable of better capturing the swash lens’s temporal variability than the variance methods. However, it was concluded that for the calculation of bed level changes the water line determined with the varying variance method, derived from data in the validation section, provided the best match to the researchers’ observations of maximum run-up, average run-up, maximum run-down and relative elevation change image in time. Finally, the observed bed level changes between consecutive profiles were quantified. In this case in total, 165 swash events with the majority of events being short than 11s were identified. Out of 165 events, 77 accretive and 88 erosive events were observed. While the majority of all these events caused bed level changes below 0.05m2, the smaller scale events did not contribute to the net morphological change of the beach slope. The events that did contribute the most to the net morphological change of the slope were larger than 0.05m2. In addition, the largest event was determined to induce bed level change which was 50% of the net morphological change over a ~30min period. Additionally, to the bed level changes along the full length of the swash zone, the point measurements were also examined. At the point of maximum accretion and erosion during 30min period, the bed level changes at these points in inter swash scale most of the time were below 2mm. Furthermore, the correlation between the length of the swash event and the change in the swash zone was examined. It was concluded, that there is no correlation between the two latter variables. This research implies that although LiDAR is capable of monitoring the morphological evolution of the swash zone in the period of 30min, the device's error is too large to deterministically analyse the bed level changes at the inter-swash scale. This occurs, because at the inter-swash scale, most of the changes that occur are below the LiDAR error margin (<~5mm).
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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