Exploring the morphological development of embryo dunes in Terschelling using unmanned aerial vehicle imagery

By Bram Schipper (the Netherlands)

Abstract
Dunes act as a flexible natural flood defence and protect low-lying areas from flooding by the sea. Due to climate change more and more man-made measures such as sand nourishments are needed to counteract coastal erosion and maintain safety of the inland. However, these measures are not always the best solution from a financial and environmental perspective. Adaptive measures like building with nature concepts can possibly help maintain and develop the coast by for example stimulating new dune formations. Natural dune development in coastal zones is the result of an interaction between vegetation and different dynamic processes influencing sedimentation and erosion. A lot of research has been done on processes and factors influencing sand supply to the dunes. However, how the development of embryo dunes is influenced by different spatial factors and dune characteristics and its relative individual impact is less known. This study explored how geomorphological development of embryo dunes can be monitored by using imagery acquired from unmanned aerial vehicles (UAV) and analysed the main drivers influencing this development. An experimental dune field called ‘Windwerk’ located on the Wadden island Terschelling in the Netherlands was monitored between May 2016 and June 2017 with six observations from a UAV equipped with a digital camera system. Windwerk consist of 32 unique dune fields where embryo dunes (young dune formations) are naturally created by planting grasses on the beach with all a different amount of vegetation and spatial design. This makes the Windwerk experiment an excellent opportunity to study different aspects and causal relations of different main drivers that are influencing early dune development. For every dataset a digital surface model (DSM) and orthomosaic was reconstructed with a 0.05m resolution by applying a structure from motion method. From this DSM, a digital terrain model (DTM) was derived where after a dune model was extracted by using a slope-based filtering technique. Based on the final dune models, different explanatory factors were related to changes between the dune fields over summer, winter and a full year. The applied method of this study was very useful for reconstructing 3D dune models from aerial UAV imagery to quantify dune volume/size changes over time. The results indicate that short-term changes are mainly expressed by the temporal sand shadows behind vegetation, whereas long-term development is expressed by accumulation of sand in between individual neighbouring dunes. During one year an overall positive change in dune volume can be found for most dune fields. However, some exposed dune fields on the edge of the study area did not develop. Summer development was mainly determined by the amount of planted grass plots and sheltering by other fields, whereas winter development was influenced by the beach elevation and terrain roughness. The applied methodology of this study can be used in comparable studies using UAV’s to monitor dunes and morphological development. The results of this study can be valuable for coastal management strategies and can support future decision making in the design and implementation of vegetation as a tool for adaptive coastal defences.

Keywords: Unmanned Aerial Vehicle (UAV); Structure from Motion; DTM extraction; embryo dune; embryo dune development; geomorphology; Windwerk; building with nature