Rapid Topographic Mapping Using Terrestrial Laser Scanning and Uav Photogrammetry: Case Study at Bryan Beach in Freeport, Texas

Coastal regions are generally vulnerable to impact from long-term coastal erosion and episodic coastal hazards caused by extreme weather events. Predictions of storm impact, coastal planning and resilience observations after natural events require high-accuracy and high-resolution coastal morphologic maps. Rapid topographic mapping is of critical importance for long-term coastal management and for quick response after natural disasters. Terrestrial Laser Scanning (TLS) techniques have been frequently applied to beach and dune erosion studies and post-hazard responses; however, TLS surveying is relatively slow and costly for rapid surveying. Furthermore, TLS surveying unavoidably retains gray areas that cannot be reached by laser pulses, particularly in areas where there is a lack of direct access. Aerial images taken by Unmanned Aerial Vehicles (UAVs) enable the mapping of coastal features quickly, safely, inexpensively, on short notice, and with minimal environmental impact. This study aims to develop an efficient method for acquiring highly accurate Digital Elevation Models (DEMs) applied in a beach and dune area by using the UAV Structure from Motion (SfM) photogrammetry technique. A workflow for conducting rapid and high-accuracy UAV photogrammetry survey in beach and dune environments was developed in this study. Two repeated UAV surveys were conducted within a 1-km long, 300-m wide beach-dune belt in Freeport in April and September 2018. DEMs derived from TLS and UAV datasets were used as the basic survey products for assessing the accuracy of UAV surveys. Our study showed the elevation accuracy of UAV photogrammetry varies from approximately 5 cm in the beach and front dune area to approximately 30 cm in the vegetated back dune area. Significant beach erosion was observed by comparing the DEMs derived from UAV photogrammetry datasets collected in April and September 2018. The shoreline retreated approximately 2.5 m on average during those five months. However, the vegetation line retreated over 3.5 m on average because of severe sand loss in the front dune area. The total volume of sand loss was approximately 12,000 m3 over the 1-km long beach and dune belt.