Bathymetry observations of inland water bodies using a tethered single-beam sonar controlled by an Unmanned Aerial Vehicle

Abstract. High-quality bathymetric maps of inland water bodies are a common requirement for hydraulic engineering and hydrological science applications. Remote sensing methods, e.g. space-borne and airborne multispectral or LIDAR, have been developed to estimate water depth, but are ineffective for most inland water bodies, because of water turbidity and attenuation of electromagnetic radiation in water. Surveys conducted with boats equipped with sonars can retrieve accurate water depths, but are expensive, time-consuming, and are unsuitable for non-navigable water bodies. We develop and assess a novel approach to retrieve accurate and high resolution bathymetry maps. We measured accurate water depths using a tethered floating sonar controlled by an Unmanned Aerial Vehicle (UAV) in a Danish lake and in a few river cross sections. The developed technique combines the advantages of remote sensing techniques with the potential of bathymetric sonars. UAV surveys can be conducted also in non-navigable, inaccessible, or remote water bodies. The tethered sonar can measure bathymetry with an accuracy of ca. 2.1 % of the actual depth for observations up to 35 m, without being significantly affected by water turbidity, bedform or bed material.