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

Abstract

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. 15We 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 20 observations up to 35 m, without being significantly affected by water turbidity, bedform or bed material.

Technical note: Bathymetry observations of inland water bodies using a tethered single-beam sonar controlled by an unmanned aerial vehicle

“…However, TLS suffers from similar limitations as lidar. Furthermore, the highly oblique scan angles of TLS make refraction effects more problematic (Woodget et al, 2015) and decrease returns from the bottom while increasing returns from the water surface (Bangen et al, 2014). Therefore, field surveys are normally performed using single-beam or multi-beam swath sonars transported on manned boats or more recently on unmanned vessels (e.g., Brown et al, 2010;Ferreira et al, 2009;Giordano et al, 2015) .…”

“…UAV measurements of water depth (i.e., elevation of the water surface above the bed) can enrich the set of available hydrological observations along with measurements of water surface elevation (WSE), i.e., elevation of the water surface above sea level (Bandini et al, 2017;Ridolfi and Manciola, 2018;Woodget et al, 2015), and surface water flow (Detert and Weitbrecht, 2015;Tauro et al, 2015aTauro et al, , 2016Virili et al, 2015).…”

“…The radar is an ARS 30X radar developed by Continental. The radar and GNSS systems are the same instrumentation as described in Bandini et al (2017), in which WSE was measured by subtracting the range measured by the radar (range between the UAV and the water surface) from the altitude observed by the GNSS instrumentation (i.e., altitude above ref- erence ellipsoid, convertible into altitude above geoid level). In this research, the radar and GNSS instrumentation are used to (i) retrieve WSE and (ii) observe the accurate position of the tethered sonar.…”

“…The accuracy of either GPS option is suboptimal for the generation of bathymetry maps; thus, more accurate measurements of the sonar position are necessary. The drone absolute position is accurately known through the differential GNSS system described in Bandini et al (2017). In order to estimate the relative position of the sonar with respect to the drone, the payload system measures the offset and orientation of the sonar.…”

“…Detailed bathymetry maps of inland water bodies are essential for simulating flow dynamics and forecasting flood hazard (Conner and Tonina, 2014;Gichamo et al, 2012;Schäppi et al, 2010), predicting sediment transport and streambed morphological evolution (Manley and Singer, 2008;Nitsche et al, 2007;Rovira et al, 2005;Snellen et al, 2011), and monitoring instream habitats (Brown and Blondel, 2009;Powers et al, 2015;Strayer et al, 2006;Walker and Alford, 2016). Whereas exposed floodplain areas can be directly monitored from aerial surveys, riverbed topography is not directly observable from airborne or space-borne methods (Alsdorf et al, 2007).…”