Evaluation of the geometry for remote river rating in hydrodynamic environments
Authors: Samboko Hubert T. , Schurer Sten , Savenije Hubert H. G. , Makurira Hodson , Banda Kawawa , Winsemius Hessel 
Published: July 2021

Abstract. Rapid modern technological advancements have led to significant improvements in river monitoring using Unmanned Aerial vehicles (UAVs). These UAVs allow for the collection of flow geometry data in environments that are difficult to access. Hydraulic models may be constructed from these data, which in turn can be used for various applications such as water management, forecasting, early warning and disaster preparedness by responsible water authorities, and construction of river rating curves. We hypothesize that the reconstruction combined with Real Time Kinematic Global Navigation Satellite System (RTK GNSS) equipment leads to accurate geometries particularly fit for hydraulic understanding and simulation models. This study sought to (1) compare open source and commercial photogrammetry packages to verify if water authorities with low resource availability have the option to utilise these without significant compromise on accuracy; (2) assess the impact of variations in the number of Ground Control Points (GCPs) and the distribution of the GCP markers on the quality of Digital Elevation Models (DEMs), with a particular emphasis on characteristics that impact on hydraulics; and (3) investigate the impact of variations in DEMs on flow estimations based on the number of GCPs used. We tested our approach over a section of the Luangwa River in Zambia. We compare performance of two different photogrammetry software packages, one being open-source and one commercial; then compare for one chosen package the performance with different GCP numbers and distributions, and finally, emphasize on the reconstruction of hydraulically important parameters. The first investigation (1) utilises the root mean square error (RMSE) method to determine if open source software performs as well as commercial software. The second task (2) aimed to assess the optimal GCP number and distribution; we generated 10 UAV based elevation models under varying GCP distribution conditions using OpenDroneMap (ODM) software. To benchmark the different DEM reconstructions we assessed the Mean Absolute Error of the elevation using the GCPs that were left out of the reconstruction. Finally (3), in order to investigate the impact of variations in DEMs on flow estimations we performed a comparison of the hydraulic conveyance across each reconstruction, as well as a comparison of the hydraulic slope against an independent estimate using an in-situ RTK GNSS tie line. Results indicate that the open-source software photogrammetry package is capable of producing results that are comparable to commercially available options. We determined that GCPs are essential for vertical accuracy, but also that an increase in the number of GCPs above a limited amount of 5 only moderately increases the accuracy of results, provided the GCPs are well spaced both in horizontal and vertical dimension. Furthermore, insignificant differences in hydraulic geometries among the various cross sections are observed, corroborating the fact that a limited well-spaced set of GCPs is enough to establish a hydraulically sound reconstruction. This is important so that future studies do not invest in procedures that may be costly, but may not contribute significantly to the improvement of desired results. The hydraulic slope was shown to be prone to errors caused by lens distortion. These errors are too large to enable use in a hydraulic model setup. We therefore recommend to combine photogrammetry results with a RTK GNSS tie line when reconstructions are to be used for hydraulic model setup.