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. Rapid modern technological advancements have led tosignificant improvements in river monitoring using unmanned aerial vehicles(UAVs), photogrammetric reconstruction software, and low-cost real-timekinematic Global Navigation Satellite System (RTK GNSS) equipment. UAVsallow for the collection of dry bathymetric data in environments that aredifficult to access. Low-cost RTK GNSS equipment facilitates accuratemeasurement of wet bathymetry when combined with subaqueous measuring toolssuch as acoustic Doppler current profilers (ADCPs). Hydraulic models may beconstructed from these data, which in turn can be used for variousapplications such as water management, forecasting, early warning anddisaster preparedness by responsible water authorities, and construction ofriver rating curves. We hypothesise that the reconstruction of dry terrainwith UAV-based photogrammetry combined with RTK GNSS equipment leadsto accurate geometries particularly fit for hydraulic understanding andsimulation models. This study sought to (1) compare open-source andcommercial photogrammetry packages to verify if water authorities with lowresource availability have the option to utilise open-source packageswithout significant compromise on accuracy; (2) assess the impact ofvariations in the number of ground control points (GCPs) and thedistribution of the GCP markers on the quality of digital elevation models(DEMs), with a particular emphasis on characteristics that impacthydraulics; and (3) investigate the impact of using reconstructions basedon different GCP numbers on conveyance and hydraulic slope. A novel methodwhich makes use of a simple RTK tie line along the water edge measured usinga low-cost but highly accurate GNSS is presented so as to correct theunwanted effect of lens distortion (“doming effect”) and enable theconcatenation of geometric data from different sources. Furthermore, wedescribe how merging of the dry and wet bathymetry can be achieved throughgridding based on linear interpolation. We tested our approach over asection of the Luangwa River in Zambia. Results indicate that theopen-source software photogrammetry package is capable of producing resultsthat are comparable to commercially available options. We determined thatGCPs are essential for vertical accuracy, but also that an increase in thenumber of GCPs above a limited number of five only moderately increases theaccuracy of results, provided the GCPs are well spaced in both the horizontaland vertical dimension. Furthermore, insignificant differences in hydraulicgeometries among the various cross sections are observed, corroborating thefact that a limited well-spaced set of GCPs is enough to establish ahydraulically sound reconstruction. However, it appeared necessary to makean additional observation of the hydraulic slope. A slope derived merelyfrom the UAV survey was shown to be prone to considerable errors caused bylens distortion. Combination of the photogrammetry results with the RTK GNSStie line was shown to be essential to correct the slope and made thereconstruction suitable for hydraulic model setup.

Evaluating low-cost topographic surveys for computations of conveyance