Grain size estimation in fluvial gravel bars using uncrewed aerial vehicles: A comparison between methods based on imagery and topography

Abstract

Grain size assessments are necessary for understanding the various geomorphological, hydrological and ecological processes that occur within rivers. Recent research has shown that the application of Structure‐from‐Motion (SfM) photogrammetry to imagery from uncrewed aerial vehicles (UAVs) shows promise for rapidly characterising grain sizes along rivers in comparison to traditional field‐based methods. Here, we evaluated the applicability of different methods for estimating grain sizes in gravel bars along a study reach in the Olentangy River in Columbus, Ohio. We collected imagery of these gravel bars with a UAV and processed those images with SfM photogrammetry software to produce three‐dimensional point clouds and orthomosaics. Our evaluation compared statistical models calibrated on topographic roughness, which was computed from the point clouds, and to those based on image texture, which was computed from the orthomosaics. Our results showed that statistical models calibrated on image texture were more accurate than those based on topographic roughness. This might be because of site‐specific patterns of grain size, shape and imbrication. Such patterns would have complicated the detection of topographic signatures associated with individual grains. Our work illustrates that UAV‐SfM approaches show potential to be used as an accessible method for characterising surface grain sizes along rivers at higher spatial and temporal resolutions than those provided by traditional methods.

Gravel replenishment downstream dams: insights from a flume experiment on morphological effects in a straight embanked channel

“…Recently, SfM photogrammetry with a Unmanned Aerial Vehicle (UAV) platform has been increasingly used for river geomorphic studies . Not only it is cost-effective , but also both topographic data and orthoimages collected from UAV surveys allow to calculate the sediment budget (erosion and deposition flux) and to retrieve spatial information of river bed roughness (if precision allows) Wong et al, 2024). This latter is one of the primary controls on sediment transport and is also essential to estimate the quality of substrate for aquatic habitat after SR restoration .…”