Coupling Unmanned Aerial Vehicle (UAV) and hydraulic surveys to study the geometry and spatial distribution of aquatic macrophytes

Aquatic macrophytes are a key component of river systems around the world. Surveys of macrophyte surface cover, cross-sectional blockage and plant/patch sizes provide data for river managers to assess instream habitat, hydraulic resistance, sediment dynamics, and the success of stream restorations. Manual surveying techniques are labour intensive, provide low spatial detail and are predominantly applied at the cross-section scale, resulting in a lack of published data on macrophyte size distributions. In this study, 1,099 Ranunculus penicillatus patches were surveyed using a UAV-mounted digital camera. Geometric properties such as patch area, length, aspect ratio and orientation were determined from the aerial orthophotos. These data were coupled with hydraulic measurements collected using an acoustic Doppler current profiler and a piezometer device. Macrophyte abundance corresponded to specific ranges of velocity, Froude number and stream power, indicating clear patterns of hydraulic habitat use (and preferential modification) by Ranunculus. At the reach scale, flow redirection around dense vegetation clusters was observed, with implications for localised sedimentation and bank erosion. The reported data can improve the design of laboratory experiments to represent Ranunculus characteristics in the field. The aerial surveying techniques can be used to efficiently estimate vegetation abundance, surface area blockage factor and also to visualise flow through patch mosaics, enabling targeted management of aquatic vegetation.