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

Abstract

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 in-stream habitat, hydraulic resistance, 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, 1099 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 then coupled with hydraulic measurements. 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.

Keywords:

• Aquatic vegetation
• UAV
• drone
• macrophyte
• aerial imagery
• orthophoto
• image analysis
• stream power
• hydraulic control
• vegetation management
• surveys
• ADCP
• piezometer
• ecohydraulics

Acknowledgments

The authors would like to thank Matthew O’Hare, Jochen Aberle, Alexander Sukhodolov and Bernhard Statzner for valuable discussions and advice during the project. The authors would also like to thank Alasdair Matheson and the Scottish Environment Protection Agency (SEPA) for providing discharge and river stage data.

Disclosure statement

There are no known conflicts of interest related to the work.

Additional information

Funding

The work was part of the research project ‘Hydrodynamic Transport in Ecologically Critical Heterogeneous interfaces’ (HYTECH), the support of which, under the European Union’s Seventh Framework Programme (Marie Curie FP7- PEOPLE-2012-ITN, European Commission [grant agreement number 316546]), is gratefully acknowledged. The work was also partially funded by the National Institute of Water and Atmospheric Research (NIWA) under the Sustainable Water Allocation Research Programme [project CDPD1706], the support of which is gratefully acknowledged.