Monitoring of avulsion channel evolution and river morphology changes using UAV photogrammetry: Case study of the gravel bed Ondava River in Outer Western Carpathians

This paper presents results from monitoring the chute cutoff in the meander bend of the Ondava River in Eastern Slovakia. An avulsion channel was formed in the central part of the meander neck during 2010 flood events, and here we describe the mechanism of evolution and post‐cutoff avulsion channel adjustment using drones, unmanned aerial vehicle (UAV) photogrammetry and field survey. Monitoring by UAV images began on 15 June 2012 with 78 processed images, followed by 259 images during April 2014 and 375 from 18 July 2014. The majority of UAV digital elevation model (DEM) errors in the study area were associated with vegetation cover, with the average vertical root mean square error (RMSE) of the UAV‐DEM on bare ground at 0.209 m compared with 0.673 m in vegetated areas. The chute cutoff was formed by floodplain headcutting during meander neck overflow and headcut migration was directed by floodplain sediment structure and land use. Although low river discharge after the 2010 floods stabilised the avulsion channel by vegetation succession, recurrent two‐yearly interval flooding increased the avulsion channel bank erosion from 36.9 m3/month (June 2012–April 2014) to 425.6 m3/month (April 2014–July 2014). We describe the mechanism of evolution and post‐cutoff avulsion channel adjustment using drones, UAV photogrammetry and field survey. We estimated erosion/deposition areas due to channel erosion. The chute cutoff was formed by floodplain headcutting during meander neck overflow, and headcut migration was directed by floodplain sediment structure and land use.