Quantifying geomorphic change in a partially restored gully using multitemporal UAV surveys and monitoring discharge and sediment production

Gully erosion in valley bottoms is a frequent process with negative consequences in the landscape. The development of new techniques and instruments allows the study of gullied channels with high spatial and temporal resolution. Here we present a detailed study of a valley bottom gully that was restored in 2017 with check dams. The channel is located in an experimental catchment, which is equipped with sensors to continuously monitor rainfall, discharge and suspended sediment concentration. The objectives of this work are (1) to analyze the effectiveness of operations carried out in the channel and (2) to elucidate the role of the former operations on the hydrological and sedimentological spatial and temporal dynamic. The methodology included the following steps: 1) field survey with a fixed-wing UAV to capture high-resolution aerial photographs and a GNSS to provide Ground Control Points (GCPs), 2) Structure-from-Motion photogrammetry to produce multi-temporal point clouds, DEMs and orthophotographs, 3) estimating topographic changes and 4) analyzing the relationship between rainfall and discharge events, sediment load and topographic changes. A spatially variable threshold was produced using a Fuzzy Inference System and considering different sources of errors. For the period 2017-2019 (i.e. after restoration activities), the gully showed a positive balance indicating accumulation of sediments (40.2 m) and hence a good performance of the restoration measures. The sediment load was reduced after dams installation, while runoff was not modified.