sUAS, SfM-MVS photogrammetry and a topographic algorithm method to quantify the volume of sediments retained in check-dams

Sediments retained in hydrological correction check dams are a useful source of information to understand soil erosion rates and sediment fluxes. Two hundred sixty nine check dams distributed in an area of 239 ha in SW Spain accumulated sediments over a period of 11 to 23 years. The aim of this work is to estimate the volume of sediments deposited in those check dams and to study the spatial variability of the accumulated sediments and its relationships with different environmental variables. The methodology included five steps: 1) flying the study area with a fixed-wing Unmanned Aerial Vehicle to capture high-resolution aerial photographs, 2) Structure-from-Motion photogrammetry, 3) processing the obtained point clouds and Digital Elevation Models (DEMs) to create the current topography and model the past soil surface, 4) estimating the volume of sediments behind each check dam using a topographic technique and 5) exploring the relationship between sediments and different environmental variables. A total of 269 check dams were identified, from which 160 were suitable to quantify the deposited sediment volume. The volume of sediments trapped by individual check-dams ranged from 0 to 108.35 m³, with an average deposition rate of 0.141 m³ ha^-1 y^-1. The 77% of the check dams retained <1 m³ of sediment. The topographic position and the size of the dam wall played a fundamental role in explaining the differences of total sediment accumulation as well as the deposition rates. Deposition rate was negatively correlated with drainage area, connectivity index, stream power index, topographic wetness index, upstream channel length and the number of upstream check dams. Conversely, deposition rate was positively correlated with the slope of the channel. Those dams located in valley bottoms with longer walls retained more sediment, while those of hillslopes with shorter check dam walls were ineffective.