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Short-term geomorphic analysis in a disturbed fluvial environment by fusion of LiDAR, colour bathymetry and dGPS surveys

Moretto, J., Rigon, E., Mao, L., Delai, F., Picco, L., Lenzi, M.A.
Catena 2014 v.122 pp. 180-195
aerial photography, color, ecological restoration, ecology, engineering, global positioning systems, lidar, models, regression analysis, remote sensing, rivers, stream channels, surveys, topography, Italy
Estimating river's underwater bed elevations is a necessary but challenging task. The objective of this study is to develop a revised approach to generate accurate and detailed Digital Terrain Models (DTMs) of a river reach by merging LiDAR data for the dry area, with water depth indirectly derived from aerial imagery for wet areas.This approach was applied along three sub-reaches of the Brenta River (Italy) before and after two major flood events. A regression model relating water depth and intensity of the three colour bands derived from aerial photos, was implemented. More than 2400 in-channel depth calibration points were taken using a differential Global Positioning System (dGPS) along a wide range of underwater bed forms.The resulting DTMs closely matched the field-surveyed bed surface, and allowed to assess that a 10-year recurrence interval flood generated a predominance of erosion processes. Erosion dominated in the upper part of the study segment (−104,082m3), whereas a near-equilibrium is featured on the lower reach (−45,232m3). The DTMs allowed the detection of processes such as riffle–pool downstream migration, and the progressive scour of a pool located near a rip-rap.The presented approach provides an adequate topographical description of the river bed to explore channel adjustments due to flood events.Combining colour bathymetry and dGPS surveys proved to represent a useful tool for many fluvial engineering, ecology, and management purposes.The proposed approach represents a valuable tool for river topography description, river management, ecology and restoration purposes, when bathymetric data are not available.