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

•Physical and Empirical relationship between channel depth and colour intensity•Factors that increase uncertainty of the final DTM•Hybrid DTMs (HDTMs) at high resolution and low uncertainty•Morphological processes in a regulated gravel-bed river after two several floods

ObjectiveEstimating 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.MethodsThis 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.ResultsThe 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 (− 104,082 m3), whereas a near-equilibrium is featured on the lower reach (− 45,232 m3). 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.ConclusionThe presented approach provides an adequate topographical description of the river bed to explore channel adjustments due to flood events.PracticeCombining colour bathymetry and dGPS surveys proved to represent a useful tool for many fluvial engineering, ecology, and management purposes.ImplicationsThe proposed approach represents a valuable tool for river topography description, river management, ecology and restoration purposes, when bathymetric data are not available.