Exploring short-term geomorphic changes and bank retreat evolution in fluvial systems

The geomorphological effectiveness of flood events may highly vary depending on several factors, including discharge, duration, frequency, ordering, environmental conditions, human interference, etc. Precise rates and patterns of change produced by flooding remain largely unknown and unpredictable and multiple sources of error undermine the achievement of accurate estimates of geomorphic variations. Focusing on channel processes, the complex control exerted by riparian vegetation at multiple scales and, in particular, on riverbank dynamics further registers a lack of understanding that may lead to possible misleading approaches to manage fluvial erosion issues. This study primarily seeks to analyse and interpret the geomorphological response of two differently-impacted gravel-bed rivers (Piave and Tagliamento rivers, Italy) to flood events slightly below bankfull discharge. Furthermore, a focus on bank erosion processes on an Australian stream characterized by extensive tree abutments and cohesive scalloped riverbanks (the King River, Victoria) will try to explain the control exerted by riparian vegetation on the evolution of bank retreat. New findings and relationships will help to create a more comprehensive conceptual model describing bank migration past forested riverbanks that will be further tested on the Piave River, offering the possibility of comparing fluvial systems with very different environmental conditions (primarily, cohesive versus non-cohesive sediment). For the detection of geomorphic variations, enhanced DoD models including a precise evaluation of flowing channels, are derived by differencing pre- and post-flood integrated DEMs (LiDAR, dGPS and colour bathymetry). The reliable estimates of change have contributed to effectively quantify planimetric and volumetric adjustments of main channels and major bank removal. On the other hand, bank migration processes were examined on the King River by surveying the characteristics of trees and bank profile spaces to uncover significant relationships explaining the retreat of vegetated riverbanks. Results on geomorphic impacts of floods have confirmed a prevalence of erosion processes in the study reaches of the Piave and Tagliamento rivers, even with a general tendency toward sediment equilibrium. Flooding has caused a nearly total reconfiguration of the main flowing channel network, contributing to erode considerable areas of stably vegetated banks. The analysis on bank migration processes has further found elements and relationships previously unconsidered in bank retreat modelling. Major findings on the King River include the proof that riparian trees mostly grow on the bank face as well as the identification of more complex patterns of hydrological erosion around riparian trees (overcutting and undercutting) suggesting tree toppling by fluvial scour than by mass failure. Further, the progression of scallop depth along riverbanks has confirmed to be fairly influenced by tree spacing, both on the King and the Piave rivers. Discussions debating the impacts of floods have highlighted the ability of events below bankfull discharge to effectively rework the geomorphological structure of river channels at all levels. River changes appear to be also influenced by the physical characteristics of river reaches and human interventions. Moreover, the new findings on bank migration have allowed to fill some gaps of knowledge related to the complex effect of trees on riverbank erosion, leading to propose a more comprehensive conceptual model describing bank retreat evolution in forested cohesive riverbanks. Further parallelisms between King and Piave rivers have demonstrated that scallop depth features similar average magnitudes in both fluvial systems, even though the average effect of tree spacing within individual morphological units (bend- or arc-level) explains better erosion development in the Piave River. The significance of the study lies on the attempt of providing a reliable approach to meaningfully evaluate river geomorphic changes, revealing the high potential of floods also below bankfull discharge to revolutionize channel forms and patterns. The enhanced understanding of crucial channel processes as the reconfiguration of main branches and the control of riparian vegetation on bank retreat can further represent a decisive help to river experts within the complicated issue of managing fluvial systems.