Large Scale Morphological Changes Due To Flash Floods In Small Streams Under Climate Change Scenarios

Morphological change in rivers is a dynamic and complex phenomenon affected by several environmental conditions and hydraulic processes, which are mainly related to the composition and the susceptibility of erosion of the river channel and watershed. While erosion occurs constantly at low rates most of the time, high flow events such as flash floods can lead to a severe increase in erosion and sedimentation rates, which can have negative effects on transportation infrastructures, residential areas and even the efficiency of hydropower projects. With the future projections of climate change showing an increase in the frequency of such events, a good understanding is important in assessing the impact they will have in already existing and planned riverside uses.

However, investigating sediment rates is a difficult task both in the field and through numerical modelling. Especially in small streams, quick events characterized by extreme flow, can produce a significant portion of the annual sediment load in the matter of a few days or hours. Satellite and drone data or acoustic/optical devices provide scarce observations and pointwise measurements respectively, thus lacking the time and spatial resolution necessary to capture the overall dynamics of the river. The development of 2D and 3D numerical models would also prove as a computationally demanding task when applied to larger scale areas such as a river reach and long simulation periods (i.e., tens of kilometers and decades). Utilizing a well-documented 1D model is a viable option due to the accessibility and low computational demands.

For these reasons, the objective of this study will be establishing a 1D sediment transport model through HEC-RAS, relying on evidences from case studies prone to hydrological quick events. The calibration procedure is deterministic parameter testing, such as sediment transport functions and cohesive factors, with the aim of reconstructing the sediment input and deposition based on the existing bathymetries and measured suspended sediment concentrations during floods.

The expected results would be assessing the sediment quantities during flood events, and the impact on the river morphology in the long term. Running various climate change scenarios, such as SSPs (Shared Socioeconomic Pathways) will provide the uncertainties of river morphology changes in the future, burden with increasing floods frequency,