Assessing the effectiveness of “River Morphodynamic Corridors” for flood hazard mapping

Abstract

In dynamic rivers the assessment of flood hazard related to geomorphological dynamics assumes crucial importance. Recently, some geomorphological approaches have been developed to assess channel dynamics in response to floods. In this study we explore the River Morphodynamic Corridors which include the current active channel and areas of the alluvial plain that may be (re)activated by channel dynamics during floods. In order to assess the effectiveness of such corridors for mapping flood hazard, we delineated the morphodynamic corridors along a river network of 74 km in the Cordevole River catchment (Italy) and compared the corridors with the channel changes that were triggered by a high-magnitude event (Vaia Storm) that hit the study area in October 2018. We observed that the morphodynamic corridors are capable to define areas where channel dynamics are most likely to occur and have proven satisfactorily effective in predicting channel widenings triggered by a severe flood. The results show that it is crucial to adopt different procedures for corridor delineation based on channel width, i.e., for small streams (width < 30 m) a more precautionary approach should be adopted that considers the entire alluvial plain as an area potentially affected by channel dynamics. The study highlighted an inherent limitation of the approach, since erosion that can affect valley slopes or fluvial terraces (i.e., widening of the alluvial plain) is not taken into account. This application represents the first validation of the River Morphodynamic Corridors approach through comparison with observed flood channel changes. Through this analysis, it can be inferred that, notwithstanding its relatively simple procedure, the approach allows a robust mapping and delineation of flood hazard due to river channel dynamics. The River Morphodynamic Corridors, when applied jointly with hydraulic model for assessing inundation processes, allow an overall assessment of flood hazard, particularly in dynamic river contexts.