Morphological response of the Magra River to a large flood event: spatial patterns and relationships with the long-term trajectories of changes

On October 25, 2011 the Magra River catchment experienced an intense flood event (return periods ranging from about 100-200 years along the Magra, and up to 500 years along some tributaries) which resulted in flooding and relevant morphological changes. The analysis of the morphological responses to the October 2011 flood provided the opportunity to investigate on the two following issues: (1) can the observed morphological changes be considered within the normal range of historical variability of this river channel? (2) Are there specific factors (e.g. stream power versus confinement) controlling the spatial pattern of morphological changes? To address these questions, the morphological changes occurred during the October 2011 flood event were analysed in detail and compared with the long-term trajectories of channel adjustments. Morphological changes occurred with the flood event were investigated by: (1) field survey, which also included the reconstruction of the peak stage profile, required to estimate the flow discharges through numerical modelling; (2) analysis of high resolution aerial photos and LiDAR before and after the event; (3) topographic survey of cross-sections. A series of multi-temporal aerial photos, cross-sections, longitudinal profiles, and field surveys were used to analyse the historical trends of channel adjustments, as well as the recent trajectories. Results of the evolutionary trends show a recent phase (last 10-15 years) of dominant aggradation and widening, following previous phases (last 100-150 years) of progressive narrowing and incision, particularly severe in the last decades of the XX Century . The flood event resulted in prevailing aggradation of channel bed alternated to erosion. Comparison of aerial photos before and after the flood event clearly shows a severe channel widening, related to two different processes: (1) bank retreat; (2) overbank sedimentation of bedload. However, comparison with the longer term evolutionary trajectory shows that the channel width is still in the range of variability occurred during the last 50-60 years. The spatial distribution of the observed morphological changes was then compared with the spatial pattern of stream power, associated to both the channel-forming discharge and the peak discharge of the event, and with the degree of confinement. Results show a weak positive correlation between widening and the examined controlling factors, suggesting a complex, non-linear response of the channel to extreme flood events.