An integrated approach for investigating geomorphic response to extreme events: methodological framework and application to the October 2011 flood in the Magra River catchment, Italy

Abstract

A high-magnitude flash flood, which took place on 25 October 2011 in the Magra River catchment (1717 km 2 ), central-northern Italy, is used to illustrate some aspects of the geomorphic response to the flood. An overall methodological framework is described for using interlinked observations and analyses of the geomorphic impacts of an extreme event.The following methods and analyses were carried out: (i) hydrological and hydraulic analysis of the event; (ii) sediment delivery by event landslide mapping; (iii) identification and estimation of wood recruitment, deposition, and budgeting; (iv) interpretation of morphological processes by analysing fluvial deposits; (v) remote sensing and geographic information system (GIS) analysis of channel width changes.In response to the high-magnitude hydrological event, a large number of landslides occurred, consisting of earth flows, soil slips, and translational slides, and a large quantity of wood was recruited, in most part deriving from floodplain erosion caused by bank retreat and channel widening. The most important impact of the flood event within the valley floor was an impressive widening of the overall channel bed and the reactivation of wide portions of the pre-event floodplain. Along the investigated (unconfined or partly confined) streams (total investigated length of 93.5 km), the channel width after the flood was up to about 20 times the channel width before the event.The study has shown that a synergic use of different methods and types of evidence provides fundamental information for characterizing and understanding the geomorphic effects of intense flood events. The prediction of geomorphic response to a flood event is still challenging and many limitations exist; however a robust geomorphological analysis can contribute to the identification of the most critical reaches.

Channel response to extreme floods: Insights on controlling factors from six mountain rivers in northern Apennines, Italy

“…Analyses of rainfall, peak discharge, channel changes, depositional features and sediment structures, wood dynamics (Lucía et al, 2015), and sediment sources were carried out by field surveys, remote sensing, and numerical modelling at different spatial scales (i.e., from catchment to cross section scale). The whole methodological framework is described in detail by Rinaldi et al (2016), while in this work we focus on delineation of spatial units, channel width changes, and potential controlling factors of channel changes. The estimation of peak discharges -used to calculate cross-sectional stream power and unit stream power -and analysis of sediment sources and of their connectivity with study reaches are also described.…”

“…Only significant factors of the two best models obtained by multiple regression analysis (see Tables 5 and 6 erosion, with sediment (volumes and size) supplied from landslides and upstream reaches becoming significant. Notable bar formation and channel aggradation were observed in several of these reaches (Rinaldi et al, 2016), and repeated avulsion processes might have occurred during the event in these aggrading subreaches. As to additional factors, large riparian trees coupled to wood jams could have played a role by occasionally reinforcing banks and, therefore, hampering channel widening.…”

“…The fact that using the pre-flood width gives better relations with the degree of channel widening (i.e., width ratio) could suggest that most width changes occurred after the flood peak. This hypothesis is supported by a video that allowed flood reconstruction along one reach of the Mangiola River (the video documents channel changes during the whole flood event; see Rinaldi et al, 2016, for more details). Statistical analyses and the video on the Mangiola River are not sufficient to validate the hypothesis that most channel widening took place after the flood peak, but the results of this work show clearly that using unit stream power based on post-flood width is not effective to explain geomorphic response, at least in streams that underwent intense widening in terms of width ratio.…”

“…We were able to address such questions in relatively small catchments (drainage areas between 8.5 and 38.8 km 2 ) because an integrated approach was deployed to study this flood event (Rinaldi et al, 2016). Besides the analysis of morphological changes, the approach includes field measurements coupled to a rainfall-runoff model to estimate peak discharges in the ungauged streams, detailed mapping of landslides and analysis of sediment connectivity, as well as information concerning other fundamental aspects of the event (e.g., sedimentological characterization of flood deposits, dynamics of large wood transport; Lucía et al, 2015).…”

The impact of extreme rainfall and flash floods on the flood risk management process and geomorphological changes in small Carpathian catchments: a case study of the Kasiniczanka river (Outer Carpathians, Poland)

“…However, it is several times lower compared to the Mediterranean region Rinaldi et al 2016). Managed reaches of the channel, allowed the assessment of a hydrotechnical infrastructure functioning, during flash floods events.…”