New tools to assess fluvial hazard in Italian streams: conceptual framework and applications

Floods are one of the major natural hazard affecting highly populated countries such as Italy. Besides hydraulic hazard (i.e. probability of inundation of a given area), geomorphological hazard due to channel dynamics should be taken into account. Channel dynamics (i.e. channel lateral mobility, changes in bed elevation, intense sediment and wood transport) may cause severe damages to human properties and infrastructures. Analysis of such dynamics is crucial to reach an overall assessment of flood hazard and an effective implementation of the EU Floods Directive (2007/60). In Italy, a methodological framework for geomorphological assessment, analysis and monitoring of streams (named IDRAIM) has been developed with the specific aim of supporting the management of river processes and integrating the objectives of the Water Framework Directive (2000/60) and Floods Directive (2007/60) (Rinaldi et al., 2013). IDRAIM builds on existing geomorphological approaches developed in other countries (with particular reference to the River Styles Framework, Brierley and Fryirs, 2005), but with a specific reference to the Italian context (e.g. wide range of physical conditions and channel morphologies, remarkable human pressures and channel adjustments). Some key characteristics of IDRAIM are: (i) the method embraces a catchment-wide perspective, by using a multi-scale, process-based hierarchical approach and using the reach (i.e. a section of river along which present boundary conditions are sufficiently uniform, commonly a few kilometres in length) as the main spatial unit of analysis; (ii) it is based on the recognition that rivers are dynamic and follow a complex evolutionary trajectory over time; (iii) it is based on an integration of GIS/remote sensing analysis and field survey; (iv) it was specifically designed to comply with the requirements of the EU Directives, but could be used for other purposes in river management (e.g., sediment management plans, evaluation of impact of engineering projects); (v) because the assessment tools are to be used by environmental or water agencies on a national level, they have been designed to be relatively simple and not excessively time consuming, but based on sound and up-to-date geomorphological concepts and approaches. IDRAIM incorporates three tools to assess channel dynamics and, therefore, geomorphological hazard: the Morphological Dynamics Index (MDI), the Event Dynamics Classification (EDC), and the Morphological Dynamics Corridors (MDC). The MDI classifies the degree of channel dynamics related to progressive changes occurring in a relatively long-term (i.e. 50-100 years), not including possible responses to extreme flood events (which are addressed in the EDC). The index is applied only to partly confined or unconfined reaches and is based on a set of 11 indicators assessing the main factors that control channel dynamics (e.g., river typology, bed and bank erodibility, past changes and present trends of adjustment). The EDC aims to investigate possible channel dynamics associated with extreme flood events (i.e. >100 years return period), and is applied to all river typologies (including confined channels). The classification is carried out by means of a guided procedure consisting of two phases: (1) assessment of expected morphological processes and changes during the event; (2) assessment of the probability of occlusion in critical cross-sections (e.g. bridges). The analysis of morphological dynamics is completed by the delineation of the Morphological Dynamics Corridors. In fact, MDI and EDC provide information on the magnitude of channel dynamics in a given reach (1-D analysis), and need to be integrated with a 2-D analysis to define the areas of the fluvial corridor that will be affected by such dynamics. This is achieved by the MDC, which represent a further development, adapted to the objectives of this framework, of the erodible corridor concept (Piégay et al. 2005), the functional mobility corridor (Rinaldi et al. 2009) and similar approaches. The MDC procedure is implemented by GIS tools and includes a reconstruction of past dynamics (i.e. historical channel positions) and a definition of possible future erosion by calculating the mean rate of bank retreat for a given reach. Three different corridors (A, B, and C) are defined: A and B are associated to progressive channel changes occurring during relatively frequent floods (similarly to the MDI), while the corridor C is associated to extreme flood events (similarly to the EDC) and does not account for existing protection structures. The tools (i.e. MDI, EDC, and MDC) were applied to several reaches in five study cases (i.e. Aurino River, Rienza River, Tagliamento River, Fella River and Magra River) for testing and the refinement of the methods. Overall these applications have shown that such tools are effective to identify and classify very different conditions in terms of geomorphological hazard, for instance from those found in steep mountain streams to those of lowland rivers. That said, we recognize that further tests will be needed to cover sufficiently the wide range of physical conditions and human pressures of Italian streams.