River basin hydrological and geomorphological parameters for the identification of vulnerable bridges

This work focuses on the definition of synthetic procedures for the damage risk assessment of infrastructures crossing the rivers. The mechanisms of the river-bridges interaction has been schematised as the combination of two instability processes: a) the lateral instability, when long term evolution of the stream or localized collapse phenomena of the banks can undermine the lateral bridge structures (piers whit shallow foundation in the floodplain, long abutments, etc.); b) vertical instability processes, when the river bed level degradation given by geomorphological phenomena at the basin scale (general scour) and at the site scale (contraction scour and local scour) can undermine the bridge foundations. A two steps procedure for the vulnerability assessment has been developed, using as a case study the roads and the railways network of two main watersheds in Tuscany. The first step of the analysis, which can be applied on a large number of river crossings, allows to identify bridges which are potentially critical, using as vulnerability parameters some hydrological and geomorphological characteristics of the catchment closed at the bridge section, such as the area and the mean slope of the catchments, the sinuosity of the stream next to the bridge, the stream order, the presence of geomorphologically dangerous areas, the rainfall mean values of the catchment area. The use of a GIS/DBMS allows to easily extract catchments characteristics related to the vulnerability of the infrastructure, and to perform the fist step of the analysis in an extensive way. In the second step, potentially critical bridges are inspected and local instability conditions are checked using a Bridge Site Inspection Form. The instability of the bridge site is then evaluated using a Risk Index, which takes into account local erosion phenomena, bridge geometry, riverbed dynamics, scour countermeasures, etc. The analysis shows that the instability magnitude, and particularly the local scour of the bridge, seem to be related to the hydrological and geomorphological parameters of the watershed. Empirical models, used for local scour forecasting, should therefore include these parameters in addition to geometric and hydraulic data of the section. The methodology here presented can be used as a rapid identification method of critical river crossings, based on the characteristics of the river basin close to the bridge, in prioritising large scale infrastructures maintenance and revision.