Monitoraggio e analisi dei processi di arretramento e dei meccanismi di instabilità di sponde fluviali

This paper reports a bibliographic review of the main aspects related to erosion processes of natural streambanks and the methods of measurement, monitoring and analysis of such processes. Riverbank retreat generally derives from a combination of several processes and mechanisms of instability, divided in two main categories: a) processes of erosion, which determine the entrainment particle- by-particle of bank material; b) mass failures, which occur by the dominant action of the gravity. Besides these two categories, a series of factors may determine a progressive weakening of the bank material, facilitating subsequent erosion or failure. Processes operating at the bank toe (removal or accumulation of material deriving from erosion or failures) are crucial in determining stabilization or further retreat. A large variety of methods suitable for measuring bank retreat are available, conveniently divided into three categories, depending on the time scale of the study. As regard the long time scale, sedimentological or botanical evidences or historical sources (topographic maps or aerial photos) are generally adopted. Methods more traditionally applied at the intermediate time scale are the topographic surveys (planimetric survey or bank profile), while terrestrial photogrammetry, metallic or photo-electronic erosion pins are the methods applicable at the short time scale. Fluvial erosion (particle-by-particle) is traditionally dealt with the excess shear stress approach. Estimation of critical shear stresses for loose sediments is possible by using formulas derived from those for bed material, while the analysis of fluvial entrainment for cohesive sediments present nowadays several difficulties, given the complexity of the phenomenon and the several factors involved. For this reason, recent researches have been addressed on the set-up and application of new techniques (jet test) for measuring in situ the erodibility parameters of cohesive sediments. The analysis of mechanisms of bank failure have been more developed compared to the fluvial entrainment because of the possibility of applying methods of stability (limit equilibrium method) already largely developed and utilised in geotechnics. First studies concerning the stability of natural streambanks appear in the international literature starting from the end of 1960s and, even more, at the beginning of 1980s. In most cases, they regard riverbanks composed by cohesive material along incised rivers of south- eastern and midwestern United States, and the bank stability is faced adopting quite simplified hypotheses and solutions. A solution for planar failure with a more realistic geometry for riverbanks, taking into account the geometrical effect of basal fluvial erosion, has been proposed by OSMAN & THORNE (1988), while DARBY & THORNE (1996) extended the previous method to take in account the river confining forces and water table hydrostatic forces. Following analysis (RINALDI & CASAGLI, 1999; CASAGLI et alii, 1999), supported by a monitoring of positive and negative pore water pressures wit hin a natural riverbank, have started to take into account partial saturation conditions and effects of negative pressures (suction) on triggering mass failures. A complete analysis taking into account these effects is possible by coupling saturated and unsaturated flow modelling with the limit equilibrium method for bank stability. It is possible to perform a seepage – stability analysis with variable bank geometry, taking also in account the changes of bank profile due to fluvial erosion at the scale of the single flow event. SIMON & COLLISON (2002) have then incorporated one of the main effects of vegetation, the soil reinforcement due to the roots, into a stability analysis, although for a simplified geometry of the bank and of the mechanism of failure.