It is proved that a small liquid stream or a triggered landslide may occasionally evolve into a debris flow. As its propagation takes place, available sediments from the bottom slope and nearby banks can be added. In addition, under certain conditions, the entrainment yields an increase of both mass and momentum, enhancing the efficiency of the resulting movement. The same efficiency can be as well sustained by bed liquefaction and mass fluidification. The first is due to an abruptly increase of fluid pore pressure of the bed when overridden, causing a falling of effective shear stresses and therefore behaving like a liquid, the latter consists of the dynamic interaction between sediments and fluid porosity inside the propagating mass, responsible of both high pore pressures and continuous modification of voids and matrix distribution. The amplification process is therefore essential for a proper hazard assessment. For example, average volumes mobilized during the 1998 events of Sarno (Italy), have been three times the initial detached volumes. The work here presented means to analyze debris flow amplification features, from the first stages up to the entrainment processes As main result, we obtain that reducing bed slope decreases the amplification to the point it is negligible. This is due to the fact that both bed liquefaction and mass fluidification are attenuated. A mitigation intervention consisting of a check dam system causes a reduction of the original bed slope, determining debris flow volumes to be stored and a stabilization of the banks at the same time. Other mitigation strategies, consisting of open dams and storing reservoirs, do not act likely, hence sustaining the effect of mass amplified. As a check dam system is realized, the slope decreases over time since sediments mobilized by small floods fill on upstream the single dam, eventually reaching the final design slope. The allocation of a check dam system, needed to decrease the amplification process, is not random but related to the position of the areas of triggering formation. Such areas are concentrated along the upstream impluvium heads, named Zero Order Basin (ZOB). We propose a one-dimentional numerical model based on a finite difference scheme. The ruling equations implemented are taken from (Egashira et al., 2000), accounting for erodible beds. Results show that exists a relationship between the original bed slopes and the amount of added volumes. Moreover, varying the different input parameters a particular temporal and spatial evolution of both debris flow and erodible bed is presented. Other numerical analyses have been carried out in (Viccione and Bovolin, 2010; 2011). Concluding, check dam systems with attained design slope of 10-15%, from the initial value ranging up to 30-35% exhibits the following advantages: - yield an efficient system control of debris flows; - help reducing liquefaction and fluidization processes, both responsible of the amplification feature; - are economically affordable.

Check dams as mitigation approach for reducing debris flow amplification

GENOVESE, MARCO;ROSSI, Fabio;VICCIONE, GIACOMO;BOVOLIN, Vittorio
2012

Abstract

It is proved that a small liquid stream or a triggered landslide may occasionally evolve into a debris flow. As its propagation takes place, available sediments from the bottom slope and nearby banks can be added. In addition, under certain conditions, the entrainment yields an increase of both mass and momentum, enhancing the efficiency of the resulting movement. The same efficiency can be as well sustained by bed liquefaction and mass fluidification. The first is due to an abruptly increase of fluid pore pressure of the bed when overridden, causing a falling of effective shear stresses and therefore behaving like a liquid, the latter consists of the dynamic interaction between sediments and fluid porosity inside the propagating mass, responsible of both high pore pressures and continuous modification of voids and matrix distribution. The amplification process is therefore essential for a proper hazard assessment. For example, average volumes mobilized during the 1998 events of Sarno (Italy), have been three times the initial detached volumes. The work here presented means to analyze debris flow amplification features, from the first stages up to the entrainment processes As main result, we obtain that reducing bed slope decreases the amplification to the point it is negligible. This is due to the fact that both bed liquefaction and mass fluidification are attenuated. A mitigation intervention consisting of a check dam system causes a reduction of the original bed slope, determining debris flow volumes to be stored and a stabilization of the banks at the same time. Other mitigation strategies, consisting of open dams and storing reservoirs, do not act likely, hence sustaining the effect of mass amplified. As a check dam system is realized, the slope decreases over time since sediments mobilized by small floods fill on upstream the single dam, eventually reaching the final design slope. The allocation of a check dam system, needed to decrease the amplification process, is not random but related to the position of the areas of triggering formation. Such areas are concentrated along the upstream impluvium heads, named Zero Order Basin (ZOB). We propose a one-dimentional numerical model based on a finite difference scheme. The ruling equations implemented are taken from (Egashira et al., 2000), accounting for erodible beds. Results show that exists a relationship between the original bed slopes and the amount of added volumes. Moreover, varying the different input parameters a particular temporal and spatial evolution of both debris flow and erodible bed is presented. Other numerical analyses have been carried out in (Viccione and Bovolin, 2010; 2011). Concluding, check dam systems with attained design slope of 10-15%, from the initial value ranging up to 30-35% exhibits the following advantages: - yield an efficient system control of debris flows; - help reducing liquefaction and fluidization processes, both responsible of the amplification feature; - are economically affordable.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11386/3120025
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