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Evaluation of open check dams within modelling chains

Open check dams are widely recognized as effective structural measures against debris flows. In the last 40 years, the Service for Torrent Control of the Autonomous Province of Trento (Italy) has built around 400 open check dams, which in many cases have made it possible to debris-flow hazard mitigation. The use of physical-based models and urban planning through hazard maps allow the technicians and scientific community to evolve the hazard assessment methodologies. However, these methodologies require quantitative analyses. The inclusion of a check dam in the modelling chain has several critical issues which are related to hydrological and hydraulic models and the effectiveness of check dams may be underestimated. To overcome these critical issues, the Service has adopted an empirical method to evaluate the effect of open check dams on hazard mitigation and the possible design alternatives by defining a reference behaviour and different behaviour scenarios. The starting data necessary to perform an analysis are: check dam and filter geometries, the geomorphological data of basin (e.g. the volume of expected event), hydrological data, i.e. the peak discharge and the hydrograph of debris flow and an estimate of the largest particle sizes in the reference event. Preserving the peak discharge and the volume of events, discharge timestep distributions are modified to reproduce a debris-flow hydrograph shape consistent with monitored field-site datasets and recorded evidence (Gregoretti et al. 2016, Marchi et al. 2020, Mitchell et al. 2022). The interaction between described debris flow and check dam is analysed by applying the equation of Mizuyama et al. 1996 to each time step until saturation of reservoir capacity and by evaluating the discharge capacity of spillway upon reaching this capacity. To define a confidence interval on the estimate of check-dam efficiency, depending on building characteristics, three other scenarios are also evaluated: the occluded filter hypothesis, the functioning hypothesis with Mizuyama et al. 1996 using the original hydrograph and the hypothesis of peak lamination by considering the volume of the deposition basin. The debris-flow hydrograph resulting from the reference scenario is used for the physically based simulations downstream of the open check dam. The methodology was applied to several/many case studies, by allowing satisfactorily to evaluate the behaviour of the open check dams. Further advances and refinements of the procedure are desirable with respect to a better and more standardized definition of debrigraph on the basis of the standardization of real debris-flow hydrograph and on a more refined description of the behaviour of filters.

Copernicus GmbH

Gabriele Bertoldi Federico Bridi Valentina Cavedon Tamara Michelini Ruggero Valentinotti

2023

Title: Evaluation of open check dams within modelling chains

Description:

In the last 40 years, the Service for Torrent Control of the Autonomous Province of Trento (Italy) has built around 400 open check dams, which in many cases have made it possible to debris-flow hazard mitigation.

The use of physical-based models and urban planning through hazard maps allow the technicians and scientific community to evolve the hazard assessment methodologies.

However, these methodologies require quantitative analyses.

The inclusion of a check dam in the modelling chain has several critical issues which are related to hydrological and hydraulic models and the effectiveness of check dams may be underestimated.

To overcome these critical issues, the Service has adopted an empirical method to evaluate the effect of open check dams on hazard mitigation and the possible design alternatives by defining a reference behaviour and different behaviour scenarios.

The starting data necessary to perform an analysis are: check dam and filter geometries, the geomorphological data of basin (e.

the volume of expected event), hydrological data, i.

the peak discharge and the hydrograph of debris flow and an estimate of the largest particle sizes in the reference event.

Preserving the peak discharge and the volume of events, discharge timestep distributions are modified to reproduce a debris-flow hydrograph shape consistent with monitored field-site datasets and recorded evidence (Gregoretti et al.

2016, Marchi et al.

2020, Mitchell et al.

2022).

The interaction between described debris flow and check dam is analysed by applying the equation of Mizuyama et al.

1996 to each time step until saturation of reservoir capacity and by evaluating the discharge capacity of spillway upon reaching this capacity.

To define a confidence interval on the estimate of check-dam efficiency, depending on building characteristics, three other scenarios are also evaluated: the occluded filter hypothesis, the functioning hypothesis with Mizuyama et al.

1996 using the original hydrograph and the hypothesis of peak lamination by considering the volume of the deposition basin.

The debris-flow hydrograph resulting from the reference scenario is used for the physically based simulations downstream of the open check dam.

The methodology was applied to several/many case studies, by allowing satisfactorily to evaluate the behaviour of the open check dams.

Further advances and refinements of the procedure are desirable with respect to a better and more standardized definition of debrigraph on the basis of the standardization of real debris-flow hydrograph and on a more refined description of the behaviour of filters.

.

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Evaluation of open check dams within modelling chains

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