Model-based approach for design and performance evaluation of works controlling stony debris flows with an application to a case study at Rovina di Cancia (Venetian Dolomites, Northeast Italy)

This paper proposes a methodology for the design and performance evaluation of debris-flow control works based on modeling the phenomenon and its interaction with the works. The reliability of such an approach depends on the trustworthy reproduction of both the phenomenon and its interaction with the structures: the former is provided by simulating all the physical processes concerning the phenomenon (runoff generation, initiation and propagation of a solid-liquid mixture) by models tested against field measurements; the latter by upgrading the propagation model for considering the non-erosive surfaces of the works overflowed or hit by debris flows and computing the forces exerted on them. This methodology is applied to a case study on the Rovina di Cancia channel (northeast Italy), frequently affected by stony debris flows. Longitudinal and transversal works are planned in a reach of the channel subject to high erosion because of its high slope and the supply of water discharge by the Bus de Diau Creek tributary. The works consist of a sectional dam at the end of the reach and in moving there the mouth of the Bus de Diau Creek. Both of them contribute to decrease bed erosion along the reach and to reduce the peak discharge and the transported sediment volume. The proposed methodology is used in the design phase to explore different geometries of the dam opening and choose its transversal position through the analysis of flow (velocity and free surface), whereas, in the evaluation phase, to estimate the reduction in the bed erosion, peak discharge and transported sediment volume provided by the works (i.e. the performance evaluation). Finally, the plot of the forces acting on the dam breakers and bottom over time allows their static dimensioning.

Impact force of a surge of water and sediments mixtures against slit check dams

Slit check dams are widely used protection structures against debris flows. The role of these structures is to trap part of the debris in order to diminish the peak of the solid discharge. However, the high volume and velocity involved induce considerable impact forces. Correspondingly, an improved estimation of the impact force is fundamental to properly design the protection structures. In order to develop an analytical expression for the impact force of a debris flow surge against a slit check dam, we have adopted a rational criterion based on the principles of mass and momentum conservation. In our formulation we have introduced a proper coefficient to account for the horizontal contraction of the streamlines near the check dam slit. This coefficient is calibrated through a series of physical experiments. Furthermore, the paper addresses the influence of a debris flow breaker located within the opening of the slit check dam. The differences between the check dam with and without flow breaker are evaluated in terms of impact forces by comparing two check dams with the same slit width and the same net slit width. By comparing the force acting in the presence of the breaker or without it, we observed that the impact is more onerous in the first case, since the incoming flow is deviated from the flow breaker to the check dam wings.