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.

Intermittency of rheological regimes in uniform liquid-granular flows.

We present a detailed analysis of a free surface-saturated liquid-granular mixture flowing over a static loose bed of grains, where the coexistence of layers dominated by collisional and frictional interactions among particles was observed. Kinetic theory was applied to the flow described above and it proved suitable for describing a realistic behavior of the collisional layers, although it failed to interpret the regions of the flow domain dominated by the frictional contacts. The paper provides a conceptual scheme with which to overcome this problem by focusing on the mechanisms governing the transition from the frictional to the collisional regime. In particular we observed that in highly concentrated flows the transition layer exhibits a typical intermittency of the dominating rheological regime, switching alternately from the frictional to the collisional one. By filtering the velocity signal, we introduced an intermittency function that made it possible to extend the validity of the equations derived from dense gas analogy, typical of the collisional regimes, also in the intermittent phase of the flow. Owing to the small values of the Stokes number, in the application of the kinetic theory we accounted for the possible variation of the elastic restitution coefficient along the flow depth.