Laura Maria Stancanelli

Propagation and deposition of stony debris flows at channel confluences

The fluid dynamics of stony debris flows generated in two small tributaries adjacent to each other and flowing into a main receiving channel was analyzed experimentally at a laboratory scale. The analysis on the propagation along the tributaries and deposition in the main channel provide information about sediment-water mobility, dangerous damming, and potential hazard. Debris flows were generated by releasing a preset water discharge over an erodible layer of saturated gravels material. As a consequence, the debris flow sediment concentration varied accordingly to the entrainment rate which, in turn, was strongly controlled by the tributary slope. The data collected by acoustic level sensors, pore fluid pressure transducers, and a load cell were used to characterize the evolution of bulk density and solid concentration of the sediment-water mixture. These two parameters were relevant to assess the stony debris flow mobility which contributes to determine the shape of sediment deposits in the main channel. The detailed bed topography surveys carried out in the main channel at the end of each experiment provided information on the morphology of these deposits and on the interplay of adjacent confluences. The influences of confluence angle, tributary slopes, and triggering conditions have been investigated, for a total of 18 different configurations. Within the investigated range of parameters, the slope angle was the parameter that mainly influences the stony debris flow mobility while, for adjacent confluences, the degree of obstruction within the receiving channel was strongly influenced by the triggering scenario.

Mutual interference of two debris flow deposits delivered in a downstream river reach

We investigate experimentally the depositions of two contiguous debris flows flowing into a main river reach. The aim of the present experimental research is to analyze the geometry and the mutual interactions of debris flow deposits conveyed by these tributaries in the main channel. A set of 19 experiments has been conducted considering three values of the confluence angle, two slopes of the tributary, and three different triggering conditions (debris flows occurring simultaneously in the tributaries, or occurring first either in the upstream or in the downstream tributary). The flow rate along the main channel was always kept constant. During each experiment the two tributaries had the same slope and confluence angle. The analysis of the data collected during the experimental tests indicates that the volume of the debris fan is mainly controlled by the slope angle, as expected, while the shape of the debris deposit is strongly influenced by the confluence angle. Moreover, in the case of multiple debris flows, the deposit shape is sensitive to the triggering conditions. Critical index for damming formation available in literature has been considered and applied to the present case, and, on the basis of the collected data, considerations about possible extension of such indexes to the case of multiple confluences are finally proposed.

Single or Two-Phase Modelling of Debris-Flow? A Systematic Comparison of the Two Approaches Applied to a Real Debris Flow in Giampilieri Village (Italy)

A comparison between the performances of two different debris flow models has been carried out. In particular, a mono-phase model (FLO-2D) and a two phase model (TRENT2D) have been considered. In order to highlight the differences between the two codes, the alluvial event of October 1, 2009 in Sicily in the Giampilieri village has been simulated. The predicted time variation of several quantities (as the flow depth and the velocity) has been then analyzed in order to investigate the advantages and disadvantages of the two models in reproducing the global dynamic of the event. Both models seem capable of reproducing the depositional pattern on the alluvial fan in a fairly way. Nevertheless, for the FLO-2D model the tuning of the parameters must be done empirically, with no evidence of the physics of the phenomena. On the other hand, for the TRENT2D, which is based on more sophisticated theories, the parameters are physically based and can be estimated from laboratory experiments.

Stony Debris Flow Debouching in a River Reach: Energy Dissipative Mechanisms and Deposit Morphology

Stony debris flows are investigated at the laboratory scale. A set of 11 experiments are carried out, to simulate the stony debris flow propagation in a typical mountain confluence with a downstream water channel. The dynamic of the tributary flow just upstream of the junction with the river, surrogated through the volumetric mixture discharge, is assessed through the analysis of velocity profiles, while observations on deposit morphology provide insight into the condition which cause the blockage of the main stream. Energy dissipation within the solid–liquid mixture is mainly controlled by collisions between grains, and a dilatant fluid model is found to approximately reproduce the velocity profile. The degree of river blockage caused by the delivered material is classified according to three configurations: (i) full blockage, whereby the entire transversal river section is dammed (100% river obstruction); (ii) no blockage when less than 60% of the transversal river section is interested by the deposit; (iii) partial blockage for intermediate damming configurations (>60% and <100% of the river obstruction).

Physical modelling of debris flow deposits in contiguous confluences

Dam formation in river channel generated by single and multiple debris flow injections from affluent basins are investigated in through flume experiments. The aim of the research is to understand the morphodynamics that governs the dam formation. A set of 14 experiments considering both single and double debris flow injections have been carried out, investigating different configurations in terms of debris flow discharge, confluence angle and water discharge along the main channel. In all cases, the debris flow consisted of a mixture of gravel and water, that reproduces stony-debris flow conditions. Results from single confluence experiments are taken as reference for better understanding the interactions between the different deposits when considering adjacent debris flow injections. The collected data are used to test the critical indexes for dam formation probability proposed in literature.