This paper verifies methods for the prediction of mean flow velocity at the reach scale in mountain streams, investigating the kinematics of a series of two small-scale artificial step-pool sequences and a transitional reach between plane-bed and step-pool under well-controlled hydraulic conditions, and improving the estimation of the energy expenditure between the step crest and the downstream pool. Experimental data were collected using three fish ladder reaches with slopes between 2.6 and 10%. Four types of field measurements were conducted: topographical surveys to extract the thalweg profiles and cross-sectional geometry of reference cross sections; grain size analyses of the bed surface; steady state runs with a given flow rate (0.005-0.234 m 3 /s), and surveying of the water profile in the most significant cross sections. The following main conclusions were reached: (i) the dominance of spill resistance at the lowest discharge (pool water depth-step height ratios of 0.4) causes primary dimensionless head losses of up to 80%, and these losses progressively decrease to approximately 40% when the water discharge and related pool water depth submerge the upstream step height. A specific predictive equation for the head loss was calibrated and then verified via data from the Rio Cordon. (ii) The verification of literature-sourced equations to predict the reach-averaged flow velocity provided suitable results for several of these equations indicating that the use of a specific step-pool equation does not appear to be crucial to achieving accurate predictions.

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Studies on susceptibility to debris flows at regional scale (100-1000 square kilometers) are very important for the protection and management of mountain areas. To reach this objective, routing models, mainly based on land topography, can be used because they can predict susceptible areas rapidly and necessitate few input data. In this research, Flow-R model is implemented to create the susceptibility map to debris flow of the Vizze Valley (BZ, North-Eastern Italy; 134 square kilometers). The analysis considers the model application at local scale for three sub-catchments and then it explores the model upscaling at the regional scale by verifying two methods to generate the source areas of debris-flow initiation. Using data of an extreme event occurred in the Vizze Valley (4 August 2012) and historical information, the modelling verification highlights that the propagation parameters are relatively simple to set in order to obtain correct runout distances. Furthermore, a double DTM filtering - using a threshold for the upslope contributing area (0.1 km2) and a threshold for the terrain-slope angle (15°) - provides a satisfactory prediction of source areas and susceptibility map within the geological conditions of the Vizze Valley

Debris flow is a gravity-driven process, which is characterized by a travelling dense surge including large boulders, and it is followed by a more fluid tail. These characteristics make difficult the measurement of the mean flow velocity by means of common hydraulic techniques. Different methods can be used at real scale and small-scale to measure the front velocity but a dedicate comparison between available methods is still lacking. This research aims to compare the front velocity measurements in the transport zone of a miniature debris flow using three devices: i) a common digital video camera (29 frames per second); ii) a high speed thermo camera (60 fps); and iii) a laser photoelectric sensors system. The statistical analysis of data has highlighted no significant differences exist between front velocities obtained by means of the video camera and the thermo camera, whereas photocells data statistically differ from those achieved via the other systems. Some lack of data recorded by photocell was documented, while the thermo camera technique did not show significant loss of information being also helpful to detect the kinematic behaviour of single particles. Finally, the tests confirmed the influence of the solid volumetric concentration in the debris-flow mechanics, which promotes, ceteris paribus, the debris-flow slowing down.

Debris flows are one of the most common geomorphic processes in steep mountainous areas. The control of their propagation on alluvial fans is fundamental; valley bottoms are usually characterised by high damage potential because they contain concentrations of inhabitants and infrastructure. It is well known that forests have a protective function in that they reduce the triggering of debris flows, as well as hinder their motion and promote deposition, but a quantitative assessment of these effects is still lacking. Using laboratory experiments that simulate debris-flow depositional processes, this research investigated the ability of forests to reduce debris-flow runout and depositional area. The experiments considered two different forest types, high forests and coppice forests, and four volumetric concentrations of sediment (0.50, 0.55, 0.60, and 0.65). The results confirmed that the sediment concentration of the flow is a key factor in determining the geometry of the deposits. On the other hand, forests can reduce debris-flow runout distance and, in general terms, affect the characteristics of their deposits. The results showed that vegetation appear to reduce debris-flow motion especially when the debris-flow kinematic load at the fan apex is low. About the sediment concentration of the mixture, high forest did not exhibit a clear behaviour while coppice forest appears to promote significant deposition at all of the tested concentrations, and this effect increases with the solid concentration (reductions in runout between approximately 20% and 30% at CV=0.50 and CV=0.65, respectively, were observed). Due to their higher tree density, in fact, coppice forests seem to have a better protective effect than the rigid trunks of high forest trees. For this last type of forest, a relationship between the H/L ratio, which represents energy dissipation, have been found and compared with the scenario without forest. IntroductionDebris flows are one of the most common geomorphic processes in steep mountainous areas (Ancey, 2001), and they are responsible for losses of human lives and economic damages on a worldwide basis every year.Their destructive potential can decrease due to processes that promote deposition and thus reduce the travel distance: i) reductions in excess pore fluid pressure (Hutchinson, 1986); ii) increases in the viscoplastic yield strength ; iii) increases in grain collision stresses (Takahashi, 1991); and iv) increases in grain contact friction and the concentration of friction at flow margins (Major and Iverson, 1999).The length travelled by a debris flow on an alluvial fan from the initiation of the flow to the lowest point of the deposits (i.e., the runout distance), is the most critical issue in the delineation of areas at risk from debris flows (D'Agostino et al., 2010), and several empirical methods have consequently been suggested to predict it (e.g., Rickenmann, 2005;D'Agostino et al., 2010;Scheidl et al., 2013). Generally, runout is recognised to depend mainly on topography (i.e., t...