Lorenzo Marchi

Suspended sediment load during floods in a small stream of the Dolomites (northeastern Italy)

Abstract Suspended sediment load was analyzed in a small, high-gradient stream of the Eastern Italian Alps which was instrumented to measure discharge and sediment transport. The ratio of suspended to total sediment yield and the relations between sediment concentration and water discharge were analyzed for seven floods which occurred from 1991 to 1996 in summer and autumn (from June to October). Different patterns of hysteresis in the relation between suspended sediment and discharge were related to types and locations of active sediment sources. The within-storm variation of particle size of suspended sediment during a major flood indicates a coarsening of transported material for increasing discharge. An analysis of grain size has shown that erosion areas on hillslopes were the main sources of suspended load.

Ten years of debris-flow monitoring in the Moscardo Torrent (Italian Alps)

Abstract Field data on debris flows are of utmost importance for improving knowledge of these hazardous phenomena and for the development of attenuation measures. In alpine basins, debris flows generally occur with a frequency high enough to create serious risk to human settlements and transportation routes, but too low to justify monitoring activity capable of providing enough data in a sufficiently short time. The Moscardo Torrent, a small stream in the Eastern Italian Alps, is an exception to this general situation since it displays a high frequency of debris flows (commonly at least one event per year). In 1989, this torrent was instrumented and the results of the following 10 years (1989–1998) of debris-flow monitoring are presented, with an analysis of collected data. The equipment installed allowed measurement of rainfall, flow stage and ground vibrations caused by debris flows. Other important debris-flow variables, that is, mean front velocity, peak discharge and flowing volume were estimated from instrumental records. Video pictures have proved to be useful for the visual interpretation of debris-flow waves and have made it possible to estimate the surface velocity of debris flows. Recorded data are compared with other experimental data sets collected and documented worldwide. Advantages and shortcomings of different types of sensors are discussed with relevance both for research monitoring purposes and for possible use in debris-flow warning systems.

An experimental station for the automatic recording of water and sediment discharge in a small alpine watershed

Abstract Rating sediment transport contributes greatly to an improved knowledge of the relationships between hydrological and sediment transport processes. In alpine watersheds the importance of these processes is not matched by a corresponding amount of available field data on sediment transport and particularly on bed load. Detailed measurements made in experimental watersheds can help collect data on sediment transport in small mountain streams. In this connection, a small watershed (5 km2) in the Italian Alps strongly affected by erosion has been equipped with an experimental station for recording water and sediment discharge data. The experimental station operates chiefly on the separation of bedload from suspended load. The two fractions are measured by different devices. The experimental equipment has been devised with different objectives: to record sediment transport data under various different hydrological conditions including flood events; to provide reliable input data for deterministic erosi...

Debris flow magnitude in the Eastern Italian Alps: data collection and analysis

Abstract The estimation of debris flow magnitude is essential for the assessment of debris flow hazard. Historical data are an important basis for evaluating magnitude and frequency of debris flows in a given geographical region. Data on debris flow magnitude concerning 130 basins of the Eastern Italian Alps have been collected from scientific and technical journals, technical reports and historical documents gathered from local archives and through field surveys carried out in the last few years. The analysis of collected data includes various approaches. Regression techniques were used to correlate debris flow magnitude to morphometric parameters and to geological characteristics of the basins. A comparison of historical magnitudes with geomorphic field estimations carried out in recent years is presented and the relations between debris flow magnitude and frequency for a few selected cases are analysed. Some proposals about the possible combined use of considered techniques are then suggested.

Video-Derived Velocity Disribution Along a Debris Flow Surge

Abstract Automated recording by videos of debris flows and the subsequent use of image processing techniques measures the surface velocity of two debris flows occurring in an instrumented basin of the Italian Alps. Mean front velocities obtained are consistent with and slightly less than average velocities derived from ultrasonic sensors measurements. A particular behavior was observed in debris flow surges: the surface velocity peak occurs behind the main front of the debris flow, whereas the velocity of the latter can be decidedly lower. This same behavior had been already revealed by other authors, through the use of laser doppler speedometers and time-lapse photography, and should be taken into proper account in the mathematical simulation of debris flow propagation. The trajectories of boulders in the debris flow front have also been examined: transversal movements toward the outer parts of the channel are mainly attributed to cross-sectional widening.

Toward the development of a stand-alone application for the assessment of sediment connectivity

The role of sediment connectivity in controlling sediment fluxes, in particular between sediment sources and downstream areas, is a key issue in the study of sediment transfer processes. In mountain catchments, in which the degree of hillslope-channel coupling and decoupling is controlled by the rugged morphology and the distribution of sediment sources, the assessment of connectivity patterns is especially useful forgiving watershed management priorities. In this work, the authors present the ongoing development and the first-stage testing phase of a free, open source and stand-alone application for the computation of the Index of Connectivity (IC), as expressed in Cavalli et al. (2013). The tool is intended to have a wide variety of users, both from the scientificcommunity and from the authorities involved in the environmental planning. Due to its open source nature, the tool can be modified and/or integrated according to the users requirements. Being also a stand-alone, easy-to-use application, the tool can help management authorities in the quantitative estimation of sediment connectivity in the framework of hazard and risk assessment. First results of the testing phase are encouraging since the modelled connectivity appears suitable to detect thepotential for the sediment to reach specific targets areas such as the main channel network.

Standardization of Methods and Procedures for Debris Flow Seismic Monitoring

Standardization of measurement procedures and their systematic performance are important goals in every field of science and are in general intensely pursued by scientists in many of their research activities. Certain phenomena, however, present particularly difficult challenges in this regard and many efforts are still needed to actually reach standardization and systematic performance of measurements. Debris flows are natural phenomena that certainly belong to this latter category. Due to their low frequency of occurrence, their short duration and their sudden and abrupt nature they are extremely difficult to be monitored. Only instrumented basins where debris flows occur with a sufficiently high frequency per year allow systematic monitoring activities. Even though during the last decades several such basins have been instrumented, field measurement data are still scanty and methods of measurement are not yet sufficiently standardized. An European Territorial Cooperation project named “Sediment management in Alpine basins: integrating sediment continuum, risk mitigation and hydropower” (SedAlp) has been recently funded within the Alpine Space Programme that has, among its tasks, to make some advancement in this direction. One of the expected outputs of the SedAlp project is a protocol on debris-flow monitoring. In this paper, in particular, we will discuss some open issues regarding the debris-flow seismic monitoring.

Rocky Headwaters in the Dolomites, Italy: Field Observations and Topographic Analysis

Rock outcrops cover large areas of alpine headwaters and are entrenched by chutes and couloirs, which are controlled by faults in bedrock. These widespread landforms play an important role in delivering sediment to lower basin slopes. High-resolution topographical data from LiDAR surveys allow investigation of morphometric characteristics and sediment transport processes in these features. Using aerial photo interpretation, field surveys, and topographic analyses of LiDAR data, this paper quantifies the morphological characteristics of rocky couloirs and their drainage basins, and the relationship between these features and the structural setting, in a study area in the Dolomites (northeastern Italy). Rock basins are characterized by small sizes (surface area < 0.066 km2) and high average basin slopes (up to 2.1 m m−1). The analysis of contributing area and local slope outlines the difference between these rock basins, and even smaller and steeper rock faces entrenched by very shallow chutes, which were defined as interbasin areas. We consider rocky couloirs and rock basins in the headwaters of the Dolomites to be part of the channel network, since channeled flow occurs in the couloirs during storms. High-intensity rainstorms trigger debris flows as evidenced from local scouring, especially in the lower parts of the couloirs. The longitudinal profiles of the couloirs are overall linear, but the high-resolution data display distinct high-slope and low-slope stretches forming steps, that may function as localized sources and sinks for debris flows. The cross-sectional widths of the couloirs do not appear related to upslope area; this may be due to both structural control on cross-sectional geometry and complex erosion of the couloir by debris flows.

Hydrometeorological characterization of a flash flood associated with major geomorphic effects: Assessment of peak discharge uncertainties and analysis of the runoff response

AbstractPostflood indirect peak flow estimates provide key information to advance understanding of flash flood hydrometeorological processes, particularly when peak observations are combined with flood simulations from a hydrological model. However, indirect peak flow estimates are affected by significant uncertainties, which are magnified when floods are associated with important geomorphic processes. The main objective of this work is to advance the integrated use of indirect peak flood estimates and hydrological model simulations by developing and testing a procedure for the assessment of the geomorphic impacts–related uncertainties. The methodology is applied to the analysis of an extreme flash flood that occurred on the Magra River system in Italy on 25 October 2011. The event produced major geomorphic effects and peak discharges close to the maxima observed for high-magnitude rainstorm events in Europe at basin scales ranging from 30 to 1000 km2. Results show that the intensity of geomorphic impacts...

Response time and water origin in a steep nested catchment in the Italian Dolomites

In this study we investigate the surface flow time of rise in response to rainfall and snowmelt events at different spatial scales and the main sources originating channel runoff and spring water in a steep nested headwater catchment (Rio Vauz, Italian Dolomites), characterized by a marked elevation gradient. We monitored precipitation at different elevations and measured water stage/streamflow at the outlet of two rocky subcatchments of the same size, representative of the upper part of the catchment dominated by outcropping bedrock, at the outlet of a soil-mantled and vegetated subcatchment of similar size but different morphology, and by the outlet of the main catchment. Hydrometric data are coupled with stable isotopes and electrical conductivity sampled from different water sources during five years, and used as tracers in end-member mixing analysis, application of the two component mixing model and analysis of the slope of the dual-isotope regression line. Results reveal that times of rise are slightly shorter for the two rocky subcatchments, particularly for snowmelt and mixed rainfall/snowmelt events, compared to the soil-mantled catchment and the entire Rio Vauz catchment. The highly-variable tracer signature of the different water sources reflects the geomorphological and geological complexity of the study area. The principal end-members for channel runoff and spring water are identified in rainfall and snowmelt, which are the dominant water sources in the rocky upper part of the study catchment, and soil water and shallow groundwater, which play a relevant role in originating baseflow and spring water in the soil-mantled and vegetated lower part of the catchment. Particularly, snowmelt contributes up to 64% ± 8% to spring water in the concave upper parts of the catchment and up to 62% ± 11% to channel runoff in the lower part of the catchment. These results offer new experimental evidences on how Dolomitic catchments capture and store rain water and meltwater, releasing it through a complex network of surface and subsurface flow pathways, and allow for the construction of a preliminary conceptual model on water transmission in snowmelt-dominated catchments featuring marked elevation gradients