Stefano Crema

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.

Flood probability quantification for road infrastructure: Data-driven spatial-statistical approach and case study applications

Climate-driven increase in the frequency of extreme hydrological events is expected to impose greater strain on the built environment and major transport infrastructure, such as roads and railways. This study develops a data-driven spatial-statistical approach to quantifying and mapping the probability of flooding at critical road-stream intersection locations, where water flow and sediment transport may accumulate and cause serious road damage. The approach is based on novel integration of key watershed and road characteristics, including also measures of sediment connectivity. The approach is concretely applied to and quantified for two specific study case examples in southwest Sweden, with documented road flooding effects of recorded extreme rainfall. The novel contributions of this study in combining a sediment connectivity account with that of soil type, land use, spatial precipitation-runoff variability and road drainage in catchments, and in extending the connectivity measure use for different types of catchments, improve the accuracy of model results for road flood probability.

Semi-quantitative method for the assessment of debris supply from slopes to river in ungauged catchments.

This paper presents an integrated empirical methodology for assessing the amount of sediment transported from slopes to the main river in absence of a sediment transport monitoring system. The amount of transported sediment is calculated through the characterization of the sediment source areas including the identification of the slope phenomena responsible for the sediment propagation to the main river: shallow landslides, channelized debris flows and deep-seated rotational slides. On this basis, several scenarios related to the climatic conditions are defined: they indicate the number of possible slope phenomena and potential volumes of mobilized unconsolidated material from sediment source areas to the main river. This methodology was finalized and tested in the Maira River basin (south-western Italian Alps) with quite good results.

Semi-automatic derivation of channel network from a high-resolution DTM: the example of an Italian alpine region

Abstract High-resolution digital terrain models (HR-DTMs) of regional coverage open interesting scenarios for the analysis of landscape, including derivation and analysis of channel network. In this study, we present the derivation of the channel network from a HR-DTM for the Autonomous Province of Trento. A preliminary automatic extraction of the raw channel network was conducted using a curvature-based algorithm applied to a 4 m resolution DTM derived from an airborne LiDAR survey carried out in 2006. The raw channel network automatically extracted from the HR-DTM underwent a supervised control to check the spatial pattern of the hydrographic network. The supervised control was carried out by means of different informative layers (i.e. geomorphometric indexes, orthophoto imagery and technical cartography) resulting in an accurate and fine-scale channel network.

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

The Rotolon Catchment Early-Warning System

Severe instability processes have affected the Rotolon catchment (Eastern Italian Alps) for long time. The first written proof of a landslide occurrence dates back to 1798. The last re-activation occurred in November 2010 when, after a cumulative rainfall of 637 mm in 12 days, a mass of approximately 320,000 m3 detached from the unstable slope and evolved into a debris flow that ran for about 4 km along the channel, threatening the villages along the stream. Since then, the design and the implementation of an early-warning system was considered of primary importance, in order to mitigate the risk for the valley inhabitants. A monitoring network consisting of 42 topographic benchmarks and 6 wire extensometers was installed and a warning system was deployed along four sections of the channel. The latter consists of 3 pendulum sections and a trip-wire section. Alarm sirens installed near the settlements are activated when the warning system is triggered by a debris-flow event, guaranteeing some minutes lead time to the local population. In this way the inhabitants could evacuate in time and reach safe areas thus reducing the risk. A Decision Support System (DSS) is also integrated with the deployed sensors, therefore technicians may support crisis response with a user-friendly instrument.

GIS tools for preliminary debris-flow assessment at regional scale

The assessment of the areas endangered by debris flows is a major issue in the context of mountain watershed management. Depending on the scale of analysis, different methods are required for the assessment of the areas exposed to debris flows. While 2-D numerical models are advised for detailed mapping of inundation areas on individual alluvial fans, preliminary recognition of hazard areas at the regional scale can be adequately performed by less data-demanding methods, which enable priority ranking of channels and alluvial fans at risk by debris flows. This contribution focuses on a simple and fast procedure that has been implemented for regional-scale identification of debris-flow prone channels and prioritization of the related alluvial fans. The methodology is based on the analysis of morphometric parameters derived from Digital Elevation Models (DEMs). Potential initiation sites of debris flows are identified as the DEM cells that exceed a threshold of slope-dependent contributing area. Channel reaches corresponding to debris flows propagation, deceleration and stopping conditions are derived from thresholds of local slope. An analysis of longitudinal profiles is used for the computation of the runout distance of debris flows. Information on erosion-resistant bedrock channels and sediment availability surveyed in the field are taken into account in the applications. A set of software tools was developed and made available (https://github.com/HydrogeomorphologyTools) to facilitate the application of the procedure. This approach, which has been extensively validated by means of field checks, has been extensively applied in the eastern Italian Alps. This contribution discusses potential and limitations of the method in the frame of the management of small mountain watersheds.

Clustering sediment connectivity maps to distinguish hillslope processes

The availability of increasingly higher resolution Digital Terrain Models (DTMs) allows to perform a detailed characterization of the morphological features on selected areas of interest. There is a growing interest, within the scientific community, for automatic DTM-based procedures to extract geomorphological features or to model surface processes. Among these, the Index of sediment Connectivity (IC), which estimates the degree of linkage that controls sediment fluxes throughout landscape, and, in particular, between sediment sources and downstream areas, has proved to be a powerful morphometric tool. In particular, IC may be used to better portray and highlight sediment dynamics and pathways at catchment scale. In this work, we compare IC maps for two alpine areas. The first area is a typical debris-flow catchment, while the second areais characterized by the presence of a large deep-seated gravitational slope deformation. Two types of numerical analysis are presented:i) the aggregation and normalization of the IC values and ii) cluster analysis. Our preliminary results show that the spatial information, which is maintained in the cluster analysis and lost in the aggregate procedure, is extremely valuable for the identification and the clustering of areas that are affected by the same predominant geomorphological process.

Landslide monitoring with an integrated platform: methodology, design and case study

Landslide monitoring is usually linked to a complex physical system with several parameters and uncertainties. Different instruments are located and set up on field, but a simple, continuous and efficient integration is compulsory for an acceptable warning support.The South-East flank of Mount Carega (Eastern Italian Alps) is affected by a complex landslide that, since 1985, is threatening Recoaro Terme. After the last re-activation on November 2010 (637 mm of intense rainfall recorded in the 12 days prior the event), a mass of approximately 320.000 m3 detached from the flank of Mount Carega and evolved into a fast debris flow that ran for about 4 km along the stream bed.A real-time monitoring system was required to detect early indication of rapid movements, potentially saving lives and property. A web-based platform for automatic and continuous monitoring was designed as a first step in the implementation of an early-warning system. Measurements collected by the automated geotechnical and topographic instrumentation, deployed over the landslide body, are gathered in a central acquisition unit. After the calibration process, data are transmitted by web services on a local server, where graphs, maps, reports and alert announcement are automatically generated and updated. All the processed information is available by web browser with different access rights.