Carlo Gregoretti

Laboratory experiments on the failure of coarse homogeneous sediment natural dams on a sloping bed.

We present the results of a systematic series of 168 laboratory experiments that examine the critical conditions for the failure of landslide dams, which obstruct the course of mountain rivers. The experiments were carried out by using three different sediment materials with a quasiuniform grain size distribution and with a flume bed slope angles that ranged between 0° and 5.71° (0–10%). Three main typologies of dam failure were observed for increasing values of the dam’s downstream-face angle: (1) overtopping; (2) headcutting, which led to the formation of an erosion channel on the dam’s downstream face that progressively migrated up to the dam crest; and (3) initial slide of large part of superficial layer of the dam’s downstream face, which was followed by headcutting. The experiments focused on the second type of failure to provide a safety criterion based on the upstream reservoir level. The quantities that govern this phenomenon have been identified and a functional relationship is proposed based on...

Experimental evidence from the triggering of debris flow along a granular slope

Abstract The instability of a granular slope, with seepage flow and low tail downstream water level, has been experimentally investigated to point out the conditions associated to the formation of a debris flow, excluding the case of the debris flow triggered by a failure due to the toe erosion. The materials employed in the experiments are uniform glass spheres (d = 0.003 m) and two different gravel (d50 = 0.007 m and d50 = 0.0022 m). Experimental runs show that the triggering of debris flow is due to the overland flow.

Runoff of small rocky headwater catchments: Field observations and hydrological modeling.

In dolomitic headwater catchments, intense rainstorms of short duration produce runoff discharges that often trigger debris flows on the scree slopes at the base of rock cliffs. In order to measure these discharges, we placed a measuring facility at the outlet (elevation 1770 m a.s.l.) of a small, rocky headwater catchment (area ∼ 0.032 km2, average slope ∼ 320%) located in the Venetian Dolomites (North Eastern Italian Alps). The facility consists of an approximately rectangular basin, ending with a sharp-crested weir. Six runoff events were recorded in the period 2011-2014, providing a unique opportunity for characterizing the hydrological response of the catchment. The measured hydrographs display impulsive shapes, with an abrupt raise up to the peak, followed by a rapidly decreasing tail, until a nearly constant plateau is eventually reached. This behavior can be simulated by means of a distributed hydrological model if the excess rainfall is determined accurately. We show that using the Soil Conservation Service Curve-Number (SCS-CN) method and assuming a constant routing velocity invariably results in an underestimated peak flow and a delayed peak time. A satisfactory prediction of the impulsive hydrograph shape including peak value and timing is obtained only by combining the SCS-CN procedure with a simplified version of the Horton equation, and simulating runoff routing along the channel network through a matched diffusivity kinematic wave model. The robustness of the proposed methodology is tested through a comparison between simulated and observed timings of runoff or debris flow occurrence in two neighboring alpine basins. This article is protected by copyright. All rights reserved.

Comparative analysis of the differences between using LiDAR and contour-based DEMs for hydrological modeling of runoff generating debris flows in the Dolomites

Citation: Degetto M, Gregoretti C and Bernard M (2015) Comparative analysis of the differences between using LiDAR and contour-based DEMs for hydrological modeling of runoff generating debris flows in the Dolomites. Front. Earth Sci. 3:21. doi: 10.3389/feart.2015.00021 Comparative analysis of the differences between using LiDAR and contour-based DEMs for hydrological modeling of runoff generating debris flows in the Dolomites

Micro-scale Landslide Displacements Detection Using Bayesian Methods Applied to GNSS Data

In this chapter, we evaluate the movement of 6 points near a landslide body, which were surveyed with GNSS receivers over time. We apply Bayesian inference to identify the areas on the ground with statistically significant vertical (downwards) shifts. Traditional statistical methods work well only when point displacements between different survey epochs are sufficiently large compared to the standard deviations of related coordinates. In such cases, coordinate differences of some points can be marked as potential displacements. The Bayesian analysis can help to improve discrimination when height differences, computed with respect to the first measurement epoch, are at the same order of magnitude as the uncertainties of the measures. After the application of the classical statistical test, one network point, close to the upper part of the landslide area, seemed to be more unstable than the remainder. In order to remove or validate the hypothesis of instability, the Bayesian statistical inference was applied, and all three of the upper group of points show significant shift, depending on the data prior parameters. This application shows that the Bayesian approach can be considered as an integration to classical statistical significance testing (e.g. z-test), reliably showing significance in vertical directional (i.e., downwards) coordinate shifts, thus supporting detection of movements having lower magnitude.

Modeling shallow water flows on general terrains

Abstract A formulation of the two-dimensional shallow water equations adapted to general and complex terrains is proposed. Its derivation starts from the observation that the typical approach of depth integrating the Navier–Stokes equations along the direction of gravity forces is not exact in the general case of a tilted curved bottom. We claim that an integration path that better adapts to the shallow water hypotheses follows the “cross-flow” surface, i.e., a surface that is normal to the velocity field at any point of the domain. Because of the implicitness of this definition, we approximate this “cross-flow” path by performing depth integration along a local direction normal to the bottom surface, and propose a rigorous derivation of this approximation and its numerical solution as an essential step for the future development of the full “cross-flow” integration procedure. We start by defining a local coordinate system, anchored on the bottom surface to derive a covariant form of the Navier–Stokes equations. Depth integration along the local normals yields a covariant version of the shallow water equations, which is characterized by flux functions and source terms that vary in space because of the surface metric coefficients and related derivatives. The proposed model is numerically discretized with a first order FORCE-type Godunov Finite Volume scheme that allows straight forward implementation of spatially variable fluxes. We investigate the validity of our SW model and the effects of the geometrical characteristics of the bottom surface by means of three synthetic test cases that exhibit non negligible slopes and surface curvatures. The results show the importance of taking into consideration bottom geometry even for relatively mild and slowly varying curvatures. By comparison with the numerical solution of vertically integrated models, we observe differences of almost 20%, in particular for the peak values and the shape of the hydrographs calculated at given sections of the fluid domain.

The debris flow occurred at Ru Secco Creek, Venetian Dolomites, on 4 August 2015: analysis of the phenomenon, its characteristics and reproduction by models

On 4 August 2015, a very high intensity storm, 31.5 mm in 20 minutes (94.5 mm/h), hit the massif of Mount Antelao on the Venetian Dolomites triggering three debris flows characterized by high magnitude. Two of them occurred in the historical sites of Rovina di Cancia and Rudan Creek and were stopped by the retaining works upstream the inhabited areas, while the third routed along the Ru Secco Creek and progressively reached the resort area and the village of San Vito di Cadore, causing fatalities and damages. The main triggering factor of the Ru Secco debris flow was a large rock collapse on the northern cliffs of Mount Antelao occurred the previous autumn. The fallen debris material deposited on the Vallon d’Antrimoia inclined plateau at the base of the collapsed cliffs and part reached the Ru Salvela Creek, covering it from the head to the confluence with the Ru Secco Creek. The abundant runoff, caused by the high intensity rainfall on 4 August 2015, mobilized about 52500 m^3 of the debris material laying on the Vallon d’Antrimoia forming a debris flow surge that hit and eroded the debris deposit covering the downstream Ru Salvela Creek, increasing its volume. This debris flow routed downstream the confluence, flooding the parking of a resort area where three people died, and reached the village downstream damaging some buildings. A geomorphological analysis was initially carried out after surveying the whole basin. All liquid and solid-liquid contributions to the phenomenon were recognized together with the areas subjected to erosion and deposition. The elaboration of pre and post-event topographical surveys, provided the map of deposition-erosion depths. Using the rainfall estimated by weather radar and corrected by the nearest rain gauge, about 0.8 km far, we estimated runoff by using a rainfall-runoff model designed for the headwater rocky basins of Dolomites. A triggering model provided the debris flow hydrographs in the initiation areas, routed downstream by means of a cell model. The comparison between the simulated and estimated deposition-erosion pattern resulted satisfactory. The results of the simulation captured, in fact, the main features of the occurred phenomenon.

Evaluating the differences of gridding techniques for Digital Elevation Models generation and their influence on the modeling of stony debris flows routing: A case study from Rovina di Cancia basin (North-eastern Italian Alps)

Debris flows are among the most hazardous phenomena in mountain areas. To cope with debris flow hazard, it is common to delineate the risk-prone areas through routing models. The most important input to debris flow routing models are the topographic data, in the form of Digital Elevation Models (DEMs). The quality of the DEMs depends on the accuracy, density, and spatial distribution of the sampled points; on the characteristics of the surface; and on the gridding method used to obtain them. Therefore, the choice of the DEMs interpolation method affects the realistic representation of the channel and fan morphology, and thus reasonably the debris flow routing modeling outcomes. In this paper, we initially investigate the performance of common interpolation methods (i.e. linear triangulation, natural neighbour, nearest neighbour, inverse distance to a power, ANUDEM, Radial Basis Functions, and ordinary kriging) in building DEMs with the complex topography of a debris flow channel located in the Venetian Dolomites (North-eastern Italian Alps), by using small footprint full-waveform Light Detection And Ranging (LiDAR) data. The investigation is carried out through a combination among statistical analysis of vertical accuracy, algorithm robustness, and spatial clustering of vertical errors, and multi-criteria shape reliability assessment. After that, we examine the influence of the tested interpolation algorithms on the performance of a Geographic Information System (GIS)-based cell model for simulating stony debris flows. In detail we investigate both the correlation between the DEMs heights uncertainty resulting from the gridding procedure and that on the corresponding simulated erosion/deposition depths, both the effect of interpolation algorithms on simulated areas, erosion and deposition volumes, solid-liquid discharges, and channel morphology after the event. The comparison among the tested interpolation methods highlights that the ANUDEM and ordinary kriging algorithms are inadequate in building DEMs with complex topography. Conversely, the linear triangulation, the natural neighbour algorithm, and the thin-plate spline plus tension and completely regularized spline functions ensure the best trade-off among accuracy and shape reliability. Anyway, the evaluation of the effects of gridding techniques on debris flow routing modeling reveals that the choice of the interpolation algorithm does not significantly affect the model outcomes.