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

Present work aims to explore the differences in hydrological modeling when using digital elevation models (DEMs) generated by points from LiDAR surveys and those digitized on the contour lines of the regional technical map (RTM) and their relevance for the simulation of debris flow triggering. Hydrological models for mountainous areas are usually based on digital elevation models (DEMs). DEMs are used to determine the flow path from each pixel, by which the basin is discretized, to the outlet. Hydrological simulations of runoff that triggered debris flows occurred in two rocky headwater basins of Dolomites, Fiames Dimai (area approximately 0.03 km2) and Cancia (area approximately 0.7 km2) are carried out using a DEM-based model designed for simulating runoff that descends from headwater areas. For each basin, the runoff is simulated using DEMs that are generated using points from LiDAR, and those digitized on the contour lines of the regional technical map, respectively. The results show that the peak discharge values corresponding to the simulations carried out using the LiDAR-based DEMs are higher than those corresponding to the simulations carried out using the RTM-based DEMs. Larger differences are observed for the Dimai basin, where the area corresponding to the RTM-based DEM is markedly smaller than the area corresponding to LiDAR-based DEM, whereas for the Cancia basin, the two areas are similar. Both the differences in the peak discharge and the basin area are due to the poor accuracy of the contour-based DEM (i.e., elevation accuracy), that is, a poor representation of the morphological features that leads to errors on the watershed divide and simplifications of the flow paths from each cell to the outlet. This result is highly relevant for estimating the triggering conditions of runoff generated debris flows. An incorrect simulated value of peak discharge can lead to errors both in planning countermeasures against debris flows and in predicting their occurrence

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 min (94.5 mm/h), hit the massif of Mount Antelao on the Venetian Dolomites triggering three stony debris \ufb02ows 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 \ufb02ow 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\u2019Antrimoia inclined plateau at the base of the collapsed cliffs and, below it, on the Ru Salvela Creek, covering it from the head to the con\ufb02uence with the Ru Secco Creek. The abundant runoff, caused by the high intensity rainfall on 4 August 2015, entrained about 52,500 m3 of the debris material laying on the Vallon d\u2019Antrimoia forming a debris \ufb02ow surge that hit and eroded the debris deposit covering the downstream Ru Salvela Creek, increasing its volume, about 110,000 m3 of mobilized sediments. This debris \ufb02ow routed downstream the con\ufb02uence, \ufb02ooding 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 \ufb02ow hydrographs in the initiation areas, after using the simulated runoff. The initial solid-liquid surge hydrographs were, then, 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

MODELLO DI INNESCO DI UNA COLATA DETRITICA ORIGINATA DALL'EROSIONE DI SEDIMENTI PER DEFLUSSO SUPERFICIALE E DELL'IDROGRAMMA SOLIDO-LIQUIDO

Le colate detritiche si originano in ambiente alpino prevalentemente per la mobiliz- zazione, causa deflusso superficiale, dei sedimenti posti al fondo di canali e corsi d\u2019acqua. Il presente lavoro propone un modello di innesco che si basa su di un mod- ello idrologico per la determinazione dei deflussi superficiali; un metodo per deter- minare le condizioni di innesco di una colata; un modello per il calcolo della portata di picco di una colata; un modello di costruzione dell\u2019idrogramma solido-liquido. Per quest\u2019ultimo si propongono due forme diverse: una che si appoggia alla forma dell\u2019idrogramma liquido ed una di forma triangolare. La prima corrisponde ad un idrogramma solido-liquido nella zona di formazione della colata, la seconda ad una colata gi\ue1 sviluppata. Si riporta infine un caso di applicazione del modello per ri- costruire lo scenario di una colata avvenuta a Fiames (Cortina d\u2019Ampezzo, BL) il 25 Luglio 200

Experimental investigation on open check dam for coarse woody debris control

The hydrodynamic behaviour of the floating woody debris is analysed in relation to trapping efficiency of an open check dam. Physical model tests have been carried out in a flume consisting of three sub-reaches with different slope: a log feeding zone, a transfer reach, a detention zone upstream of the check dam openings. The change of flow regime from supercritical to subcritical has proved to play an important role for the redistribution of the entering wood discharge. Further experiments concern the relation between log trap efficiency and inclination and openings of the check dam filter. A dimensionless variable is proposed to assess the experimental measurements and to provide a design criterion for the grid openings

10th Alexander von Humboldt Conference 2015

Four occurred events of debris flows are documented and modeled by back-analysis. The four debris flows events are those occurred at Rio Lazer on the 4th of November 1966, at Fiames on the 5th of July 2006, at Rovina di Cancia on the 18th of July 2009 and at Baselga di Pin\ue8 on the 15th of August 2010. All the four sites are located in the North-Eastern Italian Alps. In all the events, runoff entrained sediments present on natural channels and formed a solid-liquid wave that routed downstream. The first event concerns the routing of debris flow on an inhabited fan. Map of deposition pattern of sediments is built by using post-events photos through stereoscopy techniques. The second event concerns the routing of debris flow along the main channel descending from Pomagagnon Fork. Due to the obstruction of the cross-section debris flow deviated from the original path on the left side and routed downstream by cutting a new channel on the fan. It dispersed in multiple paths when met the wooden area. Map of erosion and deposition depths is built after using a combination of LiDAR and GPS data. The third event concerns the routing of debris flow in the Rovina di Cancia channel that filled the reservoir built at the end of the channel and locally overtopped the retaining wall on the left side. A wave of mud and debris inundated the area downstream the overtopping point. Map of erosion and deposition depths is obtained by subtracting two GPS surveys, pre and post event. The fourth event is that of debris flow occurred along Rio Val Molinara where runoff entrained bed sediments and routed downstream flooding the village of Baselga di Pin\ue8. The map of erosion and deposition depths of Rio Val Molinara is obtained after comparing two LiDAR data corresponding to the pre and post situation while the map of deposition depth of the village is built through carefully analysis of the photo taken after the event with field measurements. All the four occurred debris flows are simulated by modeling runoff that entrained debris flow for determining the solid-liquid hydrograph downstream the triggering areas. The routing of the solid-liquid hydrograph is simulated by a bi-phase cell model based on the kinematic approach. The comparison between simulated and measured erosion and deposition depths is satisfactory. Nearly the same parameters for computing erosion and deposition are used for all the four occurred events

GIS-based cell model for simulating debris flow runout on a fan

A GIS-based cell model, based on a kinematic approach is proposed to simulate debris flow routing on a fan. The sediment-water mixture is modeled as a monophasic continuum, and the flow pattern is discretized by square cells, 1 m in size, that coincide with the DEM cells. Flow occurs from cells with a higher mixture free surface to those with a lower mixture free surface. A uniform-flow law is used if the elevation of the former cell is higher than that of the latter; otherwise, the flow is simulated using the broad-crested weir law. Erosion and deposition are simulated using an empirical law that is adjusted for a monophasic continuum. The sediment concentration in the routing volume is computed at each time step and controls both erosion and deposition. The cell model is used to simulate a debris flow that occurred on the Rio Lazer (Dolomites, North-Eastern Italian Alps) on November 4th, 1966. Furthermore, the hydrologic and the hydraulic conditions for the initiation of debris flow are simulated, providing the solid-liquid hydrograph of the resulting debris flow. A number of simulations has been carried out with physically reasonable parameters. The results are compared with the extension of the debris-flow deposition area and the map of observed depths of deposited sediments. This comparison shows that the proposed model provides good performance. The analysis of sensitivity carried out by systematically varying the model parameters shows that lower performances are associated with parameter values that are not physically reasonable. The same event is also simulated using a cellular automata model and a finite volume two-dimensional model. The results show that the two models provide a sediment deposition pattern less accurate than that provided by the present cell model

GIS-BASED CELL MODEL FOR SIMULATING DEBRIS FLOW ROUTING AND DEPOSITION PHASES ON A FAN

A GIS-based cell model is proposed for the simulation of the routing and deposition phases of debris flow on a fan. Flow pattern is discretized by square cells, 2m size, which coincide with the DEM cells and the mixture is assumed a monophasic continuum. Flow exchange between adjacent cells is ruled by uniform flow or broad-crested weir laws and by continuity equation. Flow occurs from cells with higher surface to those with lower surface and is simulated by uniform flow law if the elevation of the formers is higher than the latter and by broad-crested weir law otherwise. Erosion and deposition are simulated using the empirical law of Egashira, adjusted for monophasic continuum. The cell model is used to simulate debris flow occurred on Rio Lazer (Dolomites, Eastern Italian Alps) the 4th of November 1966. The same event was also simulated using Flo-2D model for a comparison with a widely used model for debris flow simulation. Results of the two simulations were compared with extension of deposition area and the map of measured depths of deposited sediments. Both the model simulate quite well the extent of deposition area, whereas the deposited debris depths are better simulated by the cell model