Jošt Sodnik

Modeling of a debris flow from the Hrenovec torrential watershed above the village of Kropa

In this paper, debris-flow modeling is shown specifically on the basis of a potential debris flow from the Hrenovec torrential watershed above the village of Kropa in NW Slovenia. This site was chosen, because in this particular torrential watershed a small surficial landslide turned into a debris flow during a storm on September 18, 2007. Fortunately, the debris flow stopped inside the torrential channel above the village of Kropa. Using public available data on rainfall and topography, we developed two scenarios of debris-flow triggering with an estimated magnitude of 50,000m(3). According the first scenario, a debris flow triggers during rainfall with the 100-year return period, and according the second one, it triggers during extreme rainfall as measured during the storm on September 18, 2007. For both scenarios, we used for debris-flow modeling a commercial two-dimensional mathematical model Flo-2d. The obtained results are shown in the form of computed flow depths and velocities over a cartographic background. The results show to possible catastrophic consequences in the village of Kropa, much worse that set in by torrential flood during the storm on September 18, 2007.

Are Torrent Check-Dams Potential Debris-Flow Sources?

Check dams as a typical torrent structure behave as retention structures typically storing torrent sediments in order to stabilise the torrential bed. If such structures are damaged and partially broke down, stored torrent sediments may cause sediment–related disasters in the downstream torrent’s reaches or on torrential fans. A field study on the status of 22 check dams in the Upper Sava River in NW Slovenia with regard to their structural stability was performed to answer a question whether they should be taken as a possible debris-flow source. For this purpose we used the existing inventory (cadastre) of water infrastructure in torrents and field survey. The resulted hazard assessment showed rather high debris-flow risk for downstream infrastructure in the lower parts of the investigated torrential watersheds. That is why we should treat some check dams as “large” dams.

Topographic Data and Numerical Debris-Flow Modeling

The digital elevation models (classical DEM 5 and DEM 12.5), publically available in Slovenia, have been evaluated as a basis to prepare numerical square grids 5 × 5 m for 2D modeling of possible debris flows on torrential fans, using the model Flo-2D. Also recently available LiDAR data in their original resolution have been used, as well as their decreased resolution to the one of the numerical grid (e.g. 5 × 5 m). From our numerical results it seems obvious that the use of more precise LiDAR data over classical DEMs for numerical debris-flow modeling is fully justified. Better quality of input topographic data assures higher accuracy of results and therefore also accuracy of hazard maps produced in such a way. The LiDAR data promises better representation of torrential channels on torrential fans (narrow, deep channels) and computed results (velocities, depths) are generally better estimated. Using more precise data also increases computational times compared to using classical DEMs.

WCoE: Mechanisms of Landslides in Over-Consolidated Clays and Flysch and IPL-151 Project: Soil Matrix Suction in Active Landslides in Flysch—The Slano Blato Landslide Case

The Faculty of Civil and Geodetic Engineering of the University of Ljubljana (UL FGG), Slovenia, Europe, was voted in 2011 at the 2nd World Landslide Forum in Rome, Italy to be one of the 14 new World Centres of Excellence (WCoE) in Landslide Disaster Reduction for the period 2011 to 2014. This successful nomination followed the period 2009-2011, in which UL FGG successfully fulfilled the role as one of the WCoEs for the first time. The title of the activities of the WCoE was selected to be “Mechanisms of landslides in over-consolidated clays and flysch”.

TXT-tool 3.386-1.1: Two-Dimensional Debris-Flow Modelling and Topographic Data

Two-dimensional (2-D) debris-flow mathematical modeling is a useful und widely used tool when assessing debris-flow hazard. A high accuracy for 2-D model input parameters is essential for obtaining acceptable simulation results, especially for potential debris flows without a recent history of activity. The numerical grid in the debris-flow transport area is usually generated from available topographic data. In Slovenia, DEM5 and DEM12.5 are publicly available data. However, the morphological accuracy of those datasets is questionable because of their development methods and their low resolution. A better solution is using LiDAR-derived data with their higher resolution and a lot of options for further improvements using different methods and algorithms. 2-D debris-flow simulation results using LiDAR data, compared to less accurate DEMs, are more accurate and more useful for debris-flow hazard mapping. The modelled debris-flow depths and velocities are more accurate and better follow field conditions. Although using high-resolution topographic data results in much longer computational time, this shouldn’t be a problem for potential debris flows. For debris-flow endangered and populated torrential fans, modelling of the influence of built structures on flow is important. With adequate numerical modelling of such structures the obtained simulation results are more meaningful, with better expressed local flow conditions. Comparisons using publicly available topographic data sets also show that higher resolution (DEM5 vs. DEM12.5) doesn’t always mean better morphologic accuracy.

Identification of Landslides as Debris Flow Sources Using a Multi-model Approach Based on a Field Survey—Koroška Bela, SloveniaOpen image in new window

The landslide as a debris flow source identification is an important but often complex step in debris flow hazard assessment. Landslides are an important source of debris flows and this paper presents a multi-model approach of identification. The village of Koroska Bela in NW Slovenia has a history of debris flows and active landslides in the watershed of the Bela torrent, which presents a sediment source for potential debris flows in the future. Two models were applied for landslide identification in the watershed. A detailed field survey was carried out for model results validation. A GIS-based landslide susceptibility model was applied to identify areas susceptible to landslides and the LS Rapid triggering model was applied also to identify landslides and simulate the triggering phase. The study results show good agreement of the field survey and the models, particularly considering the completely different origin of both models and their development. These results show the possibility of applying numerical models to identify landslide sources and encourage their implementation in debris flow hazard assessment. But as with all models, a high level of expert knowledge and users’ experience is required to get useful, and what is more, reliable results.

Mechanisms of Landslides and Creep in Over-Consolidated Clays and Flysch (WCoE 2014–2017)Open image in new window

The Faculty of Civil and Geodetic Engineering of the University of Ljubljana (UL FGG), Slovenia, Europe, was voted in 2014 at the 3rd World Landslide Forum in Beijing, China to be one of the 15 new World Centres of Excellence (WCoE) in Landslide Disaster Reduction for the period 2014–2017. This successful nomination followed the period 2011–2014, in which UL FGG successfully fulfilled the role as one of the WCoEs for the second time. The title of the activities of the WCoE in this third term was slightly modified to be “Mechanisms of Landslides and Creep in Over-Consolidated Clays and Flysch”. We can divide the activities of the WCoE at UL FGG into international and national research activities. The international ones consisted of the ICL related activities with the main task of being the main organizer of this 4th World Landslide Forum 2017, international cooperation, European research activities, and bilateral cooperation. The national ones consisted of the national projects and the national research program “Water Science and Technology, and Geotechnics”. In the paper, these activities of the WCoE at UL FGG are elaborated in more detail, with a comprehensive list of publications to show the dissemination and capacity building efforts.

Concrete Torrent Check-Dams and Debris-Flow Magnitudes

Large sediment-retention dams, built in a cascade (a chain of check dams), can impose hazard (hyper-concentrated sediment flow, debris flow), if they fail during torrential flash floods or when destroyed by overtopping by a debris flow initiated on slopes or in natural torrential channels. Using estimates of specific annual sediment yields in torrential watersheds on one hand, one can use the area of the watershed and the storage volume of torrential check dams in order to estimate the maximum potential of sediment stored in the retention volume of the check dams—as a first approximation for the magnitude of the debris flows initiated by the dam failure. On the other hand, one can use debris-flow susceptibility maps to estimate likelihood for triggering of potential debris flows in torrential watersheds under investigation (we used debris-flow susceptibility map of Slovenia in the scale 1:250,000). A field study on the status of several tens of check dams in the Upper Sava River in NW Slovenia with regard to the question whether they should be taken as a possible debris-flow source was performed.