Comparison of flow resistance relations for debris flows using a one-dimensional finite element simulation model

International audienceThis paper describes a one-dimensional finite element code for debris flows developed to model the flow within a steep channel and the stopping conditions on the fan. The code allows the systematic comparison of a wide variety of previously proposed one-phase flow resistance laws using the same finite element solution method. The one-dimensional depth-averaged equations of motion and the numerical model are explained. The model and implementation of the flow resistance relations was validated using published analytical results for the dam break case. Reasonable agreement for the front velocities and stopping location for a debris-flow event in the Kamikamihori torrent in Japan can be achieved with turbulent flow resistance relations including "stop" terms which allow the flow to come to rest on a gently sloping surface. While it is possible to match the overall bulk flow behavior using relatively simple flow resistance relations, they must be calibrated. A sensitivity analysis showed that the shape of the upstream input hydrograph does not much affect the flow conditions in the lower part of the flow path, whereas the event volume is much more important

Partial Transport of a Sand-Gravel Sediment

Grains of a single size within a mixed-size bed are entrained over a range of flows. Within this range some grains exposed on the bed surface are active (entrained at least once over the duration of a transport event), while the remaining surface grains are immobile, a condition we define as partial transport. We demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand/gravel mixture. The active proportion of the bed surface increases with bed shear stress τ0. At a given τ0, 90% of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed. Mobilization of grains in a size fraction increases from 10% to 90% over a range of τ0 of a factor of 2. The bounds of this range increase with grain size Di so that at a given τ0, sizes over a range of a factor of 4 are in a state of partial transport. Fractional transport rates are independent of Di for fully mobilized fractions and decrease rapidly with Di for partially mobile fractions. Partial transport is associated with substantial transport rates of finer, fully mobile sizes, limits both the rate and size distribution of grain exchange with the bed subsurface, and may be the dominant transport condition in many gravel-bed rivers

Numerical simulation of bar and island morphodynamics in anabranching mega-rivers

Onlineopen article ©2013 American Geophysical Union.Bar and island morphodynamics in the world's largest anabranching rivers are investigated using a new numerical model of hydrodynamics, sediment transport, bank erosion, and floodplain development, operating over periods of several hundred years. Simulated channel morphology is compared to that of natural rivers and shown to be realistic, both in terms of the statistical characteristics of channel width, depth, and bar shape distributions, and mechanisms of unit bar, compound bar, and island evolution. Results demonstrate that bar and island stability may be sensitive to hydrologic regime, because greater variability in flood magnitude encourages the formation of emergent bars that can be stabilized by vegetation colonization. Simulations illustrate a range of mechanisms of unit bar generation that are linked to local bed or bank instabilities. This link may explain the reduced frequency of unit bars observed in some large anabranching rivers that are characterized by stable vegetated islands and slow rates of channel change. Model results suggest that the degree to which sand-sized bed material is carried in suspension likely represents an important control on bar morphodynamics and channel network evolution, because of its influence on sand transport direction. Consequently, differences in the partitioning of the total sand load between bed load and suspension may provide a partial explanation for contrasting styles of anabranching in the world's largest sand-bed rivers. These results highlight a need for spatially-distributed flow and sediment transport data sets from large rivers, in order to support improved parameterizations of sand transport mechanics in morphodynamic models.Natural Environment Research Council (NERC). Grant Numbers: NE/I023228/1, NE/E016022/

A probabilistic sediment cascade model of sediment transfer in the Illgraben

We present a probabilistic sediment cascade model to simulate sediment transfer in a mountain basin (Illgraben, Switzerland) where sediment is produced by hillslope landslides and rockfalls and exported out of the basin by debris flows and floods. The model conceptualizes the fluvial system as a spatially lumped cascade of connected reservoirs representing hillslope and channel storages where sediment goes through cycles of storage and remobilization by surface runoff. The model includes all relevant hydrological processes that lead to runoff formation in an Alpine basin, such as precipitation, snow accumulation, snowmelt, evapotranspiration, and soil water storage. Although the processes of sediment transfer and debris flow generation are described in a simplified manner, the model produces complex sediment discharge behavior which is driven by the availability of sediment and antecedent wetness conditions (system memory) as well as the triggering potential (climatic forcing). The observed probability distribution of debris flow volumes and their seasonality in 2000–2009 are reproduced. The stochasticity of hillslope sediment input is important for reproducing realistic sediment storage variability, although many details of the hillslope landslide triggering procedures are filtered out by the sediment transfer system. The model allows us to explicitly quantify the division into transport and supply-limited sediment discharge events. We show that debris flows may be generated for a wide range of rainfall intensities because of variable antecedent basin wetness and snowmelt contribution to runoff, which helps to understand the limitations of methods based on a single rainfall threshold for debris flow initiation in Alpine basins

Murgangmodellierung mit dynamisch-physikalischem und GIS-basiertem Fliessmodell

Während der Unwetter im August 2005 wurden in Guttannen (BE) über 500 000 m3 Geschiebe durch einen Murgang im Talboden abgelagert. Ein Rückstau bzw. eine Verlagerung der Aare aus ihrem ursprünglichen Gerinne mit anschliessender Überflutung des Dorfes waren die Folgen. Auslöser für dieses ausserordentliche Ereignis stellten nebst den intensiven Niederschlägen auch die begünstigenden hydro geologischen Verhältnisse im Anrissgebiet dar (Geologie, Geschiebepotenzial und Permafrost). Mittels eines Fliess- und Massenbewegungsmodells (MSF) sowie eines dynamisch- physikalischen Modellansatzes(RAMMS) wurde das Ereignis nachmodelliert, um einerseits Rückschlüsse bezüglich Plausibilität und andererseits Hinweise auf die Kalibrierung des Modells zu erhalten. Die Resultate korrelieren dabei gut mit den tatsächlichen Ablagerungscharakteristiken. Die Modelle können daher für unterschiedliche Fragestellungen (z.B. bei der Gefahrenkartierung) als wertvolle Unterstützung dienen. Der Einsatz von RAMMS in der Praxis steht in Kürze bevor

Surface-based Fractional Transport Rates: Mobilization Thresholds and Partial Transport of a Sand-gravel Sediment

Twenty-eight coupled observations of flow, transport, and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted, bimodal grain size distribution. These observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface. The key to our observations is the use of a sediment in which each size fraction has been painted a different color, which permits reliable, repeatable, and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface. At a given flow, the fractional transport rates may be divided into two parts: a finer-grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate, and a coarser-grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run. We define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength. Complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion ofthat fraction. Zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution, although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed. Critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude, a result strongly in contrast to many earlier observations, but consistent with our observations of incipient motion in sediments with bimodal grain size distributions

An integrated method for debris flow hazard mapping using 2D runout models

Hazard mapping of rapid mass movements in Alpine regions, especially debris flows, remains a challenging task in land-use management. Physically-based 2D runout models are capable of predicting debris-flow intensities over natural topography for use in the generation of detailed hazard maps. However constitutive hypothesis underlying the models are rarely fully tested and it is difficult to consider all natural variables. Differences in simulation results among different models are not negligible, causing uncertainty in application methodology to hazard mapping and increasing the risk if the model results are not generalized appropriately in hazard map drawing. In two cases in the Swiss Alps, a pragmatic method is proposed which accounts differences in model results, herein shown using two 2D models (FLO-2D and RAMMS) and empirical equations. This approach represents an alternative or complementary way to assess the degree of uncertainty in hazard maps

How memory effects, check dams, and channel geometry control erosion and deposition by debris flows

Debris flows can grow greatly in size and hazardous potential by eroding bed and bank material, but effective hazard assessment and mitigation is currently hampered by limited understanding of erosion and deposition dynamics. We have collected high-resolution pre- and post-flow topography for 6 debris flows over a 3 km long unconsolidated reach of the Illgraben channel in the Swiss Alps with drone-based photogrammetry. We show that the spatio-temporal patterns of erosion and deposition in debris-flow torrents are highly variable and dynamic. Check dams strongly control the spatial patterns of erosion and deposition. We identify a memory effect where erosion is strong at locations of strong deposition during previous flows and vice versa. Large sediment inputs from subcatchments initially result in new channel erosion through the subcatchment deposits and simultaneous upstream deposition, likely as a result of backwater effects. It is generally believed that erosion increases with debris-flow magnitude, but we show that there is a limit to debris-flow bulking set by channel geometry. These findings provide key guidelines for flow volume forecasting, emphasizing the importance of memory effects and the need to resolve both erosion and deposition in predictive models

A comparative 2d modeling of a debris flow e outcomes for end-users

Alpine debris flows endanger settlements and human life. Mitigation strategies based on hazard maps are necessary tools for land planning. These maps can be made more precise by using numerical models to forecast the inundated areas after carefully selecting those \u2019key parameters\u2019 (K-P) which directly affect the flow motion and its interaction with the ground surface. Several physically-based 2D models are available for practitioners and governmental agencies, but the selection criteria of model type and of the related K-P remain flexible and partly subjective. This remark has driven us to investigate how different models simulate different types of debris flows (from granular to muddy debris flows, going through intermediate types), also in cases of flows influenced by the presence of check dams and deposition basins. The main aims of the study are i) to evaluate the performances of two commercially-available models to simulate different types of debris-flows, different volumes and the capacity of simulate mitigation measures like check dams and retention basins, ii) identify a procedure to calibrate the key model parameters and define the input data, iii) identify a synthetic simple index that permits evaluation of model performanc