Debris flows can grow greatly in size by entrainment of bed material, enhancing their runout and hazardous impact. Here, we experimentally investigate the effects of debris-flow composition on the amount and spatial patterns of bed scour and erosion downstream of a fixed to erodible bed transition. The experimental debris flows were observed to entrain bed particles both grain by grain and en masse, and the majority of entrainment was observed to occur during passage of the flow front. The spatial bed scour patterns are highly variable, but large-scale patterns are largely similar over 22.5-35°channel slopes for debris flows of similar composition. Scour depth is generally largest slightly downstream of the fixed to erodible bed transition, except for clay-rich debris flows, which cause a relatively uniform scour pattern. The spatial variability in the scour depth decreases with increasing water, gravel (= grain size) and clay fraction. Basal scour depth increases with channel slope, flow velocity, flow depth, discharge and shear stress in our experiments, whereas there is no correlation with grain collisional stress. The strongest correlation is between basal scour and shear stress and discharge. There are substantial differences in the scour caused by different types of debris flows. In general, mean and maximum scour depths become larger with increasing water fraction and grain size, and decrease with increasing clay content. However, the erodibility of coarse-grained experimental debris flows (gravel fraction = 0.64) is similar on a wide range of channel slopes, flow depths, flow velocities, discharges and shear stresses. This probably relates to the relatively large influence of grain-collisional stress to the total bed stress in these flows (30-50%). The relative effect of grain-collisional stress is low in the other experimental debris flows (<5%), causing erosion to be largely controlled by basal shear stress.

Abstract. Debris-covered glaciers in the Himalaya play an important role in the high-altitude water cycle. The thickness of the debris layer is a key control of the melt rate of those glaciers, yet little is known about the relative importance of the three potential sources of debris supply: the rockwalls, the glacier bed and the lateral moraines. In this study, we hypothesize that mass movement from the lateral moraines is a significant debris supply to debris-covered glaciers, in particular when the glacier is disconnected from the rockwall due to downwasting. To test this hypothesis, eight high-resolution and accurate digital elevation models from the lateral moraines of the debris-covered Lirung Glacier in Nepal are used. These are created using structure from motion (SfM), based on images captured using an unmanned aerial vehicle between May 2013 and April 2018. The analysis shows that mass transport results in an elevation change on the lateral moraines with an average rate of -0.31±0.26 m year−1 during this period, partly related to sub-moraine ice melt. There is a higher elevation change rate observed in the monsoon (-0.39±0.74 m year−1) than in the dry season (-0.23±0.68 m year−1). The lower debris aprons of the lateral moraines decrease in elevation at a faster rate during both seasons, probably due to the melt of ice below. The surface lowering rates of the upper gullied moraine, with no ice core below, translate into an annual increase in debris thickness of 0.08 m year−1 along a narrow margin of the glacier surface, with an observed absolute thickness of approximately 1 m, reducing melt rates of underlying glacier ice. Further research should focus on how large this negative feedback is in controlling melt and how debris is redistributed on the glacier surface.

Dikes often have a long history of reinforcement, with each reinforcement adding new material resulting in a heterogeneous dike. As data on the dike internal heterogeneity is sparse, it is generally overlooked in the stability assessment of dikes. We present an object-based and process-based model simulating dike construction history on archeological dike cross, yielding similar patterns of heterogeneity as observed in real dikes, and apply it in a dike safety assessment. Model predictions improve when being based on more accurate statistics of dike buildup, or when being conditioned to ground truth data. When incorporated in a dike stability assessment, multiple model runs can be coupled to hydrological simulations and dike slope stability calculations, resulting in a probabilistic stability assessment considering internal dike heterogeneity. While high-resolution observations are still sparse, good model accuracies can be reached by combining regional information on dike buildup with local point observations and this model provides a parsimonious basis to include information of internal dike heterogeneity in safety assessments.

Abstract. Debris-covered glacier tongues in the Himalaya play an important role in the high-altitude water cycle. The thickness of the debris layer is a key control of the melt rate of those tongues, yet little is known about the relative importance of the three potential sources of debris supply to those glaciers: the headwalls, the glacier bed or the lateral moraines. In this study we hypothesize that erosion from the lateral moraines is a significant debris supply to the debris-covered tongues, in particular when the tongue is disconnected from the headwall due to glacier downwasting. To test this hypothesis eight high-resolution and highly accurate digital elevation models for the lateral moraines of the debris-covered Lirung glacier in Nepal derived from an unmanned aerial vehicle between May 2013 and April 2018 are used. The analysis shows that the lateral moraines erode at an average rate of 0.31 ± 0.26 m yr−1 during this period driven by different erosion processes. There is also a higher erosion rate observed in the monsoon season (0.42 m yr−1) than in the dry season (0.25 m yr−1). In addition the loose lower parts of the lateral moraines erode at a faster rate during both seasons. These erosion rates translate into an annual increase in debris thickness ranging from 0.17 m yr−1 when the eroded material is distributed over the entire glacier to 0.29 m yr−1 in case the material is deposited in a narrow runout zone. It is concluded that the lateral moraines provide an important source of surface debris for glaciers in an advanced state of mass loss, and the source needs to be incorporated into models of glacier evolution. Further research should focus on how large this negative feedback is in controlling the melt of the tongues and a better understanding of the redistribution of debris on the tongue is therefore required.

With up to 15% of the world’s population being protected by dikes from flooding, climate-change-induced river levels may dramatically increase the flood risk of these societies. Reliable assessments of dike stability will become increasingly important, but groundwater flow through dikes is often oversimplified due to limited understanding of the important process parameters. To improve the understanding of these parameters, we performed a global sensitivity analysis on a comprehensive hydro-stability model. The sensitivity analysis encompassed fifteen parameters related to geometry, drainage conditions and material properties. The following three sensitivity settings were selected to characterize model behavior: parameter prioritization, trend identification and interaction qualification. The first two showed that dike stability is mostly dependent on the dike slope, followed by the type of subsurface material. Interaction quantification indicated a very prominent interaction between the dike and subsurface material, as it influences both groundwater conditions and dike stability directly. Despite our relatively simple model setup, a database containing the results of the extensive Monte Carlo analysis succeeded in finding most of the unsafe sections identified by the official inspection results. This supports the applicability of our results and demonstrates that both geometry and subsurface parameters affect the groundwater conditions and dike stability.

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&lt;p&gt;Slope instability of river dikes during floods is often driven by the evolution of groundwater pressures. Despite the temporal nature of high river water levels, pressure heads during floods are often assumed to reflect steady-state seepage conditions, leading to conservative estimates of dike slope safety. Here, we investigate the influence of transient groundwater conditions that result from variable flood wave shapes on probabilistic safety estimates of slope stability. We have sampled a large number of flood waves, aiming to maximize the variability in the flood wave shapes, and used them in a modeling chain consisting of a hydrological model (MODFLOW) and a probabilistic dike slope safety assessment (FORM). We compared the resulting time-dependent probabilistic dike safety for inner (landward) slope and outer (riverward) slope stability with the current flood safety assessment in the Netherlands. This comparison showed that current methods based on steady-state and analytical solutions seem to underestimate dike safety. Other methods, based on a design discharge wave, are more consistent with the multi-flood wave dike reliability, but their error increases at extreme water levels. In line with the temporal component of variable flood water levels, the failure probability also has a strong temporal component. Our results indicate that the highest failure probability always occurs after the river water level peak, with a delay of up to 15 days for both inner slope and outer slope stability. In addition, the uncertainty in the shape of the flood wave can be as important as the uncertainty in the geomechanical material properties for explaining the variation in dike failure probabilities. Therefore, this research strongly suggests that transient-groundwater conditions as a function of variable flood wave shapes should be incorporated in dike safety assessment. As a first step, we recommend further research on the occurrence probability of the most influential waveform characteristics, being the total flood wave volume (for the inner slope) and the total water level decrease after the peak (for the outer slope).&lt;/p&gt;

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