Effects of debris-flow and bed composition on erosion and entrainment

Erosion and entrainment of material by debris flows determine debris-flow volume growth and therefore hazard potential. Recent advances in field, laboratory, and modelling studies have distilled two driving forces behind debris-flow erosion; impact and shear forces. A third factor influencing the (relative) importance of these forces is the viscosity and abundance of the interstitial fluid in the debris flow and the bed. However, how erosion and these forces depend on the composition of the debris flow itself and the composition of the bed remains unclear. Here, we present results of small-scale flume experiments with a loosely packed erodible bed that highlight the far-reaching effects of debris-flow and bed composition on erosion processes and magnitude. We quantify the effects of gravel, clay, and solid fraction in the debris flow on bed erosion. In addition, we quantify the effects of water and clay content of the unconsolidated bed on erosion by a debris flow. We show that debris flow erosion increases linearly when the gravel fraction of a debris flow is increased, which is linked to an increase in both impact and shear forces. We find that debris flow erosion, and the related forces, are non-linearly impacted by the clay and water content of the debris flow and those of the bed. For both the clay content of the debris flow and the bed, an optimum in erosion exists around a specific clay percentage that does not directly relate to an optimum in either shear or impact forces. When the water content of the bed and/or the debris flow is increased, erosion becomes largest when supersaturated conditions occur. These conditions are unrelated to the magnitude of the two erodible forces. This shows that both clay and water content affect erosion by affecting the transfer of pore pressures from the debris flow to the bed. We can therefore conclude that impact and shear forces dictate debris flow erosion in most cases but that their (relative) importance is significantly altered by the means and effectivity of pore pressure transfer from the debris flow to the bed. The latter is highly influenced by the viscosity and abundance of the interstitial fluid of the debris flow and the composition of the bed

How Bed Composition Affects Erosion by Debris Flows - An Experimental Assessment

A solid physical understanding of debris-flow erosion is needed for both hazard prediction and understanding landscape evolution. However, the processes and forces involved in erosion by debris flows and especially how the erodible surface itself influences erosion are poorly understood. Here, we experimentally investigate the effects of bed composition on debris-flow erosion, by systematically varying the composition of an erodible bed in a small-scale debris-flow flume. The experiments show that water and clay content of an unconsolidated bed significantly control erosion magnitude by affecting the transfer of pore pressure, loading conditions, and contraction-dilation behavior of the bed. As the water content increases and the bed comes close to saturation, erosion increases rapidly, whereas for clay content an optimum for erosion exists around a clay content of 3%–4%. Our results show that small variations in bed composition can have large effects on debris-flow erosion, and thus volume growth and hazard potential

How, when and where current mass flows in Martian gullies are driven by CO2 sublimation

Martian gullies resemble water-carved gullies on Earth, yet their present-day activity cannot be explained by water-driven processes. The sublimation of CO2 has been proposed as an alternative driver for sediment transport, but how this mechanism works remains unknown. Here we combine laboratory experiments of CO2-driven granular flows under Martian atmospheric pressure with 1D climate simulation modelling to unravel how, where, and when CO2 can drive present-day gully activity. Our work shows that sublimation of CO2 ice, under Martian atmospheric conditions can fluidize sediment and creates morphologies similar to those observed on Mars. Furthermore, the modelled climatic and topographic boundary conditions for this process, align with present-day gully activity. These results have implications for the influence of water versus CO2-driven processes in gully formation and for the interpretation of gully landforms on other planets, as their existence is no longer definitive proof for flowing liquids

Pulling up new chairs to the table: experiences of organising diversity and inclusion events during a pandemic

Young Women of Geosciences (YWOG) is a group at Utrecht University (the Netherlands) which aims to create an equal and inclusive working environment for all employees in the faculty of Geosciences. Now in our fourth year, with an expanding committee and increasing support from the faculty, we share some details and insights from events held during the pandemic. After several years of having primarily the same small group of people attending our events (i.e. “preaching to the choir”) our aim was to engage with more people in our faculty. We wanted to pull up new chairs to the table and hear new opinions and thoughts and so, our events were planned with this primary goal in mind. However, under changing and variable conditions due to the pandemic, the planning of events to promote diversity and inclusion became more of a challenge. We had to devise strategies to keep people engaged in diversity and inclusion topics while people became tired of online events, and were busy just dealing with the pandemic. Our primary success was a book giveaway and discussion where three books related to diversity and inclusion (some also with climate and environmental aspects) were given for free to 30 staff members. This was followed by an open online discussion about topics that arose in the books and how these issues were experienced in our own faculty. This session had the greatest number of male participants we have ever had at one of our events (despite all sessions always being open to all genders) and this led to great information sharing and discussions. We also organised two Wikipedia hackathons which aimed to improve information on Wikipedia about female and minority scientists. This event required a great deal of time and skill development which unfortunately many people were not able to commit to, which led to smaller numbers and less engagement than our book event. Overall, we managed to introduce new groups of people to our discussions and engage with a broader audience than in previous years, within a virtual environment. We attribute this in large part to attractive events and hard work on our communication strategy. We found that engagement in activities, particularly for new attendees, was highly dependent on time availability and concrete communication of details of the event. We frequently used social media to communicate about our events and significant growth of these channels in the virtual-only environment of lockdowns led to overall increased engagement. This was particularly the case on Twitter, where we have found active and supportive fellow networks to engage with, be inspired by, and inspire

Vegetation and peat accumulation steer Holocene tidal–fluvial basin filling and overbank sedimentation along the Old Rhine River, The Netherlands

In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp, and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp, likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels

Data supplement to "How debris-flow composition affects bed erosion quantity and mechanisms - an experimental assessment"

Data was generated for 98 debris flow erosion experiments in a 5.4 m long flume with an erodible loosely packed sediment bed. In the 98 experiments the bed conditions were kept constant, but the composition of the debris flow changed. Volume, water content, gravel content and clay content were varied. The paper contains all information on procedures, settings, sensors and data analyses. The data in this supplement contains the DEMs of the erodible bed, the data of the sensors in the flume and an overview of all experiments

How debris‐flow composition affects bed erosion quantity and mechanisms: An experimental assessment

Understanding erosion and entrainment of material by debris flows is essential for predicting and modelling debris-flow volume growth and hazard potential. Recent advances in field, laboratory and modelling studies have distilled two driving forces behind debris-flow erosion: impact and shear forces. How erosion and these forces depend on debris-flow composition and interact remains unclear. Here, we experimentally investigate the effects of debris-flow composition and volume on erosion processes in a small-scale flume with a loosely packed bed. We quantify the effects of gravel, clay and solid fraction in the debris flow on bed erosion. Erosion increased linearly with gravel fraction and volume, and decreased with increasing solid fraction. Erosion was maximal around a volumetric clay fraction of 0.075 (fraction of the total solid volume). Under varying gravel fractions and flow volumes erosion was positively related to both impact and shear forces, while these forces themselves are also correlated. Results further show that internal dynamics driving the debris flows, quantified by Bagnold and Savage numbers, correlate with erosional processes and quantity. Impact forces became increasingly important for bed erosion with increasing grain size. The experiments with varying clay and solid fractions showed that the abundance and viscosity of the interstitial fluid affect debris-flow dynamics, erosional mechanisms and erosion magnitude. High viscosity of the interstitial fluid inhibits the mobility of the debris flow, the movement of the individual grains and the transfer of momentum to the bed by impacts, and therefore inhibits erosion. High solid content possibly decreases the pore pressures in the debris flow and the transport capacity, inhibiting erosion, despite high shear stresses and impact forces. Our results show that bed erosion quantities and mechanisms may vary between debris flows with contrasting composition, and stress that entrainment models and volume-growth predictions may be substantially improved by including compositional effects

CO2-driven granular flows as erosional forces on present-day Mars

International audienceMartian gullies are alcove-channel-fan systems which have been hypothesized to be formed by the action of liquid water and brines, the effects of sublimating CO2 ice or a combination of these processes. Recent activity and new flow deposits in these systems have shifted the leading hypothesis from water-based flows to CO2-driven flows. This shift in thinking is supported by the low availability of atmospheric water under present Martian conditions and the observation that gully activity occurs at times when CO2 ice is present. We recently performed novel experiments that have shown that this hypothesis holds; sediment can be mobilized and fluidized by sublimating CO2 ice under Martian atmospheric pressure. However, if these flows are able to erode the underlying surface and can explain the formation of Martian gully systems over the long term remains unknown. Therefore, we present an additional series of experiments that test the capacity of CO2-driven granular flows under Martian atmospheric conditions to erode sediment. These experiments were conducted in a 4 m long flume in the Aarhus Mars Simulation Wind Tunnel. Our experiments show that CO2-driven granular flows can erode loose sediment under a range of different slopes and CO2-ice fractions. The results also show that incorporation of warmer sediment increases fluidization of the mixture, reflected by an increase in gas pore pressure in the flow. These results thus prove that morphological evolution in the gully systems on Mars can be explained by CO2-driven granular flows