Graph theory-based sediment connectivity analysis of a glacierised Alpine basin for different event scenarios

Sediment connectivity has been receiving increased attention in the last years. Several approaches have been applied to analyse where and to what extent sediment sources are connected to the main fluvial network and/or to depositional areas. Especially in mountain environments, sediment transport is temporally and spatially variable, and thus assessing sediment connectivity is challenging. Within this work, a graph theory-based approach is presented, with the aim to identify changes in space and time within the sediment transport network during the main sediment transport periods of the year.A network, built up by nodes and directed edges, was manually digitized for the Sulden/Solda river basin (Vinschgau/Venosta valley, Italian Alps). The nodes represent landforms delineated within a previously developed geomorphological map, which features 32 different landform categories and seamlessly covers the entire basin (~130 km&#178;). The directed edges are connecting nodes if sediment transport is (potentially) occurring from one to the subsequent geomorphological unit. This evaluation was made based on visual evidences from orthophotos and geomorphological as well as topographical characteristics of the respective landforms. Furthermore, a sediment transport process type was assigned to each edge.Snow and glacier melt scenarios are defined by the occurrence of specific sediment transport processes, hence activation or deactivation of the related edges. Scenarios representing potential sediment transport networks during intensive heat periods and intense rain storms are included for both melt seasons, taking into consideration the expected higher frequency of these meteorological conditions in the future decades. For example, rain storm scenarios include edges showing potential debris flow trajectories, whereas these connections are not present in scenarios representing just snow- and ice-melt events. Therefore, functional connectivity changes within the proposed sediment transport network scenarios. For all the events, graph theory measures are calculated, as e.g. the betweenness centrality index to identify &#8220;hot-spot&#8221; nodes of the sediment cascades. Furthermore, the quantity and the composition of the sediment cascades reaching the catchment outlet can be identified in order to highlight the most relevant transport processes as well as to derive the most typical sediment cascades for a specific area.The study basin is characterized by a high sediment availability due to large glacio-fluvial deposits present at the glaciers forefield and to the wide areas covered with talus deposits. However, the connectivity analysis demonstrates that a vast portion of these sediment sources is not connected to the main channel under the modelled melt runoff scenarios. Only in case of intense rainstorms talus deposits might become a coupled sediment source due to the potential occurrence of debris flows. Hence, areas connected only occasionally due to the (re-)activation of specific sediment cascades can be mapped. Additionally, a relative connectivity degree is calculated for every scenario, introducing a better comparability.

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Modelling the effects of ice transport and sediment sources on the form of detrital thermochronological age probability distributions from glacial settings

Earth Surface Dynamics ◽

10.5194/esurf-8-931-2020 ◽

2020 ◽

Vol 8 (4) ◽

pp. 931-953

Author(s):

Maxime Bernard ◽

Philippe Steer ◽

Kerry Gallagher ◽

David Lundbek Egholm

Keyword(s):

Sediment Transport ◽

Sampling Site ◽

Age Distribution ◽

Morphological Characteristics ◽

Transport Processes ◽

Surface Erosion ◽

Sediment Sources ◽

Sediment Particles ◽

The Impact ◽

Local Sampling

Abstract. The impact of glaciers on the Quaternary evolution of mountainous landscapes remains controversial. Although in situ or bedrock low-temperature thermochronology offers insights on past rock exhumation and landscape erosion, the method also suffers from potential biases due to the difficulty of sampling bedrock buried under glaciers. Detrital thermochronology attempts to overcome this issue by sampling sediments at e.g. the catchment outlet, a component of which may originate from beneath the ice. However, detrital age distributions not only reflect the catchment exhumation, but also spatially variable patterns and rates of surface erosion and sediment transport. In this study, we use a new version of a glacial landscape evolution model, iSOSIA, to address the effect of erosion and sediment transport by ice on the form of synthetic detrital age distributions. Sediments are tracked as Lagrangian particles formed by bedrock erosion, and their transport is restricted to ice or hillslope processes, neglecting subglacial hydrology, until they are deposited. We base our model on the Tiedemann Glacier (British Columbia, Canada), which has simple morphological characteristics, such as a linear form and no connectivity to large tributary glaciers. Synthetic detrital age distributions are generated by specifying an erosion history, then sampling sediment particles at the frontal moraine of the modelled glacier. Results show that sediment sources, reflecting different processes such as glacier and hillslope erosion, can have distinct bedrock age distribution signatures, and estimating such distributions should help to identify predominant sources in the sampling site. However, discrepancies between the detrital and bedrock age distributions occur due to (i) the selective storage of a large proportion of sediments in small tributary glaciers and in lateral moraines, (ii) the large range of particle transport times due to varying transport lengths and strong variability of glacier ice velocity, (iii) the heterogeneous pattern of erosion, and (iv) the advective nature of glacier sediment transport along ice streamlines. This last factor leads to a poor lateral mixing of particle detrital signatures inside the frontal moraine, and then local sampling of the frontal moraine is likely to reflect local sources upstream. Therefore, sampling randomly across the moraine is preferred for a more representative view of the catchment age distribution. Finally, systematic comparisons between synthetic (U-Th)/He and fission track detrital ages, with different bedrock age-elevation profiles and different relative age uncertainties, show that the nature of the age-elevation relationship and age uncertainties largely control the ability to track sediment sources in the detrital record. However, depending on the erosion pattern spatially, qualitative first-order information may still be extracted from a thermochronological system with high uncertainties (>30 %). Overall, our results demonstrate that detrital age distributions in glaciated catchments are strongly impacted not only by erosion and exhumation but also by sediment transport processes and their spatial variability. However, when combined with bedrock age distributions, detrital thermochronology offers a novel means to constrain the transport pattern and time of sediment particles.

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Assessment of structural sediment connectivity within catchments: insights from graph theory

10.5194/esurf-2016-55 ◽

2016 ◽

Cited By ~ 2

Author(s):

Étienne Cossart ◽

Mathieu Fressard

Keyword(s):

Graph Theory ◽

Network Structure ◽

Small Scale ◽

Potential Contribution ◽

Sediment Sources ◽

Sediment Delivery ◽

Field Reconstruction ◽

Black Boxes ◽

Sediment Connectivity ◽

Broad Scale

Abstract. To understand the sedimentary signal delivered at catchment outlets, many authors now refer to the concept of connectivity. In this framework, the sedimentary signal is seen as an emergent organization of local filiations and interactions. The challenge is then to open black boxes that remain within a sediment cascade, that requires both accurate geomorphic investigations in the field (reconstruction of sequences of geomorphic evolution, description of sediment pathways) but also the development of tools dedicated to sediment cascades modelling. More precisely the development of tools dedicated to the study of connectivity in geomorphology is still in progress, even if the graph theory offers promising perspectives (Heckmann and Schwanghart, 2013). In this paper, graph theory is applied to abstract the network structure of sediment cascades, keeping only nodes (sediment sources, sediment stores, outlet) and links (linkage by a transportation agent), represented as vertices and edges. From the description of the assemblages of sedimentary flows, we provide three main indices to explore how small-scale processes may result in significant broad-scale geomorphic patterns. First, we assess the potential contribution of each node to the sediment delivery at the outlet. Second, we measure the influence of each node regarding how this node is accessible from both sediment sources and outlet. Third, we calculate a connectivity index to reveal whether the potential contribution of a node is lower or higher than expected from its location within the network. These indices are calculated in the case of a virtual sediment cascade, but are also applied to a catchment located in southern french alps. We demonstrate that these indices are robust, and may lead to simulations. In the present case, we try to predict how a sediment cascade may be impacted by a node disruption or by a reconnection.

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A dynamic, network scale sediment (dis)connectivity model to reconstruct historical sediment transfers and river reach sediment budgets

10.5194/egusphere-egu21-8668 ◽

2021 ◽

Author(s):

Marco Tangi ◽

Simone Bizzi ◽

Kirstie Fryirs ◽

Andrea Castelletti

Keyword(s):

Sediment Transport ◽

Cascade Model ◽

River Network ◽

Transport Processes ◽

Sediment Budgets ◽

Basin Scale ◽

Connectivity Model ◽

Network Scale ◽

River Reach ◽

Sediment Connectivity

Sediment transport and connectivity are key factors for the functioning of fluvial eco-systems, and variations to these drivers deeply affect the geomorphology of the river system. Given that lags often occur in river systems, these changes may appear displaced in time and space from the disturbances that generated them. Modelling sediment (dis)connectivity and its reaction to anthropic pressures with a network-scale perspective is thus necessary to increase the understanding of river processes, to quantify real impacts and estimate future evolutionary trajectories. The CASCADE model (Schmitt et al., 2016) is a sediment connectivity model developed to address this type of research question: it combines concepts of network modelling with empirical sediment transport formulas to quantitatively describe sediment (dis)connectivity in river networks.In this work, we present a new version of the CASCADE model which expands on the original model by featuring a dynamic simulation of sediment transport processes in the network (D-CASCADE). This new framework describes sediment connectivity in term of transfer rates through space and time. It takes into consideration multiple factors that can affect sediment transport, such as spatial and temporal variations in water discharge and river geomorphological features (i.e., river gradient and width), different sediment grainsizes, sediment entraining and deposition from and in the river bed and interactions between materials coming from different sources.We apply the new D-CASCADE on the Bega River, New South Wales, Australia, which due to anthropic alterations post European colonization after 1850 including large-scale deforestation, removal of riparian vegetation and swamp drainage, has experienced significant alteration to the character and behaviour of streams, widespread channel erosion and massive sediment mobilization (Fryirs and Brierley, 2001). Our objective is to reproduce the historical sediment transfers that occurred across the network and associated river reach sediment budgets. First, we reconstructed the pre-settlement geomorphic features of the river network and the past hydrology from historical observations and expert-based reconstruction, and then modelled the sediment transport processes in the network in the last two centuries introducing the different drivers of change observed historically in the proper chronological sequence. Due to the uncertainty in the reconstruction of the historical conditions, multiple scenarios have been used.The D-CASCADE model successfully reproduces the timing and magnitude of the major sediment transfers of the last two centuries in the Bega River network from headwaters swamps to lowland river reaches and associated channel geomorphic adjustments. Using the knowledge acquired by these historical simulations, the model was also applied to provide estimations on future trajectories of sediment transport and sediment budgets at the river reach scale.With this research, we demonstrate the potential of the new D-CASCADE model to simulate and quantify at the network-scale sediment transport events generating information on sediment budget transfers from a single event to historical trajectories of the last centuries. Such knowledge paves the way to aid predictions of future impacts of basin-scale management measures and can support decision-making when designing sediment management strategies or river restoration initiatives.

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Preliminary map of erosional and depositional provinces and descriptions of sediment-transport processes in the south and central San Francisco Bay region, California

10.3133/mf515 ◽

1973 ◽

Keyword(s):

Sediment Transport ◽

San Francisco ◽

San Francisco Bay ◽

Transport Processes ◽

The South

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The Role of Fluid Mud in Sediment Transport Processes Along a Muddy Coast

10.21236/ada626141 ◽

2001 ◽

Author(s):

Gail C. Kineke

Keyword(s):

Sediment Transport ◽

Transport Processes ◽

Fluid Mud

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Field Measurements of Sediment Transport Processes in Strataform

10.21236/ada629313 ◽

1997 ◽

Author(s):

David A. Cacchione

Keyword(s):

Sediment Transport ◽

Field Measurements ◽

Transport Processes

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Sediment transport processes across the Tibetan Plateau inferred from robust grain-size end members in lake sediments

Climate of the Past ◽

10.5194/cp-10-91-2014 ◽

2014 ◽

Vol 10 (1) ◽

pp. 91-106 ◽

Cited By ~ 61

Author(s):

E. Dietze ◽

F. Maussion ◽

M. Ahlborn ◽

B. Diekmann ◽

K. Hartmann ◽

...

Keyword(s):

Grain Size ◽

Sediment Transport ◽

Tibetan Plateau ◽

Transport Processes ◽

The Tibetan Plateau ◽

Dust Deposition ◽

Northerly Wind ◽

Near Surface ◽

Depositional Processes ◽

End Members

Abstract. Grain-size distributions offer powerful proxies of past environmental conditions that are related to sediment sorting processes. However, they are often of multimodal character because sediments can get mixed during deposition. To facilitate the use of grain size as palaeoenvironmental proxy, this study aims to distinguish the main detrital processes that contribute to lacustrine sedimentation across the Tibetan Plateau using grain-size end-member modelling analysis. Between three and five robust grain-size end-member subpopulations were distinguished at different sites from similarly–likely end-member model runs. Their main modes were grouped and linked to common sediment transport and depositional processes that can be associated with contemporary Tibetan climate (precipitation patterns and lake ice phenology, gridded wind and shear stress data from the High Asia Reanalysis) and local catchment configurations. The coarse sands and clays with grain-size modes >250 μm and <2 μm were probably transported by fluvial processes. Aeolian sands (~200 μm) and coarse local dust (~60 μm), transported by saltation and in near-surface suspension clouds, are probably related to occasional westerly storms in winter and spring. Coarse regional dust with modes ~25 μm may derive from near-by sources that keep in longer term suspension. The continuous background dust is differentiated into two robust end members (modes: 5–10 and 2–5 μm) that may represent different sources, wind directions and/or sediment trapping dynamics from long-range, upper-level westerly and episodic northerly wind transport. According to this study grain-size end members of only fluvial origin contribute small amounts to mean Tibetan lake sedimentation (19± 5%), whereas local to regional aeolian transport and background dust deposition dominate the clastic sedimentation in Tibetan lakes (contributions: 42 ± 14% and 51 ± 11%). However, fluvial and alluvial reworking of aeolian material from nearby slopes during summer seems to limit end-member interpretation and should be crosschecked with other proxy information. If not considered as a stand-alone proxy, a high transferability to other regions and sediment archives allows helpful reconstructions of past sedimentation history.

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