Multi-temporal runoff-sediment discharge relationships

To understand the runoff-sediment discharge relationship , this study examined the annual runoff and sediment discharge data obtained from the Tangnaihai hydrometric station. The data were decomposed into multiple time scales through Complete Ensemble Empirical Mode Decomposition with adaptive noise (CEEMDAN). Furthermore, double cumulative curves were plotted and the cointegration theory was employed to analyze the microscopic and macroscopic multi-temporal correlations between the runoff and the sediment discharge and their detailed evolution.

Development of a Numerical Model to Analyze the Formation and Development Process of River Mouth Bars

An integrated sediment management approach that includes the recovery of the amount of declined sediment supply is effective as a fundamental solution to coastal erosion. During planning, it is essential to analyze the transfer mechanism of the sediments generated from estuaries (the junction between a river and sea) to assess the amount and rate of sediment discharge (from the river to sea) supplied back to the coast. Although numerical models that interpret the tidal sand bar flushing process during flooding have been studied, thus far, there has been no study focusing on the formation and development processes of tidal sand bars. Therefore, this study aims to construct wave deformation, flow regime calculation, and topographic change analysis models to assess the amount of recovered sediment discharge and reproduce the tidal sand bar formation process through numerical analysis for integrated littoral drift management. The tidal sand bar formation process was simulated, and the wave energy and duration of action concepts were implemented to predict the long-term littoral movement. The river flux and wave conditions during winter when tidal sand bars dominantly develop were considered as the external force conditions required for calculation. The initial condition of the topographic data directly after the Maeupcheon tidal sand bar flushing during flooding was set as the initial topography. Consequently, the tidal sand bar formation and development due to nearshore currents dependent on the incident wave direction were reproduced. Approximately 66 h after the initial topography, a sand bar formation was observed at the Maengbang estuary.

Simulating sediment discharge at water treatment plants under different land use scenarios using cascade modelling with an expert-based erosion-runoff model and a deep neural network

Excessive sediment discharge in karstic regions can be highly disruptive to water treatment plants. It is essential for catchment stakeholders and drinking water suppliers to limit the impact of high sediment loads on potable water supply, but their strategic choices must be based on simulations integrating surface and groundwater transfers and taking into account possible changes in land use. Karstic environments are particularly challenging as they face a lack of accurate physical descriptions for the modelling process, and they can be particularly complex to predict due to the non-linearity of the processes generating sediment discharge. The aim of the study was to assess the sediment discharge variability at a water treatment plant according to multiple realistic land use scenarios. To reach that goal, we developed a new cascade modelling approach with an erosion-runoff geographic information system (GIS) model (WaterSed) and a deep neural network. The model was used in the Radicatel hydrogeological catchment (106 km2 in Normandy, France), where karstic spring water is extracted to a water treatment plant. The sediment discharge was simulated for five design storms under current land use and compared to four land use scenarios (baseline, ploughing up of grassland, eco-engineering, best farming practices, and coupling of eco-engineering/best farming practices). Daily rainfall time series and WaterSed modelling outputs extracted at connected sinkholes (positive dye tracing) were used as input data for the deep neural network model. The model structure was found by a classical trial-and-error procedure, and the model was trained on 2 significant hydrologic years. Evaluation on a test set showed a good performance of the model (NSE = 0.82), and the application of a monthly backward-chaining nested cross-validation revealed that the model is able to generalize on new datasets. Simulations made for the four land use scenarios suggested that ploughing up 33 % of grasslands would increase sediment discharge at the water treatment plant by 5 % on average. By contrast, eco-engineering and best farming practices will significantly reduce sediment discharge at the water treatment plant (respectively in the ranges of 10 %–44 % and 24 %–61 %). The coupling of these two strategies is the most efficient since it affects the hydro-sedimentary production and transfer processes (decreasing sediment discharge from 40 % to 80 %). The cascade modelling approach developed in this study offers interesting opportunities for sediment discharge prediction at karstic springs or water treatment plants under multiple land use scenarios. It also provides robust decision-making tools for land use planning and drinking water suppliers.

Sediment load determines the shape of rivers

Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers that carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by Parker [G. Parker, J. Fluid Mech. 89, 127–146 (1978)], that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid–sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can transport more sediment only by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge—a quantity notoriously difficult to measure in the field.

The spatially distributed nature of subglacial sediment dynamics: using a numerical model to quantify sediment transport and bedrock erosion across a glacier bed in response to glacier behavior and hydrology

In addition to ice and water, glaciers expel sediment. As a result, changing glacier dynamics and melt will result in changes to glacier erosion and sediment discharge, which can impact the landscape surrounding retreating glaciers, as well as communities and ecosystems downstream. To date, the available models of subglacial sediment transport on the sub-hourly to decadal-scale exist in one dimension, usually along a glacier's flow line. Such models have proven useful in describing the formation of landforms, the impact of sediment transport on glacier dynamics, the interactions between climate, glacier dynamics, and erosion. However, because of the large role of sediment connectivity in determining sediment discharge, the geoscience community needs modeling frameworks that describe subglacial sediment discharge in two spatial dimensions over time. Here, we present SUGSET_2D, a numerical model that evolves a two-dimensional subglacial till layer in response to the erosion of bedrock and changing sediment transport conditions below the glacier. Experiments employed on test cases of synthetic ice sheets and alpine glaciers demonstrate the heterogeneity in sediment transport across a glacier's bed. Furthermore, the experiments show the non-linear increase in sediment discharge following increased glacier melt. Lastly, we apply the model to Griesgletscher in the Swiss Alps where we use a parameter search to test model outputs against annual observations of sediment discharge measured from the glacier. The model captures the glacier's inter-annual variability and quantities of sediment discharge. Furthermore, the model's capacity to represent the data depends greatly on the grain size of sediment. Smaller sediment sizes allow sediment transport to occur in regions of the bed with reduced water flow and channel size, effectively increasing sediment connectivity into the main channels. Model outputs from the three test-cases together show the importance of considering heterogeneities in water discharge and sediment availability in two dimensions.

Multi-criteria approaches to identify the shoreline retreat downstream of dams: the North African case

Northern African beaches are among the most vulnerable areas under the extreme climate change hazard. Mainly sedimentary low-lying platform, the coasts are supplied by terrestrial yields, which are increasingly interrupted by dams. Unfortunately, the sediment fluxes are rarely measured and monitored, so that it is quite impossible today to assess the contribution of continental sediments to the coast and its variability. The aim of our study is to determine the sampling protocol of delta sedimentation plain and nearshore seabed for better understanding of the anthropogenic driver in contrast to climate change. We adopt a multi-criteria analysis based both on the geomorphologic feature and the historic evolution from the River to the littoral plain. The shoreline evolution reveals an alarming retreat trend reaching −20 m ± 0.15 m yr−1 after the human-induced change where ∼50 % of sediment discharge has been trapped upstream the dam, including quite all the coarse material, like sand. The shoreline retreat and the decreasing sediment rate of fluvial flow are all due to the dam construction.