Probabilistic Prediction and Forecast of Daily Suspended Sediment Concentration on the Upper Yangtze River

Sediment transport in suspension can represent more than 90% of a river's total annual flux of sediment. In the case of the Yangtze River, more than 99% of the sediment supplied to the sea is suspended load. Suspended sediment is thus an important component of the total sediment load, with implications for channel dynamics, landscape evolution, ecology, and human-related activities. For hydrological management of large basins such as the Yangtze River, knowledge of the processes governing suspended sediment concentration (SSC) is essential. An analysis of the temporal variation of SSC for the Upper Yangtze basin (defined at Pingshan station) is presented here. For this purpose, a database of 50years of concurrent discharge and SSC measurements, made by the Yangtze River Commission, is used. The analysis is made using a novel probabilistic data-driven technique, the Generalized Pareto Uncertainty (GPU). This technique allows for the testing of several strategies of prediction and forecast applied to a time series of SSC and streamflow. Changing between local or seasonal variables to feed these strategies, we inferred that although the main driver of the SSC transport is flow (as reported by previous authors), sediment storage is also a major control. Furthermore, the maximum necessary time lag for forecasts made with the data is on the order of one week, which provides one indication of the time scale of the local processes of SSC transport in the Upper Yangtze. In this paper, limitations and data requirements of the GPU methodology are also discussed. Plain Language Summary In this manuscript we use a probabilistic technique to study what controls the dynamics of the sediments in suspension in Upper Yangtze River. For that, a database of 50 years of measurements of streamflow and sediments in suspension collected at Pingshan station by the Yangtze River Commission was analyzed with a new technique. Sediments in suspension represent the major portion of total sediment load in most river systems, and the Yangtze River is no exception. Fine sediment dynamics is an important component of many physical, chemical, and biological processes in rivers.

Sediment Suspension Dynamics in Turbulent Unsteady, Depth-Varying Open-Channel Flow over a Gravel Bed

Fereshteh Bagherimiyab Hunkeler

Even though flow in natural rivers and channels is generally unsteady, only a few studies on turbulent structures in unsteady open-channel flows have been carried out. In hydraulic engineering problems, unsteady flow is often approximated with concepts of steady flow, because the treatment of unsteady flow can be difficult. Many variables enter into the mathematical relationship and the differential equations cannot be integrated in closed form, except under very simplified conditions. The limited number of studies of unsteady flow may also be due to the lack of experimental equipment capable of capturing small-scale, unsteady open-channel flow dynamics. Therefore, it is important to know when unsteady flow can be approximated by steady flow concepts and which simplifications are acceptable. In most cases, especially when simulating unsteady flood flow by a steady flow approach, the calculations may not produce reliable results. The mechanism of sediment transport in rivers and open channels is governed by complicated interactions between unsteady accelerating and decelerating turbulent flow, particle motion and bed configuration. Understanding the dynamics of unsteady sediment-laden water flows and characterizing the velocity of suspended particles is essential for enhancing the predictive accuracy of sediment transport and its impact on environmental processes in the water column. In order to simulate fine sediment dynamics over an armored bed in a river during the passage of a flood wave, unsteady accelerating, and decelerating open-channel flow over a movable (but not moving) coarse gravel bed (D50 = 5.5 mm) first without and then with fine sediment were studied. A layer of fine sediment of mean particle size about 120 µm was placed on the coarse gravel bed. The thickness of the fine sediment layer on the gravel bed was varied between 4 mm and 6 mm, but it was found that the thickness of the layer had no effect on the results. Quasi-instantaneous profiles of velocity and sediment concentration were taken simultaneously and co-located. An acoustic Doppler and imaging method, using an Acoustic Doppler Velocity Profiler (ADVP) was combined with an optical method, using Particle Tracking Velocimetry (PTV) for suspended sediment particle tracking. Measurements resolved turbulence scales. Unsteadiness strongly affects the profile shape of velocity and friction velocity, particularly in the final phase of the accelerating range. Flow in the decelerating range approaches steady flow. Systematically higher friction velocities were observed in the accelerating flow than in the decelerating flow for comparable flow depth. This indicates that for the same change of relative submergence, different flow dynamics are generated during accelerating and decelerating flows. For the lowest unsteadiness (90 s hydrograph), the differences between velocity profiles in the accelerating and the decelerating ranges become small, indicating that for this unsteadiness, steady state conditions are approached. During the accelerating flow range, fine sediment suspension from the bed started in bursts and in the final phase of the accelerating flow range, a ripple pattern is rapidly created that remained nearly stationary. Thereafter, vortex shedding produced most of the sediment suspension into the water column in the form of events, making suspension intermittent. Simultaneously, sediment particles rolled along the bed following the ripple structure, thus slowly advancing the ripple pattern in the direction of the flow. However, ripple geometry and ripple shape were not altered by this process, despite the fact that flow velocities changed. Due to the ripple structure, high sediment suspension events continued to occur in bursts during the decelerating flow even though mean flow velocity and friction velocity decreased. The dynamics of sediment suspension observed in this study indicate that mean value concepts cannot be applied in unsteady flow. Fine sediment particles and hydrogen bubbles were used individually and combined as flow tracers in the acoustic measurements. When used individually, hydrogen bubbles provided full depth flow and backscattering information, whereas sediment particles traced only the lower layers of the flow, indicating sediment suspension. When both tracers were combined, hydrogen bubbles could not be distinguished from sediment particles. The intermittency was observed in the backscattering of the acoustic system. The event structure in fine sediment suspension is seen by the PTV method. PTV velocity vectors varied in speed and orientation, but were organized in large coherent packets, mainly in the near-bed layers. They also extended well above the bed, supporting the concept that coherent structure events contribute to sediment suspension over ripples. The two methods provide complementary information. ADVP measurements allow long timeseries analysis, whereas the spatial details seen in the PTV results cannot be resolved in the ADVP measurements.

EPFL2012

Prediction and forecast of Suspended Sediment Concentration (SSC) on the Upper Yangtze basin

José Pedro Gamito de Saldanha Calado Matos, Mário Jorge Rodrigues Pereira Da Franca

Sediment transport in suspension may represent 90% or more of the global annual flux of sediment. For instance, more than 99% of the sediment supplied to the sea by the Yangtze River is suspended load. Suspended load is an important component for understanding channel dynamics and landscape evolution. Sediments transported in suspension are a major source of nutrients for aquatic organisms in riparian and floodplain habitats, and play a beneficial role acting as a sink in the carbon cycle. Excess of fine sediments may also have adverse effects. It can impair fish spawning by riverbed clogging, disturb foraging efficiency of hunting of river fauna, cause algae and benthos scouring, reduce or inhibit exchanges through the hyporheic region. Accumulation of fine sediments in reservoirs reduces storage capacity. Although fine sediment dynamics has been the focus of many studies, the current knowledge of sediment sources, transfer, and storage is inadequate to address fine sediment dynamics in the landscape. The theoretical derivation of a complete model for suspended sediment transport at the basin scale, incorporating small scale processes of production and transport, is hindered because the underlying mechanisms are produced at different non-similar scales. Availability of long-term reliable data on suspended sediment dynamics is essential to improve our knowledge on transport processes and to develop reliable sediment prediction models. Over the last 60 years, the Yangtze River Commission has been measuring the daily Suspended Sediment Concentration (SSC) at the Pingshan station. This dataset provides a unique opportunity to examine temporal variability and controls of fine sediment dynamics in the Upper Yangtze basin. The objective of this study is to describe temporal variation of fine sediment dynamics at the Pingshan station making use of the extensive sediment monitoring program undertaken at that location. We test several strategies of prediction and forecast applied to the long time series of SSC and streamflow. By changing the base variables between strategies, we improve our understanding of the phenomena driving SSC. Prediction and forecasts are obtained from the various input data sets based on a novel probabilistic data-driven technique, the Generalized Pareto Uncertainty (GPU), which requires very little parametrization. Addressing uncertainty explicitly, this methodology recognizes the stochastic nature of SSC. The GPU was inspired in machine learning concepts and benefits from advances in multi-objective optimization techniques to discard most explicit assumptions about the nature of the uncertainty being modeled. Assumptions that do remain are the need to specify a model for eventual non-stationarity of the series and that there are enough observations to conveniently model the uncertainty. In this contribution, several models are tested with conditioned inputs to focus on specific processes leading affecting SSC. For example, the influence of seasonal and local contributions to SSC can be separated by conditioning the probability estimation on seasonal and local drivers. Probabilistic forecasting models for SSC that account for different drivers of the phenomena are discussed.

2017

Hydro-morphodynamics of open-channel confluences with low discharge ratio and dominant tributary sediment supply

Sebastián Guillén Ludeña

River confluences in which the tributary supplies the dominant sediment load, and the flow discharge is abundantly provided by the main river, are typically observed in mountain-river basins. The existent knowledge on the hydrodynamic, morphodynamic and sedimentary processes involved in mountain-river confluences is sparse, since most of the studies on confluence dynamics focus on low land confluences. In this context, the present research study aims to deepen the knowledge on the hydro-morphodynamics and sedimentology of mountain-river confluences inspired in those of the Upper-Rhone river basin. For that purpose twelve laboratory experiments were conducted in two experimental facilities: one located at the Laboratory of Hydraulic Constructions of the École Polytechnique Fédérale de Lausanne (LCH-EPFL), and the other one located at the Instituto Superior Técnico de Lisboa (IST-UL). The experimental setup covered a wide range of configurations including three unit-discharge ratios, two junction angles, two sediment mixtures with different gradation coefficients, and two width ratios. The experiments were performed under movable bed conditions and with continuous sediment supply to both flumes, which represents a novelty in the study of this type of confluence. Systematic surveys of bed topography and water surface were recorded at different instants during the experiments and at equilibrium condition. Additionally and also at equilibrium, point velocities were measured and the spatial and grain size distribution of the bed sediments were analyzed. These measurements allowed the analysis of the influence of the discharge ratio, junction angle, sediment gradation and width ratio on the hydro-morphodynamics of open channel confluences. The bed morphology and hydrodynamics displayed some common features that were influenced by the variation of the tested parameters. The most significant morphological features were a marked discordance between the bed elevations of the tributary and main channel, a steep bed slope in the tributary, a bank-attached bar along the inner bank of the main channel and a scour hole that extended from the tributary mouth flanking the bar. The hydrodynamics were mainly characterized by a double flow deflection to the outer bank of the main channel by the tributary inflow and the bank-attached bar, flow acceleration downstream of the confluence, a shear layer that separated each confluent flows, and eventually supercritical flow regime in the tributary depending on the parameter setup. In summary, this research study presents a broad analysis of the hydrodynamic, morphodynamic and sedimentary processes involved in confluences characterized by low discharge ratios, and where the dominant sediment load is supplied by the tributary. This analysis contributes to widen the existent knowledge on the dynamics of this type of confluence and constitutes a benchmark for further studies, as well as a valuable tool for restoration projects.

EPFL2015