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DSD-INT 2019 Fine sediments - transport in suspension, storage and supply - Franca

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Presentation by Prof. Dr. Mário J. Franca, IHE Delft & Delft University of Technology, The Netherlands, at the Delft3D - User Days (Day 3a: River morphodynamics), during Delft Software Days - Edition 2019. Wednesday, 13 November 2019, Delft.

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DSD-INT 2019 Fine sediments - transport in suspension, storage and supply - Franca

  1. 1. Fine sediments: transport in suspension, storage and supply 13th November 2019, Delft3D - User Days Mário J. Franca IHE Delft & TU Delft, The Netherlands Carmelo Juez Instituto Pirenaico de Ecología – CSIC, Spain
  2. 2. Chavarrías Borràs (2019)
  3. 3. Dey (2014), Fluvial Hydrodynamics
  4. 4. The transport of sediment in suspension is primarily governed by the equilibrium between the diffusion of turbulent eddies and the submerged weight of the particles, which is typically expressed as a function of flow turbulence and of the particle settling velocity. The advection of the fine particles in suspension is, however, driven by the drag induced on them by the mean flow. 𝜕 ҧ𝑐 𝜕𝑡 + 𝜕 ҧ𝑐 ഥ𝑢𝑖 𝜕𝑥𝑖 = 𝜕 𝜕𝑥𝑖 𝜀 𝑠𝑥 𝑖 𝜕 ҧ𝑐 𝜕𝑥𝑖
  5. 5. At the global scale, sediment transport in suspension may represent 90% or more of the annual flux of sediment (Syvitski et al., 2005); more than 99% of the sediment supplied to the sea by the Yangtze River is suspended load (Yang et al., 2002). Beyond landscape evolution and river geomorphology, fine sediment dynamics are an important component of a number of physical, chemical, and biological processes in rivers. Fine sediments thus impact the ecology of rivers (Baxter & Hauer, 2000; Findlay, 1995; Sternecker et al., 2013), sustainability of human infrastructure (Schleiss et al., 2016), and basin-level fluxes of nutrients and carbon (Boix-Fayos et al., 2015).
  6. 6. A complex imprint in the spatial-temporal distribution of fine sediments is observed in river systems, caused by temporally and spatially uneven production, storage activation, and transport of sediments in suspension.
  7. 7. Basin storage Small scale lateral storage In channel storage Reach scale lateral storage
  8. 8. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage
  9. 9. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage
  10. 10. RIVER BASIN SCALE 10
  11. 11. Pingshan (Upper Yangtze basin) 475 000 km2 (≈ 12 x Netherlands) more than 50 years of suspended sediment concentration (ssc) and discharge (Q) 11
  12. 12. suspendedsediment concentration[mg/l] discharge [m3/s] 1000 10000 Hysteresis: Q1 = Q2 ssc1 ≠ ssc2 PREDICTION STRATEGIES: Date Discharge Combined (date and discharge) WHAT IS THE REASON FOR THE ANNUAL HYSTERESIS? 13 Observations
  13. 13. Discharge alone does not govern suspended sediment transport Activation of stored sediment explains hysteresis Matos, Hassan, Lu & Franca (2018) JGR–ES. Date Discharge Combined Observations suspendedsedimentconcentration[mg/l] discharge [m3/s] 1000 10000 1000 10000 14
  14. 14. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage
  15. 15. We hypothesized that the ratio between the distal and proximal sediment deposits conditions the observations of suspended sediments
  16. 16. We investigated experimentally the control exerted by the proximal in-channel and distal fine sediment storage on the observed SSC time series A simple experiment allows for the exploration of the mechanisms responsible for shaping the magnitude and direction of the sediment hysteresis in the concentration-discharge plots.
  17. 17. Juez, Hassan & Franca (2018) WRR. Phase plots of both time-varying sediment load and flow present different hysteresis types depending on the amount of local in-channel stored sediment relative to the distal incoming sediment, constituting a good tool for inferring the origin of sediment. Clockwise hysteresis is attributed to depletion of proximal fine sediment, whether counterclockwise is attributed to dominance of distal fine sediment. The morphological evolution of the riverbed within a reach (degradation or aggradation) is controlled by the importance of the sediment availability at that location relative to the incoming sediment. The type of hysteretical loop modulated by the interaction of both the distal and proximal supply is intrinsically related with these two types of morphological processes of the riverbed. For a counterclockwise loop, aggradation of the channel is observed, and vice versa.
  18. 18. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage
  19. 19. In compound channels, the exchange of water and fine sediments between the main channel and floodplains regulates its geomorphological evolution and is crucial for the maintenance of the floodplains ecosystem. These processes depend on: ratio of water depth in the main channel and in the floodplain ratio; width ratio between the main channel and the floodplain; floodplain occupation. We examine experimentally how these are interlinked and how the deposition of sediments in the compound channel is jointly determined by them.
  20. 20. Measurements: turbidimeters, ultrasonic probe, aerial photos Turbidimeters Camera (surface PIV))
  21. 21. Juez, Schärer, Jenny Schleiss ,& Franca (2019) WRR. The morphological evolution of the floodplain is mainly controlled by the large vortices generated in the mixing interface, which promote or prevent the exchange of sediments between the main channel and the floodplain. Larger width ratios are linked with smaller vortices and less conveyance of fine sediments, with more sediments settling over the floodplain. Higher water depths over the floodplain (intermediate flow) allow the sedimentsto span wider over the floodplain with few deposition in the main channel. The frictional bottom roughness (meadow roughness) is dominant over the drag force attributed to the vertical stems (sparse and dense wood-type roughness).
  22. 22. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage
  23. 23. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage Steady state Unsteady hydrograph Shallow water equations modelling High fidelity 3D modelling
  24. 24. Wando in Yahagi river, Nagoya, Japan Fluvial restoration, Aare river, Innertkirchen, Switzerland Venoge river, Vaud, Switzerland
  25. 25. What is the impact of lateral embayments in the hydrodynamic processes as a function of flow conditions and cavity geometry. What is the amount of fine sediment trapped in the cavities and what is the celerity of this process. Steady state Unsteady hydrographTo which degree the sediment deposits within the cavities resist (fully or partially) and recover to high flow events?
  26. 26. Experimental set up: 36 configurations tested
  27. 27. Measurements: turbidimeters, ultrasonic probe, aerial photos Turbidimeters Probe Camera (surface PIV)) Sediment patterns
  28. 28. 4.8 l/s 8.5 l/s 15 l/s Longitudinal velocity ത𝑢 Spanwise velocity ҧ𝑣 Re. shear stress 𝑢′ 𝑣′ Vorticity ഥ𝜔 = 𝜕ത𝑣 𝜕𝑥 − 𝜕ഥ𝑢 𝜕𝑦 Strong correlation between flow patterns and sedimentation areas. Primary areas of sedimentation: dead areas → shelter the flow. vortical areas → complex and intricate flow patterns favor deposition.
  29. 29. o Trapping efficiency = total mass/ cavity area o Highest efficiency for middle discharge o Lower efficiency for high discharge
  30. 30. Lower efficiency for high discharge → stronger recirculation due to stronger eddies & seiche phenomenon
  31. 31. Steady state: Strong correlation between flow patterns and sedimentation areas. Primary areas of sedimentation: dead areas → shelter the flow. vortical areas → complex and intricate flow patterns favor deposition. The cavity aspect ratio (AR) ratio is the most important parameter for characterizing lateral embayments. Configurations with high AR are filled up faster and correspond to higher sedimentation at the end of experiment. Seiche phenomenon has a strong impact in the sediment trapping. Medium discharge presents the highest trapping efficiency → seiche → vertical mixing for the highest discharge Juez, Bühlmann, Maechler, Schleiss & Franca (2017) ESPL.
  32. 32. 5 geometrical configurations o Water level ( ) o Turbidimeters ( ) o Photo-recorded cavity ( ) Going unsteady
  33. 33. o Configuration 5 -
  34. 34. The complete or partial re-mobilization of sediments depends on the cavities geometry and flow unsteadiness. Low cavity aspect ratios AR (i.e., long cavities) favor trapping and sheltering of sediments, even for high flow. High aspect ratios (shorter) cavities shield sediments since they are decoupled from the central channel. Medium aspect ratios cavities allow complete sediments flushing. Larger distance between two cavities promote trapping and resistance of sediments deposits. Magnitude of high flow is the relevant parameter for changes in sediment deposits. Duration of disruptive phase has minor impact. Gradual increase of flow rate yields the same mobilization of the sediments as an instantaneous increase of the flow discharge, but with a milder tendency. Juez, Thalmann, Schleiss & Franca (2018), AdWR. Unsteady hydrograph
  35. 35. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage Steady state Unsteady hydrograph Shallow water equations modelling High fidelity 3D modelling
  36. 36. Resolution of turbulent shallow flows with sufficient accuracy in an affordable computational time. 3D small-scale vortices are modelled by means of diffusion terms, whereas the 2D large-scales are resolved. DA-URANS numerical scheme based on a high order augmented WENO-ADER scheme was used.
  37. 37. The numerical results evidence that the model accurately reproduces both longitudinal and transversal resonant waves and provides an accurate description of the flow field. This model is being used for further expansion of the experimental parameters and can reliably be used to aid in the definition of lateral cavities for diverse purposes. Navas-Montilla, Juez, Franca & Murillo (2019) JComPh.
  38. 38. Ouro, Juez & Franca (under review) AdWR Large-Eddy Simulations are used to investigate the governing 3D processes involved in mass and momentum transfer between the flow in the main channel.
  39. 39. The flow is highly 3D in the main channel, within the cavities and in the transition. A single main vortex occupied most of the volume of each cavity, with a the vertically varying location LES captures a low-frequency standing wave phenomenon even with rigid-lid approximation. The pressure gradient term is the unique contributor to flushing momentum out of the cavities whilst convection and Reynolds normal stress terms are responsible for its entraining into the cavity. Sediment deposition areas documented in the laboratory experiments are linked with the simulated hydrodynamics, which correlate with regions of low turbulent kinetic energy and vertical velocities near the bottom of the channel.
  40. 40. Basin storage In channel storage Reach scale lateral storage Small scale lateral storage Steady state Unsteady hydrograph Shallow water equations modelling High fidelity 3D modelling
  41. 41. which what’s and why’s still exist?
  42. 42. Mário J. Franca m.franca@un-ihe.org @Mario_J_Franca

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