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- 1 Tutorial Description
- 2 Suspended sediment transport and deposition
- 3 Erosion of Suspended Sediment
- 4 Suspended Sediment Particle Tracking (Optional)
- 5 Bedload Transport and Additional Sediment Fractions
- 6 Bed Layers and Armouring
- 7 3D Sediment Modelling
- 8 Summary
The TUFLOW Sediment Transport Module is a flexible and powerful bed load and suspended load sediment transport model that enables the 2D/3D simulation of sediment transport in rivers, estuaries and coastal environments. One-or-more sediment fractions can be simulated as they are distributed within the bed and transported as bed or suspended load. This fraction-based implementation allows a high level of control over sediment characteristics. For example, within a single model run, sediment fraction groups can be assigned as cohesive or non-cohesive and there is flexibility to select from a range of common sediment transport models/equations independently for each fraction. The sediment transport module is driven by the TUFLOW FV hydrodynamic model.
The tutorial will cover a number of aspects of the Sediment Transport model:
Tutorial Module Data Download
The tutorial data from which to work from, as well as a set of completed model files should be downloaded from the following page. This tutorial will build off the FMA2_Hydro_No_Structures_GIS.fvc model generated as part of the Tutorial 3. This is just a hydrodynamic only simulation but uses the GIS setup. For the purpose of this tutorial the structures are not in place.
For the purposes of the particle tracking tutorial, we'll be using the hydrodynamic model created in Tutorial Module 3 (Floodplain Application) and supplementing with additional information to specify sediment transport.
Suspended sediment transport and deposition
In the first exercise we will create a simple model of sediment transport by adding a suspended sediment input to the model and simulate its passage through the reach. The sediment input will be a fine sediment which will be allowed to deposit based on a settling velocity but no erosion will occur.
The existing FMA2_Hydro_No_Structures_GIS.fvc is a complete hydrodynamic model for a river system with its floodplain. It can be run in a hydrodynamic only mode to assess the flow hydrodynamics but as part of this exercise we’ll add some suspended sediment as an upstream boundary to the model and model it’s transfer through the river.
Specifying the TUFLOW FV Model Control Files
Open the FMA2_Hydro_No_Structures_GIS.fvc and add the include sediment command as follows into the Model Parameters block:
Include Sediment == 1,0 !(Enabled, Density coupling)
This will ensure that sediment tracers are included in the model. The second argument allows the coupling of density. We will leave set at 0 which means that water density is not affected by suspended sediment concentration.
Add the sediment control file command whose content is described in detail within Section 1.5 of the Manual:
!Sediment Files Sediment Control File == FMA2_SED_001.fvsed !Add reference to sediment control file
Modify the fvc file by changing the command from referencing the inflows.csv to steady_inflows.csv for boundary 1. And reference the FineSed time-varying data in steady_inflows.csv by adding FineSed to bc_header section.
bc == Q, 1, ..\bc_dbase\steady_inflows.csv !Flow boundary [m3/s] bc header == Date,Main_Inflow, fineSed !Column Headings for boundary data
Also change the reference for boundary 3.
bc == Q, 3, ..\bc_dbase\steady_inflows.csv !Flow boundary [m3/s] bc header == Date,Tributary_Inflow !Column Headings for boundary data
Add the following additional output parameters to the xmdf output section. This will allow us to map the bed shear stress, the sediment 1 flux representing our fine sediment, the total suspended sediment, the total deposition, the resulting bed thickness and the total sediment mass within the bed: Taub, SED_1, TSS, DEPOSITION_TOTAL, THICK and BED_MASS_TOTAL
output == xmdf output parameters == h,v,d,zb, Taub, SED_1, TSS, DEPOSITION_TOTAL, THICK, BED_MASS_TOTAL !Map Output Results Parameters
Save the TUFLOW FV control file as FMA2_SED_001.fvc
Copy and paste the steady_inflows.csv from the module data folder to the bc_dbase folder.
Open the steady_inflow.csv file, you’ll see that there is an additional column called FineSed which will provide us with our time-varying input of fine sediment in mg/l.
Close the steady_inflows.csv file.
Create a new text file in the Runs folder called FMA2_SED_001.fvsed.
Open the FMA2_SED_001.fvsed and add the following commands.
!TIME COMMANDS !_________________________________________________________________ update dt == 300. !_________________________________________________________________ !SIMULATION CONFIG COMMANDS Morphological Coupling == 0 !Turn on/off Morphological Feedback to Hydrodynamics. 1 to enable feedback, 0 to disable. Bed roughness coupling == 0 !Turn on/off Bed Sediment Feedback to Bed Roughness. 1 to enable feedback, 0 to disable. Erosion Depth Limits == 0.1, 0.2 !Depth to scale Erosion to Zero (<0.1m no erosion, >0.2m full erosion) Deposition Depth Limits == 0.1, 0.2 !Depth to scale Despoition to Zero (<0.1m no deposition, >0.2m full deposition) !_________________________________________________________________ !SEDIMENT FRACTION COMMANDS Fraction == fineSed !Sediment Fraction 1 Name d50 == 0.000002 !Median Sediment Diameter(m) particle density == 2650 !Particle Density (kg/m3) Settling model == constant !Settling Model settling parameters == 0.0002 !Settling Velocity (m/s) deposition model == ws0 !Deposition model End Fraction !End of Sediment Fraction Definition !_________________________________________________________________ !MATERIAL SPECS COMMANDS ! Default Bed Sediment properties for materials material == 0 !default material definition suspended Load Scale == 1.0 !Calibration Factor for suspended load Layer == 1 !Begin Layer Definition Dry density == 1890.!Dry Density of Sediment Fractions(kg/m3) initial mass == 1000.!Initial mass of sediment fractions (kg/m2) end layer !End layer definition end material !End material definition
Save and close the file.
Modify the batch file to run the simulation. If you have a compatible GPU card, follow the instructions here to utilise the GPU card to significantly speed up the simulation.
Once the simulation has started, open the log file and confirm that the sediment parameters are read in correctly. In this exercise we are modelling a sediment input into the model which can also deposit. No erosion is represented within this model and most options are either set as None or default. We will vary these in later exercises.
Suspended Sediment and Deposition Results Analysis
Open the results in QGIS and view the flow depth results. Note the channel in the north of the model area. The upstream of this channel is the location we have applied the steady input of fine sediment too. The sediment is applied as a boundary at the upstream end of the reach.
Select Sed_1 in the Result Type in the TUFLOW QGIS Viewer plugin, and animate results. The sediment input is added to the domain and is transported downstream to the outlet with the ocean.
The ‘Deposition_Total’ results parameter can be used to map the deposited sediment within the reach. The darker pink areas show areas of increased deposition. You’ll notice increased deposition in the location of the mid-channel and side bars within the model reach.
We can use the QGIS TUFLOW Viewer results analysis tools to determine the sediment concentration at a single point, deposition rate, or the change in bed elevation, thickness and mass as a consequence of the deposition.
You’ll see that bed thickness, the THICK results type, increases as a consequence of deposition in this model as we are not representing erosion. This change in bed thickness also slightly increases the bed elevation, Zb, in some areas of the model. The thickness of the bed is calculate as the Total quantity of bed mass (kg/m2) in each layer divided by the dry density (kg/m3). The bed mass in layer 1 is 1000kg/m2 and the dry density is 1890kg/m3. The resulting initial bed thickness is therefore 0.5291m.
Using the Plot Cross-Section/Long Plot from Map Output tool, select bed elevation, Zb and water level, H, and digitize a line across the width of the channel.
This will provide a cross-section showing the water level at the current timestep as well as the bed elevation at the start of the simulation and the bed simulation at the current point in time, Zb. The water level and Zb parameter can be animated. Note that there are some interpolation effects within the generation of the section which make it look as if the instantaneous bed elevation is lower than the bed elevation, we do not have erosion represented within the model. In this instance we have only added fine sediment fraction as a suspended sediment input into our model and simulated it’s deposition. We will add erosion in the next exercise.
Erosion of Suspended Sediment
In this part of the tutorial, we will now represent bed erosion within the model to allow any deposited fine sediment to also be picked up by the flow . This mechanism is important for fine grain bed sediment which are entrained by flow to become suspended sediment.
Modifying the TUFLOW FV Control Files
Copy and paste the existing FMA2_SED_001.fvsed file and rename as FMA2_SED_002.fvsed.
In the FMA2_SED_002.fvsed, add the following two commands to the FineSed fraction commands.
Erosion Model == Mehta !Choice of Erosion model Erosion parameters == 0.1, 0.5, 0.5 !Er, taucr, alpha
These parameters will set the erosion model to the Mehta model (Also known as the Partheniades model) (other options are van Rijn 84 , Soulsby-van Rijn, Bijker, van Rijn04 erosion models, see the TUFLOW FV STM manual for more details). The Mehta erosion model uses the following shear excess equation to calculate sediment erosion flux:
Er is the erosion rate constant (g/m2s)
τbcw is combined bed shear stress due to currents and waves (N/m2)
τce is critical shear stress for erosion (N/m2)
α is a power coefficient
In the erosion parameters command, we specify the Erosion rate constant, the critical shear stress for the sediment fraction and the alpha coefficient used by the Mehta erosion model.
Save and close the FMA2_SED_002.fvsed.
Copy and paste the existing FMA2_SED_001.fvc file and rename as FMA2_SED_002.fvc.
Update the reference to the sediment control file to reference FMA2_SED_002.fvsed.
Sediment Control File == .\FMA2_SED_002.fvsed !Add reference to sediment control file
Add PICKUP_TOTAL and NETSEDRATE_TOTAL, to the list of output parameters.
output parameters == h,v,d,zb, Taub, SED_1, TSS, DEPOSITION_TOTAL, PICKUP_TOTAL, NETSEDRATE_TOTAL !Map Output Results Parameters
Save and close the FMA2_SED_002.fvc file, update the batch file and run the FMA2_SED_002 simulation.
Erosion Results Analysis
Once the simulation has completed, open the results and plot the SED results parameter for both the completed simulations so far. It can be seen that the sediment concentration for the most recent run is significantly higher than that of first simulation where only deposition was present. The river channel has a lot of energy within it and thus the fine sediment we have introduced would by and large be eroded under the flows we have added to the system.
The Pick Up Rate results parameter represents the pick up of suspended sediment and is shown in the below figure. Those areas of highest pickup (erosion) are shown in red.
The NetSedRate total represents the Pick up rate minus the deposition. A positive value represents areas of deposition and a negative value areas of erosion. We can use this to map and identify areas of net deposition and erosion.
The resulting map shows areas of erosion in red and deposition in blue. You can plot the time-series of the bed thickness, the Thick results parameter, to confirm the areas of erosion and deposition with erosion areas showing a reduction in bed thickness and deposition areas an increase.
Suspended Sediment Particle Tracking (Optional)
It is possible to couple the sediment transport functionality with the particle tracking module described in the Tutorial 6. The following page provides a simple example utilising the FMA2_SED_002 model.
So far we have represented the transport, deposition and erosion of a fine sediment input within our model. The fine sediment model is not particularly representative for a river of this nature which is characterised by coarser sediments which are subject to bedload.
Bedload Transport and Additional Sediment Fractions
In this exercise, we will add a second sediment fraction to represent gravel particles within our reach and introduce bed load transport. The representation of gravel particles is more representative of the type of channel we are representing.
Copy and paste the existing FMA2_SED_002.fvsed file and rename as FMA2_SED_003.fvsed.
Next we’ll add an additional sediment fraction to our model to represent the gravel bed present within our reach. In gravel bed rivers with high energy, a significant amount of sediment can move via bedload. Therefore, we need to add a bed load model and bed load parameters to our gravel fraction. In this instance, we will use the Meyer-Peter-Müller-Shimizu model but other are available (Meyer_Peter and Müller, Soulsby-Van Rijn, Bijker, Wilcock-Crowe, van Rijn). We do not specify an erosion model or deposition model as these are related to suspended sediment only.
Within the FMA2_SED_003.fvsed add the following section to add gravel to our model.
Fraction == Gravel fineSed !Sediment Fraction 2 Name d50 == 0.032 !Median Sediment Diameter(m) particle density == 2650 !Particle Density (kg/m3) Settling model == constant !Settling Model (m/s) settling parameters == 0.6 !Settling Velocity (m/s) Critical stress model == Soulsby !Critical Shear Stress Model Bed load model == MPM_Shimizu !Bedload Model Bed load parameters == 8.0, -1 ,1.5 !Bedload Model Parameters Er, taucr (set to -1 to use Soulsby model and factor outputs by by 1), alpha End Fraction !End Fraction Definition
The MPM-Shimizu bedload model uses the following equation:
µs and µk are the static and kinetic friction coefficient (assumed as 0.6 and 0.48, respectively)
𝜕𝑧⁄𝜕𝑛̂ is the bed slope component perpendicular to bed shear stress direction.
τ* and τ*c are non-dimensionalised bed shear stress and critical shear stress, respectively
With the bedload parameters command we specify a bedload factor, the critical shear stress and the alpha parameter. Unlike the suspended sediment model, the Mehta equation, where we specified a user-defined critical shear stress, this time we are using the Soulsby model. The Soulsby model uses an algebraic expression for the Shields curve developed by Soulsby and Whitehouse (1997) which relates the particle size to the critical shear stress. By using a critical shear stress model the shear stress required for incipient motion varies throughout the simulation based on the D50 characteristics particle size on the bed surface.
We won’t be adding a gravel input into the upstream end of our model in this instance but instead will allow the bed, comprised of a mixture of fine sediments and gravel, to erode and provide a gravel-based sediment input.
To do this, modify the following commands in the Material Specs Command in the layer 1 sub-block to represent the two sediment fractions with the bulk being sediment fraction 2.
Dry density == 1890., 1890. !Dry Density of Sediment Fractions(kg/m3) Initial mass == 50., 2950. !Initial mass of sediment fractions (kg/m2)
Finally we’ll set up the model to represent morphological coupling by changing the following command setting to 1. This allows the changing bed elevations, as a consequence of erosion and deposition, to subsequently feedback and influence the hydrodynamics (which in turn will impact sediment transport)
Morphological Coupling == 1 !!Turn on/off Morphological Feedback to Hydrodynamics. 1 to enable feedback, 0 to disable.
Save and close the FMA2_SED_003.fvsed
Copy and paste the existing FMA2_SED_002.fvc file and rename as FMA2_SED_003.fvc.
Update the reference to the sediment control file to reference FMA2_SED_003.fvsed
Sediment Control File == .\FMA2_SED_003.fvsed ! Add reference to sediment control file
Add BEDLOAD_SED_2, BEDLOAD_TOTAL, SUSPLOAD_TOTAL to the list of output parameters.
output parameters == h,v,d,zb, Taub, SED_1, TSS, DEPOSITION_TOTAL, PICKUP_TOTAL, NETSEDRATE_TOTAL, BEDLOAD_SED_2, BEDLOAD_TOTAL, SUSPLOAD_TOTAL !Map Output Results Parameters
Save and close the FMA2_SED_003.fvc file, update the batch file and run the simulation.
Bedload Results Analysis
Once the simulation is complete, open the results in QGIS and investigate the Bedload_Sed_2 results parameter. You’ll see that we have a significant amount of the second sediment which represents Gravel subject to bedload that has been eroded from the bed particularly at the beginning of the event at the upstream part of the model where we are applying the inflow. As we have not specified an initial condition, the channel bed is dry and the resulting inflow provides a wall of water which generates significant bedload. After a while the inflow reaches a steady state.
The amount of bedload and suspended sediment transport can be plotted to show the relative contribution of the various components to total sediment loads. As you can see from the below graph the bedload dominates in our simulation.
Compare the results for Bed thickness, pickup total and NetSedRate between FMA2_SED_002 and FMA2_SED_003 to assess the variation in results as a consequence of adding the additional sediment fraction. You should see thicker beds and on the whole less erosion when bedload is considered. Also compare the hydraulics for the two models, for the bedload, FMA2_SED_003 model we have turned on morphological coupling so this is a feedback into the hydraulics.
Bed Layers and Armouring
We now have a model with an input of fine sediment at the upstream part of our river reach which is able to deposit and re-erode. Our bed is currently made up of gravel only which is subject to bedload transport. So far we have represented a single bed layer with no bed armouring. Bed armouring is an important component which is common in gravel bed rivers such as the one in our reach, and protects fine sediments in the lower part of the bed.
Copy and paste the existing FMA2_SED_003.fvsed file and rename as FMA2_SED_004.fvsed.
In the Simulation Config Commands add the following command and parameters to allow bed armouring to be represented.
Armour layer thickness == 0.05, 0.2 !Specify Min and Max Armour Layer Thickness(m)
This command controls the amount of material within the active layer. If the updated active layer thickness is less than the specified minimum, then mass is exchanged from the underlying layers in order to address this shortfall. Otherwise, if the updated active layer thickness is more than the specified maximum value then mass is exchanged to the next layer.
In the Material Specs Commands, modify the section to now look as follows. This adds two material layers, a default one which we will have a single layer of erodible mass which will be applied to the floodplain and one to represent the channel which will have two layers comprises of differing ratios of fine sediment and coarser gravels.
!MATERIAL SPECS COMMANDS ! Bed Sediment properties for each material. material == 0 ! default material definition Suspended Load Scale == 1.0 !Calibration Factor for suspended load Layer == 1 !Begin Layer Definition Dry density == 1890.,1890 !Dry Density of Sediment Fractions(kg/m3) Non erodible mass == 1000., 1000 !Non Erodible mass of sediment fractions (kg/m2). In this instance there is no erosion of material 0 end layer end material material == 1 ! River material definition Suspended Load Scale == 1.0 ! Calibration Factor for suspended load Nlayers == 2 !Number of bed layers Layer == 1 !Begin Layer Definition Dry density == 1890., 1890 !Dry Density of Sediment Fractions(kg/m3) initial mass == 50., 2950 !Initial mass of sediment fractions (kg/m2) end layer !End layer definition Layer == 2 !Begin Layer Definition Dry density == 1890.,1890 !Dry Density of Sediment Fractions(kg/m3) initial mass == 1000,1000 !Initial mass of sediment fractions (kg/m2) end layer !End layer definition end material !End material definition
With the initial mass command we are making a best estimate of the composition of the bed. We will run the model with the steady inflows for a 24 hour period, a period in which we hope to achieve an equilibrium state (in reality this may take much longer). The equilibrium state will be saved as a restart file for subsequent use with an unsteady model run. To save the bed composition, we need to save a restart file within our FMA2_SED_004.fvsed file.
To do so, add the following command to write a restart file at 12 hour intervals.
Write restart dt == 12 !Write Restart File Interval (hrs)
Save and close the FMA2_SED_004.fvsed file.
Copy and paste the existing FMA2_SED_003.fvc file and rename as FMA2_SED_004.fvc.
Update the reference to the sediment control file to reference FMA2_SED_004.fvsed
Sediment Control File == .\FMA2_SED_004.fvsed ! Add reference to sediment control file
Add D50_Layer_1, D50_Layer_2 to the list of output parameters. This provides mapped results outputs for the characteristic sediment size in each layer. Also add the
output parameters == h,v,d,zb, Taub, SED_1, TSS, DEPOSITION_TOTAL, PICKUP_TOTAL, NETSEDRATE_TOTAL, THICK, BED_MASS_TOTAL, PICKUP_TOTAL, NETSEDRATE_TOTAL, BEDLOAD_TOTAL, SUSPLOAD_TOTAL, D50_Layer_1, D50_Layer_2 !Map Output Results Parameters
Save and close the FMA2_SED_004.fvc file, update the batch file and run the simulation.
Bed Layers Results Analysis
Once the simulation is complete, use the QGIS TUFLOW Viewer plugin to investigate the Total bed mass, bed thickness and D50 particle size for the bed layers 1 and 2. Also add BED_MASS_LAYER_1_SED_1, BED_MASS_LAYER_1_SED_2, BED_MASS_LAYER_2_SED_1 and BED_MASS_LAYER_2_SED_2, this will add the bed mass in each layer for each sediment fraction. It's the relative composition of these which determine the characteristic sediment size in each layer of the bed.
The layer 1 sediment fraction masses are shown in the below figure. As can be seen the finer sediment fraction (1) is increasing and this is a consequence of the deposition that is occurring in this location.
The removal of coarser sediment leads to a reduction in the characteristics bed sediment size.
Generally speaking throughout the model, we have an equilibrium state within the bed mass which can then be used for subsequent simulations. This equilibrium state is a much better representation than the initial estimate of the initial mass and provides a more stable bed sediment regime.
Using the Bed Restart File
From the graphs and the Zb parameters, you’ll notice that we achieve some level of equilibrium bed mass from which we can run subsequent simulations on.
Copy and paste the existing FMA2_SED_004.fvsed file and rename as FMA2_SED_004a.fvsed.
Remove or comment out the following command:-
Write restart dt == 12 !Write Restart File Interval (hrs)
Replace with following command to read in the bed composition from the previous run.
Bed restart file == log\FMA2_SED_004_bed.rst !Read Bed Restart File
Save and close the FMA2_SED_004a.fvsed file.
Copy and paste the existing FMA2_SED_004.fvc file and rename as FMA2_SED_004a.fvc. Change the reference to the sediment control file to FMA2_SED_004a.fvsed.
Sediment Control File == .\FMA2_SED_004a.fvsed ! Add reference to sediment control file
Update the reference in the boundary conditions block to reference the inflows.csv file for both the discharge boundaries. In this instance we will apply an unsteady upstream flow boundary to our model. We will also remove the FineSed input from the model. The boundary conditions block should look as follows.
! BOUNDARY CONDITIONS bc == Q, 1, ..\bc_dbase\inflows.csv ! Flow boundary [m3/s] bc header == Date,Main_Inflow ! Header information end bc bc == WL, 2, ..\bc_dbase\tide.csv ! Water level boundary [m] bc header == Date,level ! Header information end bc bc == Q, 3, ..\bc_dbase\inflows.csv ! Flow boundary [m3/s] bc header == Date,Tributary_Inflow ! Header information end bc bc == WL, 4, ..\bc_dbase\tide.csv ! Water level boundary [m] bc header == Date,level ! Header information end bc
Save and close the FMA2_SED_004a.fvc file, update the batch file and run the simulation. As the simulation is for 48 hours rather than 24 hours, the simulation will take a little longer to run.
Once complete investigate the results using the results parameters shown previously. You’ll see in this instance that the unsteady event is one that extends out of bank on to the floodplain. On the floodplain we get deposition of suspended sediment but no erosion. This is a result of the spatial variation represented by the different material blocks that we added.
Investigate the bed mass and the D50 for the various layers, you should see that the starting point reflects the bed restart file and matches the bed mass and layer D50 at the end of the previous simulation. You'll also see how the characteristic sediment size varies between the channel and the floodplain with the channel characterised by coarser gravels and the floodplain by fine grain sediment. As the floodplain bed is set to non-erodible, there is no inputs into the bedload and therefore suspended sediment is dominant. This has an impact upon the bed thickness on the floodplain which is much less than that in the channel.
3D Sediment Modelling
TUFLOW FV has the capability to model sediment transport in both coastal and riverine environments in 2D and 3D. Both suspended sediment and bed load transport can be simulated including support for consolidated and unconsolidated sediments. The sediment transport simulations can be run utilising GPU card technology for significant run time gains whilst the TUFLOW Viewer QGIS plugin provides an excellent tool for the analysis and processing of TUFLOW FV sediment results. The sediment transport functionality within TUFLOW FV can also be used in conjunction with the particle tracking module and the water quality module which are covered in the following tutorials:-