CMS-Flow Sediment Transport: Difference between revisions
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=Hard Bottom= | =Hard Bottom= | ||
See the [CMS/ | See the [[CMS/Hard Bottom|CMS Hard Bottom page]] for more information. | ||
= Model Check = | |||
The CMS-Flow Model Checker, accessed from the CMS-Flow | Model Check... menu item, includes a check to ensure that no invalid hard bottom specifications exist in the grid. An invalid specification may be created, for example, by setting an infeasible hard bottom scalar value in the Edit Window or adjusted the grid's geometry without updating the hard bottom. It is suggested that the model checker be used prior to running CMS-Flow. | The CMS-Flow Model Checker, accessed from the CMS-Flow | Model Check... menu item, includes a check to ensure that no invalid hard bottom specifications exist in the grid. An invalid specification may be created, for example, by setting an infeasible hard bottom scalar value in the Edit Window or adjusted the grid's geometry without updating the hard bottom. It is suggested that the model checker be used prior to running CMS-Flow. | ||
Latest revision as of 18:03, 21 January 2025
The sediment transport controls are located in the Transport section of the CMS-Flow Model Control window as shown in the figure below. The sediment transport is activated by going to the Transport section of the CMS-Flow Model Control and checking the box labels Calculate sediment transport. The CMS card used to turn on or off the sediment transport is described in the table below.
Table 1. CMS-Flow card used for activating the sediment transport calculation.
Card | Arguments | Default | Range | Description |
---|---|---|---|---|
CALC_SEDIMENT_TRANSPORT | CHARACTER | OFF | ON | OFF | Turns on or off the sediment transport calculation. |
Transport model
There are currently three sediment transport models available in CMS: (1) Equilibrium total load, (2) Equilibrium bed load plus advection-diffusion for suspended load, and (3) Non-equilibrium total load. The first two models are selected by unchecking the checkbox which says "Use non-equilibrium transport" and selecting either "Total load" for the first model, or "Advection-diffusion" for the second next to input item named "Formulation". The third model is selected by checking the box "Use non-equilibrium transport".
Table 2. CMS-Flow cards related to the transport model.
Card | Arguments | Default | Range | Description | Versions |
---|---|---|---|---|---|
SED_TRAN_FORMULATION | CHARACTER | NET | WATANABE | LUND_CIRP | A-D | NET | Selects the sediment transport model. | >1.0 |
Note that the when selecting the equilibrium total load model, the SED_TRAN_FORMULATION card is set to either WATANABE or LUND_CIRP depending on the transport formula chosen. When selecting the equilibrium A-D model, the transport formula is specified through the concentration profile formula (described below).
1. Equilibrium Total load
In this model, both the bed load and suspended load are assumed to be in equilibrium. The bed change is solved using a simple mass balance equation known as the Exner equation. More information on the this model can be found here.
2. Equilibrium Bed load plus Advection-Diffusion Suspended Load
Calculations of suspended load and bed load are conducted separately. The bed load is assumed to be in equilibrium and is included in the bed change equation while the suspended load is solved through the solution of an advection-diffusion equation. Actually the advection diffusion equation is a non-equilibrium formulation, but because the bed load is assumed to be in equilibrium, this model is referred to the "Equilibrium A-D" model.
More information on the this model can be found here.
3. Non-equilibrium Total Load
The non-equilibrium sediment transport algorithm (NET) simulates non-cohesive, single size sediment transport and bed change using a Finite Volume method and includes advection, diffusion, hiding and exposure, and avalanching. NET sediment transport is calculated with a non-equilibrium bed-material (total load) formulation. In this approach, the suspended- and bed-load transport equations are combined into a single equation and thus there is one less empirical parameter to estimate (adaptation length).
Additional information on NET can be found here.
All of the previously mentioned models account for hard bottom and effect of the bed slope on bed load.
Time Steps
The sediment transport time step is the time step at which the sediment transport equation is solved. In the case of the equilibrium total load model, then the sediment balance equation (Exner equation) is solved every morphologic time step. The morphologic time step is the time step at which the bed elevation is updated. The CMS-Flow
Table 3. CMS-Flow cards used for setting the sediment transport and morphologic time steps.
Card | Arguments | Range | Description | Version | |
---|---|---|---|---|---|
SED_TRAN_CALC_INTERVAL | REAL | greater or equal to hydro time step for explicit scheme, or equal hydro time step for implicit scheme | Time step used for transport equation | <=v3.75 | |
MORPH_UPDATE_INTERVAL | REAL | greater or equal to hydro time step for explicit scheme, or equal hydro time step for implicit scheme | Time step used for updating bed elevation | <=v3.75 |
- Important Note:
- When using the implicit solution (CMS versions 4.0 and greater), the sediment transport and morphologic time steps are set to the hydrodynamic time step. Therefore the above cards are ignored. The reason for this is because the implicit hydrodynamic time step is already big and using larger time steps for sediment transport and bed change is not necessary.
Transport Formula
The nearbed sediment concentation or concentration capacity are calculated with one of the following transport formula:
- Lund-CIRP (2006)
- Van Rijn (1984,2007)
- Watanabe (1987)
- Soulsby-van Rijn (1997) (>=V4.0)
Table 4. CMS-Flow cards used for setting the sediment transport formula.
Card | Arguments | Default | Range | Description | |
---|---|---|---|---|---|
NET_TRANSPORT_CAPACITY | CHARACTER | LUND-CIRP | LUND-CIRP | VAN_RIJN | WATANABE | SOULSBY | Selects the transport formula. Note that SOULSBY is only available in v>=4.0 | |
TRANSPORT_FORMULA | CHARACTER | LUND-CIRP | LUND-CIRP | VAN_RIJN | WATANABE | SOULSBY | Selects the transport formula. Note that SOULSBY is only available in v>=4.0. | |
SED_TRANS_FORMULATION | CHARACTER | LUND-CIRP | LUND-CIRP | A-D | WATANABE | NET | Selects the transport formula for the equilibrium total load model. Does not specify the transport formula for the equilibrium A-D and non-equilibrium total load models. | |
CONCENTRATION_PROFILE | CHARACTER | LUND-CIRP | LUND-CIRP|EXPONENTIAL| ROUSE| VAN_RIJN | Selects the concentration profile to be used either in the equilibrium A-D or total load nonequilibrium models. | |
A_COEFFICIENT_WATANABE | REAL | 0.1 | 0.05-0.5 | Empirical coefficient which goes into the Watanabe transport formula. |
- Important Notes:
- Different transport formula may produce very different results in morphology change.
- The Lund-CIRP does well in predicting the surf zone sediment transport but tends to overestimate the transport rates near the wetting and drying limit and in deep water (>10 m).
- The van Rijn transport formula tends to underestimate the transport for conditions near the critical shear stress of motion. The formula also tends to underestimate the transport close to the shoreline.
- The Soulsby-van Rijn transport formula also
Scaling Factors
Transport Scaling Factors
The bed and suspended transport scaling factors multiply directly by the transport capacity or near-bed sediment concentration calculated from the transport formula. These factors should be used to calibrate sediment transport rates and due to the large uncertainty in the transport formula, it is generally acceptable to use scaling factors in the range of 0.5-2.0.
Table 5. CMS-Flow cards used for setting the bed and suspended load scaling factors.
Card | Arguments | Default | Range | Description |
---|---|---|---|---|
BED_LOAD_SCALE_FACTOR | REAL | 1.0 | 0.5-2.0 | Calibration factor for bed load transport capacity formula |
SUSP_LOAD_SCALE_FACTOR | REAL | 1.0 | 0.5-2.0 | Calibration factor for suspended load transport capacity formula |
Morphologic Acceleration Factor
The morphologic acceleration or scaling factor is directly multiplied by the calculated bed change at every time step and is intended as a means of speeding up the computational time. It is only recommended for periodic boundary conditions or conditions that do not change rapidly over time. The morphologic acceleration factor is set in the Sediment tab of the CMS-Flow Model Control window in SMS.
Table 6. CMS-Flow card used for setting the morphologic acceleration factor.
Card | Arguments | Default | Range | Description |
---|---|---|---|---|
MORPH_ACCEL_FACTOR | REAL | 1.0 | 1-100 | Morphologic acceleracion factor. Directly multiplies by calculated bed change. |
- Note: The morphologic acceleration factor is NOT a calibration parameter. It should only be used in cases with periodic forcing and boundary conditions and even then it should be used with caution. It is NOT recommended to use larger values than 20-30.
Schmidt Number
The sediment mixing coefficient is calculated as the eddy viscosity divided by the Schmidt number. For simplicity the Schmidt number is assumed to be constant and the default value is 1.0. The Schmidt number can only be changed using the advanced card described below.
Table 7. CMS-Flow card used for setting the Schmidt number.
Card | Arguments | Default | Range | Description | Versions |
---|---|---|---|---|---|
SCHMIDT_NUMBER | REAL | 1.0 | none | Controls the sediment mixing strength | v>=4.0 |
- Note: The Schmidt number should NOT be used as a calibration number and showed only be changed in sensitivity analysis or model testing.
Sediment Characteristics
The sediment characteristics are set in the Sediment tab of the CMS-Flow Model Control window. The sediment characteristics are the porosity, density, shape, and fall velocity.
Table 7. CMS-Flow card used for setting the Schmidt number.
Card | Arguments | Range | Default | Description | Versions |
---|---|---|---|---|---|
SEDIMENT_POROSITY | REAL | 0-1 | 0.4 | Sets the sediment porosity | |
SEDIMENT_DENSITY | REAL | none | 2650 | Sets the sediment density in kg/m^3 | |
SEDIMENT_FALL_VELOCITY | REAL | 4.0e-4 - 0.4 | none | Sets the sediment fall velocity to a constant in m/s | v>=3.5 |
SEDIMENT_FALL_VELOCITIES | INTEGER REAL REAL ... | none | none | Sets the sediment fall velocity to a constant in m/s for multiple grain size classes. The first number is the number of size classes and is followed by the fall velocities for each size class in ascending order. | v>=4.0 |
SEDIMENT_FALL_VEL_FORM | CHARACTER | SOULSBY| WU-WANG | SOULSBY | Sets the sediment fall velocity formula. | v>=4.0 |
SEDIMENT_FALL_VELOCITY_FORMULA | CHARACTER | SOULSBY| WU-WANG | SOULSBY | Sets the sediment fall velocity formula. Same as SEDIMENT_FALL_VEL_FORM. | v>=4.0 |
SEDIMENT_COREY_SHAPE_FACTOR | REAL | none | 0.7 | Sets the Corey shape factor which is used in the Wu-Wang sediment fall velocity formula. | v>=4.0 |
- Important Notes:
- It is NOT recommended to use the sediment fall velocity, porosity, density or shape factor as calibration parameters. These parameters should be estimated using field or literature data.
- The sediment porosity and density are assumed constant for the whole domain and all grain size classes. For most coastal applications these assumptions are reasonable but need to be taken into consideration.
Avalanching
Avalanching is the process of sediment sliding when the critical angle of repose is reached. In CMS, avalanching is simulated using a mass conservative relaxation method which limits the bed slope to the critical angle of repose. For most coastal applications, the critical angle of repose is never reached, so it is not needed. The CMS-Flow cards used for specifying avalanching, and its options, are described in the below.
Table 8. CMS-Flow cards related to avalanching.
Card | Arguments | Range | Default | Description |
---|---|---|---|---|
USE_AVALANCHING | CHARACTER | ON | OFF | ON | Turns On or Off the avalanching. |
RESPOSE_ANGLE | REAL | none | 32º | Specifies the angle of repose in degrees. |
AVALANCHE_MAX_ITERATIONS | INTEGER | none | 200 | Specifies the maximum number of iterations used in the implicit solution scheme. For the explicit solution scheme, the avalanching is calculated every transport time step for one iteration. |
Bedslope term
The bedslope term accounts for the effect of gravity on sloped beds. The larger the bed slope coefficient, the more sediment tends to move downslope, thus smoothing the solution. The CMS-Flow used to specify the slope coefficient is described in the table below. The bedslope coefficient is set in the Sediment tab of the CMS-Flow Model Control window in SMS 11.0.
Table 9. CMS-Flow card used for setting the bedslope coefficient.
Card | Arguments | Default | Range | Description | Versions |
---|---|---|---|---|---|
SLOPE_COEFFICIENT | REAL | 1.0 | 0-5 | Bed slope coefficient which controls enters a diffusion term which moves sediment down slope |
- Note: It is recommended that the bed slope be set to 0.1 for the nonequilibrium total load model to avoid excessive smoothing.
Ramp Period
The option is available to not calculate the morphology change during the ramp period. The best practice is the start the model simulation so that the time when the ramp period ends corresponds to the time of the measured bathymetry. This avoid the initial bed erosion (although slight) of the bed. This also facilities calculating simulation statistics such as transport rates and residual currents.
Table 10. CMS-Flow card used to turn On or Off bed updating during the ramp period.
Card | Arguments | Range | Default | Description | Versions |
---|---|---|---|---|---|
CALC_MORPH_DURING_RAMP | CHARACTER | ON | OFF | ON | Turns On or Off the morphology change calculation during the ramp period | v>=4.0 |
Total Load Correction Factor
The total load correction factor accounts for the nonuniform vertical profile of sediment concentration and current velocity and produces temporal lag in between the flow and sediment transport. The factor is used in the nonequilibrium total load sediment transport formula. The factor is obtained by integrated the
Table 11. CMS-Flow cards related to the total load correction factor.
Card | Arguments | Default | Range | Description |
---|---|---|---|---|
TOTAL_LOAD_CORR_FACTOR_CONSTANT | REAL | 0.3-1.0 | none | Sets the total load correction factor to a constant. |
CONCENTRATION_PROFILE | CHARACTER | LUND-CIRP | VAN_RIJN | EXPONENTIAL | ROUSE | none | Sets the concentration profile to be used either in the pickup and deposition functions or the total load correction factor calculation. |
Boundary and Initial Conditions
In the case of the Equilibrium Total Load sediment transport model, all boundaries are set to the equilibrium transport rate. For the Equilibrium Bed Load plus Advection Diffusion model, the suspended load is specified as the equilibrium concentration at inflow cells and a zero gradient at outflow cells. For the Total load nonequilibrium sediment transport model, the sediment concentration is set to the equilibrium concentration at inflow cells and a zero gradient boundary condition is applied at outflow cells.
In the case an initial conditions file is NOT specified both the hydrodynamics and sediment concentrations are initialized as zero. If an initial conditions file is specified, than the initial sediment concentrations are read in. If an initial conditions file is specified but without the sediment concentration, than the initial sediment concentration is set to the equilibrium concentration.
Table 12. CMS-Flow cards related to the boundary conditions
Card | Arguments | Default | Range | Description | Versions |
---|---|---|---|---|---|
NET_LOADING_FACTOR | REAL | 1.0 | 0.5-2.0 | Used to specify under- or overloading at sediment inflow boundaries. Only for NET. | 3.5=>v<=4.0 |
SEDIMENT_INFLOW_LOADING_FACTOR | REAL | 1.0 | 0.5-2.0 | Used to specify under- or overloading at sediment inflow boundaries. | >=4.0 |
CALC_MORPH_DURING_RAMP | CHARACTER | ON | ON | OFF | Determines whether to calculate the morphology change during the ramp period | v>=3.5 |
Hard Bottom
See the CMS Hard Bottom page for more information.
Model Check
The CMS-Flow Model Checker, accessed from the CMS-Flow | Model Check... menu item, includes a check to ensure that no invalid hard bottom specifications exist in the grid. An invalid specification may be created, for example, by setting an infeasible hard bottom scalar value in the Edit Window or adjusted the grid's geometry without updating the hard bottom. It is suggested that the model checker be used prior to running CMS-Flow.
Variable D50
This cell-specific parameter allows for the single-sized sediment transport model to make a hiding and exposure correction factor. More information on the use of variable grain size (D50) can be found here.
Table 13. CMS-Flow cards related to the variable D50 feature.
Card | Arguments | Default | Range | Description |
---|---|---|---|---|
TRANSPORT_GRAIN_SIZE | REAL | none | none | Transport grain size in mm. The transport grain size is the sediment size which is eroded, transported, and deposited. When this card is specified the D50 dataset is used to make a hiding and exposure correction to the critical shear stress. |
HIDING_EXPOSURE_COEFFICIENT | REAL | 1.0 | none | Hiding and exposure coefficient. |
Important Notes
- If the no transport grain size is specified, then the transport grain size is calculated based on the mean grain size of the whole domain.
- If the card CONSTANT_GRAIN_SIZE is used, then the D50 dataset will be ignored.