CMS-Flow Sediment Transport

SMS 10.1

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 labeles Calculate sediment transport. The CMS card used to turn on or off the sediment transport is described in the table below.

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".

Card	Arguments	Default	Range	Description	Versions
SED_TRAN_FORMULATION	CHARACTER	NET	WATANABE | LUND_CIRP | A-D | NET	Selects the sediment transport model.	>1.0
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
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

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.

Transport Formula

The nearbed sediment concentation or concentration capacity are calculated with one of the following transport formula:

1. Lund-CIRP (2006)
2. Van Rijn (1998)
3. Watanabe (1987)
4. Soulsby-van Rijn (1997) (>=V4.0)

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.

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.

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 Scaling Factor

The morphologic 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.

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 scaling 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.

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

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	v>=3.5
SEDIMENT_FALL_VELOCITY_FORMULA	CHARACTER	SOULSBY-WHITEHOUSE | WU-WANG	SOULSBY-WHITEHOUSE	Sets the sediment fall velocity formula.	v>=3.5

Bedslope term

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

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.

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

CMS-Flow Model Control window showing the location where the hard bottom dataset is specified.

Hard Bottom is a morphologic constraint that provides the capability to simulate mixed bottom types within a single simulation. This cell-specific feature limits the erodibility of the constrained cells down to a specified depth below the water surface. During sediment transport calculations, exposed hard bottom cells may become covered through deposition. By default, CMS-Flow cells are fully-erodible cells with no specified hard bottom depth (inactive cells; denoted by the CMS-Flow null value of -999.0). Hard bottom only needs to be specified only for computational (ocean) cells.

Within the CMS-Flow Model Control window, the hard bottom dataset can be created from the Sediment tab. If the dataset does not exist, it can be created using the Create Dataset button. If a dataset exists (created using the Data Calculator) which represents the intended hard bottom specifications, the Select Dataset.. button can be used to select such dataset and copy the values to the hard bottom dataset.

When specified, cell hard bottom depths will appear in the Project Explorer as a scalar dataset beneath the CMS-Flow grid. This dataset can not be deleted, though it can be edited like any other dataset. A CMS-Flow simulation must contain the hard bottom dataset (even if it is not specified) so SMS will create a defaulted (inactive cells) dataset if it does not already exist when saving the simulation. The hard bottom dataset can created, edited, viewed and verified using the following SMS interface features.

SMS Project Explorer showing Hard bottom dataset

Hard Bottom Specification

Although the hard bottom dataset can be edited (when its the active dataset) by selecting a cell (or group of cells) and changing the scalar (S) value in the Edit Window, an user-friendly window exists which provides specification options. With the Select Grid Cell tool active, make a selection, right click to bring up the tool menu and choose the Specify Hard Bottom... option. This will open the CMS-Flow Hard Bottom Specification window.

The following options are provided in the Hard Bottom Specification window:

• Use bathymetric cell depth - Sets the cell hard bottom depth to be the cell geometry value thereby creating an exposed non-erodible condition. If multiple cells were selected, then each cell will use its respective bathymetric depth.
• Specified distance below bathymetric cell depth - Sets the cell hard bottom depth to be the cell geometry value plus the specified distance thereby creating a sediment-covered non-erodible condition. The distance is limited to positive values to ensure the hard bottom depth is greater than the geometry value. The cell can provide sediment for transportation, however, the amount of erosion is limited. If multiple cells were selected, then each cell will use its respective bathymetric depth.
• Specified depth - Sets the cell hard bottom depth to the specified depth thereby creating a sediment-covered non-erodible condition similar to specified distance. The depth is limited to greater than the geometry value. If multiple cells were selected, then the depth is limited to greater than the largest geometry value and all cells will have the same value.
• Unspecified - Resets to an inactive hard bottom condition. The cell hard bottom depth is set to the CMS-Flow null value. If multiple cells were selected, then all cells will be reset.

If no cells are selected when opening the Hard Bottom Specification window, then all computational (ocean) cells will be used. If a selection of only non-computational cells, then specification cannot occur. If a selection contains computational and non-computational cells, then the specification will only apply to the computational cells.

If multiple computational cells with differing specifications are selected, the window will not display a selected specification type and the OK button will be disabled. This is to protect the previous specifications from being overwritten by mistake. The OK button will be enabled when an option is selected. The minimum hard bottom depth of the multiple computational cells selected will be displayed in the Depth edit field and the minimum hard bottom depth minus the maximum geometry depth of the multiple computational cells selected will be displayed in the Distance edit field.

Display Options

The hard bottom dataset (when its the active dataset) will only display the cells with hard bottom specified if the Ocean cell display option is turned on. Inactive hard bottom cells are not displayed.

CMS-Flow includes hard bottom symbols to differentiate specifications. On the Cartesian Grid page of the Display Options window (when CMS-Flow is the active model), the Hard bottom symbols check box controls the display of symbols that will appear in hard bottom cells (even if the hard bottom dataset is not active). If this is turned on, then the user must be aware of the individual symbol settings accessed by clicking on the Options... button. The Options... button displays the CMS-Flow Hard Bottom Symbols window.

Hard bottom symbols can be selected for three hard bottom specification types:

• Non-erodible - Displayed in exposed hard bottom cells (cell hard bottom depth is equal to cell bathymetric depth).
• Erodible to specified depth - Displayed in sediment-covered hard bottom cells (cell hard bottom depth is greater than cell bathymetric depth).
• Invalid specification - Displayed in hard bottom cells where the hard bottom depth is less than cell bathymetric depth (the geometry is below the erosion limit).

If the Hard bottom symbols check box is turned off, no symbols will be displayed and the individual settings cannot be accessed, however, the individual settings will not be changed.

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.

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	0.7	0.6-1.0	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.