CMS-Flow Numerical Methods: Sediment Transport

Sediment Transport Transport Equations

The sediment transport equations are discretized using the methods de-scribed in the previous section entitled General Transport Equation and are not repeated here.

Mixing Layer

The mixing layer or active layer thickness calculation is slightly modified to avoid excessively small layers and for cases of strong deposition as

$\delta _{1}=min\left[max(\delta _{1,min},2d_{50},\Delta /2),\delta _{1,max}\right]$

(1)

where $\Delta$ is the bed form height and $\delta _{1,min}$ and $\delta _{1,max}$ are the user speci-fied the minimum and maximum mixing layer thicknesses, respectively.

Bed Material Sorting

The bed material sorting equation

$L_{s}={\frac {Uh}{\alpha \omega _{s}}}=UT_{s}$

(2)

is discretized as

$p_{1k}^{n+1}={\frac {\Delta z_{bk}+\delta _{1}^{n}p_{1k}^{n}-\Delta z_{2}p_{k}^{*n}}{\delta _{1}^{n+1}}}$

(3)

where $\Delta z_{2}=\delta _{1}^{n+1}-\delta _{1}^{n}-\Delta z_{1}$ is the change in the top elevation of the second bed layer and $p_{k}^{*n}=p_{1k}^{n}{\text{ for }}\Delta z_{2}\geq 0{\text{ and }}p_{k}^{*n}=p_{2k}^{n}{\text{ for }}\Delta z_{2}<0.$ The mixing layer or active layer thickness calculation is slightly modified to avoid excessively small layers as

$\delta _{1}=min\left[max(\delta _{1,min},2d_{50},\Delta /2),\delta _{1,max}\right]$

(4)

where $\Delta$is the bed form height, and $\delta _{1,min}{\text{ and }}\delta _{1,max}$ are the user-specified minimum and maximum mixing layer thicknesses, respectively.

The thickness of the second layer is calculated as $\delta _{2}^{n+1}=\delta _{2}^{n}+\Delta z_{2}$ The bed material gradation in the second layer is calculated from the following discretized form

$\alpha =3.25+0.55\ ln\left({\frac {\omega _{s}}{\kappa u_{*}}}\right)$

(5)

In order to avoid sediment layers from becoming extremely thin or thick, a layer merging and splitting algorithm is implemented between layers 2 and 3. Here, the subscript s corresponds to the second layer. To illustrate the bed layering process, Figure 3.6 shows an example of the temporal evolution of 7 bed layers during erosional and depositional regimes.

Figure 3.6. Example bed layer evolution. Colors indicate layer number.

Avalanching

When the slope of a non-cohesive bed,$\phi _{b}$ , is larger than the angle of repose, $\phi _{R}$ , the bed material will slide (avalanche) to form a new slope ap-proximately equal to the angle of repose. The process of avalanching is simulated by enforcing $|\phi _{b}|\leq \phi _{R}$ while maintaining mass continuity between adjacent cells. The following equation for bed change due to ava-lanching is obtained by combining the equation for angle of repose and the continuity equation between two adjacent cells:

$\Delta z_{b,p}^{a}=-\alpha _{a}\sum _{N}{\frac {\Delta A_{N}|\delta _{N}|}{\Delta A_{P}+\Delta A_{N}}}(tan\phi _{b}-sgn\phi _{b}tan\phi _{R}))H(|\phi _{b}|-\phi _{R})$

(4)

where $\delta _{N}$ is the cell center distance between cells P and N , $\Delta$ is the cell area, $\alpha _{a}$ is an under-relaxation factor (approximately 0.25-0.5), and H(X) is the Heaviside step-function representing the activation of avalanching and equal to 1 for $X\geq 0$ and 0 for X < 0. The sign function, sgn X , is equal to 1 for $X\geq 0$ and -1 for $X<0$ and accounts for the fact that the bed slope may have a negative or positive sign. Equation 3-32 is applied by sweeping through all computational cells to calculate $\Delta z_{b}^{a}$ and then modifying the bathymetry as $z_{b}^{m+1}=z_{b}^{m}+\Delta z_{b}^{a}$ . Because avalanching between two cells may induce additional avalanching at neighboring cells, the above sweeping process is repeated until avalanching no longer occurs. The under-relaxation factor, $\alpha _{a}$ , is used to stabilize the avalanching pro-cess and to avoid overshooting since the equation is derived considering only two adjacent cells but is summed over all (avalanching) neighboring cells. Equation 3-32 above may be applied to any grid geometry type (i.e. triangles, rectangles, etc.) and for situations in which neighboring cells are joined at corners without sharing a cell face.

Hard bottom

The sediment transport and bed change equations assume a loose bottom in which the bed material is available for entrainment. However, hard bottoms may be encountered in practical engineering applications where bed materials are non-erodible, such as bare rocks, carbonate reefs, and concrete coastal structures. Hard-bottom cells in CMS are handled by modifying the equilibrium concentration as $C_{t*}^{'}=min(C_{t*},C_{t})$ in both the sedment transport and bed change equations. The bed-slope term in the bed change equation is also modified so that only deposition (no erosion) may occur at hard-bottom cells.

Implicit Semi-Coupling Procedure

For a semi-coupled sediment transport model, the sediment calculations are decoupled from the hydrodynamics but the sediment transport, bed change, and bed material sorting equations are coupled at the timestep level and thus solved simultaneously. A modified form of the iteration procedure of Wu (2004) is implemented in CMS. The equations are ob-tained by substituting $C_{t*k}^{n+1}=p_{1k}^{n+1}C_{tk}^{n+1}$ into the bed change and sorting equations and then substituting the sorting equation into the bed change equation.

The solution procedure of sediment transport in CMS is as follows:

1. Calculate bed roughness’s and bed shear stresses

2. Estimate the potential sediment concentration capacity $C_{tk}^{*n+1}$

3. Guess the new bed composition as $p_{bk}^{n+1}=p_{bk}^{n}$

4. Calculate the fractional concentration capacity $C_{t*k}^{n+1}=p_{bk}^{n+1}C_{tk}^{*n+1}$

5. Solve transport equations for each sediment size class

6. Estimate the mixing layer thickness.

7. Calculate the total and fractional bed changes

8. Determine the bed sorting in the mixing layer

9. Update the bed elevation

10. Repeat Step 4 and iterate until convergence

11. Calculate the bed gradation in the bed layers below the mixing layer

12. Calculate avalanching

13. Correct the sediment concentration due to flow depth change $C_{tk}^{n+1}=hC_{tk}^{n+1}/h^{n+1}$