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Improved Sedimentation & Scour Model

This Development Note highlights additions to the Sedimentation & Scour model to be released in FLOW-3D version 9.4.

The upcoming release of FLOW-3D version 9.4 contains a new and improved sedimentation and scour model. This model replaces the existing sediment scour model in FLOW-3D and contains a number of enhancements:

Multiple Sediment Species

The ability to simulate up to ten sediment species in one simulation will be possible. Each species can have its own particle diameter, density and sedimentation properties. The existing model in Version 9.3 and earlier versions allowed only a single sediment species, which required users to specify median or "worst-case" sedimentation parameters for simulations.

Addition of Scour Potential Model

It will be possible to compute the potential for sediment erosion even in simulations where the sedimentation & scour model is not turned on. Therefore, it will be possible to predict where erosion is most likely to occur without the computational expense needed to solve the additional transport of sediment. Additionally, this model can be used as a diagnostic tool with the sedimentation & scour model to visualize the shear stress on packed bed interfaces.

Isosurface of packed sediment downstream
Figure 1: Isosurface of packed sediment, colored by shear stress

Figure 1 shows a simulation of scouring downstream of a 10 m high weir. The overlying water is not visible, and the packed sediment is composed of three uniformly distributed sediment species, each of sizes 2.5, 6.9 and 10.5 mm diameter and specific gravity of 2.65. Both the sedimentation & scour and scour potential model are used in this simulation—the scour potential model allows users to output the local shear stress at solid walls and packed sediment interfaces.

Bed-Load Transport

Bed-load transport has been added to the model. In the studies of sediment transport, there are two main modes of motion: suspended-load transport and bed-load transport. Suspended load is what was simulated in the earlier models and predicted that the sediment was entrained at the packed-bed interface and carried into the bulk fluid whereby it is carried downstream. With bed-load transport, the sediment does not become entrained in the bulk fluid, but rather rolls and/or bounces over the packed bed interface. Any size sediment can undergo bed-load transport, but generally only finer sediment particles undergo suspended-load transport. Therefore, we now have the ability to predict the motion of larger sediment species than before.

Bed-load transport without and with sediment
Figure 2: Two-dimensional slices down the centerline showing the packed sediment volume fraction.

Figure 2 shows results from using the bed load transport model in simulations of erosion downstream of a 10 m weir. In the image at left, bed-load transport was not considered. Here, the erosion is presumed to be solely due to entrainment of the sediment into the water. However, because of the presence of gravel (10.5 mm sediment grains), which is not able to be entrained, there is significantly less total erosion than shown in the image on the right, where bed-load transport was considered.

Non-Linear Drift Flux

Non-linear drift flux is now standard. In the previous model, the drifting of sediment was assumed to follow a linear drag model, where the drag experienced by each particle was a linear function of the relative velocity of the particle to the surrounding fluid. Now a non-linear drag model with a drag coefficient for each sediment species has been implemented.

All of these enhancements make FLOW-3D's new sedimentation and scour model more useful and powerful than ever.