TUFLOW FV

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I am applying a flow boundary condition to my TUFLOW FV model. This boundary represents inflow from a hydrological catchment model (which is separate to TUFLOW FV). This upstream inflow boundary includes a main channel and the left and right floodplains, which are obviously much shallower compared to the main channel.

When I run the model with default settings in the fvc file for the Q boundary, I get very high velocities upon on the floodplain which don't look realistic.

Why is this?

Share on other sites There are two ways apply a flow boundary condition to a TUFLOW FV model (actually there are 4 ways, but sub-types 2 and 4 relate to boundary conditions specific to 3D):

1. Flow is distributed according to the width of each individual cell face along the nodestring (by setting “sub-type == 1” in the fvc input control file).
• If sub-type == 1, then the flow (Qi) entering each of the (i = 1,…, n) cells along the boundary is distributed from the total flow (Qtot) according to the width (wi) of each cell face: 1. Flow is distributed according to the width and depth of each individual cell face along the nodestring (by setting “sub-type == 3” in the fvc input control file).
• If sub-type == 3, then the flow (Qi) entering each of the (i = 1,…, n) cells along the boundary is distributed from the total flow (Qtot) according to the width (wi) of each cell face and also the depth (hi) in each cell: The logic for this formulation is derived from the Chezy equation describing friction flow;

Q = AC(RS)0.5

where Q is flow, A is area (width w * depth h), C is the Chezy coefficient, R is hydraulic radius (approximately equal to depth h) and S is slope. From this is a proportionality between flow Q and water depth h:

Q ≈ h1.5

What does this mean for a model simulation? It is important to consider the flow distribution along inflow boundaries that have a significant variation in bed levels across the nodestring; a common example is a boundary condition representing a floodplain and main channel, illustrated as follows. For this boundary condition:

• Application of a sub-type == 1 will result in significantly higher velocities on the floodplains compared to the main channel.
• In comparison, application of a sub-type == 3 will distribute the flows so that there is more flow in deeper water, less flow in shallower water, and a generally uniform velocity distribution. This specification is recommended for the majority of inflow boundary conditions in overland and riverine situations.

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