Jump to content
TUFLOW FV
Sign in to follow this  
CFN

Application of a Q boundary over a channel with a floodplain

Recommended Posts

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 this post


Link to post
Share on other sites
Guest Chris Nielsen

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:

post-5-0-10622500-1364273982_thumb.gif

 

  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:

post-5-0-52546300-1364274099_thumb.gif

 

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.

 

post-5-0-69315200-1364274715_thumb.gif

 

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.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Sign in to follow this  

×
×
  • Create New...