Jump to content
TUFLOW Forum
JamesC

Fence Blockage

Recommended Posts

Trying to model a wooden paling fence with slats that allows water to pass through for an urban development. Any suggestions other than increased roughness?

Hi James,

Have you seen the publications part of the TUFLOW web site? There's one from 2008 called 2D Modelling Approaches for Buildings and Fences. There's lots of information in there about modelling fences and buildings.

Cheers

Share this post


Link to post
Share on other sites

Hi Phil,

I have looked at the reference publication and there's not a lot of explicit modelling advice. I'm modelling some timber paling fences where there is not complete flow blockage.

Initially I used the 2d_zsh but there is no allowance for porosity, only complete blockage. So I tried 2d_fcsh to change the flow width of the cell sides where the fence is located, however I find the results aren't as expected. In one case I specified a pBlockage of 95% yet the results viewed in SMS still show a fair amount of flow depth and velocity crossing the fence.

What is your preferred approach for fences with partial blockage?

Cheers,

Andrew

Share this post


Link to post
Share on other sites

Hi Andrew,

By using the flow constrictions, with only a percentage blockage you will still get a high velocities through the fence (but restricted flows). By adding a percentage blockage you have blocked part of the flow width, but and not increased the roughness or added any losses. In reality there will be additional losses associated with the turbulence around the palings.

The Hydraulics of bridge and waterways (Bradley, 1978) which can be found online: http://www.fhwa.dot.gov/engineering/hydraulics/pubs/hds1.pdf

The relates the area blockage by piers (and pier shape) to an additional formloss. For a J (fraction blockage) of 10% the is likely to be significant losses, see the attached image (apologies the figure seems to be split between pages in this version). This can be added to the 2d_fcsh (or 2d_fc) objects as a form loss coefficient (FLC), which is a applied as an energy loss based on the dynamic head (V2/2g). For more details see section 4.7.2.1 of the 2010 TUFLOW manual.

This will act to restrict the velocities associated with the flows through the fences.

As to the preferred approach I'm not sure how to answer that one, I think it would be interesting to see some recorded data! As we are ultimately trying to replicate reality with the modelling.

Regards

Phillip

post-220-0-20936700-1321490614_thumb.jpg

Share this post


Link to post
Share on other sites

Hi Phil,

Thanks for the reply. Based on the US-FHA reference, Figure 7 (incremental backwater coefficients for piers) includes values of the pier obstruction ratio (J) up to 0.18. I'm looking at a value of around 0.9 for a modern timber paling fence so you could reasonably assume a value of 1.0 through extrapolation to represent the additional form loss caused by such a fence (applied using FLC_below_Obvert for 2d_fcsh).

I have searched a few other hydraulic references and can't find anything else for guidance. As you said, calibration data is what we need. Did anything come out of the work in Newcastle?

Also, I have some acoustic fences in my model that are impervious to flow. I initially modelled these as 2d_fcsh with a pBlockage of 100 however flow still seems to cross the fence. I have reverted to 2d_zsh which does the job, but I'm confused why the 2d_fcsh didn't work as expected.

Regards,

Andrew

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.


×
×
  • Create New...