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David Crompton

1d Channel vs 2d domain

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Hi All

 

We are currently working on a existing model that was run with a 10 m and a 1d channel to defining the creek lines. We now plan to refine the modelling extent to focus a reported flooding problem and reduce the cell size down to 2m as we also have detailed survey of the site.

 

My initial thoughts would be to remove the 1d channel that was previously used to define the creekline and now use a 2d surface to better define the terrain and flow conveyance in the area on interest . I would think a 2d approach would provide a better representation of flood impacts than 1d channel?

 

Therefore my question is do we continue with the 1d channel (with updated cross section information) as this was the adopted approach in the previous assessment or flip this to a 2d only model now that we have improved survey information and will be running at significantly reduced cell size.

 

Does a 2d only Approach provide an improved results? 

 

Thanks

David

 

 

 

 

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David,

The key question is "How wide is your creek?".

In order for 2d flows to be properly represented, I would say you would need at least 5 cells across your creek - in other words, if you creek is not at least 10m wide then I would not consider this option (think also of your possible % error in representation of width with 2m cells). A wide, shallow creek would lend itself more to 2d than a deep, narrow creek.

2d modelling of channels is great if you are interested in the flow patterns within the channels and/or where such flow patterns do impact on conveyance/water levels/1d assumptions (of uniform water surface across section and average velocity for section being meaningful). For example if you have large stagnation zones, islands, tight bends (although note for bends, 2d is better than 1d, but still doesn't capture the full 3d effects that lead to superelevation on the outside of the bend).

However, if your channel is relatively simple and/or with many structures (which are easier to represent in 1d) and it is exceedence of the overall conveyance capacity that is important, then you will probably be better leaving the creek in 1d. 1d models are good for calculation of overall conveyance (provided they are properly set-up of course, with sufficient cross-sections!).

Hope this helps,

Kate

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Hi David,

If you've got:

- the data to describe the channel well

- sufficient resolution the represent the channel (4-6 cells minimum measured perpendicular to flow), exactly as Kate said already

then I would say yes, go fully 2D!

 

I'd say having the data for the channel is normally the tricky bit...

 

If you're all in 2D, you avoid limitations involved with transfer of flows over boundaries and momentum issues, not to mention it does explicitly represent bends and the like which may be completely ignored in a 1D model (though again, see what Kate said on this). Plus your processing afterwards is probably easier, and most clients will find it more comfortable conceptually, if they wish to know about such things. It may even run faster as you'd have the option of using adaptive timestepping on a 2D only model. And the downsides? Well, maybe stability problems if your channel gets really interesting at some point (you may need to reduce your timesteps, or courant limit, a bit if you've any tight bends), but other than that, none that I can think of right now... I certainly wouldn't let the fact a past study used 1D elements put you off going fully 2d.

ESTRY structures can be linked in as and where required without too many problems, so I wouldn't let those put you off either.

 

I hope that helps your thinking, but as ever my views are my own, and I'd welcome hearing from others, especially if they can think of things I've missed!

PHA.

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