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how to define weir boundary

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The outlet of my simulated lake was a weir. When the water level was bigger than 130m, it wiil has outflow. I only define this weir in fvc file as follow: 

structure == Nodestring, 4                          !Weir                          
  Flux function == weir                             !Weir flag 
  Properties == 130,1.0                                                        !Weir elevation, weir coeffecient
end structure

But when I run the model, only very small inflow can make the water level of lake increase sharply. So I suspect a problem with the weir.

I need your help to define a outflow boundary as a weir. 

Thanks very much.

Best wishes.



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Hi Lei,

Thanks for posting. There are a few options to fixing your problem. The key is that the model by default will extrapolate internal water levels into the 'ghost cells' downstream of your weir. This means that the weir has the same water level either side causing complications.

There are three options to force the model to do what you want.

1. You can add extra cells downstream of your weir and set a water level boundary around those cells to ensure they are always lower than your weir crest.

2. You can just add a water level boundary to the same nodestring as your weir is on. This is the same as solution 1 except the water level boundary will apply to the ghost cells so you cannot extract results from the ghost cells to confirm, though it doesn't require changing your mesh.

3. Instead of a weir structure you can specify an HQ bc boundary on your nodestring. HQ bcs require a comma delimited file of water levels and corresponding flow rates (make sure that you use negative flows for outflows). You can specify a range of water levels H and then use flows specified by Q = CBhd2/3  where hd is the height of water above your weir crest, not necessarily the same as the H column. The model will then interpolate the flows linearly between the water levels that you choose. ie. for a 100m wide channel with a weir crest at -2m and a weir coefficient of 1.



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