Dam break // wet bed
Consider a horizontal and frictionless channel, which is 1000 m in lenght. A dam site is located at 500 m. The initial upstreamwater depth is 10 m, and downstreamwater depth is 6 (or 2 or 0.1 m).The vertical dimension of gaz layer is 3m. At time t=0, the dam is broken instantaneously.
I use a space steps of 10 m (100 cells )in horizontal direction and 1 m (10 cells in upstreamwater, 6 cells in downstreamwater and 3 cells in gaz layer) in vertical direction. A time step is 0.3 s. I work with a (2D) model and using implicit scheme for time and hybrid scheme for convection terms. I use SEM method(Scalar Equation Method) in PHOENICS code for this case but I don't have a best results compared with analytical solution so, 1 Is it the problem in the time step (CFL condition). 2 The simulation shows that the gaz layer has an influence on the freesurface profile; How I should impose the vertical dimension of this layer. 3 What is the significance of the velocity field in the gaz layer. Thanks 
Re: Dam break // wet bed
(1). Your mesh size is (100x10), with 100 in the horizontal direction. (2). Have you looked at the solution if you arrange the mesh in (10x100), with 100 in the vertical direction? (3). There are also other combinations, such as 20x50, 40x25. (4). And then you can do nonuniform mesh in each direction. (5). By the way, I am assuming that your software and hardware are optimized for 1000 cells. (a friend of mine complained about the number of a postprocessor license in the place he was working, this world leading company has only one copy of the license. So, he has to complain very hard to get the other guy to release it from time to time. I think, he is a subcontractor, so, they can easily ask him to work second shift. I think, the company is right. With more copies of the postprocessor, the company can only generate more beautiful pictures. I still think that at least PR needs nice pictures though. Another firend of mine has a habit of generating multimillion cell mesh. In this way, he can use several computer and processors at the same time, so, it can make other engineers hard to get on the computer. Another friend of mine told me that even if he has test data, he still couldn't get his cfd calculations right.) (6). So, try some different combinations of the mesh, unless you have other reasons to use this 100x10 uniform mesh you are using.

Re: Dam break // wet bed
Dimensions of the dam, space step of 10 m (100 cells )in horizontal direction and time step of 0.3 s are given in an exemple by J.S Wang ans al : FiniteDifference TVD Scheme For Computation Of DamBreak Problems; Journal of Hydraulic Engineering/April 2000/ 253.
They don't give space step in vertical direction, so I was do many simulations with different mesh sizes in both directions horizontal and vertical and I changed the time step too in many cases . I think that the problem is in the depth of the layer gaz or in the mesh size like you asked. If you know some one who are interested a bout this kind of problem (Dambreak) please make me in contact with him. Thak you Jhon. 
Re: Dam break // wet bed
To understand your problem right, could you explain:
How fast is the shallowwater surface wavespeed in the two compartments before the dambreak? where is the gazlayer? how does your code track the surface? what is wrong about your results? 
Re: Dam break // wet bed
It's clear

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