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No Active Segments & QIN files

Fri, 12/17/2004 - 10:44am — admin
I'm modelling a shallow river system with 9 branches. Going through the preprocessor I've cleaned up my input files, but still have error messages stating that 4 of my 9 branches have no active segments and two of those branches have water level that is too low. I have tried increasing the flow through the system and made sure that WSEL for each segment is almost at the top of the computational grid but nothing has worked. Does anyonoe have suggestions on what I should try? Also, (I'm not sure if this is related or not) I have QIN files for my two most upstream segements, one for the mainstem and one for a major tributary that I am modelling. My other branchs are connected to one another by small spillways, which I don't have flow data for. The model seems to require that I provide a QIN file for each branch that is receiving water from an upstream spillway, but I thought the hydrodynamic portion of CE-QUAL-W2 could model this. I have added QIN files for each of these branches but have used dummy flow values of 0cms, but am not sure if this is the best thing to do. Greg
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Hydrodynamics
Mon, 12/20/2004 - 9:55am — Tom

First, make certain that you have NL = 1 for all of the branches. You need to adjust the INITIAL water surface elevations in the bathymetry file to ensure that you have water in the branches. Currently, the model requires a dummy qin_brx.npt file for each flow boundary condition, which a spillway qualifies as. Just set the flows to zero
Tue, 12/21/2004 - 8:37pm — admin

Thanks Tom, I've now gotten through the pre-processor with no errors and am trying to run the model. I'm having a big stability problem right at the beginning of the simulation. I have an unstable water surface elevation at all the segments in my first waterbody. I've tried decreasing the slope, increasing segment widths, changing initial water surface elevations and inflows but no luck. I now want to try and decrease the resolution of my computational grid as all my layer depths are 0.1m. I'd like to change layers of my first waterbody to 0.2m and am trying to use the merge layer tool in the GUI. I highlight all the layers in the first waterbody and then specify that I want half as many layers expecting that my segment depths will remain the same but will have half as many active layers in each segment, but what happens is that although the depth of each layer does increase by 2 the number of active layers are also increasing therefore making each segment much much deeper than it actually is. Is this what is supposed to happen? Greg
Mon, 01/03/2005 - 11:50am — Tom

Nope, that is not what is supposed to happen when decreasing the number of layers. You will need to discuss this with Scott as he is the keeper of the bathymetry editor. That said, I would suggest running at a very small timestep for the first day to allow the model to work out the kinks in the water surface elevation setup. This is a very difficult process complicated by the way you set up the slopes. A very small difference in the slope can result in a large difference in bottom elevations as you move through the grid setup.