I need to verify the units for vertical “dispersion” assigned to the hyd file export for WASP linkage (EE 11.8). I had expected units of m2/s, but it appears it may be cm2/s? I believe that “dispersion” in the hyd file is basically the vertical mixing output.
I need to verify before applying any unit conversion with the hyd output.
Thanks,
Brandon
Hi Brandon,
The dispersion should be in m2/s.
I suppose you were referring to the ABmax = 0.001 m2/s which found in efdc.wsp file. This is too large for the dispersion coefficient, but it is actually a default for the maximum value that EFDC+ will check and not allow the dispersion, at any segment and any time, higher than that (the dispersion coefficient computed by the model should never be at that high).
In addition, dispersion coefficient cannot be found in the .hyd file. It is used to compute the dispersive flow that is contained in the .hyd file, together with the advective flow.
Sincerely,
Kien
Thanks Kien, That is very helpful. I understand what you describe regarding ABmax, although I had not looked at that when considering the high dispersive flow that I am seeing in the hyd file.
Before I switched to Explorer, old versions of EFDC would return dispersive flow typically on the order of 4 to 5 m2/s (as exported to the hyd file). However, I have now looked at two separate models generated and run with Explorer that return dispersive flow (from the hyd file) on the order of hundreds to thousands of m2/s (highest around 4,500 m2/s in one instance). The hydrodynamics in at least one of these models still show strong persistent density stratification in Explorer, so I do not believe this is due to the model setup.
These calculated dispersive flows seem extremely high, and indeed result in instantaneous mixing in our external water quality model. When converted using a 0.0001 multiplier the dispersive flow seems to be much more normal (less than 10 m2/s), and do not result in extreme mixing. This is what leads me to think the units exported to the hyd file are cm2/s. Is it possible there is some unit confusion between Explorer output and the hyd export routines?
Hi Brandon,
Sorry for replying to you late.
Regarding the dispersive or advective flows, the flow unit is m3/s.
I also tried to generate the .hyd file from a Lake hydrodynamic model and obtained a much higher value of the dispersive flow (20 m3/s) compared to the advective flow (1 m3/s) at the same cell as shown in the attached photo.
However, it is exactly what EFDC calculates, also in the old versions. In the code, the dispersive flow is calculated generally as following: Dispersive = cell area * AB / layer thickness.
In my model, the cell area is about 10^4 m2, the layer thickness is about 0.5 m and the dispersive coefficient AB estimated by EFDC is sometime reach to 0.001 m2/s. All of these values give the final dispersive flow as 20 m3/s.
So please check the cell area, layer thickness and the dispersive coefficient AB in your model. To see the value of AB, you can use the function “Write EE_array.out Linkage file” from the run tab.
If you have any more questions, please feel free to ask.
Kien
