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Is it possible to model a weir within a catch basin or manhole in the the storm-sewer Haestad products?
It is not possible model a weir directly within a node, but there are a few workarounds that can be tried.
First, you can apply a start control structure on the conduit directly downstream of the catch basin or manhole. This would simulate a weir that is right where the catch basin or manhole connect to the downstream conduit. One of the control structure types that can be modeled in this way is a weir.
Enter the weir data with elevations expected for the weir within the catch basin or manhole. The flow through the conduit will not start under the elevation is above the weir elevation in the control structure. This method works best when the invert of the downstream conduit and the invert of the catch basin or manhole is the same. If the you are modeling a drop structure, you would need to adjust the weir elevation for the start control structure to account for the drop depth.
It is also possible model this as a stop control structure. The workflow is the same except that the flow through the conduit would not enter the catch basin or manhole until the elevation in the conduit is above the weir.
Another option is to model the catch basin or manhole as a small pond with a pond outlet structure using a weir. In this case, the manhole would be replaced with a pond with the same dimensions as the original manhole. The conduit that drains into the manhole would need to end with an outfall that uses the new pond as the boundary element. A pond outlet structure will need to be added as well. This will be set as the weir.
The end result for this will be similar to Option 1 above. If the manhole or catch basin is relative small in depth and/or diameter, you may need to decrease the time step and the output increment to increase the stability of the results.
In addition, this workflow would work best when there is no gutter attached to the catch basin.
The third option would be model the weir as a structure loss. If you are using the GVF-Rational solver (available in SewerGEMS, CivilStorm or StormCAD) or the GVF-Convex solver (available in SewerGEMS and SewerCAD), you could convert a rating table for the weir into a flow-headloss curve, which is one of the structure loss methods available in this solver.
If you are using the Implicit solver, you could use the Standard headloss. With the right headloss coefficient, you may be able to reasonally match the rating curve for the weir (the rating curve would need to converted to use a head drop instead of an elevation to better see the difference). This will take some trial and error and would in all likelihood end up being an approximation.
You could also use the Absolute headloss method based on the head drop seen in the weir rating table for a typical flow. This would allow you to make a conservative estimate for the headloss. In addition, if you are using the Explicit solver for Option 3, you would need to use the Absolute headloss method since the Standard headloss method is not available with this solver.