# Problem

Is it possible to model a weir within a catch basin or manhole in the the storm-sewer OpenFlows products? Basically a control structure (weir plate, orifice plate, etc) inside the structure vault.

# Solution

It is not possible model a weir directly inside a node, but the below approaches should be hydraulically equivalent:

## Option 1: conduit control structure

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 connects to the downstream conduit. One of the control structure types that can be modeled in this way is a weir.

Note: conduit control structures currently are not supported with the GVF-Rational (StormCAD) solver. The Explicit (SWMM) solver tends to work best in a model with multiple control structures. You may need to use a smaller Routing Step (in the calculation options) such as 1 second.

Enter the weir data with elevations expected for the weir within the catch basin or manhole. When the water elevation exceeds the weir elevation, flow will pass into the conduit. For example, below is a case where the weir conduit control structure elevation is set to 93 feet, which is above the top of both the incoming and outgoing pipes.

This method works best when the invert of the downstream conduit and the invert of the catch basin or manhole is the same. (in other words, "set invert to start?" is set to true, on the downstream conduit). If the outgoing conduit start invert is above the bottom of the node invert, you would need to adjust the weir elevation for the start control structure to account for the drop depth.

If you have more than one outgoing conduit (going away from the single control structure), you will need to use one of the other methods below, or add a short "fake" conduit connected between the manhole or catchbasin that contains the weir, and a "fake" node, from which the multiple outgoing pipes connect to. Then, the start control structure can be set on that "fake" conduit. Or if using the Implicit or explicit solvers, multiple conduits can leave the node and could each of their own start control structure if needed.

Note that if you're using the Implicit solver (set in the calculation options) with SewerGEMS or CivilStorm to model a conduit control structure, you may need to change to the Explicit (SWMM) solver with a small Routing Step, for more stable results.

## Option 2: pond

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 relatively 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, if the weir is inside a catchbasin, this workflow would work best when there is no gutter attached.

## Option 3: structure loss

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 method. 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 may need to superimpose the rating curve of the weir (which can be obtained from the composite outlet structure dialog) with several rating curves based on different headloss coefficients.

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.