The key problem discussed here is to keep the Force Mains full or d/D equal to 1 when the pumps turn on, it is sometimes difficult to get it to work like ICM or IWCS due to the fact that there is only one Q in the link of the SWMM 5 solution compared to the 4 or more flow points in the IWCS solution.  I have suggested in past that the user add a break node in the force main at the end so that the downstream end of the Force Main is full – however does not always work if there is a gravity main at the end of the rising force main.  The gravity main takes all of the flow coming down the long force main and the downstream node depth stays small so the Force Main is never full – this annoys the customers.    My suggestion is to increase the gravity main roughness to simulate the effect of the force main transitioning to the gravity main – this keeps the depth higher and the force main full most of the time and full when the pump turns on. 

Here are eight suggestions:

1. Incorporate a Flap Gate: Consider utilizing a Flap Gate for the rising main, making sure to employ HW Force Main Coefficients. This will facilitate control of the flow and ensure optimal operation.
2. Introduce a Break Node: Aim to insert a Break Node at the terminal point of your lengthier Force Mains. This can be accomplished by employing the Insert Manhole Tool and establishing a Surcharge Depth within the node. This step ensures that your system is designed to adapt to various flow conditions.
3. Addressing the d/D Ratio: It's important to note that the d/D values for the force main were frequently below 1. This is primarily due to the low depth at the downstream node of the Force Main, as the downstream link was quickly absorbing the flow from the Force Main. By introducing a Break Node, you can retain a fuller downstream end, effectively addressing this issue.
4. Modify the Post-Break Node Link: Upon establishing the Break Node, adjust the subsequent link to operate as a Gravity Main. Additionally, consider augmenting the roughness n value to mimic losses transitioning from the Force Main to the gravity main. This could be roughly 2 to 3 times rougher.
5. Enhance Downstream Node Depth: This adjustment to the Gravity Main will result in an increased depth at the node located at the Force Main's downstream end. Consequently, the Force Main will be full most of the time, enhancing its operational efficiency.
6. Understand Flow and Depth: In the context of the force main link, note that there's one flow point (Q) and three depths. The d value reported in the d/D graph is derived from the midpoint depth or the mean of the upstream and downstream link depths.
7. Modeling Reality: In terms of modeling reality, it's essential to understand that the force main is always full at the upstream end of the link. However, due to the low downstream depth, it may not be full at the downstream end.
8. Increase Gravity Main Roughness: Lastly, increasing the roughness in the gravity main will cause the results to more closely align with user expectations for the d/D value, thereby offering the advantage of somewhat mirroring reality. This adjustment enhances the model's accuracy and usability.

Use a Flap Gate for the rising main with HW Force Main Coefficients,

Add a Break Node at the end of your longer Force Mains with a Surcharge Depth in the node using the Insert Manhole Tool,

The reason d/D values for the force main were normally less than 1 was that the downstream node of the Force Main had a low depth, as the link downstream was taking the Force Main flow almost immediately, adding a Break Node keeps the downstream end fuller,

Change the link AFTER the Break Node to a Gravity Main and make the roughness n value higher to simulate losses from the Force Main to the gravity main (2 to 3 times rougher),

This will increase the depth of the node at the downstream end of the Force Main and force it to be full most of the time.

As mentioned, there is one Q in the force main link and three depths, the reported d in the d/D graph is from the midpoint depth or the average of the upstream and downstream link depths,

In reality (model reality) the force main is always full at the upstream end of the link but due to the low downstream depth

Adding the increased roughness in the gravity main makes the results look more like the what the users expect for the d/D value and have the advantage of somewhat matching reality

Break Node Placement in InfoSWMM