The issue at hand is to maintain the Force Mains at full capacity with a diameter-depth ratio (d/D) of 1 when the pumps turn on. In SWMM 5, there is only one flow point in the link, which can make it difficult to achieve this goal compared to ICM or ICM SE, which have four or more flow points. Previously, I suggested adding a break node at the end of the Force Main to ensure it remains full. However, this solution is not always effective 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 remains small, preventing the Force Main from being full. To address this issue, I suggest increasing the roughness of the gravity main to simulate the effect of the force main transitioning to the gravity main. This will keep the depth higher and ensure the force main is full most of the time and full when the pump turns on.
Here are some specific suggestions to achieve this:
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 typically less than 1 was that the downstream node of the Force Main had a low depth. By adding a Break Node, the downstream end remains 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.
In SWMM 5, there is only one flow point 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 (in terms of modeling), the force main is always full at the upstream end of the link, but due to the low downstream depth, it is not full at the downstream end.
By increasing the roughness of the gravity main, the results will look more like what the users expect for the d/D value and will somewhat match reality.
By following these suggestions, you can ensure that your Force Mains remain at full capacity and maintain the desired d/D ratio.
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.
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.
In SWMM 5, there is only one flow point 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.
The reason d/D values for the force main were typically less than 1 was that the downstream node of the Force Main had a low depth. By adding a Break Node, the downstream end remains fuller.
In reality (in terms of modeling), the force main is always full at the upstream end of the link, but due to the low downstream depth, it is not full at the downstream end.
By increasing the roughness of the gravity main, the results will look more like what the users expect for the d/D value and will somewhat match reality.
