A challenging issue in modeling and managing sewer force mains using the SWMM 5 system. The crux of the problem is maintaining the force main at full capacity with a diameter-depth ratio (d/D) of 1 when the pumps turn on.
The SWMM 5 system uses one flow point in the link, which can make achieving this goal more difficult compared to systems like ICM Ultimate or ICM Standard, which use four or more flow points. The single flow point makes it harder to accurately simulate the conditions in a force main, particularly at the downstream end of the main, where the water depth can be lower.
One solution that has been previously suggested is to add a break node at the end of the force main to help it stay full. But this approach can fail if a gravity main follows the force main. The gravity main can take all the flow from the force main, leading to a small downstream node depth and preventing the force main from staying full.
To counteract this, the text suggests increasing the roughness of the gravity main to simulate the effect of the force main transitioning to the gravity main. This can keep the downstream node depth higher and help ensure the force main stays full most of the time, including when the pump turns on.
Here are the specific suggestions outlined:
- Use a Flap Gate for the rising main with HW Force Main Coefficients: A flap gate allows water to flow in one direction and prevents backflow when the fluid in the pipe reverses direction. This can help maintain the full capacity of the force main.
- Add a Break Node at the end of longer Force Mains with a Surcharge Depth in the node using the Insert Manhole Tool: Adding a break node can help maintain the fullness of the downstream end of the force main.
- Change the link after the Break Node to a Gravity Main and increase the roughness value (n value) to simulate losses from the Force Main to the gravity main: Making the gravity main rougher can simulate the transition from the force main and keep the downstream node depth higher.
- The main reason why the d/D values for the force main were typically less than 1 was due to the low depth at the downstream node of the Force Main. By implementing these suggestions, you can help ensure that the force main stays full and maintains the desired d/D ratio.
- Keep in mind that 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. This means that 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 more closely align with what users expect for the d/D value and will more closely match the actual conditions in the force main.