Subject: What are the Types of Force Mains (FM) in SWMM 5?
There are five ways to model a force main in SWMM 5 for the combination of full and partial flow in the force main (Figure 1):
1. Full Flow using Darcy-Weisbach for the friction loss
2. Full Flow using Hazen-Williams for the friction loss
3. Full Flow using Manning's n for the friction loss
4. Partial Flow uses Manning's n for the friction loss for Force Main Equation options
If you use Darcy-Weisbach or Hazen-Williams then an equivalent Manning's n for a force main that results in the same normal flow value for a force main flowing full under fully turbulent conditions is calculated internally in SWMM 5 in forcemain.c
· Equivalent n for H-W is 1.067 / Hazen-Williams Coefficient * (Full Depth / Bed Slope) ^ 0.04
· Equivalent n for D-W is (Darcy-Weisbach friction factor/185) * (Full Depth) ^ 1/6
Figure 1. Types of Full and Partially Full Force Mains in SWMM 5
