Subject:   What are the Types of Force Mains (FM) in SWMM 5?

Force Mains in SWMM 5 can be modeled in several ways, depending on whether the flow within the pipe is full (the pipe is completely filled with fluid) or partial (the pipe is not completely filled). Here are the five different methods to model a force main:

1. Full Flow using Darcy-Weisbach for the friction loss: In this method, the Darcy-Weisbach equation is used to calculate friction loss when the pipe is flowing full. This equation is particularly useful for pipes of any cross-sectional shape and flow regime.
2. Full Flow using Hazen-Williams for the friction loss: This method uses the Hazen-Williams equation, which is simpler than the Darcy-Weisbach equation but is less accurate for certain types of flow. It is often used for water distribution systems.
3. Full Flow using Manning's n for the friction loss: In this method, Manning's equation is used for calculating friction loss. Manning's equation is often used for open channel flow but can also be used for full flow in pipes.
4. Partial Flow using Manning's n for the friction loss: For pipes that are not completely filled, Manning's equation can be used to calculate friction loss.

When Darcy-Weisbach or Hazen-Williams methods are used, SWMM 5 internally calculates an equivalent Manning's n to ensure consistent calculations across different flow regimes.

• The equivalent Manning's n for Hazen-Williams is calculated using the formula: 1.067 / Hazen-Williams Coefficient * (Full Depth / Bed Slope) ^ 0.04.
• The equivalent Manning's n for Darcy-Weisbach is calculated using the formula: (Darcy-Weisbach friction factor/185) * (Full Depth) ^ 1/6.

By selecting the appropriate method for modeling a force main, users of SWMM 5 can accurately simulate the behavior of these critical components of the sewer system under various flow conditions.

Figure 1.  Types of Full and Partially Full Force Mains in SWMM 5

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