Understanding the Cutoff Divider in SWMM 5's Kinematic Wave Solution

The Cutoff Divider in SWMM 5 is one of three methods (alongside Tabular and Weir dividers) to manage how water is split between two downstream paths in a sewer network. It's akin to a smart traffic system that decides which route vehicles should take based on how busy the roads are.

How It Works:

• Setting the Cutoff Flow: Let's say you set a Cutoff Flow of 2 cubic feet per second (cfs). This number is your threshold for when to start diverting flow.
• Flow Distribution:
  • Up to the Cutoff: All incoming flow up to 2 cfs will go through what's called the "undiverted link" (the main path).
  • Exceeding the Cutoff: Any flow above this threshold (e.g., if 3 cfs comes in, the extra 1 cfs) will be sent down the "diverted link" (the alternate path).
• Dynamic Flow Handling:
  • The system doesn't just look at new inflow but also considers existing flows:
    • If there's already 1 cfs in the undiverted link and 2 cfs of new water arrive, the system will immediately send the additional flow (1 cfs) to the diverted link, even though the undiverted link hasn't yet reached the 2 cfs cutoff. This ensures an immediate response to changes in flow conditions.

Key Points:

• Capacity Management: The Cutoff Divider is especially useful for managing capacity in sewer systems where one path might have limited flow capacity. Once that limit is reached, excess flow is automatically redirected.
• Dynamic Adjustment: It adjusts in real-time to varying inflows, ensuring no single path is overwhelmed, which is critical for preventing backups or overflows in the system.

Comparison to Other Divider Types:

• Tabular Divider: Uses a lookup table to determine flow distribution based on inflow rates. It's more flexible but requires pre-calculated values for each flow rate.
• Weir Divider: Mimics the physical behavior of a weir, where flow is split based on the weir's geometry and the height of the water above it, suitable for systems where physical characteristics dictate flow.

Figure 1. How the Cutoff Divider Works in SWMM 5