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

 

Elevation Relationships in SWMM 5 Hydraulic Calculations Understanding Pipe Connections Between Nodes

Elevation Relationships in SWMM 5 Hydraulic Calculations Understanding Pipe Connections Between Nodes

Elevation Relationships in SWMM 5 Hydraulic Calculations

Understanding Pipe Connections Between Nodes:

In SWMM 5, the connection of pipes between nodes is akin to constructing a miniature water slide where the height and angle of entry and exit points dictate the flow of water. Here's how it works:

1. Key Elevation Measurements:
   
   
   • Upstream Node's Invert Elevation: This is the lowest point at the start of the pipe. It's where the water enters the pipe from the node.
   • Downstream Node's Invert Elevation: This marks the lowest point where the pipe ends, where water exits into another node.
   • Pipe Length: The actual distance the pipe spans between the two nodes.
   • Pipe Offsets: These are the heights at which the pipe connects to each node relative to their invert elevations. If a pipe doesn't connect directly at the bottom (invert) of a node, this offset shows how much higher or lower the connection point is.
2. Determining Slope:
   
   
   • The slope of the pipe is not just calculated by the difference between the upstream and downstream invert elevations. Instead, SWMM 5 considers:
     
     
     • The elevation difference between the points where the pipe actually connects to each node (which might be above the invert due to offsets).
     • The length of the pipe.
   • Here's how it breaks down:
     • Effective Slope Calculation: SWMM takes the difference between the elevations at the pipe's entry and exit points (which includes the invert plus any offset) and divides this by the pipe's length. This gives you the true slope the water will experience as it flows through the pipe.
   • Formulaic Approach:
     • If the upstream connection point (invert + offset) is higher than the downstream (invert + offset), water flows downhill.
     • If they are at the same level, the slope is zero, suggesting a flat or horizontal pipe.
     • If the downstream is higher, you might have a situation where water would flow uphill, which in real-world scenarios could indicate a misconfiguration or pumping requirement.

Why It Matters:

• Flow Dynamics: The slope directly affects flow velocity, capacity, and how water navigates through the system. A steeper slope allows for faster flow; a shallower slope might lead to slower flows or even backing up if not designed correctly.
• Design and Troubleshooting: Understanding these elevation relationships helps in designing efficient systems, ensuring proper drainage, and diagnosing issues like why a pipe might not be draining as expected.

By focusing on these elevation measurements, SWMM 5 provides a detailed simulation of how water will behave in real sewer systems, allowing for better planning and management of urban drainage.

Reducing Dry Weather Flow in InfoSWMM: A Systematic Approach

Reducing Dry Weather Flow in InfoSWMM: A Systematic Approach

When modeling wastewater systems, you may need to adjust the overall dry weather flow while preserving the mean flow patterns. InfoSWMM provides an elegant solution through its pattern-based flow modification system. This method allows for precise control over flow reductions without disrupting the underlying flow distribution.

Implementation Process:

First, navigate to the Operations tab within the Attribute Browser. Here, create a new Flow Pattern that will serve as your reduction multiplier. For instance, if your goal is to reduce flows by 15 percent, enter a pattern value of 0.85 (representing 85% of the original flow). Next, connect this pattern to your system by applying it in the Node DWF DB Table. Once implemented, this adjustment will systematically reduce all dry weather flows throughout your simulation by the specified percentage while maintaining the relative relationships between flow components.

This approach offers several advantages: it preserves the temporal distribution of your flows, maintains system-wide consistency, and allows for easy modification if different reduction factors need to be tested. The method is particularly useful when calibrating models or evaluating system capacity under various flow scenarios.

Technical Note: The reduction occurs uniformly across all dry weather flows, so consider carefully whether this global approach aligns with your modeling objectives before implementation.