InfoSewer FM Split Issue

InfoSewer FM Split Issue

A common challenge in InfoSewer (and other sewer modeling software) when dealing with force main splits: how to accurately model the flow distribution at the point where a single force main diverges into two or more downstream force mains. The method you described, using a duplicate wet well, pump, and force main, is a valid and often effective workaround. Let's explore this solution and discuss other potential approaches.

The Problem: Force Main Splits in InfoSewer

• Single Upstream Link: InfoSewer, by its nature, typically allows only one upstream link connected to a wet well. This represents the physical reality that a pump draws from a single wet well.
• Flow Splitting at Junctions: When a force main discharges into a gravity sewer system at a junction (manhole), InfoSewer can handle the flow split using the standard junction logic (based on hydraulic grade lines and downstream pipe capacities).
• Force Main to Force Main Split: However, when a force main needs to split into two or more force mains, the standard junction logic is not directly applicable because the flow is entirely driven by the upstream pump, not by gravity or downstream hydraulic conditions.

Solution: Duplicate Wet Well, Pump, and Force Main

The method you described effectively addresses this issue:

1. Duplicate Wet Well: Create a new wet well node immediately downstream of the original force main's discharge point (which is likely a chamber or a manhole).
2. Duplicate Pump: Create a new pump connected to this new wet well.
   • Pump Curve: This new pump should have a fixed capacity (as you mentioned) equal to the capacity of the original pump that feeds the split. This ensures that the total flow entering the split is maintained.
   • Pump Type: It is best to have the pump type be a constant flow pump.
3. Short Force Main: Connect the new wet well to the original discharge manhole with a short force main. This link will carry the entire flow from the original force main. The purpose of this force main is to transition the flow from the new wet well to the node where you want the split to occur. This short force main can be a very small diameter pipe. The purpose of the small diameter is to keep the flow as pressurized flow.
4. Split at Junction: Now, the original discharge manhole has two inflows:
   • The short force main from the duplicate wet well.
   • One or more gravity or pressure pipes carrying flow from other parts of the system. This manhole will now act as the flow splitting point. You can connect your two (or more) downstream force mains to this manhole, along with any other gravity or pressure pipes that need to be connected.
5. 50/50 Split (or Other Ratios): In your example, you want a 50/50 split to links 25 and 35. You can achieve this by:
   • Equal Pipe Characteristics: Make sure that links 25 and 35 (and their downstream networks) have similar hydraulic characteristics (diameter, roughness, slope) so that they offer roughly equal resistance to flow.
   • Weir Splitting at the Junction: The most accurate way to force a 50/50 split is to model a weir inside the manhole to divide the flow. The weir crest length will control the split of the flow.

Advantages of this Method:

• Maintains Flow Continuity: The total flow from the original force main is preserved.
• Accurate Flow Splitting: The flow split can be controlled by adjusting the downstream pipe characteristics or, more precisely, by modeling weirs.
• Flexibility: You can model different split ratios by adjusting the downstream pipe characteristics or weir geometry.
• Relatively Simple: It uses standard InfoSewer elements (wet wells, pumps, force mains, junctions).

Other Potential Approaches:

1. Control Rules:

   • You could potentially use control rules to manipulate the flows in the downstream force mains based on the flow in the upstream force main. For instance, you could set the flow in one downstream force main to be a fraction of the upstream flow and let InfoSewer calculate the flow in the other downstream force main to satisfy continuity.
   • Complexity: This approach can become complex to set up and maintain, especially if the split ratio needs to vary dynamically.

2. Specialized Split Node (if available):

   • Some sewer modeling software packages might offer specialized node types designed to handle force main splits directly. It's worth checking if InfoSewer has such a feature, although it's not a standard element.

Important Considerations:

• Headloss: Make sure to account for headlosses in the short force main connecting the duplicate wet well and the original discharge manhole.
• Numerical Stability: Introducing short, small-diameter pipes can sometimes affect numerical stability. Monitor your model for any instabilities and consider adjusting time steps or other solver settings if necessary.
• Real-world Behavior: Keep in mind that real-world flow splits at force main junctions can be complex and influenced by factors like turbulence and momentum. The chosen modeling approach should be a reasonable approximation of the actual physical behavior.
• Verification: If possible, validate your model results against field measurements or more detailed CFD (Computational Fluid Dynamics) simulations to ensure that the flow split is being accurately represented.

In conclusion, the duplicate wet well, pump, and force main method is a practical and effective way to model force main splits in InfoSewer. It provides a good balance of simplicity, accuracy, and flexibility. While other approaches might be possible, this method is generally recommended for its robustness and ease of implementation.

SWMM 5 (and ICM SWMM and InfoSWMM) is a link-node model

SWMM 5 (and ICM SWMM and InfoSWMM) is a link-node model

The fact that SWMM 5 (and ICM SWMM and InfoSWMM) is a link-node model, where a significant portion (or even all) of a node's surface area can come from the connecting conduits, has several important implications for how the model behaves, how you should build and interpret models, and the overall accuracy of simulations.

Here are the full implications, broken down into key areas:

1. Representation of Physical Processes:

• Approximation of Storage: In reality, junctions in a drainage network often have some physical volume (e.g., manholes, catch basins). In SWMM 5, unless a node is explicitly defined as a storage node with a depth-area curve, its storage volume is approximated based on the surface area contributed by connected links.
• Simplified Geometry: The link-node representation simplifies the complex geometry of real-world drainage systems. It assumes that storage and flow transitions occur primarily at nodes, with links acting as connectors. This simplification can affect the accuracy of simulations, particularly in situations where the physical volume of junctions is significant compared to the volume of the links.
• Emphasis on Link Hydraulics: The model places a greater emphasis on accurately representing the hydraulic behavior within the links (conduits). The St. Venant equations are solved for the links, and the node behavior is derived from the link hydraulics.

2. Model Building and Parameterization:

• Node Sizing: When creating nodes that are not intended to be storage nodes, you don't need to explicitly define their surface area or volume, unless you use the Ponded Area Parameter of a Node. The model will automatically calculate the area based on connected links. However, this means that the node's storage capacity is directly tied to the dimensions of the connecting links. If the physical volume of the node is large you should use a storage node instead of a junction.
• Storage Nodes: For nodes that represent significant storage elements (e.g., ponds, detention basins), you must define them as storage nodes and provide a depth-area or depth-volume curve. This ensures that the storage capacity is accurately represented and not solely dependent on the link areas.
• Link Dimensions: Accurate representation of link dimensions (diameter, width, height) is crucial, as these dimensions directly influence the calculated node surface area and, consequently, the node's storage behavior.
• Ponded Area Parameter: SWMM 5 allows for a "ponded area" at nodes, representing the area available for temporary surface ponding above the node's maximum depth. This adds some flexibility to model surface storage that might exceed the node's normal capacity based on link areas.

3. Simulation Behavior and Accuracy:

• Numerical Stability: The way link areas are assigned to nodes (as discussed in the link flow classification) is designed to enhance numerical stability during the iterative solution of the St. Venant equations.
• Continuity Errors: Inaccurate link dimensions or neglecting the physical volume of junctions can lead to larger continuity errors, especially in systems with significant storage at junctions.
• Sensitivity to Time Step: The model's behavior can be sensitive to the routing time step, particularly if the node storage is dominated by link area contributions. Smaller time steps might be needed to accurately capture rapid changes in node depth.
• Approximation of Dynamic Effects: While SWMM 5's dynamic wave routing is sophisticated, the simplified representation of node storage can affect the accuracy of simulating highly dynamic events, such as rapid filling and emptying of junctions. 
  
  
  

4. Interpretation of Results:

• Node Depth as a Surrogate: Node depth in SWMM 5 is often used as a surrogate for the water level in a manhole or catch basin. However, it's important to remember that this depth might not perfectly correspond to the physical water level, especially if the node's storage is primarily derived from link areas.
• Storage Node Output: When analyzing storage nodes, pay close attention to the depth-area or depth-volume relationship you've defined. The reported depths and volumes will be based on this relationship, not on the link area contributions.
• Continuity Checks: Always check the continuity error reported by the model. Large errors can indicate problems with the model setup, including issues related to node storage representation.

5. Model Limitations:

• Simplified Representation of Junctions: The model's simplification of junction geometry can be a limitation in cases where the physical volume of junctions has a significant impact on the system's hydraulic behavior.
• Two-Dimensional Flow: SWMM 5 is fundamentally a one-dimensional model. It does not explicitly simulate two-dimensional flow patterns that might occur within junctions or at complex flow transitions. 
  
  
  

Best Practices to Mitigate Limitations:

• Use Storage Nodes: For nodes with significant physical storage, always define them as storage nodes with appropriate depth-area curves.
• Refine Link Discretization: In areas with complex hydraulics or significant storage at junctions, consider using shorter link lengths to better capture the spatial variation in flow and storage.
• Calibrate Carefully: If possible, calibrate your model using observed data. Pay attention to water levels at nodes and adjust storage parameters (e.g., depth-area curves, ponded area) as needed.
• Consider Alternative Modeling Approaches: For situations where the physical volume and geometry of junctions are critical, consider using a two-dimensional or coupled 1D/2D model, which can more realistically represent these features. 
  
  
  

In conclusion, understanding that SWMM 5 is a link-node model with node surface areas often derived from connected links is crucial for building accurate models, interpreting simulation results, and recognizing the model's limitations. While this approach simplifies the representation of drainage networks, it's important to be aware of its implications and take steps to mitigate potential inaccuracies, especially when modeling systems with significant storage at junctions or highly dynamic flow conditions.