Background: How SWMM 5 Handles Flow Direction and Node-Link Convention

Background: How SWMM 5 Handles Flow Direction and Node-Link Convention

SWMM 5 (Storm Water Management Model) is a dynamic rainfall-runoff simulation model used for analyzing urban drainage systems. It represents a drainage system as a network of nodes (junctions, outfalls, storage units) connected by links (conduits, pumps, weirs, orifices).

• Node-Link Convention: The core of SWMM's hydraulic calculations relies on the node-link concept. Links transport water between nodes. The direction in which a link is defined (from which node to which node) is crucial for how the model interprets flow direction.
• Flow Direction is Implicit: In SWMM, the order in which you list the upstream and downstream nodes for a link in the input file determines the assumed positive flow direction.
  • Upstream Node: The node from which flow is assumed to originate for that link.
  • Downstream Node: The node to which flow is assumed to be directed for that link.
• Positive and Negative Flow:
  • Positive Flow: A positive flow value in a link means that water is flowing in the direction you defined in the input file (from the upstream node to the downstream node).
  • Negative Flow: A negative flow value indicates that water is flowing opposite to the direction you defined (from the downstream node back towards the upstream node). The model is capable of handling flow reversals.

The Arrow Direction Question: What Happens When Nodes are Reversed?

The question addresses a common point of confusion: what happens if you accidentally reverse the upstream and downstream node names when defining a link in your SWMM input file?

Clarification and Explanation:

1. Flow Magnitude Remains the Same: If you reverse the nodes, the magnitude (absolute value) of the flow calculated by SWMM in that link will be exactly the same as if the nodes were entered correctly. The hydraulic equations are solved based on head differences and conduit properties, regardless of the defined direction.

2. Flow Sign is Reversed: The sign of the flow will be flipped. If the flow was positive before the reversal, it will become negative, and vice versa. This is because the model is now interpreting the flow direction opposite to what is actually happening in the simulation.

3. Upstream and Downstream Depths, Areas, and Node Depths are Unchanged: Crucially, reversing the node order for a link does not change the calculated water depths at the nodes or the flow depths and cross-sectional areas at the upstream end of the link, as long as the overall network and inflow locations stay the same. The hydraulic calculations are based on conservation of mass and energy, and these principles hold regardless of how you define the link direction.

4. Example Breakdown:

   • Correct Setup: Let's say you have nodes 80408A (upstream) and 80408 (downstream) connected by a link. If the flow is going from 80408A to 80408, SWMM might report a flow of +10 cubic feet per second (cfs) in the link. The node depth and the flow into the link at node 80408A will both be positive.
   • Reversed Setup: If you mistakenly define the link as going from 80408 to 80408A, SWMM will calculate the same flow rate but will report it as -10 cfs. The node depth and the flow at node 80408A will be the same, but the node depth at 80408 will show a flow of -10 cfs.
   • Why? The model still sees the water flowing from 80408A to 80408, but because you've told it that the "positive" direction is the other way, it reports the flow as negative.

Implications and Importance of Correct Node Order:

• Model Interpretation: While the underlying hydraulics are the same, reversing node order makes it much harder to interpret your results. A negative flow where you expect a positive flow can lead to confusion and misinterpretations about how your system is behaving.
• Post-Processing and Visualization: If you're using software to visualize your SWMM results (e.g., creating flow direction arrows on a map), reversed nodes will result in incorrect visualizations.
• Debugging: Identifying errors in a model with reversed node orders can be more challenging.
• Continuity Checks: The continuity errors at a node, a check of flow in versus flow out, will be affected by the change of sign.

Best Practices:

1. Double-Check Node Order: Always meticulously verify that you've defined your links with the correct upstream and downstream nodes when creating your SWMM input file.
2. Consistent Convention: Establish a consistent convention for defining flow direction in your model (e.g., always left-to-right or top-to-bottom on your schematic) to minimize errors.
3. Use a GUI: If you're using a graphical user interface (GUI) for SWMM (like EPA SWMM, PCSWMM, or others), it can help with visually verifying node-link connectivity and flow direction.
4. Visualize Results: After running a simulation, use visualization tools to check that the flow directions make sense.

Conclusion:

Reversing the upstream and downstream nodes in a SWMM link definition does not alter the fundamental hydraulic calculations, but it does reverse the sign of the reported flow, making results difficult to interpret and potentially leading to errors in visualization and analysis. Maintaining the correct node order is essential for clear communication and accurate model interpretation.