Siphon Simulation in SWMM 5, ICM SWMM, PCSWMM and InfoSWMM

Siphon Simulation in SWMM 5, ICM SWMM, PCSWMM and InfoSWMM

 This is a great explanation of how siphons are simulated in SWMM 5 and InfoSWMM. Here's a breakdown of why this approach works and a few additional thoughts:

Why this approach to Siphon Simulation in SWMM 5 and InfoSWMM Works:

• Fundamental Principles: The simulation relies on the fundamental principles of fluid mechanics:
  • Continuity Equation: This ensures conservation of mass at each node. In simpler terms, what flows into a node must either flow out or be stored, leading to a change in depth.
  • Momentum Equation: This governs the flow within the links (pipes). It accounts for forces like gravity, pressure, and friction, which determine the velocity and flow rate.
• Numerical Solution: SWMM 5 and InfoSWMM employ numerical methods to solve these equations iteratively over time. Each time step, the model calculates the flow in each link and the depth at each node based on the previous time step's conditions and the specified inflows and boundary conditions.
• Simplified Representation: The siphon is not explicitly modeled as a separate element type. Instead, its behavior emerges naturally from the interaction of the nodes (manholes) and links (pipes) based on their physical properties (elevation, diameter, length, roughness) and the governing equations.
• Head Difference Drives Flow: As stated, the key to siphon initiation and operation is the head difference between upstream and downstream nodes. Once the upstream node (MH1 in your example) fills sufficiently, a head difference develops, driving flow through the siphon, even if part of the conduit is higher than the water level at the inlet.

Key Elements and How They Contribute:

1. Inflow: Provides the water that initiates the siphon action. Different inflow types accurately simulate various scenarios (dry weather, wet weather, constant flow, etc.).

2. Boundary Condition: Defines the downstream constraint. A free outfall allows unimpeded discharge, while a fixed or time-series outfall can simulate interaction with receiving water bodies or other hydraulic structures.

3. Node Properties (Invert, Max Depth, Surcharge Depth):

   • Invert: Defines the elevation of the bottom of the node, crucial for calculating head.
   • Max Depth: Represents the depth at which the node becomes full and potentially overflows or is pressurized.
   • Surcharge Depth: An optional depth above the max depth, simulating a pressurized condition. This is important when the water level rises above the crown of the pipe.

4. Link Properties (Length, Diameter, Offsets):

   • Length & Diameter: Determine the volume and flow capacity of the pipe.
   • Offsets: Define the elevation difference between the pipe invert and the node invert. These are crucial for accurate representation of the siphon's geometry.

5. Calculated Variables (Node Depths, Link Flows, Link Depths, Cross-sectional Areas): These are the outputs of the simulation, dynamically changing with each time step based on the input data and governing equations.

Additional Considerations and Potential Enhancements in SWMM 5 and InfoSWMM

• Entrance and Exit Losses: While the basic simulation captures the main siphon behavior, adding minor losses (entrance and exit losses) at the nodes connected to the siphon can improve accuracy, especially for high flow conditions. InfoSWMM allows for these loss coefficients to be input.

• Air Entrapment: In reality, air can become trapped in a siphon, potentially affecting its performance. SWMM 5 doesn't explicitly model two-phase flow (air and water), so this is a limitation. However, if significant air entrapment is expected in a real-world scenario, you might need to consider other modeling approaches or design modifications to ensure proper venting.

• Break-Siphon Conditions: Understanding under what conditions the siphon might "break" (cease to function as a siphon) is important. This usually happens when the upstream head is insufficient to maintain flow over the high point. The simulation can help you identify these critical conditions.

In conclusion, the described method effectively leverages the core capabilities of SWMM 5 and InfoSWMM to simulate siphon behavior without requiring specialized siphon elements. By accurately defining the node and link properties and applying appropriate inflows and boundary conditions, you can model the complex hydraulics of a siphon system and gain valuable insights into its performance under various operating conditions.