Iterative Hot Start File in SWMM5

 Iterative Hot Start File in SWMM5

This is a useful technique for improving the accuracy and stability of SWMM 5 simulations, particularly when dealing with complex models or those with significant initial conditions. Here's a breakdown of why and how to use iterative hot start files:

What is a Hot Start File?

• A hot start file captures the hydraulic and hydrologic state of a SWMM model at a specific point in time. This includes things like water levels in nodes, flow rates in conduits, and soil moisture conditions in subcatchments.
• By using a hot start file, you can initialize a simulation with these pre-existing conditions instead of starting from "cold" (empty) conditions.

Why Use Iterative Hot Start Files?

• Reduce Initial Transients: Starting a simulation from cold conditions can sometimes lead to numerical instabilities and inaccuracies, especially in dynamic wave routing. These initial transients can affect the results, particularly in the early part of the simulation.
• Improve Continuity: A good hot start file helps ensure better mass balance (continuity) in the model, meaning that the total volume of water in the system is conserved throughout the simulation.
• Faster Convergence: For long simulations or those with complex interactions, using a hot start file can help the model reach a stable solution more quickly.

The Iterative Process

The steps for a systematic way to generate a refined hot start file:

1. Initial Run: Run the SWMM model with no hot start file.
2. Save Hot Start File 1: Save the state of the model at the end of the simulation as "Hot Start File 1."
3. Run with Hot Start File 1: Run the model again, this time using "Hot Start File 1" as the initial condition.
4. Save Hot Start File 2: Save the state of the model at the end of this second run as "Hot Start File 2."
5. Run with Hot Start File 2: Run the model again, using "Hot Start File 2."
6. Repeat: Continue this process of saving and using hot start files, alternating between the two files, until the initial and final storage volumes in the flow routing continuity table are nearly identical.

Indicators of Convergence

• Flow Routing Continuity Table: Pay close attention to the "Initial Stored Volume" and "Final Stored Volume" reported in this table. When these values are very close, it indicates good mass balance and that the model has reached a stable state.
• Other Model Variables: You can also monitor other key variables, such as water levels at critical nodes or flow rates in important conduits, to ensure they stabilize over successive iterations.

Benefits

• Increased Confidence: Using an iteratively generated hot start file increases confidence in the model's accuracy and stability.
• Reduced Errors: It minimizes the impact of initial transients and improves mass balance.
• Efficient Simulations: It can lead to faster and more efficient simulations, especially for long-term or complex models.

By following this iterative process, you can create a high-quality hot start file that improves the overall performance and reliability of your SWMM simulations.

 

Dynamic Wave Routing and Node Time Steps in SWMM5

Dynamic Wave Routing and Node Time Steps in SWMM5

SWMM 5 uses dynamic wave routing to simulate the movement of water through the drainage system. This method solves complex hydraulic equations to accurately represent flow and water level fluctuations. Time steps are crucial in this process, as they dictate how often these equations are solved.

Node Time Step Calculation

The node time step in SWMM 5 is variable, meaning it adjusts throughout the simulation based on the conditions at each node. The formula used for this calculation considers:

• Maximum Depth: The difference between the crown elevation (top) and invert elevation (bottom) of the node. This represents the maximum possible water depth at the node.
• Last Time Step: The time step used in the previous calculation.
• Change in Depth: The difference in water depth at the node between the current and previous time steps.

Why Node Time Step Matters

• Accuracy: A smaller node time step increases the accuracy of water level calculations, especially during rapid changes in flow, such as at the start of a storm or when a surge enters a node.
• Stability: It helps maintain numerical stability in the model, preventing errors and unrealistic results.

Relationship to Link Time Step

• Typically Smaller: The node time step is generally much smaller than the link time step (which governs flow routing in conduits). This is because water level changes in nodes can be more rapid and sensitive than flow changes in conduits.
• Importance at Simulation Start: The node time step is most critical at the beginning of a simulation, especially when starting with an empty system (no hot start file). This is when the change in depth is most significant.
• Time Step Critical Elements: If the node time step becomes smaller than the link time step, it can limit the overall simulation time step. SWMM reports these "Time Step Critical Elements" to help identify potential bottlenecks in the model.

Practical Implications

Understanding the node time step helps in:

• Model Stability: Ensuring that the model can handle rapid changes in flow and water levels, especially during critical storm events.
• Computational Efficiency: Balancing accuracy and simulation time by allowing SWMM to adjust the time step dynamically based on the system's behavior.
• Model Diagnostics: Using the "Time Step Critical Elements" report to identify nodes that might require closer attention or model refinement.

By considering these factors, modelers can build more robust and reliable SWMM models for stormwater management and flood prediction.

Figure 1. Equations for the Node Time Step.

Figure 2.  Node Time Step is critical at the beginning of the simulation.

Posted 14 ye