This is a helpful guide for determining the appropriate time step and conduit lengthening settings in SWMM 5 ICM SWMM and InfoSWMM.

This is a helpful guide for determining the appropriate time step and conduit lengthening settings in SWMM 5 ICM SWMM and InfoSWMM. Here's a breakdown of why this is important and how to use the information provided:

Why Time Step Matters

• Accuracy: In hydrodynamic modeling, the time step influences the accuracy of flow routing and water level calculations. A smaller time step generally leads to higher accuracy but longer simulation times.
• Stability: If the time step is too large, the model can become unstable, leading to unrealistic results or even simulation crashes.
• Efficiency: Finding the right balance between accuracy and simulation time is crucial.

Conduit Lengthening

• Purpose: Conduit lengthening is a technique used in SWMM to artificially increase the length of conduits (pipes). This allows you to use a larger time step while maintaining model stability.
• Challenge: Determining the optimal amount of lengthening can be tricky. Too much lengthening can distort the hydraulic behavior of the system, while too little might not provide enough stability.

The Time Step Guide

The formula provided offers a practical approach to estimate a suitable time step:

Time Step Guide (seconds) = Link Length / [Velocity + sqrt(g * Maximum Depth)]

• Link Length: The length of the shortest conduit in your model.
• Velocity: The flow velocity in the conduit.
• g: Acceleration due to gravity (approximately 9.81 m/s² or 32.2 ft/s²).
• Maximum Depth: The maximum depth of flow in the conduit.

Key Takeaways

• Wave Celerity: The formula approximates the wave celerity in the conduit, which is a critical factor in determining the appropriate time step.
• Average Time Step: The average time step used during a preliminary simulation run can be a good starting point for refining the time step and conduit lengthening settings.
• Adjustment Factor: An adjustment factor (like the 0.75 mentioned) might be necessary to fine-tune the time step for optimal performance.

How to Use This Information

1. Initial Simulation: Run a preliminary simulation with an estimated time step.
2. Analyze Results: Observe the average time step used in the simulation.
3. Apply Formula: Use the formula to calculate the "Time Step Guide."
4. Adjust and Refine: Compare the calculated value with the average time step from the simulation. Adjust the time step and conduit lengthening settings iteratively until you achieve a stable and accurate model with a reasonable simulation time.

Important Notes

• This is a guideline, and some adjustments might be needed depending on the specific characteristics of your model.
• Consider factors like the complexity of the network, the presence of pumps or other hydraulic structures, and the desired level of accuracy when fine-tuning your time step and conduit lengthening.
• Always validate your model results to ensure they are reasonable and reflect the real-world system behavior.