Identifying Instability in SWMM5, InfoDrainage, and ICM SWMM Models

Identifying Instability in SWMM5, InfoDrainage, and ICM SWMM Models

Numerical instability is a common issue in hydraulic modeling that can lead to inaccurate or unreliable results. Fortunately, SWMM5, InfoDrainage, and ICM SWMM provide tools to help you identify and address such problems. One key indicator of potential instability lies in metrics related to flow oscillations or fluctuations within the model, though the specific name and presentation might vary slightly between the software packages.

Understanding Flow Oscillations and Instability

The core concept across all three platforms is the identification of "flow turns" or rapid changes in the direction of flow change within a link (conduit or channel). These oscillations can indicate numerical instability.

How Flow Turns (or Equivalent Metrics) are Calculated

The general process, which is conceptually similar across SWMM5, InfoDrainage, and ICM SWMM, involves these steps:

1. DQ (Flow Difference): At each time step, the model calculates the difference between the new flow (Q_new) and the old flow (Q_old) in a link. This is often referred to as DQ (DQ = Q_new - Q_old).

2. Significant Change Threshold: To filter out very small, insignificant fluctuations, a threshold is applied. A flow change is only considered if the absolute value of DQ is greater than a predefined small value (e.g., 0.001 cfs in SWMM5).

3. Sign Change: The most crucial step is checking for a change in the sign of DQ between consecutive time steps. A flow turn (or equivalent instability indicator) is counted only when the sign of DQ changes from positive to negative or from negative to positive. This signifies an oscillation – the flow rate was increasing and is now decreasing, or vice versa. It is not the same as the sign of the flow.

   • Example: If DQ was +0.05 cfs at the previous time step and is now -0.02 cfs, a flow turn is counted because the sign changed from positive to negative.
   • Monotonic Flow: If the flow is consistently increasing (DQ is always positive) or consistently decreasing (DQ is always negative), no flow turns are counted, even if the flow rate changes significantly.

Interpreting Instability Indicators

• SWMM5: SWMM5 reports a "Flow Instability Index" in the output report file (RPT file). This index lists the links with the highest number of flow turns.
• InfoDrainage: InfoDrainage provides a "Continuity Error" and other stability-related metrics in its output. You may also need to visually inspect flow hydrographs to assess stability.
• ICM SWMM: ICM SWMM offers detailed simulation logs and allows you to track flow oscillations and identify areas of instability. You can also generate reports that highlight potential stability issues. The flow instability index is called the Number of Flow Turns in ICM and the Flow Turns are listed in the Node Summary Table of the node summary csv file.

General Workflow for Assessing Instability:

1. Check Output Reports/Logs: Examine the output reports or logs generated by your chosen software. Look for sections related to flow instability, continuity errors, or warnings about oscillations.

2. Focus on Problem Areas: Identify the links (or nodes in some cases) that are flagged as having the highest instability or the most flow turns.

3. Visual Inspection: The most important step is to visually inspect the flow hydrograph for the identified links. Plot flow versus time to reveal the nature of any oscillations.

   • Stable Oscillations: Small, rapid oscillations around a relatively stable average flow might not be significant and can often be ignored. This is especially true at the beginning of the simulation before flow becomes connected.
   • Unstable Oscillations: Large, erratic spikes or fluctuations indicate significant instability that needs to be addressed.

Addressing Instability

If you find significant instability, you'll need to take corrective action. Common strategies include:

• Reducing the Time Step: Using a smaller computational time step often improves stability.
• Checking Model Input Data: Ensure that your input data (conduit geometry, roughness, boundary conditions, etc.) is accurate and free of errors.
• Adjusting Routing Method: Experiment with different routing methods (e.g., dynamic wave, kinematic wave, diffusion wave) if available in your software.
• Simplifying the Model: Sometimes, simplifying overly complex parts of the model can help.
• Software-Specific Settings: Investigate any software-specific settings related to numerical stability or damping (InfoDrainage and ICM SWMM, for example, offer under-relaxation or damping options).

Key Takeaways

• Flow oscillations, or rapid changes in flow direction, can indicate model instability.
• SWMM5, InfoDrainage, and ICM SWMM provide tools to help you identify and analyze these oscillations, although the exact terminology and presentation may differ.
• Visual inspection of flow hydrographs is crucial to determine the significance of any flagged instability.
• Addressing instability often requires adjustments to model parameters, input data, simulation settings, or the use of flow damping options in the software.

By carefully analyzing the available instability indicators and examining flow hydrographs, you can ensure the reliability and accuracy of your hydraulic model results in SWMM5, InfoDrainage, or ICM SWMM.