Overview

The following error is received in the application:

2D model Unstable - Continue with 1D Simulation?

Resolution

  1. Check the Timestep:
    The 2d model time step should generally be 1/2 to 1/4 of the Grid cell size (for metric). For example, a 1m GRID would require a timestep between 0.5s to 0.25s.

  2. Locate the Instability
    1. Open the xp2D Diagnostic File: In the Layers control, go to Reporting and Diagnostics.


    2. Right-click the file under Diagnostics, go to Properties and click the Data tab.


    3. To help locate the error, ensure that Diagnostic Display Selected/All is turned on and then turn off the Display Warning Messages and Display Check Messages.

      Note: You should review these to not cause the model to fail.
    4. Zoom to model extents and look for the Orange (instability) text. The arrow will point the locations the instabilities are occurring.


  3. Troubleshoot:
    1. If you have already ruled out timestep, interrogate the location of the instability. Look for anything in the model that could cause a rapid change in water level. Things like: initial water levels, boundary conditions, steep terrain, change in manning’s roughness, 1d structures. 
    2. Once you have an idea of what is causing the issue, use the tips below to resolve the instability.


Tips on Resolving Instabilities

Boundary Conditions

  • Possible Issue: Inflow hydrograph not connected to enough cells.
    By default, a node is connected to a single cell. This forces the full hydrograph attached to the node to be pushed onto the single 2D GRID cell.


    Typically, the hydrograph should be connected to the number of cells that represent the flow width from the upstream hydrograph. To resolve this, simply connect the hydrograph to more cells by:
      • Using a 2D Rainfall / Flow Polygon
      • Using a 2D Flow Boundary
      • Using an 1D/2D Interface Line and 1D/2D Connection Line.

  • Possible Issue: Downstream boundaries unstable
    The spatial representation of the downstream boundary condition is a common cause of instabilities. Some general rules to digitising boundary conditions:
      • Should be generally drawn perpendicular to flow direction
      • Should cover the full flow width
      • Should be drawn close to the GRID extent or active area (but ensure it is within the active cells)
      • Where multiple outflow points are expected, multiple boundaries should be digitized. The model assumes the same boundary condition across the length of the polyline. And if using a head boundary, it assumes the water level is equal across the line, which is unrealistic for multiple outflows.

  • Potential Issue: Initial water level does not match downstream boundary conditions
    If a water level (head boundary) is set at the downstream boundary of the model, the initial water level should be set to match the boundary at the first timestep. This prevents a ‘surge’ of water back into the model from the boundary.

    To represent a free outfall, a 2d head boundary can be digitised with a water elevation below the surface level. In this case, no initial water level is required.

1d Structures

  • Potential Issue: Underground network representation
    By default, xp2D connects each node (with a 2d connection ponding type) to the GRID cell it is located within. Water is passed between the 1d and 2d domains through the one GRID cell. This can cause a flow constriction at the 1d node when the capacity of the inlet is greater than the volume of water in the 2d GRID cell at a timestep. Where two nodes fall within the same GRID cell, only one node will be connected to the 2D GRID. To improve stability between the 1d and 2d domains, try the following:
      • Ensure each node has a unique GRID cell connection (1 node = 1 GRID cell).
      • Where the width of flow at the interface between the 1d and 2d is greater than 1 GRID cell width, increase the number of cells that are connected to the node. This can be done using 1d/2d interface lines and 1d/2d connection lines.
      • Check pit spill crest levels are not above GRID level. This allows water to flow to the 1d as soon as the cell becomes wet.

Topography

  • Potential Issue: sharp / steep slopes in topography
    Sharp changes in topography / elevations can cause instabilities as the water levels in the cells change rapidly with high resulting velocity. The first thing to do in these circumstances is decide if the model GRID provides a good representation of the ground conditions (i.e. is there truly steep changes in grade at that location?). Often, these issues can be caused by errors in the original topography data. In this case, you can smoothen the topography manually (Use the DTM tools available in the software). If the ground representation is correct, there are some general tips to try:
      • Rotate the GRID – This does not change the base topography data, but changes the relationship of the cells and elevations.
      • Reduce the GRID – If the topography data is detailed enough, this will help provide smaller steps in water elevations.
      • Increase the GRID – If the topography data is poor, this can help smooth out the lumps and bumps in the data.

  • Potential Issue: low Manning’s roughness causing high velocities
    Very low values of roughness (such as for open water bodies) can cause high velocities and instabilities. Ensure Manning’s values are within reasonable tolerances. Some reference guidelines include: QUDM, ARR16, Brisbane City Council Open Channel Design Chow Open Channel Hydraulics.


  • Potential Issue: Deep water
    xp2D is based on the shallow water equations. This does not refer to the depth of water but the depth of water in comparison to the length of flow. In saying that, deep standing water bodies can cause instabilities. Options to resolve this include:
      • Modify the topography to reduce the depth.
      • Remove the water body from the model and use a boundary condition to represent the water body (if it is a discharge point).