---
title: "Troubleshooting Unstable xp2D Models"
canonical: "https://help.innovyze.com/space/xps/19662296/Troubleshooting%20Unstable%20xp2D%20Models"
format: markdown
---
## Overview

The following error is received in the application:

`2D model Unstable - Continue with 1D Simulation?`

![image](media://8581f289-26ea-4e08-9713-e9b5b253e19f)

## 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 <span style="color: #ff9900">xp</span><span style="color: #333333">2D</span> Diagnostic File: In the **Layers** control, go to **Reporting** and **Diagnostics**.> Macro (inline-media-image)
  2. Right-click the file under **Diagnostics**, go to **Properties** and click the **Data** tab.> Macro (inline-media-image)
  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.**
  4. Zoom to model extents and look for the Orange (instability) text. The arrow will point the locations the instabilities are occurring.> Macro (inline-media-image)
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

> Macro (legacy-content)

### 1d Structures

- ***Potential Issue: Underground network representation***  
By default, <span style="color: #ff9900">xp</span><span style="color: #333333">2D</span> 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:

> Macro (legacy-content)

### Topography

> Macro (legacy-content)


> Macro (legacy-content)