One of the most commonly asked questions to our support team is what to do if a model displays instabilities and will not run. Following an instability, the error message may report that a section has run dry or the parameter Dflood has been exceeded - often the location is not where the problem actually lies! This is because the instability can oscillate up and down the channel before creating the model failure.
What to do to stabilise your model can involve a multitude of techniques; a modeller needs to diagnose the problem and take steps to remedy it, whilst ensuring the model remains physically realistic to the site and appropriate to the scenario being modelled. Whilst by no means extensive, this article provides five tips and tricks for resolving instability issues within 1D river networks.
Please see the user manual for more details on these methods and remain especially vigilant of the potential consequences any adjustments will have on your model.
1. Be methodical when building your model
When building a new model, build your 1D network using cross-sections only, i.e. without any structures. Check the model runs successfully, and generates a good, stable set of initial conditions before adding structures. Similarly, add one structure at a time and check the model runs successfully before proceeding to the next.
Simplifying your river network as much as possible, splitting it into smaller sections, and dealing with tributaries separately are all recommended. Once you have checked through each section of your model and corrected any issues causing instabilities, combine your model one section at a time. A spill unit should be used prior to the confluence in a tributary channel to adjust the bed elevation. Each time you join a section together, run your model again to ensure it will still run.
When dealing with a complex model, controlling structures are essential, so try to simplify a series of structures where not all of them are needed by removing the ones that have no impact on the model results and are not in the area of interest.
Figure 1: The initial conditions table in the ‘Network’ panel. A good set of initial conditions should be generated at each stage of the model build.
2. Use the best data available
Using the best possible survey data is always important. Plotting a long-section of your river network can help reveal whether there are any errors in your cross section and structure data that could lead to instabilities. For example, if there are steep decreases in gradient over short distances that are not captured by a structure, then it may be that there is missing information in the survey data (i.e. a weir).
Poor conveyance is another common cause of instabilities. This is particularly the case where there is a decrease in conveyance with an increase in stage. The cross section plotting tool gives the modeller a quick reference to check that conveyance increases monotonically and smoothly with stage. A decrease in conveyance as the water level goes out-of-bank may be rectified by adding a panel marker at bank level.
After selecting your model nodes, the ‘Tabulate Cross Section Properties’ tool can also be used to show conveyance alongside other cross-section properties for a range of water levels, namely depth, area, top width and wetted perimeter. You can then plot the conveyance for multiple cross sections in one plot by selecting the ‘River section property plots’ tab and plotting conveyance on the X axis and stage on the Y axis.
Figure 2: The Flood Modeller ‘Tabulate Cross Section Property’ tool. This is located in the ‘Model Review Tools’ section of the Toolbox, alongside the ‘1D Health Check’ tool.