Diagnostics and tips for linked 1D river and 2D simulations

If you are struggling to get your integrated simulation to run to completion, the model diagnostics will help you pinpoint the cause of the problem. The locations of these messages vary:

• User interface - a pop-up window will appear to indicate a problem.
• Running a simulation via the UI - the simulation window contains a log of all the messages from the engines.
• 1D river engine - the messages are in files with extension lf1 and/or zzd.
• 2D engine - the messages are in files with extension lf2 and/or log.
• 1D urban engine - the messages are in a file with extension rpt.
• TUFLOW - the messages are in a file with extension tlf.

General tips

If time allows, test running each component of your linked model individually to check how stable each model is.

A 1D river only model may require a higher “glass wall” to account for there not being anyway for overtopping flows to go to (ensure this is changed back for the linked version).

A 2D model will need a flow boundary input to replace the 1D element, e.g. for integrating with a river network, use the link-line shapefile (or for integrating with an urban network, use the link-element shapefile) to locate the boundary flow and assign a constant flow.

Model timesteps for the 1D and 2D elements of the linked simulation are required. We recommend adopting a 1D timestep that is exactly half of the specified 2D timestep (as the 2D solver performs 2 “half timestep” calculations in the x and y directions, so 1D should correspond to this). However, if you use a different 2D timestep (possibly required to achieve stability) then this must be selected such that your 1D timestep is an integer multiple of half the 2D timestep. Please note that usually shorter timesteps are required (compared to running each element individually) to maintain stability in a linked model.

Tips on integrating river networks and 2D components

• The level link has been found to be more stable than the weir link when water levels either side of the link are similar. This would typically apply to natural banks
• The weir link would be more appropriate for a defence wall (or similar) with lower ground behind, where flow from the river network to the 2D domain is better described as weir-type flow. Using a level link here would lead to a large jump between the boundary water level and potential problems in solving the shallow water equations.
• The flow link is more appropriate where the inflow to/outflow from the 2D area covers one or a small number of grid cells and is therefore more suitable for point discharges into the 2D domain. The assumptions of where it distributes the flow to or obtains a representative water level from (in the 2D domain) increases with the number of cells. It is also more suited to situations where the hydraulic behaviour is described accurately by the node immediately upstream of the Head-Time boundary (HTBDY) unit.
• Setting up the level and weir links requires no/little extra effort on the 1D side and a small amount amount of effort on the 2D side (especially if the 1D nodes are already georeferenced and if using the link-line generator). Setting up the flow link requires the hydraulic unit defining the flow exchange to be schematized in the 1D model, attached to a dummy HT boundary; the effort required in the 2D model is similar to that for a level/weir link.

It is always recommended that you check your automatically generated link-lines carefully as further manual editing may be required to complete your model setup. Manual edits might be:

• Correcting links that overlap with the adjacent link-lines (can sometimes happen when linking around bends). Correction needed to avoid double counting of 1D model inputs to the same cells of a 2D model.
• Add additional links across reaches that the automatic tool was unable to link across, i.e. fill in any gaps.
• Positioning of link-lines for interpolate and replicate 1D section nodes.