This section summarises the advanced parameters available to the 2D solver user, which have not been previously discussed. It is strongly recommended that users do not adjust these from their default values unless they understand the function of the parameter and its likely effect on model results.
|
Parameter |
Solver |
Description |
See section |
|
Beta |
ADI/TVD |
Multiplier for the advection term to allow for velocity varying with depth. For typical floodplain flows this should be in the range 1-1.03. Can also be set to zero to switch off the advection term if this is causing problems and only an approximate solution is required. |
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|
Calculation depth |
ADI/TVD |
Water above this depth uses the simplest approach to calculating depths at cell edges: average of the water levels either side minus the ground height. Below this a more sophisticated approach is used. Users may need to increase this parameter for modelling over steep topography, typically when the ground slope is big enough to make elevation differences between cells > 0.5m. |
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Depth threshold |
ADI/TVD |
The depth below which friction is increased to reduce negative depth problems on steep drying slopes |
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Eddy viscosity |
ADI/TVD |
Factor controlling horizontal turbulent and viscous momentum transport. Typically ranges between 0 and 1. |
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Nadvit |
ADI |
Controls the number of iterations used to deal with non-linearity in the Shallow-Water equations. Increasing this may improve stability; reducing it below the default value of 2 may result in an unstable model. |
|
|
-ve depth threshold |
ADI/TVD |
Depths below this negative number are corrected by balancing with water from neighbouring cells. Increase this value if negative depths are acceptable; this may improve stability. |
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|
New flooding and drying |
ADI/TVD |
Switches on the more sophisticated approach for calculating depths at cell edges, as parameterised by Calculation depth. |
|
| Rainfall acc. Depth |
ADI/TVD |
Depth to which rain must fall before a "pulse" is added to a cell in direct rainfall models. This may need to be increased for models with very high topography; should only be adjusted if mass balance errors are encountered. |
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|
Slip wall flag |
ADI/TVD |
Specifies whether a slip or no slip condition is used at walls and dry cells. This will only have an effect on flow if a non-zero eddy viscosity is used. |
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Upwind flag |
ADI/TVD |
Switches on upwinding in the advection term calculation. Switching this off may result in a slightly more accurate solution, but may produce an unstable model when the advection term is significant. |
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|
Velocity head threshold |
ADI |
The velocity head above which advection capping is used. Choose a very high value (>=5) to switch off advection capping; choose a lower value where accelerating flows are causing instabilities. However, note that a lower value may reduce the accuracy of the model output in the areas of your model with accelerating flows. If this area is an important part of your model then you could consider using the TVD solver instead (which tends to perform better for accelerating flows). |
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|
Water depth (dry) |
ADI/TVD |
Depth threshold at which a cell becomes wet. A smaller value may improve accuracy for very shallow flows (e.g. direct rainfall), at the expense of stability. A larger value may improve stability; flows below this depth will be routed using the kinematic wave approximation. |
|
|
Water depth (friction) |
ADI/TVD |
The water depth below which the ADI scheme switches to the semi-implicit treatment of the non-linear friction terms. The default value is high to ensure the semi-implicit method is used nearly all the time. Reducing this value may help model stability for coastal flows. |
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|
Convergence criteria |
FAST solver |
Controls convergence of the 2D FAST Solver algorithm. Reduce this for a more accurate, but slower solution. |
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Discretisation
|
FAST solver |
Water level – volume curves for each depression are stored at heights separated by the Discretisation parameter. Increasing this will reduce file size, and reduce accuracy. |
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Filter |
FAST solver |
Controls a preprocessing stage to filter identical ground heights in neighbouring cells. |
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|
Merge factor |
FAST solver |
Topographic depressions are merged if the “lip” between them is below this height. Increasing this value will lead to fewer depressions, and fewer, larger flooded areas. |
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| Padding height |
FAST solver |
Depth to be filled in along flow path |
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Padding width |
FAST solver |
Width in cells of flow path |
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Pre-process DTM |
FAST solver |
Switch this on to force 2D FAST Solver to recalculate topographic depressions; switching off will cause 2D FAST Solver to use previously calculated depressions, meaning a shorter computation. Switch this to off after the first run of 2D FAST Solver, unless you change the topographic processing parameters. |
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Probable path mark-up |
FAST solver |
Switch on marking of drainage paths in model output |