Advanced parameters

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.

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.

The ADI solver

Depth threshold

ADI/TVD

The depth below which friction is increased to reduce negative depth problems on steep drying slopes

Wetting and drying

Eddy viscosity

ADI/TVD

Factor controlling horizontal turbulent and viscous momentum transport. Typically ranges between 0 and 1.

Shallow Water Hydraulics

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.

The ADI solver

-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.

Wetting and drying

New flooding and drying

ADI/TVD

Switches on the more sophisticated approach for calculating depths at cell edges, as parameterised by Calculation depth.

The ADI solver

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.

Direct rainfall

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.

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.

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).

Modifications to the advection term

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.

Direct rainfall

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.

The ADI solver

Convergence criteria

FAST solver

Controls convergence of the 2D FAST Solver algorithm. Reduce this for a more accurate, but slower solution.

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.

DTM pre processing

Filter

FAST solver

Controls a preprocessing stage to filter identical ground heights in neighbouring cells.

DTM pre processing

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.

DTM pre processing

FAST solver

Depth to be filled in along flow path

2D FAST Solver

Padding width

FAST solver

Width in cells of flow path

2D FAST Solver

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.

DTM pre processing

Probable path mark-up

FAST solver

Switch on marking of drainage paths in model output

2D FAST Solver