Link lines

Link-lines are used to connect 1D river networks and 2D modelling components. They are created using the Link Line Generator button provided on the 2D Build tab.

Linking is defined by drawing boundary lines within your active area that reference 1D river nodes. Boundary water levels are calculated along each boundary line at runtime by interpolating values between ‘start’ and ‘end’ 1D nodes referenced in your boundary line attribute table. Flood Modeller can be used to define 1D-2D link lines, automatically assigning the required shapefile attributes. Other GIS can also be used however care must be taken to ensure the correct attributes are included.

The required attributes for a link line shape file are:

• “type” – defines the type of link. Options are; level (enter “H”), flow (enter “Q”) and weir (enter “W”). These define how to calculate the passage of water across the link, i.e. by comparing levels or flows or by using a weir equation.

• “node1” – 1D network node name reference at start of link line

• “node2” - 1D network node name reference at end of link line

• “height1” – Underlying ground elevation at start of link line. This can be left set to the default value (the missing data flag; “-9999”) and then at runtime the 2D engine will calculate a composite underlying ground elevation using all entered topographic data.

• “height2” – Underlying ground elevation at end of link line. This can be left set to the default value (the missing data flag; “-9999”) and then at runtime the 2D engine will calculate a composite underlying ground elevation using all entered topographic data.

• “Factora” – optional field (i.e. can be left blank, though must be present). Applicable with flow type links only. Used in equation: Flow into 2D = (Factora x 1D flow) + Factorb

• “Factorb” – optional field (i.e. can be left blank, though must be present). Applicable with flow type links only. Used in equation: Flow into 2D = (Factora x 1D flow) + Factorb

It is recommended to leave the height fields set to their default missing data flags. This enables the same link line to be used for multiple development scenarios in which the topographic input data varies, e.g. the addition of proposed flood defences. For each simulation the 2D solver will know to calculate the link line heights using the topographic data (rather than adopt any fixed levels entered in the link line file).