What are the link lines in flood modelling?

In hydraulic flood modelling link lines (or 1D-2D links) are the digital boundaries that connect a 1D river channel model to a 2D floodplain model. They act like the “spillways” where water flows from the river channel to the floodplains as a result of overtopping.

Once the simulated water surface within the 1-D channel reaches the bank elevation defined at each link line, the excess volume of water is transferred into the surrounding 2-D grid. This enables the simulation to accurately show how water disperses throughout a site. Topography and obstacles can influence this distribution.

The accuracy of modelling is critical for a FRA. Link lines need to be validated with data collected from topographical surveys so that the model will accurately predict when flooding begins. Incorrectly defined link lines could cause inaccuracies in estimating flood depths and lead to either failing the sequential test or rejecting potential mitigative strategies.