1D-2D Flood Modeller-TUFLOW

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Introduction

TUFLOW models can be configured to dynamically link to Flood Modeller models by utilising a water level boundary to the 2D cells along the 1D/2D interface. In the 2D boundary condition (2d_bc) GIS layer, we define the location at which this link occurs. The 2D water level applied at the 2D boundary cells is calculated in the 1D Flood Modeller component. The terminology used in TUFLOW is a HX type boundary on the 2D cells, with the H indicating that a Head (water level) boundary is used and the X indicating the value is coming from an eXternal model (in this case Flood Modeller).
Depending on the water level in the surrounding 2D cells, flow can either enter or leave the HX cells. The volume of water entering or leaving the 2D boundary is added or subtracted from the 1D Flood Modeller model to preserve volume. We must connect the HX lines to the 1D Flood Modeller model. This is done using CN type lines in the 2d_bc layer, where a CN line is connected to the HX line, the water level from the 1D Flood Modeller nodes is transferred to the HX line. In between 1D nodes, a linear interpolation of water level is applied. This is shown in the figure below.

Once the water level in Flood Modeller exceeds the elevation in the boundary cell water can enter or leave the model. Similar to a Flood Modeller lateral spill or lateral inflow, the discharge is distributed laterally along the length of the HX line. Note that it is the elevation of the HX boundary cell centres that determines when the spill starts to occur and not the cross section within Flood Modeller. If there is a levee or flood defence, it is important that we use breaklines in the model to ensure that the elevations of the 2D cells are consistent with the levee crest. The next four images show a section view of the 1D/2D link and how this may progress during a flood event.



Often HX lines are located along the top of a levee (natural or artificial) or flood defence running along the river bank. When carving a 1D channel through a 2D domain, the HX line must be either on the top of the levee or on the inside of the levee (closest to the channel). If the HX line is located on the other side of the levee away from the channel, the effect of the levee on water flow is not modelled. In the image above, it can be seen that the boundary cell is along the levee and the interaction between the channel and the floodplain (1D and 2D) occurs at the correct elevation.


Building an Flood Modeller-TUFLOW 1D-2D Connection

Flood Modeller and TUFLOW will be considered linked if an Flood Modeller node in a 1d_x1D layer is snapped to a TUFLOW CN line which in turn is snapped to a TUFLOW HX line in a 2d_bc file. ‘CN’ or connection lines read the water level from Flood Modeller and transfers this to the HX line.


In the figure below, the water level is calculated in Flood Modeller at the nodes FC01.16, FC01.15 and FC01.14. These water levels are linearly interpolated along the lengths of the HX line on each of the left and right banks of the watercourse. When the water level exceeds the ZC elevation of the boundary cell, water is able to flow out onto the TUFLOW 2D floodplain.