Thank you for your query. An unidirectional flow feature is not available in TUFLOW 2D modelling at the moment. However, you can use a 2d_sapo layer to pass flow through the embankments based on the downstream flood level. The details of the "Read GIS SA PO" option is documented in the Section 18.104.22.168 of the TUFLOW manual. Basically, you need to:
Create a 2d_po point object of Type “H_” at the water level location that you want to use to control the flow rate.
Add “Read GIS PO == ….” to the .tcf file if not already there.
Create a new 2d_sa layer (call it 2d_sapo) and add two new attributes, as listed in Table 7-6 of the TUFLOW manual.
(i) PO_Type: Char of length 16
(ii) PO_Label: Char of max length 40
Digitise two SA polygons, one at the area upstream the embankments (where "ponding" happens), and another one downstream the embankments. For the attributes:
(i) Set the Name attribute to the name of the water level vs flow curve in the BC database.
(ii) Set PO_Type to “H_”.
(iii) Set PO_Label to the PO Label of the relevant 2d_po “H_” point to be used to determine the flow from the h vs Q curve.
Create a .csv file to define the stage-discharge relationships. Note that for the SA polygons upstream the embankments, the flow rate must be set as negative values to extract water, while for the SA polygons downstream the embankments, use exactly same flow rate but in positive values.
Add “Read GIS SA PO == …” to the .tbc file, and the the "Names" of the SA polygons to the BC database
Please view the _MB.csv files to check the SA in/outflow from the polygons. The extracted flow may be smaller than the inflow to the downstream, if the water upstream the embankments is "dried" by the SA polygon. In that case, you can simply use a QT inflow for the downstream SA polygon based on the actually extracted flow rate to ensure the mass balance is conserved. But this process will be iterative.
Hope this helps and please feel free to get back to us or send email queries to firstname.lastname@example.org if you have any troubles setting up the model.