TUFLOW 2D Hydraulic Structures
2D Bridges are recommended to be modelled using the following flow constriction commands/layers:
- Read GIS Layered FC Shape == 2d_lfcsh... ; or
- Read GIS Layered FC Shape == 2d_fcsh_...
The TUFLOW 2D solution automatically predicts the majority of “macro” losses due to the expansion and contraction of water through a constriction, or around a bend, provided the resolution of the grid is sufficiently fine (Barton, 2001; Syme, 2001).
Where the 2D model is not of fine enough resolution to simulate the “micro” losses (e.g. from bridge piers, vena contracta, losses in the vertical (3rd) dimension), additional form loss coefficients and/or modifications to the cells widths and flow height need to be added. This can be done by using flow constrictions.
The following should be noted when adapting structure loss coefficients from a 1D model or from coefficients that apply across the entire waterway, for example, from Hydraulics of Bridge Waterways (FHA 1973):
• The additional or “micro” losses, which may be derived from information in publications, such as Hydraulics of Bridge Waterways, need to be either:
o Distributed evenly over the FC cells across the waterway by dividing the overall additional loss coefficient by the number of cells (in the direction of flow); or
o Assigned unevenly (e.g. more at the cells with the bridge piers), however, the total of the loss coefficients should be equivalent to the required overall additional loss coefficient.
• The head loss across key structures should be reviewed, and if necessary, benchmarked against other methods (e.g. using HEC-RAS or Hydraulics of Bridge Waterways). Note that a well-designed 2D model will be more accurate than a 1D model, provided that any “micro” losses are incorporated.
• Ultimately the best approach is to calibrate the structure through adjustment of the additional “micro” losses – but this, of course, requires good calibration data!
An example of how to apply 2D FCs and a 2D FCSH to a bridge structure is shown in Figure 6 8 and Figure 6 9. The loss coefficient quoted in the figure is an example, and is not necessarily applicable to other structures. Every structure is invariably different!
When applying FCs the best approach is to view the structure as a collection of 2D cells representing the whole structure, rather than being too specific about the representation of each individual cell. A good approach is to use a 2d_po layer to extract time histories of the water levels upstream and downstream of the structure and of the flow and flow area upstream, downstream and through the structure (see Section 9.5).
Of particular importance is to check that the correct flow area through the structure is being modelled. Digitise a 2d_po QA line through the structure from bank to bank, and use this output to cross-check the flow area of the 2D FC cells is appropriate (the QA line will take into account any adjustments to the 2D cells due to FC obverts and changes to the cell side flow widths). If the overall structure flow area is not correct, then the velocities within the structure will not be correct and therefore the energy losses due to the changes in velocity direction and magnitude and additional form losses will not be well modelled.