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[[File:1D2D Model Stability 005.png|500px]]<br>
=SX Boundary Lines (2d_bc)=
2d_bc SX line features are often used to connect 1D structure to the 2D domain if the structure width is greater than one 2D cell wide (e.g. [[Tute_M02_QGIS_1d2d_Link#2d_bc_Link_as_Line_Object|Tutorial Module02]]). This configuration not only increases the number of 1D/2D linking cells, it also has the added benefit of defining the 1D/2D boundary cells at the approximate location of the inlet/outlet of the 1D culvert. An additional benefit of this approach is that the "width" of the linking cells remains similar in length and location irrespective of 2D cell size. <br>
The 1D/2D linking cells in the 5m, 2m and 1m cell size models are shown in the the figure below.<br>
[[File:1D2D_Model_Stability_006.png|500px]]<br>
The result below indicates cell size convergence is achieved, the flow oscillations have also been reduced (compared to the model using the 1d_nwk Node 1D/2D link method).<br>
[[File:1D2D Model Stability 007.png|500px]]<br>
=SX Storage Factor (2d_bc)=
A new "SX Storage Factor" feature was been added to TUFLOW in the 2017 release. As the name suggests, this feature adds additional storage to the 1D/2D linking (SX) cells. The storage factor is specified by adjusting the 2d_bc "a" attribute value. "a" is treated as a storage multiplier. For example, specifying an "a" value of 2.0 doubles the storage associated with the 1D/2D boundary. Note that an "a" value of 0.0 assumes a multiplier of 1.0.<br>
The modelling result with an "a" value of 2.0 are presented in the figure below. The flow rate is now very smooth and the difference in peak flow rates is negligible.<br>
[[File:1D2D Model Stability 008.png|500px]]<br>
Testing has shown small increases in storage usually has immeasurable effect on results, however, sensitivity testing is recommended to confirm this assumption in different models. For example, if specifying an "a" value of 2 stabilises the link, sensitivity test by increasing "a" further to 3. If there is no appreciable or unacceptable change in results, increasing the storage by a factor within these bounds can be considered to have negligible effect on results.
=SX Boundary Regions/Polygons (2d_bc)=
SX boundary regions/polygons are another feature that was added for the 2017 TUFLOW release. Polygons can now be used as a 2d_bc SX input. All cells with a cell centre within the polygon are set as SX cells. A CN line is required to connect the 1D feature to the SX polygon, snapped to any vertex on the perimeter of the polygon. An example of this configuration and the 1D/2D linking cells produced in the 5m, 2m and 1m cell size models are shown in the figure below.<br>
[[File:1D2D_Model_Stability_009.png|500px]]<br>
The modelling results are presented in the figure below. Again, cell size convergence is demonstrated and the oscillations at low flow is removed.<br>
[[File:1D2D Model Stability 010.png|500px]]<br>
=Conclusion=
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