The IFACE error occurs when SRH-2D has difficulty mapping the pressure zone. The best practice is to align the boundary condition arcs so that they form a rectangle rather than a parallelogram. When structures are skewed to the channel this can be challenging and sometimes warrants slightly changing the alignment of the BC arc such that it may not perfectly align with the bridge face. The skewed mesh elements (poor mesh quality) could also cause a stability issue. The upper example should be ok, but for the lower example try to realign the mesh elements and boundary condition arcs to form a rectangle. This should solve the IFACE error.