The flux across an infrastructure face - a building face or a road face - is calculated using an empirical formula (see General description) or the flow equations depending on the surface elevation. A building face is defined as a face in the mesh, where the element to one side of the face belongs to a building zone and the element to the other side belongs to a road zone or a material zone. Also, a face where the elements on both sides of the face belong to a building zone and where there is a difference in the level of the buildings, is defined as a building face. A road face is defined as a face in the mesh, where the element to one side of the face belongs to a road zone and the element to the other side belongs to a material zone. The structure level for an infrastructure face is defined as the maximum of the level for the two elements to the right and left of the face. If the surface elevation in both the element to the right and left of the infrastructure face is above the structure level the flow equations is applied and else the empirical formula is applied. If a building zone is excluded from the calculation the flux across the corresponding building faces are always zero and the elements in the zone is treated as dry elements.
Both for buildings and roads you have to specify the critical level difference and the weir coefficient. When the water level gradient across a structure is small, the corresponding gradient of the discharge with respect to the water levels is large. This in turn may result in a very rapid flow response to minor changes in the water level upstream and downstream. As a way of controlling this effect, a critical level difference has been introduced. The critical water level difference defines the water level difference below which the discharge gradients are suppressed. The default setting is 0.01 meter. If a structure shows oscillatory behaviour it is recommended to increase this value slightly.
For each building zone you also have to specify a runoff factor. The factor must be larger or equal to zero and lower or equal to one. This runoff factor is multiplied to the flux across a building face.