EXTRAN simulates both weir outfalls and free outfalls. Either type may be subject to a backwater condition and protected by a tide gate. A weir outfall is a weir which discharges directly to the receiving waters according to relationships given previously in the weir section. The free outfall is simply an outfall conduit which discharges to a receiving water body under given backwater conditions.
The free outfall may be truly "free" if the elevation of the receiving waters is low enough (i.e., the end of the conduit is elevated over the receiving waters), or it may consist of a backwater condition. In the former case, the water surface at the free outfall is taken as critical or normal depth, whichever is less. If backwater exists, the receiving water elevation is taken as the water surface elevation at the free outfall.
Up to 50 different head versus time relationships can be used as boundary conditions. Any outfall junction can be assigned to any of the 50 boundary conditions.
When there is a tide gate on an outfall conduit, a check is made to see whether or not the hydraulic head at the upstream end of the outfall pipe exceeds that outside the gate. If it does not, the discharge through the outfall is equated to zero. If the driving head is positive, the water surface elevation at the outfall junction is set in the same manner as that for a free outfall subjected to a backwater condition. Note that even if the tide gate is closed, water can still enter and fill an empty outfall pipe as sometimes happens at the beginning of a simulation.