External Boundaries

External boundaries in a sewer system include outfalls at the downstream ends and very first section at the upstream ends. For the upstream end boundaries, usually a simple zero flow is used as upstream boundary condition or a flow time series can be used as upstream boundary condition.

There are a few different boundary conditions users can select for the outfall at the downstream end:

A constant user-defined tail water elevation.
A user-defined water elevation time series (time-elevation curve), such as a tide surface elevation time series.
A user-defined tabular relation between the outfall water elevation and outflow discharge (elevation-flow curve), often called as single-valued rating curve or simply rating curve. Sometimes more than one outfall discharges to one receiving point; in this situation, the discharge in the rating curve would be the summation of all the flows from these discharging pipes.
A free outfall, which means that the outflow is freely discharged without any anticipated backwater effects. In this case, the model automatically applies the proper boundary equation, either a normal flow equation or a critical flow equation, to the outfall boundary based on the dynamic hydraulic condition at the boundary. The normal flow equation will be used if the flow is in supercritical condition and the critical flow equation will be used if the flow is subcritical.

In the first three cases, the control elevation h at the downstream boundary (outfall) is determined from the curves at each time step. It can be replaced by normal or critical flow elevations if it falls below those normal or critical elevations.

The dynamic model also supports boundary elements, such as ponds or storage nodes, as downstream boundaries even when there are no further outflow outlets from there. In this case, a storage equation is used as a boundary condition. If there are no outlets from these boundary elements, then these elements are treated as internal regular elements.