This is the water level occurring at the outfall or lower end of a drainage system, which can influence water levels and flows through a system due to backwater effects.

1. For a pipe system discharging to a free water body such as a lake, a river or the sea, the tailwater will be the water level occurring in this body at the time that the storm being analysed occurs. Users of DRAINS must try to determine the most likely level coinciding with the storm for normal design or analysis, and high values such as the Mean High Water Springs level in tidal waters for modelling of extreme conditions.
   
   
2. Where the drainage system catchment is significantly smaller than the catchment of the larger, receiving water body, it is likely that the rainfalls over the two catchments will differ in intensity and timing. The estimation of appropriate events to define critical conditions requires some statistical skill and knowledge of local storms.  Complex procedures for dealing with this are now available in Australian Rainfall and Runoff, 2019.
   
   
3. Where the system being analysed is a pipe system discharging into a larger pipe or trunk drain, the level to be selected should be the receiving pipe’s HGL or receiving channel’s water surface level at the junction. Hydraulic calculations may well be necessary to establish these levels, but valid results cannot be obtained unless appropriate tailwater levels are used.
   
   
4. The storage routing models available in DRAINS can be used to set tailwater levels for a pipe system, by running a detailed ILSAX model of an urban area together with a RORB, RAFTS or WBNM model of the larger catchment of which it is a part. 
   
   
5. If you require a tailwater level that varies with time, you can do this using an artificial channel at the outlet. The method is described in a worksheet of the DRAINS Utility spreadsheet.