Low-Head Air Regulated Siphons

Air-regulated siphons have significant advantages over blackwater siphons: (1) They discharge over a wide flow range with minimal change in upstream water level; and (2) they are self-regulating and maintenance free. When the head drop at a control point is small, siphons can operate by using the tailwater to provide a seal, in which case the siphon has certain of the hydraulic characteristics of both a free weir and a sluice. Such low-head siphons are in use on rivers and canals. For convenience the design process can be considered in three parts: (1) The determination of the size and basic profile; (2) the intake; and (3) the outlet. Design solely by analytical methods is not possible, so a (Froude) model study is usual, despite limitations arising out of the nonscaling of viscosity and surface tension. Most importantly, the design must ensure the free movement of air as well as water through the structure.