Novel Drop Shaft System for Conveying and Controlling Flows from High Level Sewers into Deep Tunnels

The paper describes a novel drop shaft system that relies on the use of a self-activating, non-powered and no moving part air intake control system to prevent instabilities and provide smooth transitions between air-entrained vortex flow modes and pipe full operating modes. This allows more compact drop pipe arrangements to be deployed within a single drop structure, also providing for maintenance access. These systems therefore do not require auxiliary maintenance or air shafts resulting in significant project cost savings. Further optimization of the system has involved the use of compact hydraulic break, rotary flow energy dissipation structures at the base of the vertical drop pipes resulting in a space efficient and effective system for controlling flows into deep tunnels. The operational characteristics of the novel drop shaft system including the basis for more effective flow control and elimination of unstable flow conditions are described in detail in this paper. The paper also includes case studies of its successful application in a number of deep tunnel schemes and at a wastewater treatment plant.