The Thames Tideway Sewage Tunnel does not have to have input flows free-falling to a low level hydraulic gradient (NCE last week).
The whole system can be designed as a hydraulically full inverted siphon, with energy input to create the hydraulic gradient required for any specific flow rate. This is the principle behind the new Hong Kong sewage tunnel system, now in operation.
The Tideway Tunnel proposal brought back memories of the late 1960s when as a graduate engineer I worked on the design of the then GLC's proposed Northern Trunk Sewer, almost identical in concept but never built.
We were among the first to use hydraulic computer modelling to determine flows.
The worst case scenario was a storm moving SW-NE across London, quite a likely meteorological prospect.
We considered incorporating motorised penstocks on existing large sewers, operated by realtime logic in order to hold back upstream flows and to optimise storage capacity.
In those days the IT systems were not capable of such control but perhaps it would be feasible today?
David Hall, Stile House Cottage, Menith Wood, Worcs, WR6 6UN