Kings Reach - Thames Tideway Tunnel Blackfriars Embankment CSO Diversion

Mariners are advised that from 2nd December 2024, and for a period of approximately six weeks, the Tideway Tunnel Combined Sewer Outfall (CSO) at the Blackfriars Embankment site will be diverted directly to the river. 

During this period the CSO may discharge rain-water and untreated sewage into the river whenever there is, or has recently been, rain in the local catchment area. The discharge amount and velocity will vary, but during a storm this could be around 6 knots, lessening further from the outfall. 

Mariners should note that the CSO in this location is within the Authorised Channel and effects may be felt by all craft within the channel. Less manoeuvrable vessels and human powered craft should be aware of the possibility of cross current in most of the channel and the potential of an eddying effect and adjust their passage accordingly (example modelled flow velocities are included at the end of this Notice).  

The approximate location of the CSO on the Blackfriars site is shown in red on the chartlet below:

A set of warning lights will be mounted vertically in front of a black backboard on the riverwall above the CSO and a repeater light will be mounted above the bridge pier between arches 2 and 3. The locations of the lights are shown on the chartlet (shown as a black star).

Above - CSO warning light mounted on the river wall.

Above - CSO warning light on Blackfriars Rail Bridge

The lights will flash a yellow alternating 1 second pattern, visible day and night at a range of up to 200m, whenever the CSO is discharging or likely to discharge. Tideway advise the lights will activate approximately 8-10 minutes before any discharge will occur.

CSO discharge alerts and warnings will be broadcast by London VTS on Ch 14. Additionally, the live status of each CSO (not including Putney) is available on the PLA website.

Routine VTS broadcasts will also include a warning if Tideway inform us there is a likelihood there will be a discharge from the site within the next hour.  

Human powered recreational craft, lower powered vessels and craft engaged in towing are advised to avoid the area over the slack water period (around 2 hours over low water and around 1 hour over high water), or if there has been heavy rain and to be vigilant if there is a Met Office weather warning for rain or storms in force for the London area. 

For this period, whilst construction works are still underway, the number 2 arch closure smart signal will continue to display a closed signal. See Notice to Mariners M85-23 for details.  

The CSO will be partly connected to the tunnel shortly after the diversion works commence which will then intercept some of the discharges, but any larger storms and rain downpours may still discharge directly into the river. The Tunnel is expected to be in full service by the middle of 2025. A new Notice will be issued at a later date to advise of permanent Tideway Tunnel operations. 

All mariners are advised to exercise caution when passing the site, remain outside of the works area (marked in purple on the chart) and to pay attention to warnings given. 

Further information on the characteristics of the CSO discharges, assessments of risk and potential impacts on mariners, which were undertaken by Tideway’s designers, are available on their website. Go to www.tideway.london/contact-us/document-library and follow the link to CSO DISCHARGES - RISK TO NAVIGATION. Mariners should consult these documents when passage planning. 

(This link will take you to a third-party website which does not belong to the Port of London of Authority and may have different security, privacy and accessibility standards.) 

Example discharge plume and velocities in m/sec for a 1 year return period discharge 

(1 m/sec = 1.94 knots) 

Above - LW slack (difference in depth-averaged currents) 

Example discharge plumes and velocities in m/sec for a 15 year return period discharge 

(1 m/sec = 1.94 knots) 

Above - 10 minutes before LW slack (Difference in depth-averaged currents) 

Above - LW slack (difference in depth-averaged currents) 

Above - LW slack (Depth-averaged currents) 

Above - 10 minutes after LW slack (difference in depth-averaged currents)