Lambeth Reach - Thames Tideway Tunnel - Victoria Embankment CSO diversion

Mariners are advised that from 16th July 2025 Tideway Tunnel contractors working on Victoria Embankment site will start work to divert the flow to the river through the new Combined Sewer Outfall (CSO) location.   

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. Lower powered and less manoeuvrable vessels should be aware of the possibility of cross currents and adjust their passage accordingly (example modelled flow velocities are included at the end of this Notice). Manoeuvrable motorised vessels travelling within the navigable channel will likely feel little effect.  

The Tideway Tunnel is now partly in operation pending further storm testing and this site is connected to the tunnel, meaning the expected number of discharges into the river is reduced. These will only occur if the tunnel becomes full or it is not available. At this site it is predicted a discharge into the river will happen once a year. 

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

A set of warning lights are mounted vertically in front of a black backboard at the upper end of the site, near the CSO. The location of the light is shown on the chartlet (shown as a black star). 

 
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 ordinarily activate at least 10 minutes before any discharge occurs. However, if the Tunnel is shut during a storm this may be reduced. Therefore, if the lights are flashing, human powered craft are advised not to pass directly in front of the site as the warning time may not be sufficient to clear the area before a discharge occurs.  

CSO discharge alerts and warnings will be broadcast by London VTS on Ch 14. Additionally, the live status of each CSO is available on the PLA website, www.pla.co.uk/cso-status. 

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 two hours over low water and around one 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. 

The CSO is partly connected to the tunnel which will intercept most of the discharges, but any larger storms and rain downpours may result in a discharge directly into the river. The Tunnel is expected to be in full service later in 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 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 plumes and velocities in m/sec for a 15 year return period discharge 

(1 m/sec = 1.94 knots)  

30 minutes before Low Water slack (Difference in depth-averaged currents) 

10 minutes before Low Water slack (Difference in depth-averaged currents) 

Low Water slack (Difference in depth-averaged currents) 

Low Water slack (Depth-averaged currents) 

10 minutes after Low Water slack (Difference in depth-averaged currents) 

30 minutes after Low Water slack (Difference in depth-averaged currents)