Chelsea Reach - Thames Tideway Tunnel Chelsea Embankment Foreshore CSO Diversion

Mariners are advised that from 21st February 2025 Tideway Tunnel contractors working on Chelsea Embankment Foreshore site will start work to remove the remaining cofferdam sheet piles to the front of the site. Approximately 2 weeks from this date the Combined Sewer Outfall (CSO) will be diverted directly to the river. 

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 the potential of an eddying effect and adjust their passage accordingly (example modelled flow velocities are included at the end of this Notice). Human powered recreational craft are advised not to pass the area when there is a discharge occurring. 

The location of works and approximate location of the CSO on the 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 terracing above 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 activate approximately 4.5 minutes before any discharge will occur. If the lights are flashing human powered craft are advised not to pass in front of the site as the warning time may not be sufficient to clear the area before a discharge occurs. 

Warning light mounted near CSO	Site image showing remaining piles and CSO warning light

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 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.

The CSO is partly connected to the tunnel which will intercept some 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 from Mid 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) whilst construction works are ongoing 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)

Low Water 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) 

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)