Chelsea Reach - Thames Tideway Tunnel Chelsea Embankment Foreshore CSO Diversion

Mariners are advised of the continued interim operation of the Tideway Tunnel Combined Sewer Outfall (CSO) at the Chelsea Embankment site.

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 directly in front of the area when there is a discharge occurring.

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 to roughly 6-8 a year. These will only occur if the tunnel becomes full or it is not available.

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 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 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)