Works will include cordoning off part of the gardens to "accommodate permanent structures required to operate the main tunnel".
The Chelsea Embankment Foreshore site comprises an area of Thames foreshore, a section of the Chelsea Embankment and part of Ranelagh Gardens, around the Bull Ring gates. Several ventilation columns and two electrical and control kiosks will be permanent above-ground features. A low-level sewer will connect to a shaft 45m deep with an internal diameter of approximately 12m. The shaft will transfer sewage flows from the combined sewer overflow to the main tunnel.
At this location, in a typical year, there are 26 discharges of untreated sewage into the tidal River Thames with a volume of about 280,000 tonnes - equivalent to 112 Olympic swimming pools. The Chelsea Embankment Foreshore site will control the discharges from the Ranelagh combined sewer overflow. When the tunnel is in operation it is expected that only two discharges will occur.
The 25km tunnel will cost more than £4bn, paid for by Thames Water customers, and is designed to provide capture, storage and conveyance of almost all the combined raw sewage and rainwater discharges that currently overflow into the river. It is due to open in 2023.
Those living alongside proposed sites are also concerned about the noise, disruption and loss of public space at the listed gardens resulting from construction. Others have argued that a sustainable urban drainage solution or green infrastructure would remove the need for the tunnel. The Chelsea Society said it is worried about "years of disruption, traffic congestion, noise, dirt and pollution".
A Thames Tideway representative said: "We will be working in an area of approximately 450sqm in Ranelagh Gardens from September 2017 for approximately three years. We are liaising closely with the Royal Hospital Chelsea and the RHS to co-ordinate our works."
Thames Tideway said work would continue throughout the flower show.
The Thames Tideway website says work at Chelsea will take four years. A daily average of three barges and seven heavy goods vehicles will access the site. This will rise to approximately 42 heavy goods vehicles per day at peak times, when there will be 24-hour working. Normally, work will go on for up to 57.5 hours per week.
A Code of Construction Practice site specific requirements document says access through the Bull Gate entrance for set up, public access and takedown of the Chelsea Flower Show and "shall be maintained".
Royal Hospital Chelsea said there is a plan in place to deal with disruption, as did the RHS. The Royal Borough of Kensington & Chelsea said: "Permission for the Thames Tideway Tunnel was granted in September 2014 by the secretary of state (Defra and Communities & Local Government), not us." The borough council said any queries should be directed to the RHS, Royal Hospital Chelsea or Thames Water.
In a previous planning document, Kensington & Chelsea commented: "The Royal Hospital is a grade I listed building and these works will impact its setting, which we will object to. The gardens to the Royal Hospital and Ranelagh Gardens are both listed. The embankment wall is grade II listed. There are potential implications for the setting of Ranelagh Gardens and South Gardens. Impact on views to the river should be considered."
Work on RHS Chelsea Flower Show 2016 gardens was delayed while the RHS built a "temporary layer of steel above the sewer" to protect it from the weight of show garden trees.