Dredging Chelsea Harbour marina is no easy matter

It is thirteen years since the marina at Chelsea Harbour on London’s River Thames was last dredged. That was a fairly complex operation technically but this time around the rules have changed.

The main area of concern is the pollution content of the silt. Hydrocarbons are prevalent due to the site’s previous use and traces of cadmium and mercury were also found. More than 10,000 cu metres of silt will need to be removed and the preferred method to dredge the basin is water injection dredging (WID) by vessels such as Van Oord’s Odin, or the ‘hydro dynamic’ GPS Innovation.

The Marine Management Organisation (MMO) has accepted a pre-application request to consider WID flushing of some of that silt once the contaminants have been removed, which means the remainder would be lifted by bucket dredger or backhoe into small barges that can fit through the narrow entrance lock.

The prestigious Chelsea Harbour estate was constructed in the late 1980s on a 25-acre brownfield site. The Basin, which is now the marina, was built in the 1830s for barges carrying coal to Lots Road power station, a traffic that ended in 1960, since when the basin and the lock were infilled with contaminated materials. The developers excavated the basin, reduced its size, re-puddled the dock floor and renovated the lock.

Anna Hampton, an independent marine and environmental consultant, is advising on the tender process and navigating the minefield of regulation, bureaucracy and costing. Obtaining permissions has been a slow process, taking over a year, and still no company has been contracted to do the work.

Mrs Hampton has spoken to various companies including Van Oord, GPS Marine and Land and Water. She says GPS actually employ someone in house to deal with MMO licence applications.

The MMO (a part of DEFRA) is the national licensing authority that has taken over jurisdiction from local authorities such as the PLA. It has added a new level of bureaucracy to the process because licences were until recently issued by local authorities. Although the MMO now issues licences this is still done in consultation with the PLA because they have local knowledge of the river.

But to obtain a licence you have to have an EA permit because of the Thames flood defences. The EA (also part of DEFRA) has an interest in water quality and they are responsible for the Water Framework Directive (WFD), a risk assessment tool, which is a European requirement.

So far, Mrs Hampton has worked through the MMO pre-application process and got Chelsea Harbour back to the same place with the MMO as it was two years ago with the PLA. She has repeated silt samples and analysis to MMO criteria and says “The next stage is to apply for a licence, and the MMO have indicated that Water Injection Dredging (WID) will be considered if we can show how we will deal with the contaminants – this is where the WFD comes in. To take the project forward I need to carry out a WFD to see if we can pinpoint the contaminants’ location, dig them out and use WID for the rest. Further sampling and analysis will be required.

“The only place able to analyse the samples was the MMO lab CEFAS in Lowestoft”, says Ms Hampton. “Other labs do meet the MMO criteria (including the EA lab) but these have not been certified yet. This meant that the sampling process was slow and expensive. Samples have to be taken, frozen and then couriered to the lab whilst still frozen in containers and iceboxes issued by the MMO. Peter Cowburn from Tom & Bourne and Partners did this for us and personally delivered them.”

The biggest constraint on moving the silt is the small size of the entrance lock. Maximum dimensions permitted in the lock are 24m length x 5.5m beam x 1.8m draught. Barges needed to move silt out would be too small for onward transport so there is a need to transship to bigger barges moored outside the marina.

“This will be the cleanest but most expensive option,” says Mrs Hampton. “We may need to set up a work station in the Thames because the river wall dries out at low tide. The small barges only have a small time frame to move in and out of the marina, so we might need to have quite a few barges to maximise disposal.”

On average the lock gates are only open one and a half hours each side of high water. The maintained waterline within the harbour is kept at 2.5m above the cill; high tide can be above this height but sometimes it doesn’t reach this level.

Using road vehicles to transfer from dockside to the outer quay would be messy, smelly and disruptive.

Other factors include a working hours restriction on behalf of residents, with works limited to 8am – 6pm on weekdays, 8am - 1pm on Saturdays and none on Sundays or Bank Holidays.

A PLA River Works licence will be required if a platform/mooring pontoon is needed for the barges. The local Council (LBH&F) will also have a say about works ashore when Construction Design and Management Regulations come into play.

After the permits and permissions are all in place there is still the practical problem of what to do with the 40-odd boats moored in the marina. Most will be shifted from their berths in rotation and rafted up within the marina, although some might need to be moved out into the river.

Using WID would be the least disruptive procedure but even that would take about three months. Other methods would be more prolonged.