Articles & Case Studies

The key to replacing the lock

Posted: Monday 9th November 2020

When a lock landing needed replacing on the picturesque River Great Ouse at Hemingford, construction project consultants JN Bentley together with the Environment Agency were quick to engage The Rothen Group’s services. However, following extensive rainfall resulting in navigational restrictions and flooding while work was underway, the company had to go above and beyond to resolve these flood-related issues and complete the required works.

The River Great Ouse is a picturesque stretch of water in Cambridgeshire that provides a route between Hemingford Grey and Brampton. Nestled among a rural landscape with plentiful trees and picturesque pubs, it is a treasured and well-maintained waterway that is popular with a wide variety of boaters.

As part of an Environment Agency framework to maintain and upgrade the East Anglia waterways, JN Bentley enlisted the help of marine civil engineering and maintenance company The Rothen Group (TRG). As part of the planned upgrade, TRG were tasked with breaking out and removing an expired lock landing and installing a 25-metre-long purpose-built replacement.

This would involve de-vegetation and tree pollarding works adjacent to the site, installing 8.5m cantilever piles, fabricating new steel framing for GRP composite anti-slip decking, installing an anti-slip walkway, and shuttering and concrete a new walkway behind the new piling line. Mooring bollards needed to tie up craft waiting to use the lock would also need to be fabricated before being installed. Finally, a recycled plastic fender would need to be installed as a waler onto the front of the piles, which would enjoy a longer lifespan than a traditional timber model.

Rainfall-related issues

However, when Britain experienced higher-than-average rainfall earlier this year, these planned works were delayed three times due to flooding. Specifically, the weir adjacent to the lock could not cope with the amount of water flowing over it, meaning the lock needed to be reversed into a temporary weir to allow greater flow downstream, and preventing works continuing due to dangerous conditions.

This also presented a further concern – as the lock had to be reversed three times in a short period, a scour hole developed downstream, which could potentially undermine the adjacent lock and weir structure.

Paul Wallis, Project Manager at JN Bentleys, commented: “Because the recent flooding caused by the heavy rainfall earlier this year had created a scour hole, material could be washed out and deposited further downstream.

“This, in turn, resulted in a silt bar that restricted navigation of the channel and made it impossible to carry out the work planned on the lock. We needed the channel to be restored to its original condition so that work could re-commence replacing the lock landing stage as soon as possible.”

Specialised remedial work

With peak season also approaching, the need to carry out remedial works and get the channel operating again so the lock could be replaced in time was even more crucial. As such, TRG quickly set to work dredging the silt bar, redistributing material into deeper parts of the waterway where it would not cause problems and blockages, with gravel placed atop the arisings in a teardrop shape to create new fish spawning grounds.

TRG utilised its specialist crane boat to remove the dredged materials, which features a clamshell with a 12-metre reach and four hydraulic jack legs to create a stable platform from which works can be carried out. In addition to this, the boat was fitted with internal, sealed tanks to allow more liquid material to be transported, reducing time spent carrying out dredging operations.

Paul continues: “One of the main challenges of the project was logistics and the location of the works. This was because the development site was in a remote area, away from the main road. Therefore, the materials had to be transported up the channel. However, this required the machinery used to be compact enough to fit through a narrow lock north of the site and a medieval bridge to the south, while still possessing the strength needed to install large piles. TRG’s specialist machinery prevented having to bring in separate kit to carry out the additional emergency works, which decreased the amount of time the overall task took, allowing us to get traffic going through the channel again without a long delay.”

Futureproofing

The challenge of quickly preparing the channel for upcoming work and with minimal disruption and preventing this issue again also required TRG’s specialist machinery and expertise. With this in mind, the company used large stone rip rap between 500-700mm in size to fill the scour hole. The specific friction properties of these stones meant they would not wash downstream if the river levels rose again, futureproofing the waterway.

Due to the rural location and aspiration to preserve the surroundings, the materials were loaded off site and floated to the lock via pusher tugs and hoppers. A total of 130 tonnes of stone were then placed by the crane boat in the desired areas, providing a fully functioning channel that enabled the installation of the new landing lock.

Ian Rothen, Founder of The Rothen Group, comments: “High river levels can make maintenance and development work challenging or even impossible at times. This is why having specialist equipment that can be mobilised quickly is essential. It ensures that when the conditions are favourable, work can be done straight away and finished quickly.

“Our extensive fleet paired with our marine and civil engineering expertise allowed us to support this project while having the least impact on the surroundings possible. We were able to restore the channel and future-proof it to prevent the same issue from re-occurring. This allowed improvement work to commence without the worry of having to re-address the same situation again.”




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November 2020

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