Our Work: Hemingford Lock Landing
Site: Hemingford Lock Landing
Date: January 2020 to May 2020
The Rothen Group were asked by JN Bentley and the Environment Agency to assist with upgrading a lock landing on River Great Ouse. The project involved:
Understanding access & logistical restrictions
Time & ecological constraints including fish breeding season
Managing the changing river conditions in heavy winter flooding events
Vegetation clearance works
Installing 8.5m cantilever piles off floating plant
Fabricating new steel framing for GRP composite anti-slip decking
Installing an anti-slip walkway
Shuttering and concreting a new walkway behind the new piling line
Fabricating mooring bollards
Fitting a recycled plastic fender
Emergency works as a result of flooding - installing rock armour in scour holes
When Britain experienced higher-than-average rainfall, the 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.
JN Bentleys, commented: “Because the strong streams 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 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.
JN Bentley continues: “One of the main challenges of the project was logistics and the location of the works. The site was very remote resulting in all materials needing to be floated to site. In addition the kit had to be small enough to fit through a narrow lock to the north & a medieval bridge to the south, whilst having the ability to install the large piles required. 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.”
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.”