Our Work: Changing the Shape of the River Derwent, Derby

Site: River Derwent, Derby

Date: August 2024

Introduction

Following the storm seasons of 2023 & 2024, the EA set out to review its impact on the UK’s waterways. The River Derwent in Derby was flagged after a sonar scan of its navigation identified a scour hole – a direct result of heavy rainfall pushing sediment downstream.

A lack of material in this area was causing the surrounding bank to become increasingly vulnerable, making it susceptible to undercutting erosion and imminent collapse. More worryingly, the infrastructure that was under threat included a sewage treatment plant and an ambulance depot – collapse of these areas would cause critical harm to water safety and local life. Which is why the EA made this project an urgent priority and enlisted Jackson Civil Engineering to revise the shape of the river to divert water flow away from the high priority infrastructure.

Location of the scour hole was sandwiched between buildings and a wall of greenery, meaning the only way this project could be completed is by machinery on water. With this in mind, Jackson Civil Engineering reached out to The Rothen Group for boats, experience and expertise.

The Project

• High risk flood prevention scheme to be completed within a narrow timeframe

• Large scale mainline dredging project over 6000 tonnes

• Extra measures taken to work around multiple invasive species

Approach

The plan outlined by JCE included the removal and disposal of approximately 4,370t of silt and the redistribution of 2,000t – but in order to complete this, over 3500m² of vegetation had to be removed first. This wasn’t a straight-forward job, however, since there were large pockets of Japanese Knotweed and Himalayan Balsam within the overgrowth, requiring our team to undergo a check, clean and dry policy to prevent any further spread.

Once the vegetation was removed off-site, our team began the dredging process. With a long reach 40t on the bank along with 8t diggers on our wide beam digger pontoons and two pusher tugs to transport five 35t hoppers back and forth to the offload point, we made light work of such large quantities. A safety boat was also used as the works were upstream of a weir.

The only speed bump our team encountered was the presence of Signal Crayfish, which required us to carefully monitor each hopper that was being filled and ensure that the invasive species didn’t re-enter the waterway.

With our team’s works complete, a straighter waterway has been created, preventing the growth of the scour hole.