ENSURING MINIMAL DISRUPTION TO WILDLIFE ECOSYSTEM DURING FLOOD PROTECTION SCHEME

One of the largest flood defence projects ever carried out in Scotland, the Hawick Flood Protection Scheme’s aim is to protect the town against major flood events. But when it came to a project on this scale, we needed to adopt measures to minimise the impact on the river environment and this meant a third of our work could only be carried out in the summer months. Our approach to mitigating the impact of our flood works on the Rivers Teviot and Slitrig, a rich habitat for a number of important fish species including spawning salmon and sea lamprey, requires us to take an ecologically sensitive combination of seasonal work and silt control interventions.

For this highly complex and challenging flood alleviation scheme which is due for completion by the end of 2023, we are carrying out six kilometres of flood works, two kilometres of which have to be constructed from the river channel in the period between 1st May – 30th September in the summers between 2020-2023. This was a contract constraint placed by our client Scottish Borders Council on the back of guidance and requirements from Scottish Environment Protection Agency (SEPA) and from regulatory authorities such as the River Tweed Commission, which manages salmon and other freshwater fish such as trout and sea lamprey in the river. The seasonal restriction was required in order to protect the freshwater fish as they travel upstream, with spawning occurring in winter months from mid-October and the hatching of eggs in the spring.

Getting it right

All of McLaughlin & Harvey’s method statements for working in the river had to be approved by the statutory bodies, along with the employer before we could commence work. Following initial delays to commencement due to the Covid-19 pandemic, works commenced in June 2020 with the first couple of months reviewing and agreeing the proposed methodologies with our subcontractors and our supply chain. We lost much of the first 2020 river season, due to the Covid delay, necessitating the completion date being extended from December 2022 until September 2023. This has allowed us a third river season from the first of May until the end of September 2023.

Our construction work could only start in earnest in August 2020 where we overcame challenges from the outset, due to very bad weather at the end of the month when the river rose to about two metres, significantly higher than the summer average of half a metre deep. The impact of the high water led to our temporary platforms in the river being partially washed away. It became a trial run to determine what we were up against and led to changes in our methodologies.? We adopted the lessons learned in the first summer to better plan the works for summer 2021. The team only had a five-month period to build the flood during each summer season, but fortunately the river in the summer of 2021 and 2022 was at record low levels.

In the summer of 2021, we created much more robust platforms with temporary sheet piles on the edges which protected them from high flow events.? If the water rose over the platform, then foreign aggregates would not enter the river. This was something that met the approval of SPEA and River Tweed Commissioners.

Curtains to silt

With stone platforms, it's very difficult to prevent silt and dirt coming off the stone and then flowing downstream. As a proven prevention method, we deployed a series of silt curtains with straw bales and gabion baskets. The straw bales are wrapped in a geotextile to catch all the silt that we generate from these temporary works. Once we had the stone platforms in place, we could then bring in a 50 Ton piling rig to install the temporary piles. In short, we created a three-sided cofferdam which protects our platform and allows us to install the permanent works by mitigating the risk of causing silt release into the river. Our temporary piles that we have around our platform tend to trap the silt that would be caused by excavating the floodwall foundations. The permanent piles can be driven up to 13 metres deep into the riverbed which can cause silt to rise in the adjacent riverbed. Our silt curtains are designed to catch any of that disturbance downstream.

One of the biggest issues we are continuing to face is digging below the riverbed to create the pile caps for the floodwall. This can raise an excessive amount of silt but our silt barriers and temporary piles will effectively contain 95% of that silt. We also have pumping arrangements set up and we can pump the dirty water from the excavation through silt busters and pH balancers when we're pouring concrete to try and mitigate any release downstream. ?

Engaging communication

For all the work that is carried out, we provide weekly reports to SEPA in collaboration with the client and designers. Regardless of the time of year, our work also avoids key spawning areas as vibration from piling and any silt disturbance could have a negative impact on fish spawning.

Due to our best practice approach, SEPA and the River Tweed Commissioners have allowed us to leave some of our temporary platforms in place during the winter provided they were fully protected from washout during high flows and flood events.

At a time when climate change poses new pressures for water management and has necessitated flood prevention measures at the scale seen in Hawick, we must also ensure the impact on freshwater ecosystems is kept to an absolute minimum. With the project due for completion at the end of 2023, McLaughlin & Harvey continues to work hard for its clients and the Hawick community.? Our collaborative approach, methodologies and direct engagement with SEPA during the construction of this state-of-the art flood protection system will ensure the river’s rich ecosystem will remain healthy for the long term.