When erosion threatens to undermine a structure the logical solution might be to construct robust defences to repel the threat. But wouldn’t a better solution be to remove the threat entirely?
In the case of coastal erosion that is simply not an option, as the Danish king Cnut famously demonstrated over 1,000 years ago.
But consulting engineer Baker Hicks has recently shown that a lesser threat – in this case a mountain stream – can not only be relocated, but that significant environmental benefits can result.
Cumberland Council appointed Baker Hicks, through its professional services framework, to find a solution to erosion damage that risked undermining the carriageway on the A5086 connecting the towns of Egremont and Cockermouth.
A 15% increase in rainfall in the region over the past 10 years had resulted in changes to the alignment of a small stream running alongside a 190m-long stretch of the road at Scallow Farm. As a result the road and its associated infrastructure was at risk of collapse.
“The watercourse had moved closer to the road over time and was causing under-scour to the embankment,” explains Baker Hicks associate
director Nick Wood. “This had caused a longitudinal crack to start opening up in the road surface.”
Ground investigations were carried out to ascertain the existing ground conditions and help identify the key parameters for creating a stable embankment.
“The ground conditions comprised mostly alluvium and areas of made ground with silts and clays overlying a bed of limestone at a depth of about 5m,” says Wood.
In a situation such as this, where erosion has weakened the embankment supporting one side of a road, the solution is usually to install some form of defence. In this case, Baker Hicks considered two main structural remedies: a row of steel sheet piles installed parallel to the carriageway or alternatively a supporting structure comprising stone-filled gabions.
“But there was another alternative, which was to realign the watercourse away from the road, and I’m glad we chose that option for a number of reasons,” says Wood. “First of all it’s a long-term solution – over time, sheet piles and gabion baskets would suffer from corrosion. But also relocating the watercourse allowed us to introduce more biodiversity. If we’d left the watercourse alone it would just be another ditch alongside a road.”
While the primary focus was to ensure the stability of the road for many years to come, realigning the watercourse gave Baker Hicks the perfect opportunity to introduce additional sustainable benefits into the area.
“This included promoting habitat creation, watercourse geomorphology, marsh areas doubling up as attenuation, and specifying locally sourced materials, especially for the pebble beach areas which attract specific types of nesting birds,” says Wood.
Baker Hicks used 3D modelling to design the watercourse realignment and ensure it worked with the local landform. Open-channel flow analysis identified the stream’s capacity and Hjulstrom Curves were employed to engineer and reinforce the stream-bed.
The Hjulstrom curve, named after a Swedish geographer, Filip Hjulstrom, is a simple graph that allows hydrologists to determine whether a watercourse will erode, deposit or transport sediment. “It’s basically a tried-and-tested method used mostly by lazy engineers to work out the behaviour of sediment in a given water velocity,” says Wood.
“It gives you a feel for the size of the particles that should stay in place on the bed during high rainfall.”
Armed with this information, Wood and his team began designing the new stream-bed. Rapid flows cause most erosion and so it was important to design the new alignment to control the flow and minimise the risk of scour.
Natural features, such as ponds, marshes, and gentle slopes were introduced to help manage water flow, slowing it by 40% and, consequently, reducing the risk of flooding. The designers restored the stream's natural shape with winding paths and strategically-placed boulders, along with specially designed ‘dragon’s teeth’ stones to create small waterfalls and cascades, acting as a kind of ‘traffic-calming’ for the watercourse.
These additions not only control water movement but also help oxygenate the water, encourage fish to spawn and promote a healthy ecosystem. Using its in-house carbon assessment and measurement tool, CarboniCa, Baker Hicks has calculated that prioritising flood prevention and enhanced environmental benefits, has reduced the project’s whole life embodied carbon by more than 5%.
Besides designing the new alignment, Baker Hicks also drafted the client’s tender documents to appoint the main contractor, Cumbria-based Metcalfe Civil Engineering.
Metcalfe detailed the scope of the work to be undertaken and designed a series of drainage improvements for the road, including several additional gullies and drainage outfalls.
Letting nature take its course
Creating new ponds, restoring bogs, rewilding rivers and even reintroducing beavers are among the methods being recommended by the Royal Society of Wildlife Trusts (RSWT) to reduce flood risks.
New research by consultant Stantec on behalf of RSWT and RSA Insurance, concludes that every £1 invested in natural flood management (NFM) is likely to deliver £10-worth of benefits over 30 years.
Besides the devastating impact it can have on individuals and communities, flooding is also the UK’s most expensive natural hazard, costing around £2.2bn annually. This cost is projected to rise by up to 49% by the middle of this century according to the UK’s latest climate change risk assessment.
Data collected by the Environment Agency has already shown that natural flood management is effective at reducing the overall damage from flooding. RSWT says that its new research has gone further to focus on the wider benefits of NFM, such as better habitats for wildlife, carbon storage and improvements to health and wellbeing.
The RSWT report looks at 10 natural flood management schemes created by individual Wildlife Trusts. Collectively, these schemes are calculated to have an average cost-benefit ratio of 4:1 over 10 years, rising to 10:1 over 30 years.
“One in six houses across the UK is currently at risk of flooding and climate change is leading to more frequent and heavier rainfall – and we know that this will become more severe in the future,” says Kathryn Brown, director of climate change and evidence at the Wildlife Trusts.
But she adds: “The good news is our research proves that restoring natural habitats can help us tackle the effects of climate change – and in doing so, help reverse nature declines.”
Despite the perceived benefits, public funding for NFM makes up just a small proportion of overall flood risk mitigation, according to the Wildlife Trusts. The Environment Agency’s £25m natural flood management programme is currently the biggest public fund for NFM but comprises less that 1% of the total flood and coastal erosion risk management budget for the next 12-month period.
Private investment in NFM is also very low, due to a number of barriers and a lack of confidence in the market potential, says the RSWT: “Unless these are removed and confidence is increased, perhaps through increased public funding, private investment will remain limited,” it says.
One private investor with a vested interest in promoting natural flood management is RSA Insurance. Chief executive Ken Norgrove says: “RSA has been on the frontlines of extreme weather events alongside our customers for decades and sees the devastation they can cause. Our teams are often on the ground within the first few hours of a flood helping people, business owners, and communities recover and rebuild.
“Climate change will continue to be a defining issue of our century, so it’s important for society to prepare, adapt and become more resilient. By investing in innovative natural flood management solutions, we can help safeguard people, businesses, and the environment for a sustainable future,” Norgrove says.
The report, Assessing the multiple benefits of natural flood management, makes three key recommendations:
First, an improved data and a standardised approach to project design, monitoring and valuation to increase investor confidence. Second, it says that creation of a government-led framework, including support for private finance markets and improved data collection, is essential for increasing investment.
And third, the private sector, including insurers, should be encouraged to recognise the broader economic and environmental benefits of taking an active role in natural flood management.
“Natural approaches to water management should be government’s first port of call wherever appropriate – and we need to see such benefits regularly accounted for, measured and valued by both the government and private sector in future,” says Brown.
Natural flood management can include:
•Planting trees, shrubs and hedgerows to capture rainwater before it hits the ground, slow overland flow and increase water storage below ground.
•Reintroducing beavers. Their activities store water in the landscape through the construction of dams and lodges and the digging of channels.
•Creating ponds and wetlands to store water during wet periods.
•Restoring peat bogs which can hold vast amounts of water.
•Regenerating soils and wilder grasslands. This improves soil structure, allows rainfall to penetrate the surface. Rough, tussocky grasslands are more effective at storing water than intensively managed grasslands.
•Constructing leaky dams or log diverters to release water slowly and to divert waters onto flood plains rather than straight into a river.
•Re-engineering or modifying a river’s course to restore its natural meandering shape, or to improve its ecological health, mitigates high flow events.
•Sustainable drainage schemes (SUDs) such as de-paving, ponds or vegetated ditches (bio-swales) increase water infiltration in urban areas.
Got a story? Email news@theconstructionindex.co.uk
