'Leaky dams', (also known as Engineered Log Jams), are natural dams made from tree trunks, back-pinned and encased in spilled living willow. They are designed to allow water to pass in low flow, but in flooding conditions temporary hold back and store flood water that would otherwise travel downstream. Together, all the dams installed back-up the equivalent of an Olympic swimming pools volume, around 2,500m3 or 2,500,000 litres!
Both catchments have experienced flooding in recent years. Great Sutton in Ellesmere Port recently flooded, whilst Blackbrook in St Helens has flooded three times since 2000 – with the last event on Boxing Day 2015 (Storm Eva). Both brooks are far from functioning natural ecosystems, and fail EU directives for near natural conditions in the water environment.
Existing leaky dams in the Sankey Valley (St. Helens) have been shown by Newcastle and Liverpool University to reduce phosphate concentration, as well as trap sediment, that could otherwise further reduce watercourse capacity downstream. Excessive topsoil loss into rivers can cause so-called 'muddy floods' which can deposit thick layers of mud during flooding, causing damage to property.
Both catchment areas are developing innovative and proactive methods to improve water quality and reduce flood risk, with the community onside. The wider Natural Flood Management (NFM) initiative in Blackbrook, St. Helens is featured in the forthcoming launch of 'Working with Natural Processes, Evidence Base', an Environment Agency initiative to share and promote the evidence behind NFM in the water environment.
Meanwhile, the wider Sankey Valley initiative was presented in Parliament last year, with Local Authorities in the catchment signing-up to a trans-boundary catchment plan – believed to be the first of a kind in the UK.