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An innovative approach to flood risk

Tom Palmer, project design engineer at Essex County Council (ECC), on a new scheme currently underway that will significantly reduce local flood risks by using an innovative approach to leaky dams.

The Kingsmoor area of Harlow in Essex has historically experienced flooding, with properties internally damaged and access to roads cut off. As part of its Capital Flood Alleviation Programme, ECC investigated the area further and proposed a scheme to reduce local flood risk.

Working in partnership with Harlow District Council, the Environment Agency and Thames Water, an innovative approach was devised involving the construction of eight leaky dams and pond desilting works within local woods to slow flows. These are combined with an earth bund in a playing field directly upstream of the residential area to provide larger floodwater storage.

The woodland areas of Parndon and Risden Woods are both ancient woodland and Sites of Special Scientific Interest (SSSIs), so a sensitive and sustainable approach was developed by ECC’s Place Services following consultation with Harlow District Council and Natural England to ensure compliance and minimise ecological impacts. To achieve this, dams were constructed by hand from logs sourced directly from the wood as part of its ongoing conservation management. These were moved across the woodlands and into position using heavy horses to minimise the impact on the trees and local wildlife and to avoid the use of heavy machinery.

As well as providing flood risk reduction benefits, the leaky dams will improve local biodiversity by creating new and diverse habitats. Newts, previously present within the ponds, had left the area due to the heavy silting, and by reinstating them it is hoped they will return along with other pond flora and fauna.

The earth bund and flood storage area are also within a local nature reserve and designated green belt land owned by Harlow District Council, so consultation with them has been important to inform the design and positioning of the feature to maximise storage and reduce environmental impacts.

Part of the proposed flood storage area directly affects the nature reserve, which consists of ancient grassland. To address this, rather than providing compensatory planting elsewhere, ECC is implementing a Grassland Enhancement Plan for the whole five-hectare nature reserve area to restore and improve its wildlife interest. This represents a best-practice approach to ecological mitigation, with long-term management sustained through the implementation of a conservation grazing scheme on the bund site following its completion.

The flood alleviation scheme is costing £440,000, with £42,800 being spent on the pond-desilting and leaky dams aspects. Direct financial contributions to the scheme have been received from the Environment Agency through flood defence grant in aid, with Harlow District Council facilitating the works and undertaking the future maintenance of the bund and ponds, and implementing the management plans.

The scheme will reduce flood risk to 38 properties within Kingsmoor. Construction of the leaky dams was completed by the end of November, with construction of the earth bund commencing in early 2018. The entire scheme will be complete and operational by spring next year.

Top image: Essex County Council





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