River Sediments and Habitats and the Impact of Maintenance Operations and Capital Works

Duration: July 2004 – February 2008

Colin R Thorne (UoN), Roger Bettess (HR Wallingford)

Funding

Environment-Agency/Defra Joint Research Programme (Overall Project Funding £309,672, University of Nottingham component £48,750 plus VAT).

Project overview

This project is aimed at improving our understanding of the interactions of sediments, habitats and conveyance as affected by maintenance operations and capital works constructed for flood defence. The primary objective is to carry out field trails and demonstrations whose results can be interpreted to provide information on the self-regulatory nature of conveyance response, effective river management and new approaches to maintenance and channel design, including adaptive management for flood control. At present the interaction between flow, sediment and habitats is poorly understood. If this understanding could be improved then it should be possible to integrate this knowledge into improved approaches to operational maintenance. This should lead to channels that operate satisfactorily in terms of water flow, sediment transport, morphology and habitats while providing the required standard of flood defence with minimum maintenance. The purpose of the project is therefore to improve our knowledge of the interactions between flow, sediments, habitats and the hydromorphological impact of channel maintenance. The project study sites cover a wide range of maintenance practices and stream environments in the Rivers Dearne, Eden, Harbourne, Kent and Long Eau. The improved knowledge of sediments and habitats and the effects of maintenance/capital works on them arising from this project will enable design engineers and managers to work better with the systems, leading to more sustainable management methods. This will be important at both the strategic and local scales. More targeted maintenance would not only support improved ecological quality but would also have economic and operational benefits.

Research outcomes

Under the Water Framework Directive (WFD) there is a requirement to put in place programmes of measures by 2015 to achieve ‘good ecological status’ for all water bodies, with the exception of ‘heavily modified water bodies’. There is also a requirement for no degradation. It is imperative that if these responsibilities are to be met, there needs to be far greater understanding on the effects of maintenance on sediments and habitats, and ultimately the biology that the habitats support. Through greater understanding of the inter-linked processes, flood defence management could, in the future, hold the key to achieving sustainable river rehabilitation through management practices that encourage natural processes to sustain both habitats and conveyance. The benefits to the Environment Agency of the project are therefore that it should be possible to deliver present Standards of Service in terms of flood defence and land drainage while reducing impacts on channel eco-systems and prompting habitat rehabilitation through natural processes to meet WFD requirements by 2015.


Operational maintenance often involves mechanised weed cutting across the full width of the channel that damages aquatic ecosystems.

Research conducted during this project using the new EA/Defra Conveyance Estimation System (CES) shows that it is only necessary to cut weeds over part of the channel width to achieve the necessary standard of flood capacity. This can often be achieved by hand cutting – reducing ecological impacts and carbon emissions.

Use of heavy machinery to dredge or desilt channels destroys valuable habitat features such as shoals, bars and sediment berms.

Research conducted in this project suggests that the extent and elevation of sediment bars and other features becomes self limiting when they are left to evolve naturally, reducing or even eliminating the need for maintenance dredging or desilting.