Large woody debris -A river restoration panacea for streambed nitrate attenuation?

For centuries, large woody debris has been removed from UK and European rivers to reduce potential flood risks caused by these obstacles and blockage. However, the removal of large woody debris has been shown to severely impact on ecosystem services provided by rivers, including habitat functioning and the potential to reduce nutrient and sediment loading by acting as both a physical and biochemical barriers. This caused detrimental impacts on in-stream and floodplain biota as well as society in the provision of clean drinking water.

Recent EU and UK environmental policies (notably the European Water Framework Directive) are promoting a significant shift in management practices, strongly emphasizing the reintroduction of in-stream wood to improve the resilience of lowland river ecosystems to environmental change by increasing their capacity to attenuate excess nitrate pollution. As obstacle in the river, woody debris is expected to increase the down-welling of surface water into the streambed and enhance its contact time in the very reactive streambed zone, resulting in increased chemical turnover and nitrate attenuation. However, the impacts of large woody debris on the main drivers of biogeochemical processes in the streambed, the exchange fluxes between groundwater and surface water as well as the residence time of water in the streambed are poorly understood. To improve design of large woody debris structures to efficiently remove nitrate from the streambed, river basin management and restoration programmes require scientific evidence for the effectiveness of different large woody debris designs for water quality improvement.

This project will therefore provide the scientific evidence required for assessing the efficiency of different permanent and mobile designs of large woody debris for enhancing the uptake of nitrogen in lowland rivers. Our research focuses specifically on lowland streams as these represent the majority of UK rivers under threat of critically high nitrate concentrations. The findings of this project will directly inform river restoration practice and decide whether, to what degree and with what design, large woody debris will be deployed in UK lowland streams to reduce critically high nitrate loads.

We will combine novel monitoring techniques and environmental tracer technologies with innovative numerical modelling approaches to identify the occurrence of streambed hotspots of increased biogeochemical turnover that are facilitated by large woody debris. We will quantify to what degree excess nitrate concentrations can be reduced in this reactive hotspots and what different designs of large woody debris structures best facilitate this ecosystem service. We will improve sophisticated numerical models to quantify how effective permanent and mobile large woody debris may attenuate excess nitrate from lowland rivers. By developing and simulating a set of scenarios, assuming wider ranges of environmental conditions then observed experimentally, we will predict how efficiently different large woody debris structures in lowland rivers can reduce nitrate pollution under changing environmental conditions.

Knowledge of how in-stream wood facilitates the occurrence of biogeochemical hotspots will assist river managers in adopting techniques which deliver benefits to both in-stream biota and water quality for human uses. This project, twinned with the proposed policy advice documents we will develop, will provide an ideal opportunity for the formation of science-based restoration activities which are able to deliver quantitative water quality improvements for little to no additional cost.

This project will create direct academic and societal impact by providing scientific evidence for the potential of LWD in enhancing streambed nitrate attenuation in lowland rivers. Our research results on LWD design-specific nutrient turnover will directly inform decisions of UK regulators and restoration agencies including EA/DEFRA, River Restoration Council (RRC), River and Wildlife Trusts on if, where and how to implement LWD as adaptation and mitigation strategies in national river restoration programmes. Direct links for policy implementation have been established by Co-I researcher M. Klaar as part of her Environment Agency secondment.

We will deploy an efficient knowledge exchange strategy for facilitation of the excellent research undertaken by an interdisciplinary team of expert scientists. The end-users and beneficiaries from the scientific community, industries, regulatory bodies and policy backgrounds are directly involved in the project planning and management and will be engaged via a project Advisory Board. The Advisory Board includes partners from the Environment Agency, RRC, Association of River Trusts, TrUck (Trees in the River Uck, a collaboration of EA/ Woodland Trust/ Wildlife Trust and SEW), South East Water (SEW) and Forestry Research (FR) and will meet for three main project management meetings and a tailored practitioners workshop. To optimise the scientific exchange with other UK funded research and to support the interactions between cross-theme scientific working groups, partners from collaborating CWC and other relevant projects e.g. NERC's Macronutrient Cycles Programme and Defra's DTC's will also be invited to the three main project meetings.

The international dissemination of project results will be achieved by publication in high impact scientific journals and presentation in international and national scientific conferences. These means of facilitation will be supported by the strong links to the UNESCO- IHP and FRIEND programmes (see letter of support) as well as the investigators' involvements in EGU, AGU, BHS, IAH and IAHS programme committees. The project's close links with the Birmingham Institute for Advanced Studies will support the dissemination of research to a wider scientific community as well as the interested public. The dissemination of the results will directly benefit from the UoB PI's and Co-I's expertise as working group leaders in the NERC funded Knowledge Transfer Network on "Groundwater-Surface Water Interactions and Hyporheic Zones", providing well-established links to regulatory bodies, water industries, environmental consultancies and public media.

A practitioner workshop for the design, installation and maintenance of LWD will be organised in association with the River Restoration Council to facilitate the technological developments and experiences of this project to scientists, regulators and river restoration managers.

A project website will support the efficient dissemination of the project's research results, featuring conference presentations of the project partners and providing access to freely available open source code and software produced by the project with add-ins of simplified examples. The website will support multi-directional knowledge exchange by including interactive features such as a project WIKI as well as pod- and vodcasts and a metadata catalogue. Data will additionally be made available via NERC data archives as well as through the communication gateways of project partners including the Wildlife Trust's Biological Records Centre and Environment Agency Central Data Bureau. Design outputs on LWD structures will be furthermore made available on the River Restoration Centre (RRC) web-forum.