Geomorphic and sedimentary evolution of an extreme event: testing a sediment-based palaeoflood record

This project will use the December 2015 extreme flooding in Cumbria to critically test the integrity and responsiveness of our unique, long (>600 year) lake sedimentary records of flood magnitude and frequency by quantifying the initial sedimentary event, its' diagenetic evolution and the impacts of a sediment-rich post-flood catchment. The recent flooding, following previous extreme flood events in 2005 and 2009, has highlighted the inadequacies of flood magnitude / return-frequency models developed using recorded river flows (30-50 year), which are too short to address societal requirements for accurate measures of flood risk and to address questions regarding the role of climate forcing of recent events. Lake sedimentary archives developed by our research teams at Liverpool and Durham Universities provide the length of record necessary to address this research gap. This includes a sediment magnitude (event particle size) signature at Bassenthwaite that correlates well with river discharges (for 40 years) and identifies that the largest 3 floods in 600 years have occurred in the past decade.

We have three specific aims:
1. To establish the deep water sedimentary event signature for lakes with differing water retention times (21-350 days) and to use repeat sampling into the summer period of peak organic productivity in the lake to test the evolution of this and high magnitude events in the recent past (11/2009) into the palaeoflood record.
2. To establish and monitor how post-flood conditioning of catchment-to-lake and within-lake processes modifies or augments the lake sediment flood signal for 2015 event.
3. To improve confidence in our existing palaeoflood series by placing this recent extreme event in that longer term context, and to establish flood frequency models that use integrated sediment and river flow data.

The proposed project will use the unique opportunity afforded by contrasting pre- and post-flood sedimentary records and fluxes to establish the sedimentary signature of one of NW England's most extreme floods; crucially to engender confidence in the use of our lake sediment records as genuine flood series, thus contributing to calls for enhanced flood records to accurately define flood risk in these and potentially wider UK and world river systems. Our team has existing sediment traps at Brotherswater and Buttermere, together with lake sediment surface gravity cores sampled before the flood (2012-2015) at key sites impacted by the recent events in the Eden catchment (Brotherswater and Ullswater (Glenridding)) and Cocker-Derwent catchment (Bassenthwaite and Buttermere).

Our research methodology is designed to characterise the sediment signal (grain size, geochemistry, mass accumulation) of the flood deposits and repeat this sampling at 6 week intervals to establish the progression of the event into the palaeo-record. We will also monitor the post-flood sediment dynamics in the lake (monthly sediment trapping and continuous turbidity monitoring) and the catchment to quantify the supply of sediments from catchment slopes (6 weekly UAV drone surveys) through key nodes in the sediment cascade to the lake (source-to-sink). This will enable us to contrast the sedimentary characteristics (grain size, geochemistry) of the December 2015 flood with events in the longer palaeoflood record, and incorporate these data in our novel flood frequency modelling using combined sediment and river flow records.

This provides an unparalleled opportunity to investigate directly recent flood sedimentation across a variety of lake environments. Uniquely, we have the opportunity to contrast the pre- and post-flood sedimentary records and place our developing long-term data on both flood magnitude and frequency on a robust footing, which will allow us at our stakeholder workshop to encourage national organisations, hydrologists and engineers to use these records with confidence in flood risk management.

Who might benefit from this research?
The underlying goal of this research is to rigorously test a new methodology which analyses longer-term lake sediment archives to produce more realistic appraisal of flood risk. Here we explore the impact of the December 2015 extreme floods in northwest England on lake palaeoflood sedimentary archive in four Cumbrian lakes. The December event provides us the unique opportunity to rigorously test this approach and communicate our findings to the wider academic and stakeholder communities. Our approach has strong cross-disciplinary links due to the applied nature of the work in better understanding flood hazards.
The main beneficiaries of this research will be the Environment Agency (EA), National Trust (NT), United Utilities (UU) and the Lake District National Park Authority (LDNPA) (as the principal land managers in the flooded catchments). These key stakeholders, including the Centre for Ecology and Hydrology (CEH), have supplied letters of support for the project; we seek to disseminate results from the project to the organisations and communities affected by the 2015 flooding. CEH, LDNPA, NT, UU and EA will be able to use knowledge from this work to assist in planning and future policy on the use of upland catchments. Furthermore, by examining erosion and sedimentation patterns down the whole river system it will be possible to identify sensitive areas of the catchment for targeted engineering/restoration strategies. In particular there will be a focus on fine sediment sources within lake catchments. This is particularly important in light of the Water Framework Directive (WFD) and needs of good ecological quality.

How might they benefit from this research?
Stakeholders in the region will benefit from improved prediction of future climate impacts on in-stream, riparian, floodplain and lake habitats and their likely impacts at ecosystem scales. Potentially, results from the research can also feed into wider initiatives regarding landscape sensitivity and management in the face of wider climate change. One of the Department for Environment, Food and Rural Affairs (Defra) priority topics under their cross-regional research programme 'Adapting to Climate Change' involves water resources, recognising implicitly that greater year-to-year variation in climate, new patterns of rainfall and drought, and changes to seasonal river flows will affect a wide range of 'water users' as well as residents in urban areas downstream. Further benefit is to the public at large is through provision of knowledge to promote biodiversity and maintain healthy ecosystems, most specifically for river and lake users including anglers and for public bodies such as Natural England. Local interest in these issues is already very acute.
The principal means by which we will engage with our key groups is by including them as stakeholders very much involved with the research. This is important as it ensures input during the development of the proposed research and guarantees engagement with these organisations immediately. In addition we will hold a three-day Stakeholder "After the floods" Conference where we will bring together scientists, practitioners and local community representatives to review evidence and approaches to managing future floods and post flood clear up. Output would be a discussion document and suite of presentations that would be freely available through the project website. The project web site will provide detail of our methodology / approach and data summaries for sharing with the academic community.