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

Lead Research Organisation: University of Liverpool
Department Name: Geography and Planning

Abstract

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.

Planned Impact

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.
 
Description Extreme flooding is the world's most damaging natural hazard, with river flooding affecting >100 million people each year and is projected to increase in frequency with climate warming. A challenge for flood hazard managers and insurance industries is quantifying the magnitude and risk of extreme flooding. Estimating the probability of experiencing an extreme flood is hampered by short instrumental records that may not contain these floods. We address this problem directly by presenting the first quantified lake sediment reconstruction of flood frequency and magnitude for the last 500 years, which allows a more accurate estimation of the recurrence probability of rare floods and the magnitude of extreme flooding. We show for the current phase of devastating (1990-2016) flooding in the 350 km2 Derwent catchment (NW England) the 2009 event was the largest in >400 years and had an estimated recurrence interval far larger (1:9000 year) than revealed using short term gauged records (1:200 year). The 2009 flood is part of a flood cluster (1990-2016) that is unprecedented since 1515. The extended record shows that the association of floods with climate indices has varied over time, with the recent cluster associated with warmer Northern Hemisphere Temperatures and positive Atlantic Multidecadal Oscillation.

Our repeat surveying through 2016-2017 shows the deposition of the Storm Desmond flood and how that has settled into the sedimentary palaeoflood record of four lakes across the English Lake District.
Exploitation Route We are in discussions with the Environment Agency (UK) about incorporating our palaeoflood series into flood magnitide-frequency models.
Sectors Environment

 
Description The data are being used by flood managers and organisations in Northwest England
First Year Of Impact 2017
Sector Environment
Impact Types Societal,Economic,Policy & public services

 
Description BBC News article: Cumbrian lakes hold a centuries-long flood record 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Media Press release based on an interview given at the European Geosciences Union in April 2016
Year(s) Of Engagement Activity 2016
URL http://www.bbc.co.uk/news/science-environment-37304856
 
Description Cumbria Floods Partnership: Workshop January 2017 Rheged Centre Penrith 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Professional Practitioners
Results and Impact Improving the way knowledge and experience is shared to benefit all communities was a key aim of the Cumbria Floods Partnership, set up in the aftermath of flooding caused by Storm Desmond in December 2015. The Environment Agency invited the academics to the geomorphology workshop, one of a series of knowledge sharing events, which are being recorded for use with other groups. The aim is to share the latest findings and understanding of topics critical to managing flood risk with communities and organisations across Cumbria. Prof Richard Chiverrell, from University of Liverpool who presented at the workshop said: "Talking to people living in Cumbria today shows that communities are very aware we may be experiencing a flood-rich period, and the sediments in the region's lakes allows us to assess in the context of the last 600-700 years how unusual and extreme this recent flooding is."
Year(s) Of Engagement Activity 2017
URL http://www.cumbriacrack.com/2017/01/11/geomorphology-workshop-identifies-ways-improving-flood-water-...
 
Description International Business Times Article: Those calling for bigger flood defences don't understand the complexity of UK flooding 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact Requested article and interview on the Storm Desmond flooding
Year(s) Of Engagement Activity 2015,2016,2017
URL http://www.ibtimes.co.uk/those-calling-bigger-flood-defences-dont-understand-complexity-uk-flooding-...
 
Description The Conservation article: Under water again - when will Britain learn how to manage floods? 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact Requested article about the December 2015 flooding in northwest England
Year(s) Of Engagement Activity 2015,2016,2017
URL http://theconversation.com/under-water-again-when-will-britain-learn-how-to-manage-floods-52166