Immediate geomorphological impact and fluvial system response to the Cumbria floods November 2009

Lead Research Organisation: Durham University
Department Name: Geography

Abstract

Between November 18th - 20th 2009 Cumbria, NW England was hit by an exceptional precipitation event producing an unprecedented total of 372 mm of rainfall. Over the 24 h period between November 19th-20th alone, 313 mm of rain fell at Seathwaithe. This was equivalent to a month's precipitation and set a new UK record (Met. Office, 2009, previously 279 mm). The flood was the biggest event recorded in this region and had major consequences for population and infrastructure over a large area, as well as involving loss of human life. Due to its system-wide impact, the November 2009 flooding in Cumbria presents an unparalleled opportunity to gain insight into the controls on, and impact of, an extreme flood within upland, lowland parts of a large catchment. This project aims to examine the whole river corridor of the river Derwent from the headwaters, through Bassenthwaite Lake to the Lowland agricultural/urban catchment identifying key impacts and linkages. Quantifying erosion and sedimentation during extreme events is crucial so that effective hazard management can be undertaken in the short term and long-term spatially targeted management strategies devised. To achieve this, field-data must be collected in the immediate aftermath of the event to ensure essential features of the event are fully documented. Our main objectives are to produce a rapid, structured field inventory of the immediate geomorphic impact of rainfall and flooding on fluvial systems within the River Derwent river corridor along a headwater to coast transect; provide a rapid geomorphic assessment of the role of scour and sedimentation in the vicinity of bridges to discriminate factors influencing bridge collapse and so inform future design practice; identify areas of risk for remobilisation of flood-deposited sediments; delivery of hillslope sediments and hotspots of erosion in the River Derwent to identify zones at risk from future flood events and; work with project partners (EA and LDNPA) and local stakeholders in designating catchment zones that are 'at risk' from erosion and sedimentation.

Publications

10 25 50
 
Description With regard to the original project research objectives our main achievements and outcomes are:



O1. Produce a rapid, structured field inventory of the immediate geomorphic impact of rainfall and flooding on fluvial systems within the River Derwent river corridor along a headwater to coast transect. This includes reconstruction of flood hydraulics and peak flow discharge for ungauged sections of the river (in the absence of stage records due to destruction of gauging stations), using field data and high resolution LiDAR.

We have completed structured inventories of geomorphic impacts at representative sites along the river profile from the headwaters to the lowlands. This is a novel dataset comparing flood impacts and their continuity along the river continuum. The spatial inventory in the headwaters (above Bassenthwaite Lake) was restricted due to lack of post flood imagery.

O2. Provide a rapid geomorphic assessment of the role of scour and sedimentation in the vicinity of bridges to discriminate factors influencing bridge collapse and so inform future design practice.

Work is underway to assess the flood impacts at bridges as this was the most significant impact on infrastructure by the flood. Through co-operation with Cumbria County Council we are investigating the geomorphological conditions that may have lead to bridge damage and assessing the historical context of such impacts.

O3. Identify areas of risk for remobilisation of flood-deposited sediments; delivery of hillslope sediments and hotspots of erosion in the River Derwent to identify zones at risk from future flood events.

Our work has identified along the lower Derwent flood corridor areas of erosion and deposition. This information is being used to map the risk of further instability related to these flood deposited sediment and unstable river reaches. Channel modifications (post- flood) have complicated this process.

O4. Compare post-flood field-derived elevation data, aerial photography and LiDAR data with pre-existing surveys to quantify/characterise areas of greatest geomorphic change.

There were significant challenges in assembling a pre- and post-flood image and digital terrain model mosaic. Three areas of interest were processed early in the project and results presented a t a number of conferences. However, assembling the full data mosaic for the river Derwent downstream, of Bassenthwaite Lake has taken longer than expected to process because (i) each of the 12 LiDAR strips needed to be carefully matched to each other and the ground control data, and (ii) existing software would not allow 350 high definition aerial photographs to be automatically referenced to the LiDAR-derived digital elevation model. This processing is now almost complete and will allow a full comparison of pre- and post flood impacts (erosion / deposition patterns, volumes and impacts on land cover and infrastructure. Making full use of the imagery from the UAV survey required innovative data processing and extensive validation. This has delayed the interpretation, but the results have exceeded the original brief (aerial photographs) to provide LiDAR quality elevation models and orthophotographs.
Exploitation Route Working with QuestUAV to disseminate best practice for UAV image processing and platform development for next-generation instrumentation. The main beneficiaries of this research will be the Environment Agency (EA) and the Lake District National Park Authority (LDNPA) (as the principal land manager in the Upper catchment).

UAV processing - other academics, industry (primarily consultancies: monitoring, forestry, agriculture)

Other key users are Flood Action Groups (FAG) in the catchment affected (Cockermouth, Keswick already involved with the project) and lessons passed to other FAGs in northern England via a one-day workshop (Living with Floods) held at Newcastle University in February 2012.
Sectors Environment

 
Description Publications from this work are still only in draft form. There potential impact will be providing local stakeholders (National Park, National Trust, United Utilities, Environment Agency) with the geomorphological knowledge to support future decision making.
First Year Of Impact 2014
Sector Environment,Leisure Activities, including Sports, Recreation and Tourism
Impact Types Policy & public services

 
Description Cumbria Floods Geomorphology Knowledge Sharing Event 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Knowledge sharing event between scientists and practitioners involved in the 2015 Cumbria Floods recovery
Year(s) Of Engagement Activity 2017