Quantitative comparison of slope and river sediment dynamics in response to an upland extreme flood event

The aim of this project is to make a quantitative assessment of slope and river sediment dynamics in response to an extreme upland flood which occurred in the Coledale Valley in the Lake District, Northern England on 23-25th October 2008. This event caused widespread disruption over much of the region, and captured national attention when extreme weather conditions prompted a full scale emergency response to extreme local and regional flooding; and widespread erosion of hillslopes and river channels. Erosion during extreme events in upland and mountain regions dominates erosion over the short-term and conditions longer term landscape development. However, little is known about these events and in particular, the relative contribution of slopes and river channels to erosion. This is important because future climate change scenarios suggest an increased frequency of high intensity storm events which can remobilise old contaminated sediment stores typical of many upland industrialised landscapes. Quantifying erosion during extreme events is therefore crucial so that effective hazard management can be undertaken in the short term and long-term spatially targeted management strategies devised. When documenting flood events an urgent response is required so that key evidence of the flood is not lost as subsequent rainfall erodes landslide scars and debris deposits; and floodplain high water marks (wrack lines) and sediments are rapidly masked by vegetation or disturbed by clean up operations. Capturing 'fresh' evidence means that processes can be catalogued directly and not inferred at a later date. The project is split into three phases: intensive fieldwork; data compilation and management; and GIS analysis/sediment budget construction. This will enable us to (1) clearly establish linkages and pathways of sediment flux in upland catchments; (2) determine the relative importance of bare and vegetated slopes in contributing sediment to upland river channels; and (3) evaluate the significance of storage in controlling the downstream flux of sediment. The project will build on previously funded NERC research in the same catchment by using existing data sources and scientific understanding to evaluate the significance of this event. This includes a unique catchment geomorphic baseline survey, of a similar large flood from January 2005, which already includes a GIS describing the topography, soil, geomorphology and vegetation of the catchment and an extensive data base of soil properties (geotechnical properties, bulk density, etc.). The occurrence of two very large upland floods in 2005 and 2008 in the same catchment allows direct comparison between the floods in the same valley in terms of sediment dynamics and the persistence of flood related geomorphic features in the landscape. In particular we can identify useful characteristics for differentiating landslide scars of differing age which are closely clustered in time. Establishing 'best practice' for identifying evidence from a single flood is very important for extreme event analysis and statistics which rely on 'substantially complete' inventories of erosion features from single events.