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Monday, 18 February 2013

Next seminar: Monday 18th February

A Tale of Two Extremes- floods, droughts and salmon spawning habitat 
Ian Pattison, Loughborough University
Room 311, 4pm

The UK is experiencing a greater frequency of flow extremes, illustrated by the low river flows early in 2012 followed by widespread flooding. It is therefore key that floods and droughts are not investigated separately but studied as a continuum. The primary drivers of flow extremes are the climate and weather patterns, but the impact can be modified by catchment processes such as rural land management. A physically based continuous simulation hydrological model, CRUM3, is used to consider the impact of soil compaction and land cover on both high and low river flows, with the results interpreted in terms of the driving hydrological processes operating throughout the catchment. The hydrological modelling was applied to the Dacre Beck (38 km2) sub-catchment, and the results upscaled to the whole Eden catchment (2400 km2), Cumbria, UK, using hydraulic modelling. The scenario of heavy compaction produced a peak discharge 65% higher than light compaction and the minimum flow was 86.8% lower for the heavily compacted soil. This preliminary finding adds weight to the multiple benefits of rural land management and its potential widespread adoption.
             Land use changes are also thought to have increased fine sediment delivery to our river systems, which degrade salmon spawning gravels and decrease egg survival rates. The main hypotheses used to explain this being (a) fine sediment reduces gravel permeability and intra-gravel flow velocities; and (b) intra-gravel O2 concentrations decrease due to reduced supply and increased consumption by organic sediments. The SIDO (Sediment Intrusion and Dissolved Oxygen)-UK model is a physically based numerical model which stimulates the effect of fine sediment intrusion on the abiotic characteristics of the salmonid redd, along with the consequences for egg development and survival. This model has been used to assess the impact of varying sediment inputs upon the sediment intrusion rates, abiotic redd characteristics and fish egg survival rates. This will be used to comment on the relative importance of each of the hypotheses put forward to explain the impact of sediment on egg survival. Results indicate that the physical process of blockage, especially by larger sand particles, is more significant than the Sediment Oxygen Consumption mainly associated with silts and clays. These findings have implications for how we manage the sediment delivery problem.