Woody debris effects on upland river hydromorphological processes

Recent flood events in Yorkshire and in the UK more widely have highlighted the potential role of upland river management in reducing peak flood flows downstream. Natural accumulations of large wood (>1 m length) in rivers are often associated with physical habitat changes (depth, velocity, organic matter retention) which can have benefits for moderating flow peak timing and magnitude downstream. However, such features are rare because, for centuries, wood has been removed from many rivers to aid navigation and 'improve' flow conveyance. Recommendations for wood management/reintroduction in UK headwater rivers are available from the Environment Agency but these are based largely on lowland river studies. The extent to which these are transferrable to upland localities is unclear given the typically steeper gradients, coarser substrata and flashier flow regimes of upland rivers. Introducing wood structures and promoting natural accumulations in upland rivers could have benefits for delaying flow peaks to downstream areas via modification of channel morphology, and with localised flood risk less of an issue given sparse human populations and typically low intensity farming in these areas cf. lowland areas. However, there has so far been very little research in the UK on the effects of wood placement in upland rivers. A more complete understanding of upland river hydromorphological response to woody debris is vital if river managers are to develop more effective restoration and management schemes.
This studentship will address the need to develop a quantitative evidence-base of wood-morphology-hydrology interactions in upland rivers. The central question to be answered is does reintroduction of wood into upland rivers lead to significant changes in river morphology that reduce the magnitude, and delay the timing, of peak flows further downstream? The student will address questions such as: where should wood be placed and/or allowed to accumulate in upland rivers to optimise benefits? and, which types of structures have the largest influence on upland river processes? In particular, we seek to constrain better the influence of reach hydromorphology (i.e. considering stream depth, width and roughness relative to wood diameter/length) on restoration success.

The project will predominantly integrate observational and experimental approaches. One component will be monitoring of existing sites with woody debris in upland rivers of northern England. The student will benefit from access to the River Cover, Yorkshire Dales, where existing hydrological (discharge, temperature) and geomorphological (bathymetry, valley morphology) data are available. Here, BACI style experiments using fixed wood structures have been established since 2014, and further trials are planned to compare other structures vs. natural accumulations. Using Structure-from-Motion surveying, coupled with object-detection software employed through-water, novel insights into wood geometry, geomorphological response, and distributed surface grain size will be monitored from patch to river-scale without disturbing sediments, and related to hydraulic characteristics using velocity profiling and tracer techniques. This primary research will be contextualised widely via surveys of restoration practitioners (e.g. River Restoration Centre, EA) including CASE partners, to document experiences of using woody debris, restoration design and river characteristics, and assemble an up to date database of UK river restoration projects that include wood. Coupling novel geomorphological surveying approaches with established hydrological monitoring techniques and broad contextual meta-analysis will, for the first-time, provide new significant insights into the potential for using wood in upland river management.