Getting to the root of floodplain dynamics: Modelling interactions of water flow and riparian vegetation
Riparian vegetation has a substantial impact on the hydraulic, geomorphological, and ecological functioning of rivers and their adjacent floodplains. The increased propensity for flooding seen in many catchments means there is an urgent need to better inform the management of floodplains in rural and urban environments. High intensity rainfall events are becoming more frequent and occurring at all times of the year. However, the seasonal nature of riparian vegetation in temperate climate regions means that flood risk can be strongly influenced by vegetation growth, decay and management.
Improved understanding of the interactions of water flow on riverine ecosystems (ecohydraulics) is particularly important during high-flow conditions. Numerical modelling has been used to understand these ecohydraulic interactions, but is limited by the complexity of natural vegetation and the lack of empirical data. This studentship will use a recirculating flume tank in the Sorby Environmental Fluid Dynamics Laboratory to design instrumented experiments that seek to capture the dynamics of flow through and around riparian vegetation, focusing on flow-vegetation interactions at the plant-scale.
Uniquely, The Sorby lab has a pair of tanks that have transparent bases and sides to allow monitoring of 3D flow fields at a range of scales. 3D flow fields will be measured using state-of-the-art LED illuminated particle tracking techniques. Optical tracking can also be used to monitor the motion of vegetation allowing quantification of the complete structure of fluid interaction around vegetation at a range of scales. Analysis of plant motion dynamics, turbulent flow structures, and applied drag forces allows several research questions to be posed, such as: are generalized velocity profiles suitable in numerical modelling of complex plants? How do changes in plant orientation (posture and aspect), the presence of leaves, and the transfer of drag to the bed influence downstream velocity fields? How does seasonality impact drag coefficients? These experiments can be used to help validate numerical simulations aimed at improving the process-understanding of flow-vegetation interactions and predict patterns of erosion and deposition of sediment and pollutants on floodplains. The findings of this project can be integrated with industry standard hydraulic models used in catchment management, to improve the accuracy of predictions in vegetated catchments.
The lab work will be supplemented and validated by fieldwork measurements. One of the potential locations is the river Skell located within the UNESCO World Heritage Site of Studley Royal, which incorporates the ruins of Fountains Abbey. This is an unusually well instrumented catchment. Land management and climate change have accelerated flood impacts on the landscape in this flashy catchment, and there have been a series of damaging flood events in the last couple of decades, the largest of which occurred in July 2010. Riparian vegetation is a significant control of flow distribution around the Abbey ruins and is highly variable with the seasons and with flow stage. Another potential field site is the river Hull, north of Hull. The river Hull is a chalk stream, an extremely rare riparian habitat. Chalk streams are ideal for detailed hydrological measurements as they are characterised by extremely low sediment concentrations and they typically host very large volumes of riparian vegetation (Ranunculus sp). This allows excellent visualisation of the hydraulic processes and the interaction with complex natural vegetation. This site would allow detailed measurement of flow fields in and around vegetation, using either a custom field optical measurement system or an array of acoustic transducers.