Turbulence and energy dissipation in soft fluvio-tidal flood defences

Photograph of a salt marsh between the sea and homes at Lytham, NW England

Project Aim

Aquatic vegetation impacts on local and boundary flow resistance and thus can strongly modify flow fields by altering the mean flow and increasing turbulence. This has several implications for the local environment, including raising water levels and modifying the local shear stress that may induce scour around stems or patches and promote sediment redistribution. As such, both submerged and emergent vegetation have been described as ecosystem engineers of aquatic ecosystems. Following DEFRA’s (2005) Making Space for Water strategy, there has been a shift towards the employment of soft fluvio-tidal flood defences, where managers have attempted to implement a more holistic approach to managing flood and coastal erosion risks in England. In the UK, soft defences include sea grass meadows, salt marshes and sand dunes. However, current design procedures lack clear guidance on methods or parameters to account for flow-vegetation interactions, leading to uncertainty as to the spatial extent, optimal locations and the efficacy of schemes to provide desired flood risk benefit. This PhD therefore aims to improve our understanding of flow-vegetation interactions in estuarine environments through fundamental research questions including:

  1. How does saltmarsh vegetation development and succession impact mean and turbulent flow fields, energy transfer and dissipation?
  2. To what extent are plant and patch morphology and turbulent flow fields, including coherent flow structures, modulated by short (e.g. caused by waves), medium (e.g. caused by diurnal tidal variations) and longer term (e.g., lunar tidal cycles and seasonal growth and decay) temporal variability?
  3. How does canopy motion interact with flow fields to drive turbulence and turbulent mixing and furthermore to dissipate wave and current energy?

Approach

This project will adopt an interdisciplinary approach employing field work, physical scaled experiments and numerical modelling. Ffield measurements will quantify turbulent flow fields and vegetation properties including patch, plant, stem and leaf morphologies and flexible rigidities at previously established outer estuary managed realignment locations in the Humber estuary: Chowder Ness, Paull Holme Strays and Welwick, together with the new site at Skeffling. Physical modelling will be conducted at the Total Environment Simulator, Hull. This unique facility is equipped with state-of-the-art flow measurement and stress/strain instrumentation. Finally, a coupled Computational Fluid Dynamics- biomechanical model will be used to explicitly simulate the motion of plants and quantify the two-way feedbacks between turbulent flow fields and plant motion. By unpicking the interactions between flow, canopy motion and bidirectional energy transfer, we aim to inform a new method for use in large-scale and long-term simulations of coastal and estuarine morphodynamics and management options.

Training on each research component will be provided by the advisory team.

Partners

JBA Trust and JBA Consulting are supporting this project as CASE partners, through the input of JBA Consulting Technical Director, Dr. Kate Bradbrook. The student will benefit from one 3-month duration placement and site visits, which will enable the student to make connections in the public and commercial sectors.

Together with their joint venture partners Bentley, JBA are the appointed consultants by the Environment Agency for the Skeffling Managed Realignment scheme. The Environment Agency have agreed to facilitate and support site visits to Skeffling and existing Managed realignment sites in the Humber.

Benefits to student

In addition to the training provided by the DTP scheme, the appointed student will benefit from extensive learning in the fields of coastal and estuarine processes, flood management, physical modelling and numerical modelling. The student will be encouraged to attend at least one multidisciplinary international conference (e.g., AGU, EGU) and at least two thematic conferences (e.g. ISE, RCEM) at which the student will gain contacts in the academic community.

Suggested reading

Boothroyd, R.J., Hardy, R.J. et al. (2017) Modeling complex flow structures and drag around a submerged plant of varied posture, Water Resources Research, 53, 2877–2901. doi:10.1002/2016WR020186

Houseago, R.C. et al. (in review) On the turbulence dynamics induced by surrogate seagrass canopies, Journal of Fluid Mechanics.

Lei, J. and Nepf, H. (2019) Blade dynamics in combined waves and current. Journal of Fluids and Structures, 87, 137-149. doi:10.1016/j.jfluidstructs.2019.03.020

Marjoribanks, T.I., Lague, D., Hardy, R.J. et al. (2019) The role of flexural rigidity and shoot reconfiguration in determining sediment deposition behind saltmarsh vegetation patches, Journal of Geophysical Research, Earth Surface, 124. doi:10.1029/2019JF005012

Möller, I. et al. (2014) Wave attenuation over coastal salt marshes under storm surge conditions, Nature Geoscience, 7(10), 727-731. doi:10.1038/NGEO2251

Temmerman, S. et al. (2013) Ecosystem-based coastal defence in the face of global change, Nature, 504(7478), 79-83. doi:10.1038/nature12859

Thomas, R.E. and McLelland, S.J. (2015) The impact of macroalgae on mean and turbulent flow fields, Journal of Hydrodynamics B 27(3), 427-435.

Thomas, R.E., Johnson, M.F., Frostick, L.E., Dijkstra, J.T., McLelland, S.J. et al. (2014) Physical modelling of water, fauna and flora: Knowledge gaps, avenues for future research and infrastructural needs, Journal of Hydraulic Research 52(3), 311-325. doi: 10.1080/00221686.2013.876453