IAP-25-137

Peat soil crusts: formation, desiccation and modelling of transport by wind

The aim of this project is to better understand how soil crusts form on bare peat surfaces and undertake experiments to quantify the mechanics and hydrodynamics of crust erosion by wind. Results from the experiments will be used in a Computation Fluid Dynamics (CFD) model to determine the relationship between surface crust roughness and wind flow dynamics in the boundary layer.

Soil crusts occur worldwide and provide important functions for ecosystem health, hydrology and erosion control. However, peat soil crusts have not been investigated in any detail, despite their importance in controlling surface roughness, rates of erosion and near-surface carbon flux (Evans & Warburton, 2007; Foulds & Warburton, 2007; Smith & Warburton, 2018).

Bare peat soil crusts tend to form during periods of surface desiccation (drought). Prolonged dry periods lead to desiccation cracking of the peat surface which is then susceptible to erosion by wind.
Observations and recent climate modelling show that the world is getting windier. For example, in the UK the number of extreme windstorms will more than double this century and the summers will be drier increasing the frequency of surface wind erosion. Wind erosion of peat contributes directly to peat (carbon) loss but also keeps eroding surfaces bare, preventing revegetation by natural recolonization and/or restoration interventions. Understanding the processes of peat crust formation and the mechanisms of wind erosion therefore has important implications for peatland management.

The key research questions are:
1) What are the particular hydro-meteorological conditions that lead to peat crust formation and desiccation cracking?
2) What factors influence the entrainment and erosion of surface peat crusts by wind?
3) What can small-scale physical models and CFD modelling inform us about the importance of these processes?
4) At the catchment scale, how important is wind erosion of bare peat surfaces in the overall carbon budget?

This project addresses the problem of wind erosion of peat soil crusts using three integrated methods: field monitoring, small-scale wind tunnel experiments and CFD modelling. Results from these strands of research are then upscaled to demonstrate the application of the new understanding at the catchment scale. This will be the first time they these methods have been used to study the erosion of peat soil crusts.

The main research objectives are:
(O1) (i) Measure the transport of detached peat crusts at a bare peat field site to characterise thresholds of movement and how these are controlled by wind strength, local peat physical characteristics, surface roughness and the surface soil moisture regime. (ii) Survey local topography and peat roughness using terrestrial laser scanning and Structure-from-Motion photogrammetry.
(O2) Use a small-scale wind tunnel to carry out experiments to better quantify near surface turbulent flow and isolate the key controls on peat entrainment and transport in both a natural field setting (01) and for an analogue model of peat crusts.
(O3) Use the results from the field measurements and small-scale wind tunnel experiments to develop a numerical model that reproduces the turbulent flow generated by complex peat crust topography and provides mean flow properties that can be used to predict threshold conditions for wind erosion.

Fieldwork will focus on Moss Flats, which is a 0.5 ha area of bare ombrotrophic blanket peat in Northern England (54°40’49’’N, 2°22’38’W, 614 m). The bare peat surface consists of a mosaic of flats, terraces, pedestals, wind ridges and haggs representing an assemblage of typical erosional and deposition forms associated with an active area of upland bare peat.

A small portable wind tunnel will be constructed to look at peat crust entrainment in an ‘analogue model’, which uses felt swatches (20 mm hexagons) to simulate a desiccated peat crust. By altering the spacing and local roughness different entrainment threshold velocities were observed and ‘kite transport’ leading to surface crust stripping was directly simulated. The wind tunnel will be applied in both the field and laboratory to understand kite transport which leads to surface crust stripping. Laboratory experiments, over three-dimensional printed surfaces will enable precise thresholds of entrainment to be determined.

Numerical modelling of turbulence and shear stresses generated by the complex peat crust topography and mean flow properties will be undertaken in a CFD framework to create a model to simulate the flow over complex fixed bed topography for the range of conditions measured in the field and wind -tunnel experiments. Model simulations will be conducted using a RANS CFD scheme using an existing modelling framework.

Results from these methods will be upscaled to predict wind erosion of bare peat, at the catchment scale, for a range of future climate change scenarios under the Met Office Hadley Centre Climate Programme, UK Climate Projections 2018 (UKCP18) framework.

Year 1

Literature review and project design
Visit field site – survey and collect topography and set up data acquisition system
Set-up small-scale wind tunnel and carry out laboratory experiments using felt media
Learn existing CFD modelling framework

Year 2

Continue field site monitoring to the end of year 2
Set-up small-scale wind tunnel and carry out field experiments on natural crusts
Use field and experiment data in CFD to simulate different aerodynamic scenarios
Drafting of paper outlines and initial paper I

Year 3

Complete CFD modelling
Submission of first paper for publication
Complete drafts of two other papers (II & III) and preparation of the final thesis by publication

Year 3.5

Complete final thesis submission of papers for publication

A Training Needs Analysis (TNA) will be conducted to evaluate the student’s skills and design technical training bespoke to the project but also to enhance the general skills portfolio. Key skills will include fieldwork training / risk assessment; GIS & quantitative analysis of DEMs; experimental design; physical modelling set-up; and CFD modelling techniques. Full advantage will be taken of IAPETUS training opportunities e.g. modules in Introduction to modelling in Python and Advanced statistics in R. The thesis will be completed as a series of three publications. The student will attend Department, University and IAPETUS training courses to enhance skills in scientific writing with additional bespoke support from the supervisory team. The student will also be expected to take an active part in the research culture of the Department and IAPETUS Programme attending Department seminars / workshops and The IAPETUS Residential Research Training Week and annual conference. The student will also be encouraged to attend national and international conferences top present their research and network with the wider scientific community.