A vegetation roughness model for wind flow simulations over coastal dunes

Coastal dune systems are important structures that protect people, infrastructure, and land from storm surges and inundation events likely to become more frequent and severe in the future. However, increasing populations living near the coast, expensive and sometimes short-term geoengineering solutions, resource constraints and public expectations are just some of the potential conflicting pressures that can be put on these coastal dune systems. Adding to this complexity is the interaction of wind and sand with the dune system. Current understanding of coastal dunes comes from site specific field studies and some numerical simulations. It can therefore be very difficult for coastal managers to develop plans that are proactive and long term in the absence of information about their situation. Although the interaction of wind flow and sediment transport determines the development of coastal dune landforms, the accurate predictions of wind flow, sand transportation and dune dynamics remain some of the big challenges in the field of aeolian research. Vegetation cover on these dune systems adds another layer of complexity to the predictions and work to quantify its effect is difficult. Technological developments, such as the development of high-resolution ultrasonic anemometry, combined with detailed field experiments have enhanced our understanding of boundary layer flow over dunes, and thus advance recent efforts to model the interaction between dune geomorphology, airflow dynamics and sand transport. Increasingly numerical simulation, particularly Computational Fluid Dynamics (CFD), can be used to complement and enhance field-based measurements of wind flow and sand transport. If vegetation is accounted for in these models, a constant surface roughness value is used. However, the flexible nature of the dune plants means that they typically exhibit lodging, a bending over of the plant as the wind blows or sometimes a waving motion such as that seen in a field of wheat. Therefore, the surface roughness from the coastal dune vegetation varies with wind speed and its dynamic nature is not captured using a constant roughness value in CFD simulations. To understand the effect a dynamic vegetation surface roughness vegetation has on wind flow and sediment transport over dune systems, a more nuanced numerical representation of the surface roughness is needed that is responsive to plant parameters such as species, distribution and density of cover. Individual plants cannot be modelled so a roughness parameter is used at the dune surface to account for some of the effects of the vegetation being present. Work has been undertaken through taking field measurements to determine the relationship between wind speed and roughness height for several typical dune plants. Preliminary models have then been developed to describe for a range of wind speeds how roughness height varies with wind speed. These dynamic dune vegetation roughness models were implemented into a CFD code, ANSYS Fluent. Simulations of an idealised dune structure with dynamic vegetation roughness models were undertaken and compared against results using current methods for representing dune surface roughness. It is shown that using a dynamic vegetation roughness on the surface of a dune changes vertical wind profiles along the dune, the shape and size of speed up jets at the dune crest and other aspects of the flow. The implications for sand transport over and past dunes and subsequent coastal management will be discussed.