Coupled hydrodynamic-meteorological models for Gulf coastal outfall studies

Cresswell, D. and Wood, M. (2016) Coupled hydrodynamic-meteorological models for Gulf coastal outfall studies. In: Arabian Coast 2016, 20-23 November 2016, Dubai, UAE.

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Introduction. The Arabian Gulf is a complex hydrodynamic environment with currents and water levels driven by a combination of tidal, meteorological and buoyancy effects. As a region of rapid industrial and economic development, the coastal waters of the Gulf are under increasing pressure from the effects of marine outfalls and other pollutants. It is important that environmental impact assessments and planning studies for proposed coastal discharges use the best available technology to model pollutant mixing and dispersion. The authors present the findings of their latest studies using advanced coupled hydrodynamic-meteorological models in the prediction of coastal flow and pollutant dispersion assessment in the Arabian Gulf. Hydrodynamics of the Arabian Gulf. The hydrodynamics and circulation of the Arabian Gulf have been the subject of many studies over the years. Reynolds (1993) estimated that over much of the Gulf the relative influence of tides, winds and buoyancy effects on the kinetic energy in currents are approximately 100:10:1 respectively. In general, therefore, the strongest influence on currents and water levels is the tide. However, the Gulf contains a number of amphidromic points, or amphidromes, where one or more constituents of tidal water level fall close to zero. Two amphidromes offshore in the Gulf are shown in Figure 1. Near these points, the relative influence of winds can be much more important, and the authors have observed that tidal effects on direction and circulation of coastal currents can be overcome by stronger winds for periods of several days. For coastal outfalls and other sources of pollution, especially those near amphidromes, it is vital that meteorological effects are accurately represented in hydrodynamic models of mixing and dispersion. Pollutant transport models for environmental impact assessment. The potential dispersion of effluent is most commonly assessed using hydrodynamic models. A summary of model techniques is given by Roberts et al (2010) and Wood et al (2014). Models of flows in the Arabian Gulf are usually driven by tidal boundary conditions and meteorological forcing. Meteorological forcing. Meteorological information is typically available in the form of in situ measurements (e.g. airport data or site-specific meteorological station), or reanalysis datasets such as ERA-Interim (Dee et al, 2011). These provide either very local scale winds (perhaps valid for a few kilometres around a particular site), or larger scale circulations on the scale of 100 km or more. Local effects such as land-sea breeze can clearly be seen in station-based observations throughout the Arabian Gulf, and often dominate the local atmospheric circulation. Land-sea breeze is a thermally driven convection cell driving winds that are typically oriented perpendicular to the coastline. Modelling this requires a high resolution atmospheric model and an accurate coastline. The authors demonstrate an application of the Weather Research and Forecast model (WRF, Skamarock et al. 2008) to downscale winds over the Arabian Gulf. ERA-Interim boundary conditions – with an effective resolution of approximately 75km – are downscaled to 8km over the Arabian Gulf. The downscaled winds are validated against in-situ and satellite observations and found to be consistent with the regional circulation, while also providing good agreement with local coastal observations. Sea breeze signals are reproduced well and the local orientation of the wind to the coast at many sites is captured. Coupled modelling approaches. Accurate multi-scale effluent plume dispersion simulations should include winds effects over a range of scales. In the author’s experience, studies encountered in the Arabian Gulf typically only use either local scale winds, or coarser regional wind fields. To support important environmental impact assessments and planning studies for coastal discharges, the authors have applied the downscaled wind techniques described above to generate accurate hydrodynamic and pollutant dispersion simulations. This has the advantage over typical modelling approaches that multiple wind scales and effects can be resolved within a single model. For example, the improvement in orientation of the wind to the coastline in the downscaled winds shows benefits in modelled long-shore drift. The authors are able to include local sea breezes that may affect movement of plumes towards or away from sensitive coastal habitats, as well as the regional prevailing wind patterns that influence wider-scale hydrodynamics and transport. Applications. We show an application of the new techniques for a safety-critical outfall development in Abu Dhabi.

Item Type: Conference or Workshop Item (Paper)
Subjects: Coasts > General
Divisions: Coastal
Depositing User: Unnamed user with email
Date Deposited: 02 Apr 2020 09:51
Last Modified: 02 Apr 2020 09:51

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