Cumulative effects of multiple marine outfalls: issues for coastal zone management

Introduction. Rapid and large-scale industrialisation of coastal areas is an on-going challenge for both the environment and the design of efficient marine outfalls. National planning strategies (in GCC countries for example) often identify coastal regions in which multiple industries can be developed. Examples include so-called Free Zones and Industrial Cities, and may involve the construction of ports, factories, refineries, power/desalination facilities, etc. Such zoning optimises land use, but can lead to the release of multiple marine discharges within a small area of coastal waters. This raises issues for the environment, regulation, modelling and engineering. Issues. High pollutant loads along a small section of coastal waters can cause significant local ecological stress, and multiple pollutants can act synergistically on flora and fauna. Pollutants may accumulate locally, particularly if the waters are poorly flushed. The author has studied several sites where multiple thermal outfalls have caused significant seawater warming, effectively raising the ambient temperature over a region. Significant pollutant build-up can reduce the efficiency of outfall designs, by limiting the availability of fresher ambient seawater for dilution. Communication. The cumulative effects of marine outfalls are often exacerbated by a lack of reliable information-sharing between neighbouring industries, plants and facilities. Data on discharge constituents and release rates is rarely available and so it can be difficult to plan and optimise new outfalls. In extreme cases this can mean that Environmental Impact Assessments and engineering studies for each outfall are effectively carried out in isolation. Management and mitigation. Several measures may help to mitigate the cumulative effects of neighbouring discharges. Common outfalls are built to release combined wastewater from industrial zones, confining high pollutant concentrations to specific areas. These areas may be identified from baseline marine and/or hydrodynamic model studies as low ecological risk, or as regions of rapid water exchange. Examples along the Gulf include outfalls at Sohar Industrial area (Oman), and Ras Laffan Industrial City (Qatar). Common outfalls can also help to mix “complimentary” effluents. For example warm water discharges from refineries/power stations can be combined with cooler discharges from LNG regasification plants. Smaller volumes of highly concentrated effluent (e.g. wastewater) can be combined with larger cooling water discharges to effectively pre-dilute and reduce harmful pollutant concentrations. Regional planning studies. Regional planning studies can be supported by hydrodynamic modelling. used to successfully managing the effects of multiple outfalls. Computational modelling studies can be carried out to determine optimum outfall locations, and discharges can then be coordinated appropriately, accounting for in-combination effects. Management plans must be supported by open information sharing between industries / operators, which might be coordinated by the site owner, overseen by the environmental regulator. The author presents examples of such optimisation studies and how environmental impact can be reduced without significant increases to project costs