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Modelling of Dredged Material Disposal: Mersin Bay Case Study

Sadighrad, Ehsan
Tuğrul, Süleyman
Salihoğlu, Barış
Management of disposal of solid materials dredged from harbours, shipping channels and coastal environments for different purposes is essential for sustainable land/marine ecosystems, keeping economic values and human quality of life along coastal areas. Therefore, comprehensive and detailed investigations of geo-chemical properties of organic/inorganic pollutants in dredged materials and principal physical and bio-geo-chemical properties of the selected disposal sites are commonly a prerequisite for any planned dredging activity. These appropriate control measures should be taken to avoid or reduce unwanted impacts of dredged materials at dumping sites and to meet specific environmental objectives. This study focuses on major physical factors/processes (under different current velocity, stratification, barge volume/speed and direction during dumping) that govern the amount of dredged solid materials (sediment in different sizes) that go into suspension and the parameters which affect the short term spreading of sediments dumped into the upper layer (4-10 m depth range) of disposal areas. The STFATE model developed in the USA for this purpose is widely applied by environmental scientists in other seas. We have also adapted this multi-layer model to simulate spreading of sediments (composed of sand, silt, and clay) at disposal point (depth: 50m and 150m) on NE Mediterranean shelf. This model simulates short-term spreading of disposed materials which is divided into three phases; the convective phase which is the time from disposal to the bottom impact, the dynamic collapse which describes how the material spreads at the bottom after impact and the passive diffusion phase which is a long term process and not included in our study. The model results indicate critical roles of ship speed/direction under different current and sediment compositions during the short-period (2-3 hours) from dumping until sedimentation on the bottom. According to the model results, sandy particles sink much faster and reach the bottom in about 5 minutes whilst smaller fractions of sediments spreads over an area of about 7400 m(2) (in 10 m depth below surface) and then dispersed on the bottom of disposal site in 60-120 minutes under changing factors such as current and barge velocities at the time of disposal, and sediment composition. These factors also determine the thickness of disposed materials on the bottom, varying between 25 cm to 2 m depending on barge volume and other physical factors in each disposal. Model results also imply that dumping of non-toxic dredged materials mainly influence bio-optical properties of merely upper layer waters and benthic fauna of disposal site selected on the shelf having low biomass and fisheries.