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Rio Tinto AlcanEnvironmental Impact StatementSouth of Embley Project360 0315>. 270z225CCw180135asa)2 9045Mean Dirbuoy = 0.8875 Mean Dirport + 10045 90 135 180 225 270 315 360Mean Dir, SoE port siteFigure 6-15Modelled Mean Wave Direction at Proposed Port vs Measured at Buoy10080604020IINE E SE S SWWave direction (propagating from)NWNFigure 6-16Modelled Wave Direction Frequency of Occurrence at Proposed PortSite6.1.5 Sediment MovementThe beach either side of the proposed port site is relatively straight as shown in Plate 6-1.Longshore transport of beach sand occurs during storm events and the extent depends onwave height, direction, and storm duration. Calculations, using modelled wave data, show thatover time there has been a net movement of approximately 12,000m3 of beach sand percyclone season transported towards Boyd Point. The largest single sand movement event of86,000m3 towards Boyd Point occurred during Tropical Cyclone Dominic in April 1982.Mobilisation of fine sediments surrounding the dredge area is a function of the cohesive or noncohesivenature of the material and the bed stresses that are able to be developed underprevailing current and wave conditions. The material within the dredge area mainly consists ofcohesive sediments at least out to about 4km from shore where sediments increase significantlyin sand content (refer Section 6.5.4.2). Once the current and wave conditions overcome thecritical bed-shear stress, sediments will begin to move.Normally the critical bed-shear stress is not greater than 0.3N/m2 for a weakly consolidatedmud and 1.2N/m2 or greater for a moderately consolidated mud (mud is the combined silt andclay fractions).Section 6 Marine Page 6-14
Rio Tinto AlcanEnvironmental Impact StatementSouth of Embley ProjectCalculations show that the critical bed-shear stresses are exceeded when significant waveheights are larger than 1m during a neap tide, assuming the bed is weakly consolidated. Duringspring tides, the peak flood and ebb current forces alone are almost enough to initiate sedimentmotion and any additional wave action exceeds the critical bed-shear stress associated withfresh deposits of fine sediments. Larger waves will result in high concentrations of the fine bedmaterial in suspension and higher turbidity of the waters around Weipa and the Project area.Changes in sediment movement as a result of the wharf trestle structure and berth dredgingarea are discussed in Section 6.9.1.Plate 6-1View at Beach Level Looking South (towards proposed port site)6.2 Marine Protected Areas, Conservation Areas and Biodiversity AreasSection 7.2 discusses protected areas, conservation areas and biodiversity areas from CapeYork to the Project area. These relate largely to terrestrial habitats and wilderness areas whichmay include riverine and estuarine habitats including the Embley and Hey Rivers.There are currently no similar protected, conservation or biodiversity areas specific to themarine environment. However, there is a current proposal by the Queensland Government todeclare a fish habitat area (FHA) under the Fisheries Act 1994 over the northern portion ofAlbatross Bay. Public consultation on the matter is currently being conducted. The area ofAlbatross Bay concerned is that north of 12° 36' 00" south. The existing Albatross Bay spoilground and Port of Weipa entrance channel are south of the boundary of the proposed FHA.The Australian Government is currently completing the marine bioregional planning process forthe North Marine Region which covers more than 715,000 square kilometres of ocean coveringthe Gulf of Carpentaria, Arafura Sea and the Timor Sea as far west as the Northern TerritoryWestern Australian border. Within the region, five "Areas for Further Assessment" have beendefined. "Areas for Further Assessment" are not the proposed boundaries for new marinereserves. Instead, they are intended to aid further analysis of information at a more detailedscale and assist in the design of new marine reserves. Based on ecological information andinformation obtained from key stakeholders on human activities within the "Areas for FurtherAssessment", the location and zoning arrangements of a new network of marine reserves willbe developed. The Project area is not included within any of the five areas being furtherassessed.Section 6 Marine Page 6-15
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