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Rio Tinto AlcanEnvironmental Impact StatementSouth of Embley Project65731300 ////.////85721}GG ///////////.,., '/////1,- '/------- '\-:',..'--///////// i:////// " M1,, -..,-//// pl i17/61/611111/1/1857"" /////////////////////:.V.4f.85700cc//////////////////////////// /.////////i///////// p/Vlif7/// f'8569000 / 0/ / Z.,,-"'.1--- .g"' / si./..- :. 0..5 mes85680013560090 562000 564000 E:ECCC EEMCC E7CCCC E72CCC21:00:00 1940442007 Time Step 1646 of 6760.>Figure 6-8Predicted Peak Flood Tidal Currents at Proposed Port Dredging Area35730008572000857100C85700008569000856800056000C 5E2000 56400C1 2:?C:00 20(04(2007 Time Step 1fiagwpo.5E6000 E.:8CCC E7CCCC E72CCCFigure 6-9Predicted Peak Ebb Tidal Currents at Proposed Port Dredging AreaThe supply of sediment to this part of the coast appears to originate from:nearby river systems such as the Embley and Archer River catchments;dunes and indurated sand and clay cliffs; andoffshore marine sediments.During severe storms, scarping or lowering of the existing beach profile is expected. The typeof response will depend on the presence and depth of firmer underlying strata. During severestorms, when surge is combined with high tide, wave run-up will also erode the cliff at the backof the beach, transporting sediment into the littoral system.Coastal process modelling was undertaken to assess the potential for shoreline change due tothe construction of the proposed wharf and dredging. In particular, two aspects were analysedin detail to determine the influence on coastal processes:potential long-term changes to the alongshore profile and back-beach brought aboutby the required deepening of the inshore area through dredging and a qualitativedescription of the potential influence of the wharf trestle structure; andSection 6 Marine Page 6-8
Rio Tinto AlcanEnvironmental Impact StatementSouth of Embley Projectpotential for upper beach profile sediments reaching the berths due to cross-shoretransport during storm events.A discussion of these potential impacts is provided in Section 6.9.1.6.1.3 Currents and TidesWeipa is the closest standard port to the Project area with sufficient tidal data for analysis. Thetidal signal at Weipa is predominantly diurnal (daily cycles) with short periods of semi-diurnaltides at times of neap tides.The tidal planes at the Weipa tide gauge located at Humbug Point (1.752m above LAT) areshown in Table 6-1. Heights are also shown in Australian Height Datum (AHD).Table 6-1Weipa Tidal PlanesTidal Plane Metres above LAT Metres above AHDHAT (Highest Astronomical Tide) 3.38 1.63MHHW (Mean High High-Water) 3.0 1.2MLHW (Mean Low High-Water) 2.2 0.4MSL (Mean Sea Level) 1.85 0.10MHLW (Mean High Low-Water) 1.5 0.3MLLW (Mean Low Low-Water) 0.7 1.1LAT = Lowest Astronomical TideAHD = Australian Height DatumThe tide prediction does not always fully explain the tidal observations at Weipa. Meteorologicalconditions acting over time can transfer additional energy to tidal signals, resulting in higher orlower water levels than the predicted values.The tidal signal in the Project area is almost identical to Weipa, with a slight phase shiftresulting in the high tide being slightly later than at Humbug Point.Current data at the proposed port site was collected over the 2006-07 and 2007-08 periods.Measurements were taken with an ADCP located in 10 to 14m of water at Boyd Point. TheADCP recorded data at six-minute intervals and currents were measured at every half-metredepth over the water column. Little variation of current was observed with depth. Average peakspring tide currents were approximately 0.25 meters per second (m/s). Average peak neap tidecurrents were 0.15m/s. Currents are predominantly aligned with the coast, typically between210 and 220°N during flood tides and between 30 and 40°N during ebb tides. The depthaveragedmeasured current speed and direction over a four-week period during AprilMay 2007is presented in Figure 6-10.Section 6 Marine Page 6-9
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