Civil Engineering May 2021 | Vol 29 No 4

28 May 2021 Civil Engineering resultant DMP were therefore predomi- nantly based on an extensive review of international literature and prior experi- ence in similar dredging projects. Results Figure 7 and Figure 8 respectively indi- cate the predicted extents of aesthetic impacts and acute impacts on corals and seagrasses as a result of elevated water column turbidity caused by dredging, reclamation and dredge spoil disposal activities undertaken during southeast monsoon conditions. Hatched areas indicate areas where the thresholds of concern, respectively for aesthetic and acute impacts of water column turbidity, have been exceeded. Both results indicate that impacts are mainly limited to the port basin and areas surrounding the dredge spoil disposal site. During the first dredging phase, the dredging and reclamation impacts are limited to the port area, with a risk of acute impacts on the corals over a very limited area on the western shore of Ile de Romainville. During the second dredging phase, that includes dredge spoil disposal at the ex-quarry site, visual impacts are predicted to occur near Pointe La Rue and alongside the airport. Chronic impacts on corals and seagrasses in the port basin are slightly less extensive than for the first phase, mainly because of the smaller dredging footprint. Recommendations The only realistic mitigation measure to limit the impact of dredging and associated activities is to limit the total suspended sediment concentrations in the return flows from the reclamation and dredge spoil disposal areas. Alternatively, the dredging rate, and therefore the rate of sediment release into the environ- ment, could be reduced. There is scope within the dredging and construction programme for this to occur, however this is likely to significantly increase costs. Given that the thresholds of concern adopted for the study are not site-specific, an adaptive management approach has been recommended in the DMP that allows for these thresholds to be adjusted based on knowledge acquired during the execution and monitoring of the dredging activities. DREDGE MANAGEMENT PLAN Objectives The DMP and associated monitoring activities are key components of the envi- ronmental authorisation for the project. Broadly speaking, the DMP developed for the project is a document specifically focussed on the management of dredging activities and both informs and is informed by the relevant Environmental Impact Assessment studies and associated Environmental Management Plan (EMP) for the project. The key objectives of the DMP developed for this project are as follows: Q Q Provide a well-motivated and suf- ficiently comprehensive description and understanding of the proposed dredging activities. Figure 7 The predicted extents of aesthetic impacts on corals and seagrasses (based on sediment concentration at the water surface) y-coordinate (km) 9 492.5 9 490.0 9 487.5 9 480.0 9 482.5 x-coordinate (km) 340.0 337.5 335.0 332.5 330.0 327.5 9 485.0 Zone of non-compliance 0–2 2–5 5–10 10–15 15–20 20–30 30–50 50–80 80–100 100–150 >150 Sediment concentration (mg/ℓ) 99 percentile