Civil Engineering May 2021 | Vol 29 No 4

Civil Engineering May 2021 27 sediment-dwelling or aquatic organisms in the water column during dredging or dredge spoil disposal activities. Furthermore, based on the geo- technical data obtained as part of this project, it was estimated that a significant quantity of the material to be dredged comprised fines (conservatively estimated to be 65%), interspersed with chunks of coral, but with no evidence of rock to be blasted. Dredge spoil disposal site Based on the volume of dredged material to be spoiled and the quality thereof, a dredge spoil disposal site was selected. The selection process comprised a number of iterations as a result of evolving characteri- sation of dredge material, changing engi- neering designs (and spoil volumes) and inputs from local parties. The site selection process comprised a multi-criteria assess- ment of the on-land and offshore disposal sites considered and included: Q Q Environmental considerations (i.e. seabed and benthic impacts, dispersive nature of the site, proximity to marine protected areas, etc.) Q Q Beneficial use impacts (visual impacts, interference with recreation and/ or fishing activities and interference with port operations and other marine traffic) Q Q Technical considerations (distance of spoil site from dredge areas, possible infrastructure requirements to contain dredge spoil, possible ground improve- ment activities required and coordina- tion between dredging and quay wall construction activities). Based on the above considerations, the ex-quarry adjacent to the airport was selected as the disposal site, with the discharge of return flows into the disper- sive marine environment adjacent to the site. Figure 6 indicates the dredging and spoil sites. Dredging methodology Due to the requirement that the port remains operational during construction, the quay wall will be constructed in phases. Firstly, the southern end of the quay will be constructed and reclaimed, while the existing quay in the north remains operational. The construction in the northern area will follow the comple- tion of construction of the southern area. Since there is no space for temporary stockpiling, the phased quay wall construc- tion approach implies that dredging of the basin will also be phased. Ultimately, it is the contractor’s responsibility to derive a suit- able construction methodology. However, in order to inform the dredging assessment, WSP derived a possible dredging sequence and technology to be employed, based on the information at hand. A critical assumption was that the first phase will consist of only dredging for reclamation, while the second phase would entail dredging for reclamation as well as for disposal. DREDGING ASSESSMENT Background WSP undertook a dredge modelling study to characterise the impact of dredging, disposal, and reclamation activities on the environment. The assessment was undertaken for the most likely dredge description and a representative range of environmental conditions that included both monsoon seasons. Modelling approach A coupled wave-hydrodynamic model, using the Deltares modelling suite, was set up, encompassing the area of interest in terms of potential dredging impacts. This 3D barotropic model includes wind-, wave- and tidally-driven currents and mixing processes. The model was set up to represent the following sediment loadings: Q Q Dredging related sediment loadings such as propeller wash, suspension of sediments at the dredge head/ bucket, and hopper overflow (lean mixture overboard). Q Q Return flows from the quay wall recla- mation area and dredge spoil disposal site into the marine environment. Two fractions of fines were modelled, representing sediment loading due to dredging activities (clayey silt) and return flows (predominantly clay), respectively. This approach provided a diagnostic capability to determine which of these two vectors of impact are of greatest con- cern and consequently would need to be mitigated or more closely managed during construction. The sediment loadings modelled assumed fairly restrictive limits on hopper overflows and the assumption of well-managed return flows from the backfill operations at the quay wall and from the dredge spoil disposal site. Key vectors of potential impact The key vectors of potential impact in the modelling study are deemed to be: Q Q Elevated water column turbidity due to sediments suspended by dredging- related activities, potentially leading to aesthetic impacts and reduced light infiltration through the water column, ultimately inhibiting growth of marine organisms. Q Q Sedimentation of dredging-related fines on the seabed that could result in smothering of corals and seagrasses. Thresholds of potential concern related to these impacts needed to be identified for the analysis and presentation of the modelling results. However, literature pertaining to earlier dredging campaigns in Seychelles is both limited and contains few detailed references to environmental monitoring and dredging performance criteria. The adopted thresholds of concern in the modelling assessment and Figure 6 Dredging and spoil sites BASIN RECLAMATION DISOSAL AIRPORT ± 10 km