Abstract
The increasing applications for the conduct of offshore mining or dredging in the Philippines has drawn interest to review how the removal of significant quantities of sand and/or gravel offshore can induce modifications on the hydrodynamic regime of nearshore areas. Alarming effects of offshore dredged-induced wave transformation are the changes in sediment transport pattern and the drawdown of the beach resulting from infilling of the dredged pit by cross-shore sediment transport. For instance, changes on the water depth over a dredged area can change the speed of wave propagation and consequently lead to wave refraction. Similarly, lowering the bank of sediments could increase wave heights which could hypothetically extend up to the coastline. Relative to this is the need to establish identification of the depth of closure (DoC) or closure depth beyond which, sediment transport or morphodynamic processes is very small or nonexistent. Ideally, offshore dredging activities are conducted beyond the DoC to minimize the impact of offshore dredging to coastal areas. These ideas are applied to provide an initial assessment of the potential impacts of offshore dredging to coastal areas in in Manila Bay and similar system. Coastal impact study of Manila Bay arising from offshore dredging, however, is complicated by the accelerated sea-level rise affecting the bay since the 1960’s and the changing land use upland of Manila which may have significant effect on the coastal sediment budget.
Keywords: offshore dredging; depth of closure; wave field modifications
1.0 Introduction
The extent of coastal modification especially in major cities of the country has been substantial in the past years. Coastal alteration in Manila Bay Region alone has now been increased by more or less 10% through reclamation and seabed quarrying specifically along its eastern periphery. These undertakings are projected to continue that may eventually include up to 50% disturbance of the bay, given the numerous major projects in the pipeline and the proposed projects in Manila Bay. The same situation is likely to be experienced in the other parts of the Philippines. Unfortunately, the aggregate demand from proximal land-based quarries are declining, hence offshore sources via seabed quarrying/dredging are being eyed as an alternate source. Such strategy is seemingly cheaper, as the sources of materials for reclamation projects are nearer.
Seabed extraction/dredging of aggregates can have considerable physical impacts on the coastal environment. It changes the form of the ocean floor and consequently modifies the hydrodynamic processes surrounding the disturb area. Cooper and Brew (1998) noted three principal receptors that could be influenced directly by marine aggregate extraction. These are (a) the seabed and its sediments; (b) the sediments suspended within the water column; and (c) the coastline. Changes on the seafloor and coastal areas could be a direct consequence of physical change associated with the extraction process itself (covering both dredging and screening of sediments) or through indirect consequences of the extraction, causing changes to the wave, tide and sediment regimes operating across and beyond the dredged area (Cooper and Brew, 1998). Hence, the geographic extent of such changes needs to be predicted in order to lessen if not totally control the damage on adjoining coasts or other offshore structures.
This paper highlights coastal modification processes that may arise due to offshore dredging. In the absence of offshore studies showing the causal relationship between offshore dredging and coastal erosion in the country, learnings from offshore activities in other countries were utilized based on the published literatures. Effects of dredging offshore of Manila Bay to coastal areas are then explored, taking into consideration the current condition of bay.
2.0 Wave field modifications and shoreline response
Previous studies (e.g., Kojima et al., 1986; Demir et al., 2004) demonstrated that offshore dredging could accentuate changes in coastal areas. The physical impacts of offshore dredging include (a) beach and sand bank drawdown; (b) changes in wave condition; (c) changes in sediment transport; (e) changes in tidal current and; (f) coastal erosion. Beach drawdown directly related to dredging is likely when offshore dredging is conducted within nearshore areas that do not go beyond the closure depth. To note, closure depth in the theoretical depth along a beach profile, where sediment transport is very small or non-existent (Razak and Khan, 2020).
Kojima et al (1986) suggested that dredged holes at depths less than 30 m were observed to fill with sediment in less than 1 year. Erosion along the shoreline was also observed, although the portion attributable to the presence of the dredged pits could not be isolated. Demir et al (2004), on the other hand, demonstrated that wave transformation around and across the dredged hole indirectly affects the longshore sediment transport patterns. This is made by altering the breaking wave conditions. The typical features of shoreline change due to refraction are the leeside erosion landward of the pit and the edge deposition on either side of the eroded area (Fig.1). Demir et al (2004) added that the shoreline response is extremely sensitive to the water depth at the dredge location. To this end, they recommended for the pit to be located in the deepest location possible. If the dimensions must be increased to dredge a larger volume and the direct effects are insignificant, Demir et al (2004) suggested to increase the longshore length of the pit.