Design Process of a Sandy Convex Shaped Beach Layout

Abstract

Abstract:The planned Colombo Port City (CPC) development, shown in Figure 1, comprises 450 acres of reclaimed land, 3 km of offshore breakwater, two additional breakwater revetments and a central canal. In its final stage, the reclamation will be closed off by a sandy beach at the seaward side. This beach will be partly sheltered by the offshore breakwater and lagoon in front of it. The beach layout is convex, implying that all beach angles are offshore directed. This layout with respect to erodibility, poses multiple design complexities. These problems will be tackled by a converging design approach, focusing on reduction of risks and increasing knowledge (from measurements and modelling) at the one hand and a highly adaptable design at the other hand. This engineering management approach is described in the present paper. Sediment transport along the beach is influenced by the complex hydraulic climate in the lagoon area: the combination of wave overtopping and transmission through the breakwater, waves diffracting around the breakwater heads, local waves and residual currents. The anticipated sensitivity to beach erosion should not negatively affect the development. Therefore, to quantify the beach stability, the hydraulic climate inside the breakwater has been assessed by numerical modelling to form a basis for the spatially distributed sediment transport computations. To acquire a reliable translation from the offshore wave and current climate to the climate within the lagoon area, extensive physical and numerical model studies have been performed. The preliminary beach stability analysis indicates that mitigation measures will most probably be required. This requirement, as well as assessment of the type of mitigation measures, is key to the adaptive engineering approach that has been adopted here. The adaptive approach aims at arriving at a practical design for the beach to secure the functional (public) requirements within economical (maintenance) and practical (constructability) boundaries. A groyne scheme is a relative simple and adaptive way to stabilize an unstable beach, whilst providing flexibility as it can be implemented in a phased way and be adapted rather simply when required. Along with the design, we focused on optimization of the construction strategy and sustainable material usage. The present paper presents the numerical analysis part of the iterative design process, which has not yet been completed. As such, this paper is the launching paper regarding the CPC beach stability, providing a baseline for the design, and will be followed up by further paper(s) at a later stage of the design and construction.