Design Optimisation of Offshore Breakwater Based On 2D Model Study

Abstract

Offshore breakwater design involves different design aspects such as offshore wave height, 100 year wave return period, storm surge, global sea level rise, structural stability, economic viability and the construction feasibility. For the present study a offshore breakwater, located at 8 km away from the shore is proposed to protect marine facilities located in the Gulf of Mannar. The main objective of this study is to design a offshore breakwater and to ascertain the hydraulic stability of the breakwater against design and overload waves using physical model study. The proposed offshore breakwater facilitates to cater for larger vessels up to 185,000 DWT. Design water level is -18.0 m Chart Datum (CD) with a crest height of +8.4 m CD and toe berm at -6.0 m CD respectively. The hydraulic stability of the proposed breakwater was validated for the 100% design return waves and for an overload of 120% of waves against the analytical design parameters chosen. The proposed breakwater cross section was subjected to a 2D physical model test in order to assess the stability of armour layer, overtopping discharge, transmission coefficient and toe stability during high and low water levels. The cross-section has a crest width of 4.1m and top level of +8.4 m CD, accommodating 17 rows of 4 m³ Accropode on the sea side slope (1:1.33) and toe top level of -6.0 m CD. The rear side armour consists of 3 layers of 1-3 tonnes rock armour placed in a slope of 1:1.5. The breakwater trunk section at -18 m CD bed level was tested in the 2D flume. The 2D model setup was constructed on a scale of 48.36 for the model study. The 2D model study shows negligible damage in all stability aspects. Hence the final cross section is finalised for the onsite construction.