Tripoli breakwater reconstruction - evaluation and modification 1982 NEDECO design

Abstract

This study comprises the design of the reconstruction of the main breakwater of the Tripoli Harbour in Libya. Tripoli harbour is one of the oldest harbours of North Africa situated on the Southern coast of Mediterranean Sea. The harbour as it existed in 1972 was built at the beginning of this century and was protected by two main breakwaters. The original construction of these main breakwaters dates back to the Spaniards. The main breakwater on the North, known as the Spanish Mole, extended some 1900 m East North-East. A special feature of the harbour was the existence of a line of reefs north of the main breakwater about 100 m on the sea side, working as natural protection from wave attack. In the sixties, the increasing oil-based economic resources of Libya necessitated an expansion of the port facilities. Sir Bruce White, Wolfe Barry & Partners submitted a design of the breakwater locating the new breakwater some 100 m seaward from the Spanish Mole. The main breakwater was designed to be constructed in two stages; stage 1B, 2190 m long with a backfill (built 1972-1976, see Figure 2) and stage 2A, 2520 m long without any backfill (built 1974-1980). Significant wave heights of 4 and 4.5 m were adopted for the design of stages 1B and 2A respectively which were based on certain wave data available up to 1971 and 1975. Already during construction it became evident that the breakwater would not fulfil its design criteria. Two major storms in 1981 with a significant wave height of 9.2 m destroyed large parts of the breakwater. In deep-water sections (over 7m deep) almost all wave walls were broken and a large part of the 19-ton tetrapods damaged in the actual situation (see Figure 4). The direct hinterland of the breakwater is left at the mercy of overtopping water and spray. The two main roads on the landfill were completely eroded by overtopping and venting water.Netherlands Engineering Consultants (NEDECO) made a complete redesign of the breakwater in 1982 after the breakwater failure (see Figure 3).Meanwhile, during the last 16 years, extreme storms have further deteriorated the breakwater. Frequent nuisance of overtopping water and spray is no exception.The Harbour Authorities have asked for a complete evaluation and modification of the 1982 NEDECO design, because some criteria and boundary conditions have been changed significantly. In this study the new redesign criteria have been determined and based on these criteria a proposal for reconstruction has been worked out. The complete study of the Tripoli Breakwater reconstruction has been divided into four parts. These are: 1. Preliminary Investigations and Data Collection 2. Update Environmental Conditions 3. Preliminary Design 4. Optimization of the Reconstruction Design The approach used in the different sections is further described below and presented in a diagram.