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Deltares | R&D Highlights 2015 Delta Infrastructure
Further reading:
Van Gent (2014), Oblique wave attack on rubble mound breakwaters, Coastal Engineering, 88, 43-54 alex.capel@deltares.nl T +31(0)88 335 8034 marcel.vangent@deltares.nl T +31(0)88 335 8246 ivo.vanderwerf@deltares.nl T +31(0)88 335 8323
The administration of the city of Jakarta, Indonesia, has decided to construct an archipelago of artificial islands in the Jakarta Bay. The islands will house new residential areas for the city’s fast-growing population. Because they are located in the sea, sea defences are required for protection against flooding. After the preliminary design phase, sea defences are usually tested and optimised on the basis of hydrodynamic performance and structural stability. Recent design guidelines based on tests by Deltares showed that considerable savings can be obtained with respect to the required rock size by taking into account oblique wave attack. However, some aspects of the design were outside the validity limits of the guidelines. The design therefore required verification in physical model tests.
Physical model tests were performed for two different clients on two different islands in the multidirectional Delta Basin at Deltares. The structures had two aspects in common: both were attacked by waves at a very oblique angle, and the structures were founded on very soft subsoil. The unconventional structures were adopted due to the soft subsoil and seismic activity in the area. The slopes of the sea defences were much shallower than normal. The combination of the unconventional structure and the oblique wave angle required new physical model tests. A complicating factor was that the level of the reclaimed islands is higher than the sea level. However, the very soft subsoil means that considerable settlement can be expected, resulting in considerable subsidence, taking the islands below sea level and transforming them into a polder. A number of large 3D physical models were constructed in the Delta Basin, including a part of the seabed in front of the structure. The scales ranged from 1:40 to approximately 1:8. These models were meant to simulate various parts of the islands, including two corners and a number of straight sections with complicated transitions. The design storms were simulated with these models and calibrated wave conditions. Optical measurements were taken during the tests: both photographic analysis and 3D laser measurements were employed to determine the damage to the structure caused by the simulated storm. In addition, the amount of water flowing over the structure due to the wave action and the waves was measured. The test results showed where the damage occurred on the structure, when it occurred in the storm sequence and the amount of damage. This information allowed our clients to optimise their designs and demonstrate design stability. Physical model of the
north-west corner of one of the islands before testing. During testing
Digital terrain model made by the laser scanner
