Tidal analysis of the South China Sea

Tidal analysis of the South China Sea

Akdag, C.

Stelling, G.S. (mentor)
Battjes, J.A. (mentor)
Steeghs, H.J.M.G. (mentor)
Gerritsen, H. (mentor)
Booij, N. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

1996-10-01

The region of South China Sea and the Indonesian waters, covering an area of about 9 million km2, is characterised by a very complex tidal behaviour. This region, which will be called the South China Sea throughout this report, is entangled between the Pacific Ocean and the Indian Ocean. Therefore the tides in the South China Sea are driven by these two oceans. Beside the co-oscillating nature of tides from two different oceans, the geography and the topography of the South China Sea are other essential elements in the complex tidal behaviour. Deep sea basins as well as shallow areas exist in the region. The geography is such, that the whole South China Sea can be thought of as existing of separate basins with each basin having its own tidal characteristics. The complexity ofthe tides of the area is described by various authors. Tidal modelling of the area appears to be a hard task: no complete reliable hydraulic model ofthe area is constructed yet. Results of hydrodynamic models by different investigators seem to be widely different. In June 1993, DELFTHYDRAULICS designed a tidal model ofthe South China Sea as well. The South China Sea Model, as it was called, comprised the southern parts of the South China Sea, the Sunda Shelf, Macassar Strait, Malacca Strait, Celebes Sea and the Sulu Sea. The results of this model so far are not completely satisfactory. So further calibration seems necessary. One of the new topics of research that was suggested within the line of the South China Sea Model, was the construction of a more simplified representation of the study area. Before useful calibration of the model, this would help to study and understand basic mechanisms that control the tidal behaviour of this area. The representation should be as simple as possible but still incorporate the essential features of the hydrodynamic behaviour of the area. It should be suited as an analysis tool and produce insight into the elementary mechanisms at low expenditure in terms of time and computational effort. This report describes a few steps to obtain this basic understanding. First the tidal behaviour of the South China Sea is thoroughly analysed once more, to get a clearer picture of the system's behaviour that has to be represented or reproduced. The complexity of the tides of the area and the reasons for this are summarised and, if possible, put into a figure. Second the results of the various (numerical) models are discussed to find the reason for of difficulties with the tidal modelling. As described above, hydrodynamic modelling of the area turned out to be very difficult; results produced so far by different investigators do not always match. Even in the number of and rotating directions around amphidromic points sometimes differ. Also the "South China Sea Model" results until now were analysed in detail.The third item of the study was an investigation of the applicability of a I-D box model approach to stimulate the complex tidal behaviour of the study area. This approach was chosen in order to understand the sensitivity of the tidal behaviour in relation to various relevant parameters such as topography, modelling of passages, behaviour of some specific basins etceteras. As a start this approach was applied to the Sulu Sea region only, leaving the total South China Sea area to be covered later once the method had shown its usefulness. The Sulu Sea was modelled as a connection of four basins, of which the outer basins represented the narrow passages, and the inside basins represented the deep sea basins of the Sulu Sea itself. At the open boundaries (at the two edges), water levels were given as boundary conditions and transition conditions of mass and momentum were prescribed between the basins. The reason for the choice of the Sulu Sea was a logical consequence of the major modelling difficulties there and its assumed influences for the rest of the South China Sea Model area. Finally some new calculations were made with the South China Sea Model. These calculations were for some part the logical consequence of the study results of this project and for some part originating from ideas of former studies. One of the interesting calculations is for instance the model calculation with tide generating forces introduced in the model for the first time.

tidal behaviour
numerical modeling

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Student theses

master thesis

(c) 1996 Akdag, C.