Abstract
Short-period sea level changes around the islands of Mauritius and Rodrigues in the central Indian Ocean have been analysed to determine the favoured seiche periods. The largest amplitudes are found inside the harbours of Port Louis, Mauritius (7 and 20 minutes) and Port Mathurin, Rodrigues (25 minutes), but these amplitudes are much smaller just outside the harbours, and the characteristic periods are found only locally, confirming that local topography controls the periods of the seiching. The extent and energising of these seiching phenomena calls for further investigation. Associated seiche currents, potentially much stronger than tidal currents, could influence harbour shipping movements. Analysis of the sea level data showed that there are significant spatial variations in the amplitudes and phases of the tides around Mauritius Island, but the around-island tidal variations are much smaller for Rodrigues Island.
Highlights
Seiching is the oscillation of bodies of water at natural periods, controlled by the depth and horizontal dimensions of those bodies
Short period oscillations of sea level are commonly observed at coastal locations around many oceanic islands, including and perhaps, especially, in the Indian Ocean
0 gh(x) where L= length of basin; h= depth of water in basin; n= number of nodes. When this formula is applied for Port Louis, there are no oscillations at the gauge outside the harbour at the period of 20 minutes. It seems that Port Louis harbour acts as a quarter wave resonator with a node formed at the mouth of the harbour and an antinode at the head of the harbour
Summary
Seiching is the oscillation of bodies of water at natural periods, controlled by the depth and horizontal dimensions of those bodies. Short period oscillations of sea level are commonly observed at coastal locations around many oceanic islands, including and perhaps, especially, in the Indian Ocean. These oscillations can have periods lasting a few minutes to more than an hour, and amplitudes sometimes greater than 0.2 m. At Mahe in the Seychelles oscillations of periods of 10 and 50 minutes were found to be superimposed on the larger tidal oscillations of sea level (Pugh, 1979),. Shorter period seiching at Aldabra with periods from 3 to 5 minutes, were correlated with high swell, reaching a maximum between June and August, the season for maximum Southern Ocean storms, and may have been due to surf beat phenomena (Pugh, 1979)
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