Tidal studies on Aldabra

Abstract

The oceanic tide at Aldabra possesses a very large range for an atoll owing to the influence of the Mozambique Channel. The mean spring-tide range, expressed as 2 ( M 2 + S 2 ), is 2.74 m. This is much greater than the 0.5 m common to the majority of central Pacific atolls, and the 0.7 to 1.7 m for Indian Ocean atolls. Ten tide-recording stations were set up by members of the expedition. Foxboro—Yoxall tide-gauge records were obtained from most sites, though there are few accessible rock ledges near deep water suitable for the instrument, especially in the lagoon. Visual records were used extensively in a detailed study of the tidal system in Passe Houareau. Tidal predictions for Aldabra found in Admiralty Tide Tables possess marked lagoonal characteristics. They originate from ‘Grand Poste’, a site well within Grande Passe. It is recommended that records for Passe du Bois be taken as standard since they most closely represent the oceanic tide. Empirically derived tide-predicting graphs are presented for key stations around the atoll, using Kilindini as standard port. Reduction in tidal range is pronounced in the lagoon and is conspicuous even near the mouths of the major channels. Within Passe Houareau spring-tide amplitudes may be a mere 1.2 m only 500 m from the channel mouth where the oceanic range is 3.3 m. Time lag increases lagoonwards more rapidly at Passe Houareau than at either Passe du Bois or Grande Passe, and is greater at low water than at high because of the very shallow nature of the lagoon floor. The greatest recorded phase lag occurs at the head of Bras Cinq Cases, where high water is delayed by 4 1/4 h at major spring tides: at neaps the whole region is dry at high water. Harmonic analyses are presented for one oceanic and two lagoonal stations. Computer synthesis of neap tides in the lagoon demonstrates a monthly masking effect produced by prominent shallow water harmonics. This results in the lagoon being virtually tideless on the smallest neap tides. Strong higher harmonics are indicated by visual records for Ilot Marquoix at the eastern end of the lagoon. These are characterized by a plateau-like high water, where the level commonly remains stationary for over 3 h. Lagoon tide curves are markedly asymmetrical, and often show linear ebb profiles, indicating that the lagoon behaves as a simple water-filled basin in many areas. Drainage off lagoon platforms is slow, water level falling by only 3 to 5 cm h -1 . This results in the tide ebbing for to 10 h out of the 12 1/2 h cycle. Towards spring tides more water enters the lagoon through Passe Houareau than can drain away before the next tide. Water level gradually builds up until at major springs a foot of water covers the lagoon platform at low tide. Extreme low water coincides with neap tides. Because of this ponding effect significant differences in insolation are experienced by marine bottom communities. On oceanic platforms extreme low water coincides with spring tides and occurs at midday: in the lagoon, platforms are most exposed between 06h00 and 08h00 and are always covered at noon. The effects of solar insolation are therefore minimal throughout much of the Aldabra lagoon, but are at a maximum around the coast. Tidal currents were measured in each of the Passes. In the western channels maximum values of 1.5 m s -1 (3 knots) were recorded in Passe du Bois. In Passe Gionnet a peak ebb value of 3.7 m s -1 (7.2 knots) was attained at high springs, with 3 m s -1 (6 knots) sustained for over 2 h. Flow/ebb current reversals are rapid, and in Passe Gionnet accompanied by the development of standing waves. Many channels are floored with trains of reversing sand megaripples. Elsewhere scour is appreciable; a wire mesh shark cage on the floor of Passe du Bois being undercut by 25 cm in 2 weeks. Transitory megarippled sandbanks occur on the eastern flank of Passe Houareau, and bottom facies throughout the lagoon are sculptured by the strong tidal scour.