Atoll Lagoon Flushing Forced by Waves
Water level and current measurements from two virtually enclosed South Pacific atolls, Manihiki and Rakahanga, support a new lagoon flushing mechanism which is driven by waves and modulated by the ocean tide for virtually enclosed atolls. This is evident because the lagoon water level remains above the ocean at all tidal phases (i.e., ruling out tidal flushing) and because the average lagoon water level rises significantly during periods with large waves. Hence, we develop a model by which the lagoons are flushed by waves pumping of ocean water into the lagoon and gravity draining waler from the lagoon over the reef rim. Thai is, the waves on the exposed side push waler into the lagoon during most of the lidal cycle while water leaves the lagoon on the protected side for mosl of the tidal cycle. This wave-driven through flow flushing is shown to be more efficient than alternating tidal flushing with respect to water renewal. Improved water quality should therefore be sought through enhancement of the natural wave pumping rather than by blasting deep channels which would change the system to an alternating tide-driven one.
Keywords: Aloll lagoon (lushing; Hydrodynamics; Water quality; Wave pumping; Modelling; Wave set-up; Pearl fanning.
There are two natural sources of power for flushing atoll lagoons: tides and waves. Their relative importance depends on the topography of the atoll rim as well as on the local wave and tide climates. If the atoll has wide reef passes, which are deep compared with the wave height and the tidal range, the flushing will he generated mainly by the tide and the lagoon water level will oscillate within the range of the ocean tide as shown in Fig. Ih. If, on the other hand, the atoll has an almost unbroken rim of living coral growing to a few decimetres above mean sea level (MSL), the flushing will be driven by the waves as shown in Fig. la. That is, the side facing the largest waves will have large amounts of water pushed over the reef rim while water will drain to the ocean on the leeward side. The tide will modulate this process to a degree that depends on Atidc/ff, i.e., the ratio between the tidal amplitude and the wave height, Fig. la.
The Manihiki and Rakahanga atolls (Fig. 2) are located in the northern group of the Cook Islands. Both atolls consist of several wide reef flats which arc elevated above MSL (mean sea level). Consequently, the lagoons are virtually disconnected from the ocean (Solomon, 1997). This configuration is quite different from the other atolls, also located in the northern group, which have several deep reef passes that allow the ocean tide to drive the lagoon flushing. However, similar to Penrhyn, Cook Islands, the lagoon is used to grow black lipped oysters for their black pearls, which is the primary source of income for the Manihiki populace (McKcnzic, 2004). The water quality within the lagoon is therefore of great importance to the profitability of this industry. To increase the pearl yield from the lagoon, the number of oysters has been increased leading to several episodic large scale oyster deaths from disease directly linked to poor water quality (Sharma et al., 2001; McKenzie, 2004). This reduction in water quality has also increased the number of shells rejecting the seed (nucleus for pearl development) or dying after being seeded. One method to overcome these problems is improving water quality by increasing lagoon flushing. This should however be done in a way which is in harmony with the natural system. This paper aims to provide understanding of the natural system before proposing methods to enhance the flushing.