Bikini and nearby atolls, Marshall Islands; oceanography (physical) : Adjustment of Bikini atoll to ocean waves

Abstract

Four and one-half months early in 1946 were spent in the field at Bikini and Rongelap Atolls to study the circulation systems within the lagoons. Later, in the laboratory, synoptic studies were made of kinimatically similar models based on early field data. The results of these studies were then tested against further observations made in the field. It was found that both lagoons exhibited a primary circulation (overturning winddriven circulation) and a secondary circulation (rotary circulation composed of two counter-rotating compartments). The direction of the primary circulation and the division between the counter-rotating compartments of the secondary circulation were always contained in a vertical plane having roughly the azimuth of the prevailing wind. The vigor of both circulations varied with the average wind strength taken over the previous 24 hours. Perturbations of the primary and secondary modes of circulation were produced by tides and waves and by the North Equatorial Current, which surrounds the atolls of the northern Marshall Islands group. Seasonal changes also were noted: the circulation of the winter months, when the northeast trade winds prevail, showed a marked stability and strength which was lacking in the summer season, when the atolls are on the edge of the belt of doldrums. In addition to the qualitative synopsis of the circulations, numerical data are presented in tabular form which show the volumetric exchanges of water between the lagoons and the ocean. An estimate is made of the rates of refreshment and overturning of each lagoon. A chart of the values of horizontal diffusion in the surface plane is given for Bikini atoll. NOTE.-Contribution from The Seripps Institution of Oceanography, New Series No. 700.

Wave and Tidally Driven Flow Dynamics Within a Coral Reef Atoll off Northwestern Australia

“…For wave‐driven reefs, wave forces (radiation stress gradients) generated by breaking waves increase the mean water level (wave setup) in the surf zone, establishing pressure gradients that drive flow across reef flats and into a lagoon, with water eventually returning back to the ocean through channels in the reef (Figures 1a and 1b; Gourlay, 1996; Lowe et al., 2009a; Munk & Sargent, 1954; Symonds et al., 1995; Young, 1989). For tidally driven reefs, tides create sea level gradients between the reef and offshore that drive oscillatory flows in and out of a reef over a tidal cycle (Figures 1a, 1c and 1d; Dumas et al., 2012; Green et al., 2018; Gruber et al., 2019; Lowe et al., 2015).…”

Part 4. Submarine topography and shoal-water ecology

“…w To be sure there is also a conspicuous coincidence of groove concentration with prevailing wind and surface current directions(Munk and Sargent, 1954; Emery, Tracey, and Ladd, 1954, p. 142), but this may be because these factors have also favored the formation of islands, today or in the past.…”

Mapping the susceptibility of reefs to rubble accumulation across the Great Barrier Reef

“…Windward, exposed reef slopes are generally steeper, resulting in rubble accumulating as talus at the slope's foot in deeper waters beyond the depth limits of this study(Montaggioni et al 2009). Reef slopes facing north, or west are typically shielded from the south-easterly trade winds-induced currents that affect the GBR for nine months of the year, such that they tend to have gentler pro les and accumulate more rubble.Spur-and-groove systems on reef slopes with high wave exposure, which have been frequently linked to signi cant rubble accumulation(Duce et al 2014;Kan et al 1997;Munk and Sargent 1954), cannot be differentiated in our analysis due to resolution constraints in the bathymetry and geomorphic data layers. Despite some mapping efforts at the Capricorn Bunker Group (Duce et al 2014), there is currently insu cient high-resolution data on the spatial distribution of spur and groove structures at a GBR scale.…”