Predicting the impact of sea-level (SL) rise on coral reefs requires reliable models of reef accretion. Most assume that accretion results from vertical growth of coralgal framework, but recent studies show that reefs exposed to hurricanes consist of layers of coral gravel rather than in-place corals. New models are therefore needed to account for hurricane impact on reef accretion over geological timescales. To investigate this geological impact, we report the configuration and development of a 4-km-long fringing reef at Punta Maroma along the northeast Yucatan Peninsula. Satellite-derived bathymetry (SDB) shows the crest is set-back a uniform distance of 315 ±15 m from a mid-shelf slope break, and the reef-front decreases 50% in width and depth along its length. A 12-core drill transect constrained by multiple 230 Th ages shows the reef is composed of an ∼2-m thick layer of coral clasts that has retrograded 100 m over its back-reef during the last 5.5 ka. These findings are consistent with a hurricane-control model of reef development where large waves trip and break over the mid-shelf slope break, triggering rapid energy dissipation and thus limiting how far upslope individual waves can fragment corals and transport clasts. As SL rises and water depth increases, energy dissipation during wave-breaking is reduced, extending the clast-transport limit, thus leading to reef retrogradation. This hurricane model may be applicable to a large sub-set of fringing reefs in the tropical Western-Atlantic necessitating a reappraisal of their accretion rates and response to future SL rise.
Many shallow-water acoustic applications (e.g., naval mine-hunting sonars, acoustic Doppler current profilers) involve devices operating in the frequency range 50–300 kHz over path lengths up to several hundred meters. At these ranges and frequencies the thermoviscous absorption of sound energy in the boundary layers of suspended particles leads to a significant acoustic attenuation. Although the viscous absorption mechanism is well known, little appears to have been done to quantify its effects over the longer path lengths now of operational interest. Additionally, scattering effects, although dominant at much higher frequencies, contribute to the total acoustic energy loss. Preliminary calculations of the acoustic attenuation due to scattering and viscous absorption by suspended particulates are presented here for particle concentrations typically encountered and at sonar frequencies of interest. This attenuation is compared with calculations of the attenuation due to seawater alone and found to be a significant contribution to the total attenuation, possibly giving an additional attenuation of 3 dB over a total path length of 100 m at 100 kHz for a particle concentration of 0.2 kg m−3. Concentrations of this order or even greater are common in coastal and estuarine waters. The effect of distributions of particle sizes on the acoustic attenuation are also investigated. Plans for future work are briefly described.
The presence of mineral particles in suspension results in excess acoustic attenuation which may be significant for high-frequency (tens to hundreds of kilohertz) sonar systems operating in shallow water. Laboratory measurements of absorption at 50-150 kHz in dilute suspensions of spherical, highly non-spherical and natural marine sediment particles are presented and compared with predictions of models for visco-inertial absorption by spherical and spheroidal particles. The methods are validated by the good agreement obtained between the predictions of the spherical model and measurements made with glass spheres. Good agreement is obtained between measurements with plate-like kaolin particles and the predictions of the model for oblate spheroids. Both the spherical and spheroidal models give the approximate magnitude of the attenuation in suspensions of natural marine sediment particles over the frequency range of the measurements.
The visco-thermal absorption of sound by suspended particulate matter can be reliably measured using a reverberation technique. This absorption may have an adverse effect on the performance of sonars operating at 50-300 kHz in coastal waters where suspensions are often present in significant concentrations. A series of experiments has been performed to study the viscous absorption by suspensions in the frequency range of 50-150 kHz. In the test volumes employed, the effect is small. It is therefore measured by taking the difference in reverberation times of a volume of water with and without particles. This greatly reduces the effect on the measurement of the other sources of absorption. Even so, it is necessary to design the experiment to characterize and minimize acoustic losses which occur at the surfaces of the container, the hydrophones, and their cables, and losses associated with bubbles and turbulence. These effects are discussed and results for particulate absorption for suspensions of spherical glass beads are presented and compared to theoretical predictions. Measured absorption agrees well with that predicted by theory for concentrations above 0.5 kg/m 3 and up to 2.0 kg/m 3
a b s t r a c t MONK Ò and MCBEND are Monte Carlo software packages for: nuclear criticality and reactor physics; and radiation shielding and dosimetry applications, respectively. The codes are actively developed, maintained and supported by AMEC's ANSWERS Ò Software Service in line with the ANSWERS vision of providing easy-to-use software that meets the current and emerging needs of the user community. This paper summarises the current status of MONK and MCBEND and the recent developments which have been carried out to the codes, and their supporting nuclear data libraries and visualisation package.
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