Three-Dimensional Seismic Response Analysis of Zhubi Reef in the South China Sea

Reef islands are valuable terrestrial resources for ocean exploitation and utilization but face the serious threat of earthquake disasters. This study takes Zhubi Reef in the South China Sea as the research object and establishes a three-dimensional (3D) reef-seawater model to investigate the seismic response of Zhubi Reef. The 3D local topography and the effect of fluid-solid interaction are comprehensively considered. The artificial boundaries of fluid and solid domains are adopted to simulate the wave radiation and absorption of the semi-infinite seabed and the infinite seawater layer. The boundary substructure method is used to input the seismic waves into the numerical reef models. The results indicate that the seismic responses, including the peak values of the ground motions and the acceleration response spectra, are significantly amplified over Zhubi Reef. The acceleration response spectra on the reef flat shift closer towards the low-period direction compared with those of the seismic waves input from the subsea bedrock. In addition, the 3D topographic effect on the seismic response of Zhubi Reef is studied through a comparative analysis between two-dimensional (2D) and 3D reef models. The distribution laws of the peak response in the middle region of the reef flat calculated by the 2D and 3D models agree well with each other, and the differences are obvious in the edge areas where the local 3D topographies change drastically.

Numerical Analysis of the Flow Effect of the Menger-Type Artificial Reefs with Different Void Space Complexity Indices

Based on fractal theory, a regular fractal is used to construct symmetrical reef models (e.g., cube and triangle reef models) with different fractal levels (n = 1, 2, 3). Using the concept of fractal dimension, we can better understand the spatial effectiveness of artificial reefs. The void space complexity index is defined to quantify the complexity of the internal spatial distribution of artificial reefs models under different levels. The computational fluid dynamics (CFD) flow simulation approach was used to investigate the effects of void space complexity on the flow field performances of the symmetrical artificial reef models. The upwelling convection index (Hupwelling/HAR, Vupwelling/VAR), wake recirculating index (Lwake/LAR, Vwake/VAR) and non-dimensionalized velocity ratio range were used to evaluate the efficiency of the flow field effect inside or around artificial reefs. The surface area and spatial complexity index of artificial reefs increase with increasing fractal level. The numerical simulation data shows that the Menger-type artificial reef models with a higher spatial complexity index have better flow field performances in the upwelling and wake regions. Compared to the traditional artificial reef models, the upwelling convection index (Vupwelling/VAR) and recirculating index (Vwake/VAR) of n = 3 fractal cube artificial reef increase by 37.5% and 46.8%, respectively. The efficiency indices of the upwelling region and wake region around the fractal triangle artificial reef model are 2–3 times those of the fractal cube artificial reef model when the fractal level is 3.

High-resolution topography of the uplifting Huon Peninsula (Papua New Guinea) reveals high interstadial sea-levels over the past ~400 ka

<p>Quaternary sea-level curves provide crucial insights to constrain tectonic and climatic processes, but require calibration with geological constraints that are particularly scarce for cold periods prior to the last glacial-interglacial cycle. To derive such constraints, we re-visit the Huon Peninsula in Papua New Guinea, which is a classic coral reef terrace (CRT) site that was used for the earliest relative sea-level (RSL) curves. We use digital surface models calculated from 0.5m Pleiades satellite imagery to improve RSL constraints, and unlike previous studies, we find that large-scale tilting of the terrace sequence is generally N-directed. This implies that RSL estimates are several meters higher than previously thought for most highstands over the past ~125 ka. We use the large-scale geometry of the terrace sequence to estimate sea-level highstands up to ~400 ka, and our results suggest that global mean sea-level curves derived from oxygen isotopes consistently underestimate sea-level during the relatively cold Marine Isotope Stages 3, 5a, 5c, 6, 9a and 11a, up to ~10-20 m. We use coral reef models to show that our age interpretation is consistent with the overall terrace sequence morphology, and fits between models and topography improve when adjusting sea-level highstands according to our findings.</p>

Species-specific artificial reef models for lobster (Homarus gammarus Linnaeus 1758)

The objective of this study is to determine the possible artificial reef models preferred by lobster species (Homarus gammarus Linnaeus, 1758). For this purpose, eight artificial reef models at different shapes and sizes have been designed. These models include flat, grilled, U-type, cowl, fireplace chimney brick shapes made from the materials of wood, concrete and metal. Artificial reefs were deployed at 5, 10, 20 m water depths and 500 m from the shore on the coast of Erdek Ocaklar Bay in Marmara Sea. Then, monthly monitoring was conducted and SCUBA equipment were used during underwater monitoring. The time for deploying artificial reefs on the sea bottom (effort) was 233 minutes/diver and the time spent underwater by the divers was calculated as 540 minutes/diver. As a result of the study, 32 individuals of H. gammarus were observed in/around the artificial reefs. A significant increase was observed in the number of lobsters when the temperature dropped below 14°C. However, it was determined that lobster individuals preferred only concrete artificial reef models at the depth of 20 m. In addition, it was observed that small individuals preferred 40×80 cm flat concrete model (24 individuals) and large individuals preferred U-type concrete model (8 individuals). Therefore, it has been proposed to the usage of concrete artificial reef models in order to protect and ensure the sustainability of the reared lobster stocks and to release them into these artificial reefs while introducing them to the natural environments. In this context, this study will shed light on future studies.

Brazilian coral reefs in a period of global change: A synthesis

Brazilian coral reefs form structures significantly different from the well-known reef models, as follows: (i) they have a growth form of mushroom-shaped coral pinnacles called "chapeirões", (ii) they are built by a low diversity coral fauna rich in endemic species, most of them relic forms dating back to the Tertiary, and (iii) the nearshore bank reefs are surrounded by siliciclastic sediments. The reefs are distributed in the following four major sectors along the Brazilian coast: the northern, the northeastern and the eastern regions, and the oceanic islands, but certain isolated coral species can be found in warmer waters in embayments of the southern region. There are different types of bank reefs, fringing reefs, isolated "chapeirões" and an atoll present along the Brazilian coast. Corals, milleporids and coralline algae build the rigid frame of the reefs. The areas in which the major coral reefs occur correspond to regions in which nearby urban centers are experiencing accelerated growth, and tourism development is rapidly increasing. The major human effects on the reef ecosystem are mostly associated with the increased sedimentation due to the removal of the Atlantic rainforest and the discharge of industrial and urban effluents. The effects of the warming of oceanic waters that had previously affected several reef areas with high intensity coral bleaching had not shown, by the time of the 2010 event, any episodes of mass coral mortality on Brazilian reefs.

Experimental Study on Fish Attractive Effects of Artificial Reef Models

In order to observe the attraction effects of the artificial reef models on Alepocephalus, the behavior and distribution of Alepocephalus in the experimental flume under the presence of five reef models with different shapes and structures were studied using experimental ethology method, and the attraction effects of different reef models were compared. The results indicate that the distribution of Alepocephalus is rather dispersed when no reef model is put into the marked area of the flume, and they prefer to stay in relatively dark area. In this case, the Alepocephalus distribution ratio of the reef marked area is 7%. When reef models are put into the marked area, Alepocephalus shows obvious positive taxis to them. And the Alepocephalus distribution ratio of the reef marked area increased to a level between 16 % and 29 %. In addition, the apertured cubical reef model shows the best attraction effect with a distribution ratio of 29 %.