Physical processes in an Indian Ocean atoll

Most climate projections suggest that sea level may rise on the order of 0.5–1.0 m by 2100; it is not clear, however, how fluid flow and sediment dynamics on exposed fringing reefs might change in response to this rapid sea-level rise. Coupled hydrodynamic and sediment-transport numerical modeling is consistent with recent published results that suggest that an increase in water depth on the order of 0.5–1.0 m on a 1–2 m deep exposed fringing reef flat would result in larger significant wave heights and setup, further elevating water depths on the reef flat. Larger waves would generate higher near-bed shear stresses, which, in turn, would result in an increase in both the size and the quantity of sediment that can be resuspended from the seabed or eroded from adjacent coastal plain deposits. Greater wave- and wind-driven currents would develop with increasing water depth, increasing the alongshore and offshore flux of water and sediment from the inner reef flat to the outer reef flat and fore reef where coral growth is typically greatest. Sediment residence time on the fringing reef flat was modeled to decrease exponentially with increasing sea-level rise as the magnitude of sea-level rise approached the mean water depth over the reef flat. The model results presented here suggest that a 0.5–1.0 m rise in sea level will likely increase coastal erosion, mixing and circulation, the amount of sediment resuspended, and the duration of high turbidity on exposed reef flats, resulting in decreased light availability for photosynthesis, increased sediment-induced stress on the reef ecosystem, and potentially affecting a number of other ecological processes.

Non-hydrostatic numerical modelling of nearshore wave transformation over shore-oblique sandbars

Pulau (P.) Semakau comprises a narrow, low‐lying island, surrounded by a wide fringing coral reef of late‐Holocene age. The modern reef flat comprises a gently sloping surface related to modern mean low water neap tide level. Six sediment facies are recognised below the line of high water of which three, adjacent to the island, are composed of terrigenous minerals and rock fragments and three are autochthonous carbonates sediments developed since local sea‐level still stand (c. 6,500 years BP). The fringing reefs of P. Semakau and Singapore differ from many of the reef forms recorded from Peninsular Malaysia waters and the Gulf of Thailand. The Singapore reefs have wide, well‐developed intertidal reef flats, lack lagoons and reef crests, and have a very steep reef slope. In contrast to most other fringing reefs of the region, the Singapore reefs have developed in a low wave energy, meso‐tidal environment lacking strong environmental gradients.

Tide Effects on Wave Attenuation and Wave Set-up on a Caribbean Coral Reef

Reef profiles are remarkably consistent in general design because they are essentially biological constructions responding to environmental factors. All have reef flats, a horizontal expanse at low tide level that develops when the reef can no longer build upwards and extends horizontally. This is commonly the largest expanse and is the only part that early naturalists could examine until the advent of scuba diving relatively recently. However, it is always relatively depauperate because of the extremes of heat, salinity, and light that affect it. To seaward is the reef crest, commonly constructed by calcareous red algae that are the only form of life that can withstand the severe waves that pound the reef. To seaward of the crest, the reef slopes downwards, and is where environmental conditions are most benign and where the greatest biodiversity is found. The rugosity or 3-D structure of a reef is a key factor in reef biodiversity because it provides a vast number of niches for the huge diversity of life. Reefs are constantly subjected to erosion from burrowing and tunnelling forms, whose activity creates sand and sediment. This is then forced into crevices or pushed up onto the island behind it, so this erosion is also the starting point for further reef and island construction. Expanses of sand are associated with all reefs, and are home to another rich assemblage of species.

Kang, Y.K. and An, S.H., 2018. Development of New Coastal Erosion Countermeasure Using Wave Energy Control Method. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1516–1520. Coconut Creek (Florida), ISSN 0749-0208.Coastal erosion has become a social issue around the world, including Korea. A Considerable number of coastal erosions have occurred along the East Sea in Korea, primarily caused by port and harbor developments. Countermeasures have generally utilized wave breaks with wide–low–crest submerged breakwater (so-called artificial reefs), which are coastal constructions emulating the wave absorbing effect of natural reefs with good wave–protection performance. However, these result in secondary damages such as scouring near openings and erosion towards the rear because of strong currents generated by wave setups. This study suggests a new countermeasure for coastal erosion by controlling wave energy differentiated by incident wave, i.e., wave direction control method (WDCM) to prevent the critical coastal erosion expected from a massive harbor expansion project planned along the east coast of Korea. The WDCM attempts to balance disparities in wave energy caused by harbor expansion to restrain imbalanced wave energies that result in imbalanced littoral drift. To accomplish this purpose, low crest structures are obliquely deployed at 250–300 m from the coastline to manipulate the wave energy by modulating incident wave direction. This new deployment method avoids excessive interference with the incident waves, and prevents resilience by leaving cross-shore sediment transport unhindered. In order to determine the effectiveness of the proposed WDCM, numerical computations were conducted evaluating its performance regarding coastal erosion prevention.

Field measurements show rough fore reefs with spurs and grooves can dissipate more wave energy than the reef crest

Field observation data of water level at three river mouths are collected and analyzed in order to find quantitative relationship between wave set-up height at a river entrance and wave characteristics. At a relatively bigger river mouth which is protected by two jetties, wave set-up cannot be observed in the river entrance, whereas at smaller river mouths, distinct water leve l rise above tidal elevation can be observed during the period high waves. An empirical expression is obtained for the relationship between wave set-up height and deep water wave height, which shows slight difference from a previous expression for wave set-up at the shoreline on a sloping beach. Introduction There have been a lot of observations of wave set-up caused by breaking waves, though, most of them have been limited to laboratory flume experiments ( e.g., Bowen et al., 1968; Battjes and Janssen, 1978 ). The reason of the scarcity of field observations might be its considerable difficulty in the surf zone, especially during high waves. In recent years, several field observations have been reported on wave set-up height in a river entrance ( Hanslow and Nielsen, 1992 ; Hanslow et al., 1996 ; Santoso et al., 1998 ; Tanaka and Shuto, 1992 ). Among these, Hanslow and Nielsen (1992) obtained very little set-up in a river entrance in Australia, whereas Tanaka and Shuto (1992) found that the wave set-up height measured at the Nanakita River mouth attained to 10-20% of the deep water wave height. The difference 1 Professor, Department of Civil Engineering, Tohoku University, 06 Aoba, Sendai 980-8579, Japan. Phone & Fax:(+81)-22-217-7451, e-mail: tanaka@ tsunami2.civil.tohoku.ac.jp 2 Professor, Department of Civil Engineering, Nihon University, Nakagawara, Tokusada, Tamura-machi, Koriyama 963-8642, Japan. 3 Civil Engineer, Construction Department, OBAYASHI Co. Ltd., 2-15-2 Kounan, Minato-ku, Tokyo 108-8502, Japan. between the results from these groups indicates that the occurrence of wave set-up has distinct dependence on the geometry of river entrance. In the present study, the measurement and analysis of wave set-up height is carried out at three river mouths with different geometry in Japan and the height is formulated in terms of the deep water wave quantities. Study Area and Data Collection Measured time-variations of water level at three river mouths ( see Table 1 and Figure 1 ) are collected and analyzed. Rivers in Japan are classified into class A and B according their dimensions and its importance. The former are governed by the national government, whereas the latter by prefectural government. The Natori River in Table 1 belongs to the former category and, subsequently, has larger catchment area as compared with other two rivers investigated in the present study. There are two jetties at the river entrance as depicted in Figure 1 to protect sediment intrusion into the mouth by waves. The water level variation in the mouth is measured every one hour by the Sendai Construction Office, Ministry of Construction. More detail of the study area can be found elsewhere ( Tanaka et al., 1996 ; Tanaka et al., 1997 ). On the other hand, the Nanakita River is classified into class B river as shown in Table 1. It is located about 9km north to the Natori River as seen Figure 1 and there is a jetty on the left-hand side of the mouth which limits the northward migration of the mouth, while the movement towards to the south is not limited. A comprehensive investigation on river mouth topography change has already been made one of the authors ( Tanaka et al., 1996 ; Tanaka and Ito, 1996 ). The water level variation in the river entrance is measured by means of an automatic water level gauge shown in Photo 1 every 5 minutes. The Natsui River is also rank B river, though, its has about three times larger catchment area as compared with the Nanakita River as seen in Table 1. Another distinct difference from other river mouths in Table 1 is the absence of a jetty at the river mouth. A water level gauge has already been installed at Suga and Yokokawa Stations in Figure 1 by the Fukushima Prefectural Government and the data has been being obtained every three hours. Table 1 Characteristics of study area. class catchment area (km) length of river (km) number of jetties Natori River A 984 55 two Nanakita River B 229 45 one Natsui River B 749 67 none Figure 1 Location map of observation sites. Photo 1 Measuring station of water level at the Nanakita River mouth. JAPAN SENDAI .. IWAKI

Study on wave-induced setup over fringing reefs in the presence of a reef crest

Algal turf assemblages of the northern Great Barrier Reef, Australia, were sampled to determine the nutritional value of detritus and algae. Samples were collected with a suction apparatus across an exposure gradient from (1) the reef crest at highly exposed outer barrier reefs, (2) the reef crest of moderately exposed midshelf reefs, and (3) the reef slope of sheltered midshelf reefs. The biomass of algae and detritus decreased from sheltered midshelf reefs to moderately exposed midshelf reefs to highly exposed outer barrier reefs. This decrease was significant only for detritus (P < 0.005). Wave energies were calculated across the exposure gradient with the wave model WAMGBR. Detrital mass was inversely correlated with predicted wave energies and fitted a polynomial relationship (P < 0.001) and explained 52.8% of the variation. A similar relationship was also found between algal mass and wave energy (P < 0.001) but only explained 30.0% of the variation. The nutritional value of samples in protein amino acids and starch was assessed. The amino acid composition of detritus and algae was similar and not considered nutritionally different, whereas the concentration of protein amino acids was significantly (P < 0.001) higher in detritus (21.2 ± 2.0 mg g−1) than in algae (11.8 ± 1.0 mg g−1). Starch content was significantly (P < 0.05) higher in algae (7.7 ± 0.9 mg g−1) than in detritus (6.0 ± 1.0 mg g−1). These results demonstrate that detritus is a potentially valuable food source to grazing fishes on coral reefs.

Gravity wave (GW) momentum flux spectra help to uncover the mechanisms through which GWs influence momentum transfer in the atmosphere and provide crucial insights into accurately characterizing atmospheric wave processes. This study examines the momentum flux spectra of GWs in the troposphere (2–14 km) and stratosphere (18–28 km) over Koror Island (7.2°N, 134.3°W) using radiosonde data from 2013–2018. Utilizing hodograph analysis and spectral methods, the characteristics of momentum flux spectra are discussed. Given that the zonal momentum flux spectra of low-level atmospheric GWs generally follow a Gaussian distribution, Gaussian fitting was applied to the spectral structures. This fitting further explores the seasonal variations of the zonal momentum flux spectra and the average spectral parameters for each month. Additionally, the GW energy is analyzed using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) satellite data and compared with the results of the momentum flux spectra from radiosonde data, revealing the close negative correlation between wave energy and wave momentum for stratospheric GW changing with time. The findings indicate that the Gaussian peak shifts more eastward in both the troposphere and stratosphere, primarily due to the absorption of eastward-propagating GWs by the winter tropospheric westerly jet and critical layer filtering. The full width at half maximum (FWHM) in the stratosphere is larger than in the troposphere, especially in June and July, as the spectrum broadens due to propagation effects, filtering, and interactions among waves. The central phase speed in the stratosphere exceeds that in the troposphere, reflecting the influences of Doppler effects and background wind absorption. The momentum flux in the stratosphere is lower than in the troposphere, which is attributed to jet absorption, partial reflection, or the dissipation of GWs.

Measurements of waves, water levels, currents, and winds at a nearshore site on the east coast of the United States coincided with the passage of Hurricane Gloria in September 1985. These measurements are described and compared to numerical model results of winds, waves, and wave induced setup and longshore current at the site. The model hindcasts reproduce the measured wind and wave height at three locations and the wave setup and longshore current at five gages close to shore. The wave setup component is found to be a relatively small percentage (approximately 10 percent) of the total mean water level anomaly due to the storm nearshore. This is attributed to an upper limit on wave energy nearshore, controlled by the depth, which in turn results in limitation on the gradient of wave energy and hence, wave setup and wave induced longshore current.

Owing to the interactions among the complex terrain, bottom materials, and the complicate hydrodynamics, typhoon waves show special characteristics as big waves appeared at the high water level (HWL) and small waves emerged at low and middle water levels (LWL and MWL) in radial sand ridges (RSR). It is assumed that the mud damping, sandy bed friction and wave breaking effects have a great influence on the typhoon wave propagation in this area. Under the low wave energy, a mud layer will form and transport into the shallow area, thus the mud damping effects dominate at the LWL and the MWL. And high Collins coefficient (c around 1) can be applied to computing the damping effects at the LWL and the MWL. But under the high wave energy, the bottom sediment will be stirred and suspended, and then the damping effects disappear at the HWL. Thus the varying Collins coefficient with the water level method (VCWL) is implemented into the SWAN to model the typhoon wave process in the Lanshayang Channel (LSYC) of the RSR, the observed wave data under “Winnie” (“9711”) typhoon was used as validation. The results show that the typhoon wave in the RSR area is able to be simulated by the VCWL method concisely, and a constant wave breaking coefficient (γ) equaling 0.78 is better for the RSR where wide tidal flats and gentle bed slopes exist.

The influence of slow time variations of the Brun-Väisälä frequencyNupon the energy of internal gravity waves is investigated. It is found that, when time variations inNare produced by a mean deformation field (reversible mean state), the wave energy can vary in either direct or inverse proportion, depending on the wavenumber orientation. WhenNchanges owing to a certain type of irreversible process, the wave energy varies with only inverse proportionality.The nocturnal planetary boundary layer (NPBL) provides an example whereN=N(z, t). The full initial/boundary-value problem for anN(z, t) similar to the climatological mean for theNPBLis solved.

Clonal populations are often characterized by reduced levels of genotypic diversity, which can translate into lower numbers of functional phenotypes, both of which impede adaptation. Study of partially clonal animals enables examination of the environmental settings under which clonal reproduction is favoured. Here, we gathered genotypic and phenotypic information from 3,651 georeferenced colonies of the fire coral Millepora platyphylla in five habitats with different hydrodynamic regimes in Moorea, French Polynesia. In the upper slope where waves break, most colonies grew as vertical sheets ("sheet tree") making them more vulnerable to fragmentation. Nearly all fire corals in the other habitats are encrusting or massive. The M. platyphylla population is highly clonal (80% of the colonies are clones), while characterized by the highest genotype diversity ever documented for terrestrial or marine populations (1,064 genotypes). The proportion of clones varies greatly among habitats (≥58%-97%) and clones (328 clonal lineages) are distributed perpendicularly from the reef crest, perfectly aligned with wave energy. There are six clonal lineages with clones dispersed in at least two adjacent habitats that strongly demonstrate phenotypic plasticity. Eighty per cent of the colonies in these lineages are "sheet tree" on the upper slope, while 80%-100% are encrusting or massive on the mid slope and back reef. This is a unique example of phenotypic plasticity among reef-building coral clones as corals typically have wave-tolerant growth forms in high-energy reef areas.