Shoreline changes in a rising sea level context: The example of Grande Glorieuse,Scattered Islands,Western Indian Ocean

This paper provides baseline data on absolute and relative sea level variations and shoreline changes in the Scattered Islands region of the Indian Ocean, based on aerial image analysis, satellite altimetry and field observations and in situ measurements from the 2009 and 2011 TAAF scientific expeditions. The analysis shows the importance of regular observations and monitoring of these islands to better understand reef island responses to climate stressors. We show that Grande Glorieuse Island has increased in area by 7.5 ha between 1989 and 2003, predominantly as a result of shoreline accretion: accretion occurred over 47% of shoreline length, whereas 26% was stable and 28% was eroded. Topographic transects and field observations show that the accretion is due to sediment transfer from the reef outer slopes to the reef flat and then to the beach. This accretion occurred in a context of sea level rise: sea level has risen by about 6 cm in the last twenty years and the island height is probably stable or very slowly subsiding. This island expansion during a period of rising sea level demonstrates that sea level rise is not the primary factor controlling the shoreline changes. This paper highlights the key role of non-climate factors in changes in island area, especially sediment availability and transport. We also evidence rotation of the island, underscoring the highly dynamic nature of reef islands.     

Wave and tidal flushing in a near-equatorial mesotidal atoll

Most of the atolls found worldwide are under microtidal regimes, and their circulation mechanisms are widely documented and well known. Here, we describe the flushing mechanisms of a small-sized mesotidal atoll, based on water-level, wave and current data obtained during two different periods (total of 60 d). Rocas is the only atoll in the South Atlantic Ocean and is built primarily of coralline algae. Two reef passages connect the atoll lagoon to the ocean. Synchronous current profilers were deployed at the two reef passages, one inside and one outside the atoll, to characterize the influence of tides and waves on the circulation. Results showed that wind waves drove a setup on the exposed side of the atoll and that currents were predominately downwind, causing outflow at both reef passages. Waves breaking on the windward side supplied water to the atoll causing the lagoon water level to rise above ocean water level, driving the outflow. However, unlike microtidal atolls, at Rocas Atoll the water level drops significantly below the reef rim during low tides. This causes the reef rim to act as a barrier to water pumping into the lagoon by waves, resulting in periodic activation of the wave pumping mechanism throughout a tidal cycle. As result, inflow occurs in the wider passage during 27% of each tidal cycle, starting at low tides and reversing direction during mid-flood tide when the water level exceeded approximately 1.6 m (while overtopping the atoll’s rim). Our findings show that tides play a direct role in driving circulation on a mesotidal atoll, not only by modulating wave setup but also by determining the duration of wave pumping into the lagoon.

     

Wave reduction in a mangrove forest dominated by Sonneratia sp.

Based on a field observation at the Vinh Quang coast in northern Vietnam, the characteristics of wave reduction due to the drag force of one mangrove species, Sonneratia sp., were quantitatively analyzed. The reduction rate of sea waves in this area changed substantially with the tidal phase, due to the unique vertical configuration of Sonneratia sp. At the shallow range of water depth, since the shape of pneumatophores of Sonneratia sp. tapers off upward, the effect of drag force by these roots on the wave reduction decreased with the increase in the water level, resulting in a decrease in the rate of wave reduction. On the other hand, when water levels rose above the height of thickly spread branches and leaves of these trees, the rate of wave reduction increased again with an increase in the water level. Further, at this high range of water level, the rate of wave reduction depended strongly on the incident wave height. These results indicate that the thickly grown mangrove leaves effectively dissipate huge wave energy which occurs during storms such as typhoons, and protect coastal areas. Referring to the past studies, our results suggest that the hydrodynamic knowledge in various mangrove conditions such as the vertical configuration of mangrove species, their vegetation conditions, water depth, incident wave condition and the relationships between these factors should be further accumulated and then quantitatively formulated in order to protect coastal areas from severe sea waves.     

On the circulation in the Puerto Morelos fringing reef lagoon

For a period of 22 months beginning in September 2003, an array of four current profilers were deployed on the Puerto Morelos fringing reef lagoon, a microtidal Caribbean environment characterised by the influence of the Yucatan Current (YC) and a Trade Wind regime. The dataset includes water currents, bottom pressure, and surface waves complemented with coastal meteorological data and surface currents from an acoustic Doppler current profiler moored 12 km offshore. Normal circulation conditions consisted of a surface wave-induced flow entering the lagoon over a shallow reef flat and strong flows exiting through northern and southern channels. This wave induced flow was modulated by a low-frequency sea level change related to a geostrophic response to the YC variability offshore, with tidal and direct wind forcing playing additional minor roles. Under extended summer low-wave height conditions, together with a decrease in sea level from the intensification of the offshore current, the exchange of the lagoon with the adjacent ocean was drastically reduced. Under normal wave conditions (H _S = 0.8 ± 0.4 m, mean ± SD), water residence time was on average 3 h, whereas during Hurricane Ivan’s extreme swell (H _S = 6 m) it decreased to 0.35 h.     

A physical derivation of nutrient-uptake rates in coral reefs: effects of roughness and waves

Mass-transfer rates between water and benthos are derived based on the dissipation of energy by the benthic communities of coral reefs. Roughness of the benthic communities causes currents and waves to dissipate energy on reef flats at rates which far exceed ocean values of energy dissipation. The derivation here shows that first-order rate constants for nutrient uptake are (1) proportional to energy dissipation to the 0.25 root, (2) proportional to the bottom shear stress to the 0.4 root, and (3) proportional to current speed to the 0.75 root (decreasing to the 0.4 root under extreme wave activity). The shear stress, thus nutrient uptake, is positively correlated to the large-scale roughness, and to excess wave height (above the breaking height) of incoming waves. These causal relationships between nutrient-uptake rates and dissipation of energy support the general observations of reef zonation and reef metabolic rates, and are the paramount reason that coral reefs can maintain high productivity in low-nutrient tropical waters.     

Currents and flusing of Britomart reef lagoon,Great Barrier Reef

Currents, sea levels and weather were measured for a 60 d period at a number of sites in and near Britomart reef, Great Barrier Reef. The tidal currents were primarily semi-diurnal in character. The lowfrequency currents were due to a simple balance, in the longshore direction, between the wind stress and the bottom frictional stress, and, in the cross-shelf direction, between the bottom frictional stress and the pressure gradient generated by the passage of wind-driven continental shelf waves. The bottom friction coefficient, for the nonlinear drag law, is estimated to be of the order of 0.02. By contrast, the currents over the surrounding continental shelf are primarily controlled by continental shelf wave dynamics and are not dominated by friction. It thus appears that the water bodies in, respectively, the reef matrix and the Lagoon of the Great Barrier Reef are relatively uncoupled. Tidal and low-frequency currents contributed roughly equally to the flushing of Britomart reef lagoon with an estimated flushing time of the order of 4 d.     

Physical processes in an Indian Ocean atoll

 Detailed measurements of water levels, and tide and wave-induced currents were undertaken to examine physical processes and their relationship with morphology in the Cocos (Keeling) Islands, a medium sized atoll in the Indian Ocean. Results indicate that the atoll structure controls both lagoon circulation and the spatial pattern of energy distribution. Lagoon circulation is tide dominated (currents 16–31 cms^-1) with flushing (2–5 days) of the lagoon occurring through the deep leeward passages. Wave- and tide-driven unidirectional flows through shallow passages (26–65 cms^-1) are important mechanisms of ocean to lagoon water exchange and contribute up to 24% of the lagoon neap tide prism. Reef flats are dominated by wave energy (maximum velocity 140 cms^-1, east) with measurements of the attenuation of wave energy between reef flats and shallow lagoon (80–90%) conforming to measurements from fringing and barrier reefs. Spectral analysis shows that the characteristics of wave energy vary on different sectors of the atoll, with gravity wave energy dominating the east, and infragravity wave energy dominating the southern reef flat and passages. Wave setup at the reef crest is considered to be responsible for an identified 0.1 m higher water level in the southern as opposed to eastern and northern atoll, which promotes higher reef flat growth. Transmission of gravity waves across reef flats requires threshold water depths of 0.65 (east) and 0.70 m (south). The higher southern reef is an effective filter of gravity wave energy for most tidal elevations. Differences in the type and magnitude of physical processes within the atoll are discussed with relation to geomorphic development on Cocos. Accepted: 28 February 1998     

The large-scale influence of the Great Barrier Reef matrix on wave attenuation

Offshore reef systems consist of individual reefs, with spaces in between, which together constitute the reef matrix. This is the first comprehensive, large-scale study, of the influence of an offshore reef system on wave climate and wave transmission. The focus was on the Great Barrier Reef (GBR), Australia, utilizing a 16-yr record of wave height from seven satellite altimeters. Within the GBR matrix, the wave climate is not strongly dependent on reef matrix submergence. This suggests that after initial wave breaking at the seaward edge of the reef matrix, wave energy that penetrates the matrix has little depth modulation. There is no clear evidence to suggest that as reef matrix porosity (ratio of spaces between individual reefs to reef area) decreases, wave attenuation increases. This is because individual reefs cast a wave shadow much larger than the reef itself; thus, a matrix of isolated reefs is remarkably effective at attenuating wave energy. This weak dependence of transmitted wave energy on depth of reef submergence, and reef matrix porosity, is also evident in the lee of the GBR matrix. Here, wave conditions appear to be dependent largely on local wind speed, rather than wave conditions either seaward, or within the reef matrix. This is because the GBR matrix is a very effective wave absorber, irrespective of water depth and reef matrix porosity.     

Distant Storms as Drivers of Environmental Change at Pacific Atolls

The central Pacific Ocean with its many low lying islands and atolls is under threat from sea level rise and increased storm activity. Here, we illustrate how increasing frequency and severity of large scale storm events associated with global climate change may be particularly profound at the local scale for human populations that rely on lagoon systems for provision of a variety of goods and services. In August 2011 a storm originating in the Southern Ocean caused a large amplitude ocean swell to move northward through the Pacific Ocean. Its arrival at Palmyra Atoll coincided with transient elevated sea surface height and triggered turnover of the lagoon water column. This storm-induced change to the lagoon reflects long distance connectivity with propagated wave energy from the Southern Ocean and illustrates the increasing threats generated by climate change that are faced by human populations on most low-lying Pacific islands and atolls.     

Water level and currents of tidal and infragravity periods at Tague Reef, St. Croix (USVI)

A two-week study, at Tague Reef, St. Croix, USVI investigated the magnitude and spatial variation of tides, sea level differences, infragravity waves, and unidirectional cross-reef currents on a modern coral reef. Infragravity oscillations of water level (∼ 27 min period) of 1–2 cm height correlate with a quarter wavelength resonance over the shelf. Particle displacements associated with these waves may be important to the dispersive characteristics of the reef environment. Estimates of cross-reef mass transport per unit width ranged from 0.058 to 0.032 m²s ^-1. Sea level differences across the reef (1–4 cm) varied at diurnal and infragravity periods with contributions from wave set-up, and a small contribution from cross-shelf wind stress to the observed sea level differences. The quadratic bottom friction coefficient over the reef was estimated at 0.06–0.2, 20–70 times greater than on open shelves, reflecting the reef’s extreme bottom roughness. Accepted: 28 February 1998     

Holocene coral reef accretion in Hawaii: a function of wave exposure and sea level history

 In the high Hawaiian Islands, significant accretion due to coral reef growth is limited by wave exposure and sea level. Holocene coral growth and reef accretion was measured at four stations off Oahu, Hawaii, chosen along a gradient in wave energy from minimum to maximum exposures. The results show that coral growth of living colonies (linear extension) at optimal depths is comparable at all stations (7.7–10.1 mm/y), but significant reef accretion occurs only at wave sheltered stations. At wave sheltered stations in Hanauma Bay and Kaneohe Bay, rates of long term reef accretion are about 2.0 mm/y. At wave exposed stations, off Mamala Bay and Sunset Beach, reef accretion rates are virtually zero in both shallow (1 m) and deeper (optimal) depths (12 m). At wave sheltered stations, such as Kaneohe Bay and Hanauma Bay, Holocene reef accretion is on the order of 10–15 m thick. At wave exposed stations, Holocene accretion is represented by only a thin veneer of living corals resting on antecedent Pleistocene limestone foundations. Modern coral communities in wave exposed environments undergo constant turnover associated with mortality and recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. Breakage, scour, and abrasion of living corals during high wave events appears to be the major source of mortality and ultimately limits accretion to wave sheltered environments. Depth is particularly important as a modulator of wave energy. The lack of coral reef accretion along shallow open ocean coastlines may explain the absence of mature barrier reefs in the high Hawaiian Islands. Accepted: 14 May 1998     

Wave-energy distribution and hurricane effects on Margarita Reef,southwestern Puerto Rico

Wave measurements at Margarita Reef in southwestern Puerto Rico show that wave height decreases as waves travel across the forereef and into the backreef. Wave spectra reveal the presence of two wave trains impinging on the reef during the study: trade-wind waves and locally generated seas. Significant wave height calculated from the spectra show an average reduction of 19.5% from 20- to 10-m isobaths and 26% from 20- to 5-m isobaths. The significant wave height decreases an average of 82% for waves traveling across the reef crest and into the backreef. Wave-energy reduction is 35% from 20- to 10-m isobaths and 45% from 20- to 5-m isobaths. Energy loss across the reef crest is 97% which translates into the formation of strong across-the-reef currents capable of moving coarse sediment. Refraction diagrams of waves impinging on the reef from the SE provide a display of wave energy distribution around the reef. The transmission coefficients calculated for trade-wind waves and locally generated seas have means of 18% and 39%, respectively. A wave height model with negligible energy dissipation, produces wave height estimates that are, in general, within the ±15% error bands. Results of wave-energy changes from this study were applied to waves representative of hurricane conditions at the reef. Aerial photographs of the reef before and after the passage of hurricanes were compared to assess the reef changes. Changes observed in the photographs are interpreted as products of sediment transport by hurricane-generated waves. The patterns of change agree with the refraction diagrams suggesting that waves were the main agents of change at margarita Reef during severe storms. Correspondence to: A. Lugo-Fernández}     

Numerical modeling of the impact of sea-level rise on fringing coral reef hydrodynamics and sediment transport

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.     

Holocene reef evolution in a macrotidal setting: Buccaneer Archipelago,Kimberley Bioregion,Northwest Australia

This study uses information derived from cores to describe the Holocene accretion history of coral reefs in the macrotidal (up to 11 m tidal range) Buccaneer Archipelago of the southern Kimberley coast, Western Australia. The internal architecture of all cored reefs is broadly similar, constituting well-preserved detrital coral fragments, predominantly branching Acropora, in a poorly sorted sandy mud matrix. However, once the reefs reach sea level, they diverge into two types: low intertidal reefs that maintain their detrital character and develop relatively narrow, horizontal or gently sloping reef flats at approximately mean low water spring, and high intertidal reefs that develop broad coralline algal-dominated reef flats at elevations between mean low water neap and mean high water neap. The high intertidal reefs develop where strong, ebb-dominated, tidal asymmetry retains seawater over the low tide and allows continued accretion. Both reef types are ultimately constrained by sea level but differ in elevation by 3–4 m.     

An integrated habitat enhancement approach to shoreline stabilization for a Chesapeake Bay island community

Shore protection and habitat enhancement along a residential island werethe main goals of this shoreline study. The physical and geological factorsnecessary to design shoreline stabilization structures capable of confidentlysupporting suitable and stable habitat enhancement/restoration substrate areemphasized since this area of study generally may be unfamiliar to wetlandresource managers. Erosion along the targeted shoreline is influenced by aunidirectional wave field from the south-southwest. Results of our analysesshowthat a headland control system comprised of headland breakwaters could be usedsuccessfully to stabilize the existing shoreline and provide resource managersflexibility in habitat restoration decisions. Headland breakwaters are designedto diffract wave energy so that shore planform equilibrium is attained and canbe sized and positioned to maximize the length of stabilized shoreline.Maximization of the new shoreline length provides increased subaerial,intertidal, and subaqueous environments for flexible habitat restorationalternatives. The final restoration design developed through this study willcreate approximately 69,000 m² of new habitat includingstable beach, dune, tidal marsh, scrub shrub, and submersed aquatic vegetation.An additional 2,000 m² of rock substrate habitat isprovided directly by the headland control structures.     

Oyster reefs as natural breakwaters mitigate shoreline loss and facilitate fisheries

Shorelines at the interface of marine, estuarine and terrestrial biomes are among the most degraded and threatened habitats in the coastal zone because of their sensitivity to sea level rise, storms and increased human utilization. Previous efforts to protect shorelines have largely involved constructing bulkheads and seawalls which can detrimentally affect nearshore habitats. Recently, efforts have shifted towards "living shoreline" approaches that include biogenic breakwater reefs. Our study experimentally tested the efficacy of breakwater reefs constructed of oyster shell for protecting eroding coastal shorelines and their effect on nearshore fish and shellfish communities. Along two different stretches of eroding shoreline, we created replicated pairs of subtidal breakwater reefs and established unaltered reference areas as controls. At both sites we measured shoreline and bathymetric change and quantified oyster recruitment, fish and mobile macro-invertebrate abundances. Breakwater reef treatments mitigated shoreline retreat by more than 40% at one site, but overall vegetation retreat and erosion rates were high across all treatments and at both sites. Oyster settlement and subsequent survival were observed at both sites, with mean adult densities reaching more than eighty oysters m(-2) at one site. We found the corridor between intertidal marsh and oyster reef breakwaters supported higher abundances and different communities of fishes than control plots without oyster reef habitat. Among the fishes and mobile invertebrates that appeared to be strongly enhanced were several economically-important species. Blue crabs (Callinectes sapidus) were the most clearly enhanced (+297%) by the presence of breakwater reefs, while red drum (Sciaenops ocellatus) (+108%), spotted seatrout (Cynoscion nebulosus) (+88%) and flounder (Paralichthys sp.) (+79%) also benefited. Although the vertical relief of the breakwater reefs was reduced over the course of our study and this compromised the shoreline protection capacity, the observed habitat value demonstrates ecological justification for future, more robust shoreline protection projects.     

Wave-current interactions on a shallow reef (Nicaragua,Central America)

Measurements of wave height and currents associated with normal trade-wind conditions have been made on a linear reef that parallels the northern and northeastern coast of Great Corn Island, eastern shelf of Nicaragua, Central America. Analyses indicate that waves breaking over the reef crest generate lagoonward flow normal to the reef. Average reef-normal flow was in the range of 10 to 20 cm/s; however, individual wave surges reached values of up to 180 cm/s. The strength of the over-the-reef flow is modulated by the tide. Lagoon currents are weak (2–5 cm/s) and change direction with the tide as the lagoon fills and drains. Long-period oscillations in water level (30 s to 20 min) and in the current were observed, and may be important in transporting fine-grained sediments out of the reef-lagoon system. Strong, short-duration surge currents ( <5 s) transport coarse sediment from the breaker zone to the seaward margin of the backreef lagoon.     

Monitoring austral and cyclonic swells in the “Iles Eparses” (Mozambique channel) from microseismic noise

We deployed five broadband three-components seismic stations in the Iles Eparses in the south-west Indian Ocean and on Mayotte Island, between April 2011 and January 2014. These small and remote oceanic islands suffer the effects of strong ocean swells that affect their coastal environments but most islands are not instrumented by wave gauges to characterize the swells. However, wave action on the coast causes high levels of ground vibrations in the solid earth, so-called microseismic noise. We use this link between the solid earth and ocean wave activity to quantify the swells locally. Spectral analyses of the continuous seismic data show clear peaks in the 0.05–0.10 Hz frequency band (periods between 10 and 20 s), corresponding to the ocean wave periods of the local swells. We analyze an example of austral swell occurring in August 2013 and a cyclonic event (Felleng) that developed in January 2013, and quantify the ground motion at each station induced by these events. In both cases, we find a linear polarization in the horizontal plane with microseismic amplitude directly correlated to the swell height (as predicted by the global swell model WaveWatchIII), and a direction of polarization close to the predicted swell propagation direction. Although this analysis has not been performed in real time, it demonstrates that terrestrial seismic stations can be efficiently used as wave gauges, and are particularly well suited for quantifying extreme swell events. This approach may therefore provide useful and cheaper alternatives to wave buoys for monitoring swells and the related environmental processes such as beach erosion or coral reef damages.     

The Potential for Created Oyster Shell Reefs as a Sustainable Shoreline Protection Strategy in Louisiana

Coastal protection remains a global priority. Protection and maintenance of shoreline integrity is often a goal of many coastal protection programs. Typically, shorelines are protected by armoring them with hard, non‐native, and nonsustainable materials such as limestone. This study investigated the potential shoreline protection role of created, three‐dimensional Eastern oyster (Crassostrea virginica) shell reefs fringing eroding marsh shorelines in Louisiana. Experimental reefs (25 × 1.0 × 0.7 m; intertidal) were created in June 2002 at both high and low wave energy shorelines. Six 25‐m study sites (three cultched and three control noncultched) were established at each shoreline in June 2002, for a total of 12 sites. Shoreline retreat was reduced in cultched low‐energy shorelines as compared to the control low‐energy shorelines (analysis of variance; p < 0.001) but was not significantly different between cultched and noncultched sites in high‐energy environments. Spat set increased from 0.5 ± 0.1 spat/shell in July 2002 to a peak of 9.5 ± 0.4 spat/shell in October 2002. On average, oyster spat grew at a rate of 0.05 mm/day through the duration of the study. Recruitment and growth rates of oyster spat suggested potential reef sustainability over time. Small fringing reefs may be a useful tool in protecting shorelines in low‐energy environments. However, their usefulness may be limited in high‐energy environments.     

Mapping and analysis of the physical vulnerability of coastal Tamil Nadu

Abstract

This is an integrated study to classify Tamil Nadu Coast in terms of its vulnerability to coastal processes. It is accomplished through estimating Coastal Vulnerability Index (CVI) for the entire stretch of the said coast using weighted averages of six parameters, viz., geomorphology, shoreline changes, coastal slope, and relative sea level rise, mean significant wave height and mean tidal range. Thus, CVI evaluates the state of the coast in terms of level of risk hovering on it as and when it is affected by oceanic hazards. The target of this study is to evaluate the vulnerability status of coastal stretches of 35 taluks of Tamil Nadu. Twenty one percent of the coastal stretch is under very high vulnerability level. Following this is 36% stretch under high vulnerability state. Another 34% coastal stretch is in moderate setting. Low vulnerability state prevails about 6%. Further, it is learnt that there are only 2% of Tamil Nadu coastal region coming under the least vulnerable category. The study revealed the very high vulnerability of Nagapattinam coastal region. The present study may also help create awareness among people about marine natural hazards such as coastal erosion, inundation, loss of life and their properties.