Fore-reef Slope Research Articles

2DH Numerical Study of Solitary Wave Processes around an Idealized Reef-Fringed Island

In order to better understand the role of coral reefs around an isolated island in mitigating tsunami hazards, this study performed a horizontally two-dimensional (2DH) numerical study of tsunami-like solitary wave propagation and run-up around an idealized reef-fringed island. The shock-capturing Boussinesq wave model, the FUNWAVE-TVD is used in the present study and well-validated with existing experimental data for its robustness in predicting 2DH solitary wave processes around an island. Based on the validated model, the typical solitary propagation process around the reef-fringed island and the effects of morphological and hydrodynamic parameters on the maximum run-up heights were systematically investigated. It is found that coral reefs can effectively reduce maximum run-up heights around an isolated island. The reef flat’s water depth, reef flat width, and reef surface roughness are the main factors affecting maximum run-up heights around an island, while the fore-reef slope has little impact. For the idealized reef-fringed island in this study, sea-level rise will cause coral reefs to lose their protective capability on the lee side, and the presence of coral reefs may even enhance tsunami hazards around an island when the reef flat width is very narrow or coral bleaching happens.

Effects of internal waves on bottom thermal structures and turbulent mixing in the Xisha Islands

Using velocity and high-resolution temperature mooring data from the fore-reef slope of Yongxing Island in the northwest South China Sea (water depth of 69 m), we examine the effects of internal waves on the temporal variations in temperature, bottom mixed layer (BML) and turbulent mixing. The diurnal tide is found to be the dominant tidal force and the baroclinic tide is highly active, which would account for the 11 d abnormal spring-neap cycle in the barotropic tidal current. During the ebb period (tidal elevation decreases), the bottom diurnal baroclinic current transports cold water upslope, resulting in a decrease in temperature, and vice versa during the flood period. The BML thickness Hbml widely varies around 1.5m, approximately 2% of the water depth. The bottom turbulent mixing is not so active, indicated by the bulk dissipation Eɛ of 10 mWm−2 and turbulent diffusivity κz of 2×10−4m2s−1. Both Hbml and Eɛ approximate a log-normal distribution, demonstrating strong intermittency. The high-frequency (ω) internal bores can increase Hbml by four times and enhance the turbulent mixing by one order, which should be responsible for the slow cascade of Hbml(∼ω−1.5) and Eɛ(∼ω−1.0). In the downslope phase, Hbml is about 30% thicker, and turbulent mixing is enhanced by 3 times stronger than those in the upslope phase. It is revealed that under the forcing of internal waves, turbulent mixing corresponds to a thick BML with Hbml∼〈κz〉0.25, and stratification (N) has a significantly negative correlation with the development of BML, Hbml∼〈N〉−1.8.

Numerical investigation of infragravity wave hydrodynamics at fringing reef with a permeable layer

In tropical and subtropical coastal regions, coral reefs are abundant and play a vital role in maintaining ecosystem balance. Additionally, they effectively dissipate a significant amount of wave energy that propagates from the open sea towards the coastline, providing coastal areas with protection against wave impacts. Consequently, numerous scholars have conducted extensive research to investigate the hydrodynamic characteristics of wave propagation and transformation over coral reef topography. However, previous studies have often simplified the bottom boundary of coral reefs as impermeable layers, neglecting the fact that the coral reef consists of a permeable canopy structure in the actual marine environment. To fill the knowledge gap of previous research, this study is based on the Non-Hydrostatic Wave Model Solver (NHWAVE) to numerically simulate the propagation process of random waves over permeable coral fringing reefs. The study takes a comprehensive approach by considering the influences of various factors on the hydrodynamic characteristics of random waves over the fringing reef, including incident wave height, reef-flat water depth, peak wave period, permeable layer thickness, permeable layer porosity, and median diameter of the permeable layer. This paper focuses primarily on analyzing the variations in sea-swell wave height (HSS), infragravity wave height (HIG), and mean water level (MWL) along the fringing reef, while conducting a comparative analysis between fringing reefs with a permeable layer and impermeable fringing reefs. The findings reveal that the presence of a permeable layer reduces the shallow water deformation of waves on the fore-reef slope and mitigates the wave-breaking phenomenon near the reef edge, thereby significantly reducing the sea-swell wave height (HSS), infragravity wave height (HIG), and wave setup on the reef-flat. Furthermore, the permeable layer's existence also leads to a decrease in the maximum wave run-up height on the back-reef slope. The research findings of this study can further enhance our understanding of the hydrodynamic characteristics of infragravity waves over fringing reefs, which is significant for studying the impact of random waves on coastal areas and understanding the protective mechanisms of coral reefs in coastal regions.

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

Disturbance-induced rubble accumulations are described as “killing fields” on coral reefs as coral recruits suffer high post-settlement mortality, creating a bottleneck for reef recovery. The increasing frequency of coral bleaching events, that can generate rubble once coral dies, has heightened concerns that rubble beds will become more widespread and persistent. But we currently lack the tools to predict where rubble is most likely to accumulate. Here, we developed a modelling framework to identify areas that are likely to accumulate rubble on forereef slopes across the Great Barrier Reef. The algorithm uses new high-resolution bathymetric and geomorphic datasets from satellite remote sensing. We found that 47 km of reef slope (3% of the entire reef surveyed), primarily in the southern region, could potentially reach 50% rubble cover. Despite being statistically significant (p < 0.001), the effects of depth and aspect on rubble cover were minimal, with a 0.2% difference in rubble cover between deeper and shallower regions, as well as a maximum difference of 0.8% among slopes facing various directions. Therefore, we conclude that the effects of depth and aspect were insufficient to influence ecological processes such as larval recruitment and recovery in different coral communities. Maps of potential rubble accumulation can be used to prioritise surveys and potential restoration, particularly after major disturbances have occurred.

Focused wave group propagation over fringing reef and its impact on the vertical wall mounted on a reef flat: Experiment and RANS modeling

This paper presents flume experiments and numerical simulations of focused wave group propagation over an idealized fringing reef profile and its impact on the vertical wall mounted on reef flat. The large transient wave group is produced over the steep fore-reef slope by focusing JONSWAP wave trains. The numerical simulations are performed using the compressible two-phase OpenFOAM model. Wavelet and FFT analysis of the surface elevation time histories clearly show wave focusing process and the generation of higher harmonics due to the nonlinear wave-wave interactions. For the high peak frequency cases, the low frequency wave motion increases gradually after wave breaking, but the peak frequency motion still dominates energy spectrum partially due to wave defocusing and reformation of broken waves. However, the magnitude of low frequency wave motion is higher than those around peak frequency and high harmonics for the lower peak frequency cases. It is found that the contribution of low frequency wave motion to the total force exerted on the wall is comparable to or higher than peak frequency wave motion. The effect of geometry characters of fringing reef and focusing position on the maximum horizontal force on the vertical wall is investigated utilizing the validated OpenFOAM model.

THE INFLUENCE OF CORAL REEF SPUR AND GROOVE MORPHOLOGY ON WAVE ATTENUATION

Wave energy dissipation on coral reefs provides protection from coastal hazards to over 200 million people globally (Ferrario et al. 2014). The mechanisms of wave energy dissipation over the complex morphologies of the fore-reef slope have typically been overlooked (Duce et al. 2016). Here we focus on wave attenuation by the elongated troughs and depressions typical of fore-reef slopes known as spurs and grooves (SaGs).

Defining multi-scale surface roughness of a coral reef using a high-resolution LiDAR digital elevation model

A decline in coral reef surface roughness may indicate that a coral reef ecosystem is approaching functional collapse. This is because surface roughness underpins many critical ecosystem metrics, such as, live coral cover and high fish biomass. Yet, we understand very little about how surface roughness changes at ecosystem scales due to the limited spatial extents of coral reef ecological surveys. Here, we investigate the surface roughness of a coral reef across multiple spatial scales on a high-resolution Light Detection and Ranging (LiDAR) derived digital elevation model (DEM, 0.25 m cells). We found that complex features dominated regions with high roughness (e.g., spur and groove systems on the reef slope) and increased the average roughness of geomorphic zones that are otherwise flat and featureless (e.g., lagoonal patch reefs in the sandy deep lagoon). Each geomorphic zone had a unique signature with high values of roughness usually expressed over finer spatial scales and, conversely, lower values of roughness observed over broader spatial scales. We allocated each geomorphic zone into one of four roughness equivalent habitats (REHs 1–4) that share similar surface roughness properties but are not necessarily geographically contiguous regions. These results identify fine scale features and geomorphic zones that are important for providing roughness to coral reef systems, such as spur and groove systems and the fore-reef slope respectively. They also suggest that morphodynamic processes operating at broader spatial scales influence the physical structure of coral reef ecosystems. The quantification of coral reef surface roughness is becoming increasingly important due to the wider availability of high (<1 m) and hyper (<0.1 m) resolution DEMs. Our approach and findings here can be used to provide greater surface textural information for common coral reef geomorphic zones and aid future management and research efforts, such as, monitoring of coral reef ecosystem response to environmental change and reef restoration and adaptation programs.

Experimental Investigation of Wave Propagation and Overtopping over Seawalls on a Reef Flat

A series of physical model tests were performed to investigate the wave propagation, pressure, and wave overtopping processes for vertical and sloped seawalls on the reef flat. For both regular and irregular waves, the effects of incident wave height, wave period, relative distance, fore reef slope, and still-water depth on wave propagation, wave pressure distribution, and mean wave overtopping discharge were investigated. The peak pressure increased with the increase in incident wave height and decreased with the increase in relative distance and fore reef slope. The mean wave overtopping discharge over the vertical and sloped seawalls increased with the increase in incident wave height, wave period, and still-water depth, but it was significantly reduced with the increase in relative distance and fore reef slope. Modified empirical formulas are proposed for predicting the wave force and mean wave overtopping discharge over the vertical and sloped seawalls on the reef flat for both regular and irregular waves.

Effects of bottom roughness on wave transmission across a submerged reef

This paper presents a laboratory study to investigate the effects of bottom roughness on wave transmission across a submerged coral reef that has a steep fore-reef slope. Experimented models were carried out in wave flumes for four different bed roughness levels: including a smooth bed and three roughness heights. Our investigations showed that there is a significant dissipation of energy in short-period waves (SS-waves), owing to bottom friction, compared with that of infra-gravity waves (IG-waves). In addition, the study found that the IG-wave heights are increased slightly as waves propagate shoreward from the reef-edge surfzone, indicating that a significant portion of energy was transferred from SS-waves to IG-waves. Empirical formulae of wave energy dissipation due to bottom friction and non-linear transfer of energy among the SS- and IG-waves have been derived; these allow for the straightforward determination of wave heights and wave periods on the submerged reef. These local wave parameters can be used to design offshore structures on submerged coral reefs.

Numerical Investigation on Hydrodynamic Processes of Extreme Wave Groups on Fringing Reef

The low-lying reef islands distributed in the tropical and subtropical coastal regions are highly vulnerable to the devastating damages of surges and waves during the severe weather events. Over the past two decades, extreme waves have caused tremendous loss and damages to the tropical and subtropical coastal regions. Previous research has focused on the wave hydrodynamics of tsunami waves, as well as regular and irregular waves on the fringing reefs. The complex wave hydrodynamics of extreme waves on the fringing reefs are rarely studied. By applying the nonhydrostatic numerical flow solver (NHWAVE), transformation and breaking process of the crest- and trough-focused wave groups on the fringing reef are analyzed in this study. Influences of the major factors, i.e., water depth, significant wave height, peak wave period, forereef slope and backreef slope, and ridge width, are discussed in detail. The results show that there are complex interactions between the fringing reef and the focused wave group. Breaking waves of high intensity can form at the reef crest. Meanwhile, due to the wave breakings at the reef crest and bottom friction of the reef flat, the local wave height can be effectively reduced. Within the complex wave hydrodynamics of focused waves on the fringing reef, most of the wave energy can be dissipated. In addition, hydrodynamic difference between the crest- and trough-focused waves on the fringing reef is very limited. The research results of this study will further help researchers to better understand the wave hydrodynamics of extreme waves over the fringing reefs.

Modeling wave processes in a reef-lagoon-channel system based on a Boussinesq model

The reef-lagoon-channel system is very typical on the low-lying reef-lined coasts in tropical and subtropical areas. Comparing to the well-studied one-dimensional horizontal (1DH) fringing reefs, little is known about the alongshore wave runup variation and its implication for coastal flooding in such a two-dimensional horizontal (2DH) reef configuration. To better understand the wave processes in an idealized reef-lagoon-channel system, a numerical model is presented in this study, which is based on a set of fully nonlinear Boussinesq equations. Wave breaking is addressed by a shock-capturing scheme and bottom friction is formulated by the Manning frictional law. The adopted model is first validated by a published laboratory dataset in view of wave spectrum, wave height, wave setup, mean current as well as wave runup on the back-reef beach. Subsequently, the effects of hydrodynamic (incident wave height, incident wave period, reef-flat wave level) and reef morphological (fore-reef slope, cross-shore reef-flat width, channel width and frictional coefficient) parameters that are not fully considered by the laboratory experiments are investigated through the numerical simulations. Finally, the model is used to analyze the infragravity (IG) wave resonant modes in the system, and the effects of channel on such modes are also demonstrated.

Numerical simulation of interaction between wave-driven currents and revetment on coral reefs

The stability of a revetment breakwater is vital for the safety of islands and reefs formed with reclaimed sand under the action of a complex marine environment. The interactions in wave-reef-revetment systems are complex and difficult to accurately simulate; in addition, it is difficult to describe the dynamic wave pressures distribution by empirical formula, although these parameters are important for the safety of reefs and revetments; hence, more studies are required. In this study, an investigation of the interaction between the wave-driven current on the reef flat and rear revetment breakwater is performed, and the hydrodynamic pressure on the revetment is discussed. A two-dimensional numerical model is established using the Navier–Stokes equation to explore the interaction and calculate the pressure distribution on the revetment, which is verified by an experiment designed and implemented in a wave flume. The simulation illustrates that the wave-driven current is stratified in the vertical direction, and the velocity at the surface is approximately 2–3 times that of the average value, and the maximum flow velocity can reach 13 m/s. By varying the parameters of the geomorphology of the coral reef, the results suggest that for steeper fore-reef slopes, longer reef flats, and larger relative distances, the peak value of the pressure increases consistently. The detailed and sophisticated hydrodynamic pressure distribution on the revetment is obtained, and the numerical results indicate that the fifth-order polynomial can match the regulation of pulsating pressure at the wave surface with the relative submerged water depth.

Reef-rim structure and building history, Rangiroa, an uplifted Atoll, French Polynesia: The role of morphotectonics and extreme marine hazard events

Reconstructing the responses of low-lying, atoll-rim islands to future environmental changes due to global warming requires a robust knowledge of the building history of reef rims through time. In the northwest of Tuamotu Archipelago, atolls have experienced opposite vertical motions (uplift versus subsidence) in response to differential tectonic constraints undergone by the underlying oceanic crust. As a result, during Pleistocene high sea levels, some atolls remained emerged whereas others remained flooded. Based on two electrical resistivity tomography profiles across the northern rim-island of the uplifted Rangiroa Atoll and on U/Th dating of corals clasts collected from unconsolidated sand-rubble deposits within two wells through the same island area, the internal structure of the atoll-rim and its mode and timing of building are tentatively reconstructed. The irregularly shaped, upper boundary of the basal resistive unit is interpreted as residual, pinnacle-shaped reliefs of Miocene age. At the seaward rim side, the uppermost highly resistive layers, an about 5 m-thick, were assumed to be reef-related, partly consolidated material emplaced during the Holocene. Thicker resistive layers that infill cavities in the Miocene basement, in the central part of the atoll rim, refer to the fresh water lense in porous, more or less consolidated material (phosphorites?) presumably emplaced during the Mio-Pliocene. In northern Rangiroa, the age of the oldest coral clasts trapped into the atoll-rim islands ranges between about 11,000 and 8000 calendar years when sea level was approximately 60 to 20 m below its present position. This suggests that coral-clast supply from the adjacent forereef slopes was triggered by extreme wave-surge events (presumably, tsunamis generated by island flank failures). The earliest island initiation phase is thought to start by approximately 6000 cal. yr and to be achieved by about 3000 cal. yr. By constrast, on the nearby subsiding Takapoto Atoll, the rim island began to initiate not earlier than 2500 cal. yr. At Rangiroa, morphotectonics is assumed to have played a major role in controlling the internal rim architecture and, associated to extreme marine hazard events, to have promoted the earlier settlement of low-lying, rim islands through an earlier infilling of accommodation space as compared to nearby subsiding atolls.

Smoothed Particle Hydrodynamics simulations of reef surf zone processes driven by plunging irregular waves

As waves interact with the slopes of coral reefs and other steep bathymetry profiles, plunging breaking usually occurs where the free surface overturns and violent water motion is triggered. Resolving these surf zone processes pose significant challenges for conventional mesh-based hydrodynamic models, due to the rapidly-deforming nature of the free surface and associated flows. Yet the accurate prediction of these surf zone hydrodynamics is critical for predicting a wide range of nearshore processes driven by wave breaking (e.g., wave dissipation and energy transfers; mean water levels and currents; and wave runup). In this study we assess the ability of the mesh-free, Lagrangian particle-based numerical modelling approach Smoothed Particle Hydrodynamics (SPH) based on DualSPHysics, to simulate the fine-scale hydrodynamic processes driven by irregular wave transformation over a fringing reef profile, by comparing results against detailed experimental observations from a physical modelling study. To greatly improve the computational efficiency, the SPH model was coupled to the mesh-based multi-layer nonhydrostatic wave-flow model SWASH. With this coupled approach, SWASH was used to efficiently simulate the evolution of non-breaking waves from the wavemaker up to the fore reef slope, with the SPH model then used to simulate the detailed hydrodynamic processes over the reef from just offshore of the breakpoint to the shoreline. The SPH model was able to accurately reproduce the complex free surface deformations during plunging breaking, the spectral evolution of waves across the reef flat (including nonlinear wave shape), the mean water levels and currents, and wave runup at the shoreline. Using the long duration simulations (>400 wave periods), the model was able to reproduce the full range of wave motions over the reef (from sea-swell to infragravity frequencies), including the increasing dominance of low frequency waves towards the shoreline and the large cross-reef standing wave motions excited by the reef geometry.

On the runup parameterisation for reef-lined coasts

The degradation of coastal ecosystems in recent years, combined with more intense storms and greater sea levels associated with climate change, are likely to increase vulnerability to coastal flooding along reef-lined coasts. Therefore, there is a need to accurately predict extreme water levels to identify areas with high vulnerability and implement mitigation measures. Runup parameterisations allow a rapid assessment of coastal vulnerability at a regional to global scale, however these formulations are primarily developed for beaches. Hydrodynamic forcing and reef geometry are key parameters for the estimation of coastal flooding in reef environments. The present study aims to develop runup parameterisations for an idealised 2DV reef-lined coast profile using a widely validated nonlinear non-hydrostatic numerical model (SWASH). The numerical model is employed to simulate different combinations of wave conditions, water levels, and reef geometries. A machine learning (ML) approach, in the form of genetic programming, is used to identify the most suitable predictors for wave runup based on the numerical results. Analysis of runup results suggests that runup parameterisations can be improved for reef environments by incorporating the crest elevation, lagoon width, reef flat depth, and forereef slope. A dimensional and non-dimensional parameterisation that include reef geometry are presented. Further research efforts should be devoted to incorporate the effects of bed roughness and three-dimensional processes in this framework that were not taken into account in the present work.

Atoll-dependent variation in depth zonation of benthic communities on remote reefs

The distribution and organisation of benthic organisms on tropical reefs are typically heterogenous yet display distinct zonation patterns across depth gradients. However, there are few datasets which inform our understanding of how depth zonation in benthic community composition varies spatially among and within different reef systems. Here, we assess the depth zonation in benthic forereef slope communities in the Central Indian Ocean, prior to the back-to-back bleaching events in 2014–2017. We compare benthic communities between shallow (5–10 m) and deep (20–25 m) sites, at two spatial scales: among and within 4 atolls. Our analyses showed the variation in both major functional groups and hard coral assemblages between depth varied among atolls, and within-atoll comparisons revealed distinct differences between shallow and deep forereef slope communities. Indicator taxa analyses characterising the hard coral community between depths revealed a higher number of coral genera characteristic of the deep forereef slopes (10) than the shallow forereef slopes (6). Only two coral genera consistently associated with both depths across all atolls, and these were Acropora and Porites. Our results reveal spatial variation in depth zonation of benthic communities, potentially driven by biophysical processes varying across depths and atolls, and provide a baseline to understand and measure the impacts of future global climate change on benthic communities across depths.

Numerical modelling of wave transformation and runup over rough fringing reefs using VARANS equations

Computational Fluid Dynamics (CFD) models are gaining popularity recently in efforts to understand the complex dynamics of wave interaction with coral reef profiles. To provide an alternative approach to the current CFD models to account for the reef surface with large bottom roughness, a 3D numerical wave tank based on the CFD tool OpenFOAM® is developed in this study. The VARANS equations are solved for two-phase incompressible flow with the k-ω SST model for the turbulence closure and VOF method for tracking the free surface. The reef surface with high friction is modeled by using a porous media model in the VARANS equations. The numerical model is first validated with laboratory experiments. The model is then applied to evaluate the impacts of forcing condition (wave and water level), reef morphology (reef-flat width, fore-reef slope, and back-reef slope), and reef roughness on wave transformation and wave runup. Moreover, an empirical formula is derived from the numerical results to predict wave runup. Subsequently, the simulated cross-reef variations of energy dissipation, wave spectrum, wave shape parameters (skewness and asymmetry) and nonlinearity parameter (Ursell number) are discussed for both the smooth and rough reefs. Finally, the cross-reef variations of the TKE and its dissipation rate are also examined for both reefs via the numerical simulations.

Massive cryptic microbe-sponge deposits in a Devonian fore-reef slope (Elbingerode Reef Complex, Harz Mts., Germany)

A massive occurrence of microbial carbonates, including abundant sponge remains, within the Devonian Elbingerode Reef Complex was likely deposited in a former cavity of the fore-reef slope during the early Frasnian. It is suggested that the formation of microbial carbonate was to a large part favored by the activity of heterotrophic, i.e., sulfate-reducing bacteria, in analogy to Quaternary coral reef microbialites. The Elbingerode Reef Complex is an example of an oceanic or Darwinian barrier reef system. In modern barrier reef settings, microbialite formation is commonly further facilitated by weathering products from the central volcanic islands. The Devonian microbialites of the Elbingerode Reef Complex occur in the form of reticulate and laminated frameworks. Reticulate framework is rich in hexactinellid glass sponges, the tissue decay of which led to the formation of abundant micrite as well as peloidal and stromatactis textures. Supposed calcimicrobes such as Angusticellularia (formerly Angulocellularia) and Frutexites, also known from cryptic habitats, were part of the microbial association. The microbial degradation of sponge tissue likely also contributed to the laminated framework accretion as evidenced by the occurrence of remains of so-called “keratose” demosponges. Further typical textures in the microbialite of the Elbingerode Reef Complex include zebra limestone, i.e., the more or less regular intercalation of microbial carbonate and cement. Elevated concentrations of magnesium in the microbialite as compared to the surrounding metazoan (stromatoporoid-coral) reef limestone suggests that the microbialite of the Elbingerode Reef Complex was initially rich in high-magnesium calcite, which would be yet another parallel to modern, cryptic coral reef microbial carbonates. Deposition and accretion of the microbialite largely occurred in oxygenated seawater with suboxic episodes as indicated by the trace element (REE + Y) data.

HIGH CORAL DIVERSITY IS COUPLED WITH REEF-BUILDING CAPACITY DURING THE LATE OLIGOCENE WARMING EVENT (CASTRO LIMESTONE, SALENTO PENINSULA, S ITALY)

The Oligocene, well known as the apex of Cenozoic reef growth, is a crucial period of time to investigate the mutual relationship between coral reef construction and coral diversity and their link with palaeoclimate and palaeoenvironmental changes. Here we provide a complete characterization of the Upper Oligocene reef complex of the Castro Limestone (Salento Peninsula, S Italy), which is one of the best-preserved Oligocene coral reefs of the Mediterranean region. By combining facies analysis with the first detailed characterization of its coral fauna, we show that the Castro Limestone has both a rich scleractinian coral fauna (25 genera and 41 species) and a large reef volume, and it represents a luxuriant fringing reef formed within the euphotic zone in clear water conditions facing the open sea. The coral fauna differs both in its composition and in its proportions among reef palaeoenvironments, ranging from the shallow back reef to the fore reef slope, and its stratigraphic and palaeogeographic distribution testifies to the persistence of a cosmopolitan Tethyan fauna in Oligocene time, with the Mediterranean and Indo-Pacific provinces being more closely connected than the Mediterranean and the Caribbean. The age of the Castro Limestone is here reassigned to the middle-late Chattian, which coincides with the Late Oligocene Warming Event (LOWE) when atmospheric CO2 values declined. We suggest that the strong reef-building capacity of the Castro Limestone, coupled with high coral diversity, was not hampered by warming conditions but most probably promoted by the reduced pCO2 and a suitable local/regional physiographic setting.

Predicting tsunami-like solitary wave run-up over fringing reefs using the multi-layer perceptron neural network

Modeling of tsunami wave interaction with coral reefs to date focuses mainly on the process-based numerical models. In this study, an alternative machine learning technique based on the multi-layer perceptron neural network (MLP-NN) is introduced to predict the tsunami-like solitary wave run-up over fringing reefs. Two hydrodynamic forcings (incident wave height, reef-flat water level) and four reef morphologic features (reef width, fore-reef slope, beach slope, reef roughness) are selected as the input variables and wave run-up on the back-reef beach is assigned as the output variable. A validated numerical model based on the Boussinesq equations is applied to provide a dataset consisting of 4096 runs for MLP-NN training and testing. Results analyses show that the MLP-NN consisting of one hidden layer with ten hidden neurons provides the best predictions for the wave run-up. Subsequently, model performances in view of individual input variables are accessed via an analysis of the percentage errors of the predictions. Finally, a mean impact value analysis is also conducted to evaluate the relative importance of the input variables to the output variable. In general, the adopted MLP-NN has high predictive capability for wave run-up over the reef-lined coasts, and it is an alternative but more efficient tool for potential use in tsunami early warning system or risk assessment projects.