Mean Wave Conditions Research Articles

The peculiar wind and wave climatology of the Arabian Gulf

Based on a 40-year hindcast, we analyze the climatology and long-term trends of wind and waves in the Arabian Gulf. We identify and characterize four dominant wind systems, and their related mean wave conditions and long-term trends. Focusing on the summer Shamal wind, we observed a mild decrease in the number of Shamal events with time, whereas their intensity shows little to no variation. Further, we revealed a tendency toward an extension of the summer Shamal season from August to September. On a short time scale, the related wind fields show an evident daily oscillation associated with the daily heating of the Arabian Peninsula, with wind speeds peaking at 3:00 p.m. local time (UTC+3) on land and 12 h later on the sea. The long-term analysis conducted herein reveals limited, but significant, trends in surface pressure and related west-to-east pressure gradients in August. In September, the observed increasing number of Shamal storms is associated with an increase of the longitudinal local pressure gradient.

Long-term assessment of wave conditions and wave energy resource in the Arctic Ocean

It was recently shown that the Arctic has been warming much faster than the rest of the globe during the last decades. This warming has reduced the ice extent significantly, which will strongly impact the wave climate in the Arctic regions, thus affecting the design of marine structures, operations, and energy resources. This study focuses on the higher latitudes, and uses the advanced wave hindcast NORA3, which covers a big part of the North Atlantic and the whole Arctic Ocean, to analyze the spatio–temporal properties of wave height and wave energy flux during the last three decades. The most energetic waves in the Arctic Ocean are observed in the Greenland Sea and the Barents Sea. The study shows that the substantial diminishing of sea ice in the Arctic induces local and regional changes in both mean and extreme wave conditions. In the Arctic Ocean the changes in extreme wave height are more pronounced compared to changes in mean wave conditions. The results also indicate a strong positive trend in the extreme wave heights in the Arctic regions of the Barents Sea, the Kara Sea, the Laptev Sea, the East Siberian Sea, the Chukchi Sea, and the Beaufort Sea.

Modeling Morphodynamic Impacts and Optimization of Marine Hydrokinetic Arrays in Shallow Offshore Environments

Marine hydrokinetic (MHK) devices hold the promise of expanding renewable energy production by tapping into the power of waves and currents for electricity generation. However, these devices remain in the developmental stage, necessitating research to understand their environmental impacts, lower operational costs, and prevent equipment failures. In this study, we investigate various MHK array configurations to gain insights into their effects on wave patterns, water flow, and sediment conditions, considering both short-term and long-term morphodynamic changes under average and extreme conditions in shallow offshore environments. Our objectives encompass understanding the influence of mean and extreme environmental conditions on MHK devices, evaluating their impact on the bathymetry of the ocean floor, and exploring the role of different array configurations in morphodynamic evolution. Our findings, based on modeling these devices as static lumps, reveal that sediment erosion downstream of MHKs increases by 50% after one year of average conditions. When accounting for the absorption of 30% of the energy by MHK devices, downstream sediment deposition surges by almost 125%. Moreover, alterations in MHK arrays, such as spacing, size, and number, result in noticeable changes in sedimentation magnitudes during storm conditions. While long-term mean wave conditions have minimal effects on sedimentation, extreme wave conditions, akin to large storm events, bring about significant alterations in ocean floor bathymetry, potentially leading to costly maintenance of the MHK arrays. Our research provides a valuable framework for site analysis, enabling the estimation of maintenance needs and the optimization of array configurations to minimize sedimentation-related issues.

Seasonal changes in beach resilience along an urbanized barrier island

Beach width, dune height, and vegetation coverage are key parameters to assess beach resistance and resilience to storms. However, coastal development often causes beach ecosystem degradation due to poor coastal management. We propose a Coastal Resilience Index from Remote Sensors (CRIfRS) for urbanized coasts based on aerial photogrammetry. The study area, located along a 7.8 km stretch of coast on a barrier island, is characterized by persistent alongshore sediment transport and the presence of coastal structures and beach-front houses. Contrary to previous studies, we focus on anthropogenic perturbations (coastal urbanization and coastal structures), instead of hydrodynamic conditions (storms), since erosion in this region is mainly associated with alongshore sediment transport gradients induced by coastal structures. Thus, the CRIfRS is based on the relation of three indicators that affect the beach functionality for coastal protection: beach width, coastal structure influence area, and vegetation coverage. The CRIfRS was divided into five categories: Very Low resilience (VL), Low resilience (L), Medium resilience (M), High resilience (H), and Very High resilience (VH). The CRIfRS presented an important spatial and temporal variability due to changing environmental conditions and the deployment of new coastal structures. For the study period, the percentage of the coast within the VL and L resilience classification increased, whereas the percentage of the coast classified as M, H, and VH resilience decreased. During the winter storm season, the resilience increased mainly due to the cross-shore transport whilst during mean wave conditions (i.e., sea-breeze conditions) the long-shore transport becomes more persistent and thus the coastal structures play an important role interrupting the sediment flux. Additionally, the CRIfRS trajectory shows an overall increase of the L resilience and an overall decrease of the H resilience values. This study highlights the important role of anthropogenic perturbations on the assessment of coastal resilience for highly urbanized coasts. The CRIfRS can help to improve the coastal management by assessing the coastal protection capability of beaches considering both natural and anthropogenic factors.

Future behavior of wind wave extremes due to climate change

Extreme waves will undergo changes in the future when exposed to different climate change scenarios. These changes are evaluated through the analysis of significant wave height (Hs) return values and are also compared with annual mean Hs projections. Hourly time series are analyzed through a seven-member ensemble of wave climate simulations and changes are estimated in Hs for return periods from 5 to 100 years by the end of the century under RCP4.5 and RCP8.5 scenarios. Despite the underlying uncertainty that characterizes extremes, we obtain robust changes in extreme Hs over more than approximately 25% of the ocean surface. The results obtained conclude that increases cover wider areas and are larger in magnitude than decreases for higher return periods. The Southern Ocean is the region where the most robust increase in extreme Hs is projected, showing local increases of over 2 m regardless the analyzed return period under RCP8.5 scenario. On the contrary, the tropical north Pacific shows the most robust decrease in extreme Hs, with local decreases of over 1.5 m. Relevant divergences are found in several ocean regions between the projected behavior of mean and extreme wave conditions. For example, an increase in Hs return values and a decrease in annual mean Hs is found in the SE Indian, NW Atlantic and NE Pacific. Therefore, an extrapolation of the expected change in mean wave conditions to extremes in regions presenting such divergences should be adopted with caution, since it may lead to misinterpretation when used for the design of marine structures or in the evaluation of coastal flooding and erosion.

Interannual wave height variability in the Yellow Sea

This paper investigates the interannual wave climate variability in the Yellow Sea (YS) using the Simulating Waves Nearshore (SWAN) wave model. Interannual variations of both the monthly mean and extreme wave conditions are weaker than their seasonal counterparts. Meanwhile, both exhibit significant seasonal variations. The interannual variability of the seasonal mean wave condition is strongest in winter and weakest in summer. It basically decreases both northward and shoreward in all four seasons. However, the longitudinal position of the maximum variability shifts zonally with seasons. On the other hand, the interannual variability of the seasonal extreme wave condition is strongest roughly in fall, but weakest in winter in most regions of the YS except in the southeastern YS, where the largest variability appears in summer. As for the spatial distribution, the largest variability occurs in the middle YS in spring, while in the southeastern YS in other three seasons. Both of them are dominantly controlled by the local sea surface wind forcing with possibly minor contributions from the western Pacific Ocean. The climate variability modes possibly at work are also discussed.

Future Reef Growth Can Mitigate Physical Impacts of Sea‐Level Rise on Atoll Islands

AbstractWe present new detail on how future sea‐level rise (SLR) will modify nonlinear wave transformation processes, shoreline wave energy, and wave driven flooding on atoll islands. Frequent and destructive wave inundation is a primary climate‐change hazard that may render atoll islands uninhabitable in the near future. However, limited research has examined the physical vulnerability of atoll islands to future SLR and sparse information are available to implement process‐based coastal management on coral reef environments. We utilize a field‐verified numerical model capable of resolving all nonlinear wave transformation processes to simulate how future SLR will modify wave dissipation and overtopping on Funafuti Atoll, Tuvalu, accounting for static and accretionary reef adjustment morphologies. Results show that future SLR coupled with a static reef morphology will not only increase shoreline wave energy and overtopping but will fundamentally alter the spectral composition of shoreline energy by decreasing the contemporary influence of low‐frequency infragravity waves. “Business‐as‐usual” emissions (RCP 8.5) will result in annual wave overtopping on Funafuti Atoll by 2030, with overtopping at high tide under mean wave conditions occurring from 2090. Comparatively, vertical reef accretion in response to SLR will prevent any significant increase in shoreline wave energy and mitigate wave driven flooding volume by 72%. Our results provide the first quantitative assessment of how effective future reef accretion can be at mitigating SLR‐associated flooding on atoll islands and endorse active reef conservation and restoration for future coastal protection.

Nearshore hydrodynamics, beachface cobble transport and morphodynamics on a Pacific atoll motu

Measurements of wave transformation, beachface cobble transport and shoreline morphology change were undertaken over a 14month period at Fatato Island, a cobble and sand ‘motu’ on the southeast rim of Funafuti Atoll, Tuvalu. Results document significant cobble transport and beach profile change on Fatato during a year in the absence of any significant high energy wave events. Changes in beachface volume were measured on 11 across-shore beach profiles, and show marked variations of +7.1 and −7.01m3 between profiles and between survey periods. Net volumetric beachface change over the 14month experiment was approximately zero, indicating a balance between erosion and deposition and net conservation of sediment volume on the ocean shoreline. Cobble movement was measured using Passive Integrated Transponder (PIT) tags inserted in 300 randomly selected clasts (mean b-axis=75mm) on the mid-beachface. Tagged particles were re-surveyed after 6 and 14months, achieving 76% recovery, and identify large excursions of clasts on the beachface and dominant southwest alongshore transport, with an average net movement of 68m after 14months. A simple agent based probabilistic model was developed to simulate cobble movement and explore the coupling between hydrodynamic and shoreline processes. Model simulations reveal: predominant south-westerly cobble transport is driven by mean wave conditions associated with easterly trade winds, and; long-period swell events that approach the reef from the south drive episodic north-easterly transport reversals and significant onshore transport that can strand particles on the upper beach. Results present the first observations of morphodynamic behaviour of an atoll motu and highlight the highly dynamic nature of motu shorelines under mean energy conditions. Such results challenge conventional perspectives that motu are storm adjusted landforms that require extreme events to stimulate geomorphic change and inform future projections of island change.

Wave transformation and shoreline water level on Funafuti Atoll, Tuvalu

AbstractThe influence of sea swell (SS) waves, infragravity (IG) waves, and wave setup on maximum runup (Rmax) is investigated across different tidal stages on Fatato Island, Funafuti Atoll, Tuvalu. Field results illustrate that SS waves are tidally modulated at the shoreline, with comparatively greater wave attenuation and setup occurring at low tide versus high tide. A shoreward increase in IG wave height is observed across the 100 m wide reef flat at all tidal elevations, with no tidal modulation of IG wave height at the reef flat or island shoreline. A 1‐D shock‐capturing Green‐Naghdi solver is used to replicate the field deployment and analyze Rmax. Model outputs for SS wave height, IG wave height and setup at the shoreline match field results with model skill >0.96. Model outputs for Rmax are used to identify the temporal window when geomorphic activity can occur on the beach face. During periods of moderate swell energy, waves can impact the beach face at spring low tide, due to a combination of wave setup and strong IG wave activity. Under mean wave conditions, the combined influence of setup, IG waves and SS waves results in interaction with island sediment at midtide. At high tide, SS and IG waves directly impact the beach face. Overall, wave activity is present on the beach face for 71% of the study period, a significantly longer duration than is calculated using mean water level and topographic data.

Wave climate of the Hellenic Seas obtained from a wave hindcast for the period 1960–2001

The wave climate of the Hellenic Seas and particularly the climate extremes are investigated by means of a 42-year (1960–2001) model hindcast. The wave model, implemented over the Mediterranean basin, is forced by high-resolution winds generated upon downscaling of the ERA40 reanalysis. It is shown that the quality of the hindcast is overall satisfactory; however, extreme wave heights in the Aegean Sea are consistently overestimated. Accordingly, corrections to the original data are applied. The results show that the highest mean wave conditions are located east and west of Crete Island where the northerly air flow exits the Aegean Sea. Extreme waves are the highest outside the Aegean Sea, mainly in the southern Ionian Sea and south of Crete. Nevertheless, high waves also develop around the exits of the Aegean Sea and N-NE of the Cyclades islands. Despite a milder extreme wave climate in the Aegean Sea due to short fetch distances, the mean wave height range is very similar to that of the Ionian Sea. Moreover, in summer, the two seas exhibit similar extreme wave height conditions with the highest extremes found around the exits of the Aegean Sea to the Levantine basin. Storms of a longer duration are also observed in the Aegean Sea. The analysis of long-term trends in the wave climate shows that mean and extreme wave climate as well as the average intensity of extreme events have decreased in the Hellenic Seas. Nevertheless, this decrease has not been monotonic. A turning point is located around year 1981 with the mean and extreme wave height mostly increasing before this year and decreasing afterwards.

Numerical modelling of the nearshore wave energy resources of Shandong peninsula, China

In the present work, in order to investigate the nearshore wave energy resources, the third-generation wave model SWAN is utilised to simulate wave parameters of the Shandong peninsula in China for 16 years (1996–2011). The wind parameters used to simulate waves are obtained by the Weather Research & Forecasting Model (WRF). The modelling results of wave are validated by observation data. The spatial distributions of significant wave height and wave energy density are analysed under both extreme and mean wave conditions. The wave energy resources of the Chengshantou headland, with the highest wave energy density, the Langyatai headland and the Yellow River Delta are also studied in detail. For the above three sites, the mean month averaged wave energy is investigated, the wave energy resources are characterised in terms of wave state parameters, and wave energy roses are introduced. The values of extreme high and time-averaged nearshore wave energy density are 296 kW/m and 5.1 kW/m respectively.

CHRONIC BEACH EROSION INDUCED BY COASTAL STRUCTURES IN CHELEM, YUCATÁN

The Yucatan coastline has been experiencing beach erosion during the past few decades; the erosion has reached critical points at some locations such as the Chelem beach, located near the Progreso Pier. Despite this problem, only few studies have been devoted in order to investigate the role of coastal structures on explaining the high erosion rates reported at this location. Therefore, the aim of this work is to evaluate the effects of the Progreso Pier on the wave transformation and alongshore sediment transport in the study area. Field surveys were conducted in a monthly basis in order to estimate the erosion rates and wave conditions with an ADCP installed at 8 m water depth. The field information confirmed the high erosional trends (O(1)m/year) that explain coastal infrastructure damage and property losses. The wave measurements were employed as the forcing of a wind-wave numerical model (MIKE 21 SW). The numerical model is implemented in the study area for two different scenarios, with and without the pier; this was to estimate nearshore wave conditions. Subsequently, the nearshore wave climate at 5 m water depth is employed for the calculation of alongshore sediment transport rates in 5 points that are representative of the littoral drift along the study area. The modeling results show that the pier acts as a large scale wave-sheltering structure that induces important alongshore sediment transport gradients under mean wave conditions, decreasing the capacity of the waves to recover the beach. On the other hand, during winter storms, when the direction of the waves is from the NNW, the structure does not seem to play an important role on wave transformation into the study. As a result, the Progreso Pier enhances beach erosion in the Chelem area by inducing algonshore gradients in sediment transport and decreasing the beach recovery capability

Uncertainty in wave energy resource assessment. Part 2: Variability and predictability

The uncertainty in estimates of the energy yield from a wave energy converter (WEC) is considered. The study is presented in two articles. The first article considered the accuracy of the historic data and the second article, presented here, considers the uncertainty which arises from variability in the wave climate. Mean wave conditions exhibit high levels of interannual variability. Moreover, many previous studies have demonstrated longer-term decadal changes in wave climate. The effect of interannual and climatic changes in wave climate on the predictability of long-term mean WEC power is examined for an area off the north coast of Scotland. In this location anomalies in mean WEC power are strongly correlated with the North Atlantic Oscillation (NAO) index. This link enables the results of many previous studies on the variability of the NAO and its sensitivity to climate change to be applied to WEC power levels. It is shown that the variability in 5, 10 and 20 year mean power levels is greater than if annual power anomalies were uncorrelated noise. It is also shown that the change in wave climate from anthropogenic climate change over the life time of a wave farm is likely to be small in comparison to the natural level of variability. Finally, it is shown that despite the uncertainty related to variability in the wave climate, improvements in the accuracy of historic data will improve the accuracy of predictions of future WEC yield.

Wave energy and the upper depth limit distribution of Posidonia oceanica

Abstract It is widely accepted that light availability sets the lower limit of seagrass bathymetric distribution, while the upper limit depends on the level of disturbance by currents and waves. The establishment of light requirements for seagrass growth has been a major focus of research in marine ecology, and different quantitative models provide predictions for seagrass lower depth limits. In contrast, the influence of energy levels on the establishment, growth, and maintenance of seagrasses has received less attention, and to date there are no quantitative models predicting the evolution of seagrasses as a function of hydrodynamics at a large scale level. Hence, it is not possible to predict either the upper depth limit of the distribution of seagrasses or the effects that different energy regimes will have on these limits. The aim of this work is to provide a comprehensible methodology for obtaining quantitative knowledge and predictive capacity for estimating the upper depth limit of seagrasses as a response to wave energy dissipated on the seafloor. The methodology has been applied using wave data from 1958 to 2001 in order to obtain the mean wave climate in deep water seaward from an open sandy beach in the Balearic Islands, western Mediterranean Sea where the seagrass Posidonia oceanica forms an extensive meadow. Mean wave conditions were propagated to the shore using a two-dimensional parabolic model over the detailed bathymetry. The resulting hydrodynamics were correlated with bottom type and the distribution of P. oceanica. Results showed a predicted near-bottom orbital velocity of between 38 and 42 cm s-1 as a determinant of the upper depth limit of P. oceanica. This work shows the importance of interdisciplinary effort in ecological modeling and, in particular, the need for hydrodynamical studies to elucidate the distribution of seagrasses in shallow depths. Moreover, the use of predictive models would permit evaluation of the effects of coastal activities (construction of ports, artificial reefs, beach nutrient-input, dredging) on benthic ecosystems.

Wave Reflection from Nearshore Depressions

This study employs an existing finite-difference model based on the hyperbolic form of the modified mild slope equation (MMSE) to investigate wave reflection near bathymetric depressions such as dredged borrow pits and nearshore canyons. First, the model is tested for numerical limitations on the higher order bottom slope and curvature terms using idealized cases of a simple depth transition and a symmetric trapezoidal trench, with comparisons of the MMSE to both the traditional mild slope equation (MSE) solution and a shallow water analytic solution. It is demonstrated that the model gives accurate solutions on slopes as steep as 1:1, and that the solutions from all three models agree in the shallow water region. However, for waves in intermediate depths, predicted wave reflection from nearshore depressions is shown to differ significantly between the MMSE and MSE models. Next, geometrical data from a wide range of existing and proposed borrow pits and a submarine canyon are gathered and analyzed for whether wave reflection is an important process near realistic nearshore depressions. The geometric data show that realistic nearshore depressions lie within the tested range of the MMSE model and that borrow pits are generally not in shallow water, which means it is important to use a MMSE-type model to calculate reflection from these features. In addition, storm conditions on average lead to a 50% increase in reflection coefficient in comparison to the mean wave conditions, due to the increase in wave period. Finally, the results also indicate borrow pit design criteria that can be used to ensure minimal reflection.

Offshore wave climate, Perth (Western Australia), 1994 - 96

The offshore wave climate of Perth (Western Australia) was analysed by using 2.5 years of non-directional 20-min wave data collected from March 1994 to August 1996. The mean wave conditions are characterized by a significant wave height (Hs) of 2.0 m and a spectral mean wave period (Tm) of 8.8 s. However, considerable annual variation in the wave conditions is experienced because of a distinct seasonality in the regional wind regime. During summer, daily sea breezes generate moderate seas (ambient Hs 1 to 2 m; Tm <8 s). During winter, frequent storms associated with mid-latitude depressions generate heavy seas and swell (ambient Hs 1.5 to 2.5 m; Tm >8 s). A low-amplitude background swell (Hs ~0.5 m), generated distantly in the Indian and Southern Oceans, is present all year round. Analysis of extreme wave conditions (Hs >4 m) indicates that, on average, 30 storms are experienced annually, and the storms are most frequent and intense during July. Estimates of extreme Hs, based on all available offshore wave data (12 years, 1975–96), for 1- and 100-year return periods, are 6.7 m and 9.8 m, respectively.