Mean Water Level Research Articles

Active tectonics and its impacts over groundwater systems in the parts of Tamil Nadu, India

The relation between the fracture systems and the groundwater movement has been well demonstrated all over the world. Though studies have been carried out on the morphology, chronology and the dynamics of the fractures and their control over groundwater in hard rock aquifer systems, the role of active tectonics over groundwater has not been studied much and the groundwater has been targeted accordingly. In this connection, in the present study, database was generated on the annual mean water levels of 1800 observation wells for the years 1975, 1980, 1985, 1990 and 1995 from the state of Tamil Nadu, the overall mean water levels were worked out for the 5 years for these 1800 wells and digital elevation model was generated using GIS on such overall mean water level. The same indicated that groundwater levels occur as alternately arranged East–West trending elliptical domes/ridges and elliptical basins/valleys, from Cape Comorin in the south to Chennai in the north. Though these groundwater valleys and ridges broadly follow the cymatogenic arches and deeps, the groundwater valleys significantly coincide with Ramanathapuram and Chennai post collision East–West trending cymatogenic arches and the groundwater ridges coincide similarly with the cymatogenic deeps/grabens. The digital elevation models generated for the mean water level data individually for the above 5 years, for a test site in the parts of northern Tamil Nadu in proximity to Chennai arch region, indicated similar alternately aligned E–W local groundwater ridges and valleys. The axes of these E–W trending groundwater ridge and valley features of 1975 gradually attained an hourglass shape in 1995 with the axes of the southern groundwater valleys attaining northerly convexity and the northern valleys the southerly convexity. In addition, the groundwater cone of 60 km radius seen amidst many groundwater valley features in the DEM in proximity to Chennai arch region appears to have gradually shrunk from 1975 to 1995. All these indicate that these are the reflections of ongoing tectonic arching and deepening due to the still active northerly to north north-easterly oriented compressive force which has originally drifted the Indian plate towards northerly. From this, it can be further surmised that instrument-based finer water level monitoring could give indications on active tectonics, from which not only potential groundwater zones can be located but also seismicities can be predicted.

Estimating surfzone wave transformation and wave setup from remote sensing data

The spatial distribution of wave roller dissipation is derived from optical remote sensing observations in a laboratory setting and is used to estimate wave transformation and radiation stress forcing through the surfzone. The methodology relies on direct measurements of the size of individual wave breaking rollers in an irregular wave field via remote sensing. The wave roller measurements are used to calculate the roller energy, roller dissipation, and the roller component of the radiation stress. These hydrodynamic quantities then serve as input into the wave energy flux and cross-shore momentum balances in order to derive the wave height transformation and mean water level profiles. The accuracy of the methodology is shown to be very good through comparison with in situ data. In addition, the mean water level profile reproduces the transition zone lag and maximum water level at the most shoreward measuring point. Overall, it is demonstrated that the methodology can be successfully applied to irregular waves and can be used to estimate both wave transformation and radiation stress forcing through the surf zone.

Assessment of groundwater quantity and quality and saltwater intrusion in the Damghan basin, Iran

This study describes the groundwater quantity and quality conditions in the Damghan aquifer in Iran. The quantitative analysis of data obtained from observation wells indicates overexploitation of groundwater during recent years, which has resulted in deterioration of water quality. The mean water level has declined about 7.4m between years of 1966 and 2010. The hydrochemical facies of water collected from sampling wells were investigated though Piper and Chadha diagrams, and the general dominant type of water in the study area was determined as Na-Cl. The quality assessment examined the suitability of groundwater for drinking and irrigation purposes. Compared to the World Health Organization (WHO) guidelines for drinking water, all regions were found to have unpotable groundwater. Furthermore, unsuitability of groundwater for agricultural applications due to high salinity was observed through analysis of major quality indicators. The saltwater intrusion was investigated by ionic ratio analyses and was determined to be the main factor contributing to high salinity and deterioration of the groundwater quality in the Damghan basin.

The impact of Hurricane Katrina on the coastline west of New Orleans, USA

Forms of coastline are constantly modified by such driving forces as waves, sea level change, storms and winds. Among various driving forces, the effects of hurricanes can be the greatest and fastest. It is critical to continuously observe coastline changes in order to improve our understanding on the role of hurricane storms. One way is to make frequent field measurements along the coast before and after hurricane events. This is clearly labor-intensive at the regional scale. Remotely sensed data with sufficiently high acquisition frequency and spatial resolution can serve the purpose. Our question is if freely accessible Landsat data can be used to solve this problem. We used a 180-km long coast west of New Orleans that was affected by Hurricane Katrina as a case study. The storm surge induced by Hurricane Katrina caused well-known catastrophic damages in human lives and economic values. However, to what extent it has changed the coastline at the regional scale is an open question. We used Landsat time-series data (1984–2015) to estimate the rate of coastline change. First, the shortwave-infrared (1.55 mm) band of each Landsat image was selected to separate water and land. Water body detection was carried out using a simple threshold-based method. The optimal threshold was determined with a method based on the maximum inter-class variance criterion. Second, the boundary of the ocean water body was delineated as the coastline. Third, a linear regression model between water level and coastline position was established and an interpolation was conducted to remove the impact of clouds or shadows. To better analyze the impact of hurricanes on the coast, we removed the variation of water levels due to waves. Each average coastline positon was corrected to the long-term mean water level. The change rates were estimated during different observation periods. The results demonstrate that both the position and change rate of the coastline changed dramatically after the hurricane. Before Hurricane Katrina, 39% of the coastline advanced to ocean and 61% retreated to land, and the regional average change rate was - 2.53 m/a. After Hurricane Katrina, 27% of the coastline advanced to ocean and 73% retreated to land, and the regional average change rate was - 3.58 m/a. After Hurricane Katrina 87% of the coast suffered from serious erosion and the coastline shifted to land by 14.91 m during the hurricane. Further, we collected hurricane records in the study area from the National Hurricane Center and selected greater than level one hurricanes. We conducted an analysis of regime changes for coastline positions and found three regime turning points. We discovered that the regime turning points detected from Landsat time-series data matched well with the hurricane records. We conclude that there is a profound relationship between hurricane and coastline change. Landsat-level remote sensing data with 30 m spatial resolution can effectively estimate coastline changes caused by hurricanes in sediment dominated coastal areas. Coastline information derived from Landsat images could be used to assess impacts of hurricanes on the coast and to develop post-disaster reconstruction and restoration plans.

Long term marine corrosion of cast iron bridge piers

Cast iron piers of a disused 90 year old multispan railway bridge located close to the Pacific Ocean were extensively sampled for remaining wall thickness to determine corrosion loss and pit depth. From this, a corrosion loss model for the full 90 years was developed. In addition, the statistics for uncertainty in corrosion loss were obtained. Corrosion varied with elevation relative to mean water level and was negligible in the atmospheric zone, about 2–3 mm in the immersion zone and 5–6 mm in the splash and lower tidal zones. This variation is consistent with accelerated low water corrosion. It indicates that water pollution occurred sometime during the life of the bridge. Maximum pit depths were determined and analysed using extreme value statistics. The corrosion model for such long term exposure and the related statistical results are unique and important for assessment of remaining life of the many other cast iron structures still in existence in many parts of the world.

Comparison of simulated platform dynamics in steady/dynamic winds and irregular waves for OC4 semi-submersible 5MW wind-turbine against DeepCwind model-test results

The global performance of the 5 MW OC4 semisubmersible floating wind turbine in random waves with or without steady/dynamic winds is numerically simulated by using the turbine-floater-mooring fully coupled dynamic analysis program FAST-CHARM3D in time domain. The numerical simulations are based on the complete second-order diffraction/radiation potential formulations along with nonlinear viscous-drag force estimations at the body\'s instantaneous position. The sensitivity of hull motions and mooring dynamics with varying wave-kinematics extrapolation methods above MWL(mean-water level) and column drag coefficients is investigated. The effects of steady and dynamic winds are also illustrated. When dynamic wind is added to the irregular waves, it additionally introduces low-frequency wind loading and aerodynamic damping. The numerically simulated results for the 5 MW OC4 semisubmersible floating wind turbine by FAST-CHARM3D are also extensively compared with the DeepCWind model-test results by Technip/NREL/UMaine. Those numerical-simulation results have good correlation with experimental results for all the cases considered.

Analytical approach for determining the mean water level profile in an estuary with substantial fresh water discharge

Abstract. The mean water level in estuaries rises in the landward direction due to a combination of the density gradient, the tidal asymmetry, and the backwater effect. This phenomenon is more prominent under an increase of the fresh water discharge, which strongly intensifies both the tidal asymmetry and the backwater effect. However, the interactions between tide and river flow and their individual contributions to the rise of the mean water level along the estuary are not yet completely understood. In this study, we adopt an analytical approach to describe the tidal wave propagation under the influence of substantial fresh water discharge, where the analytical solutions are obtained by solving a set of four implicit equations for the tidal damping, the velocity amplitude, the wave celerity, and the phase lag. The analytical model is used to quantify the contributions made by tide, river, and tide–river interaction to the water level slope along the estuary, which sheds new light on the generation of backwater due to tide–river interaction. Subsequently, the method is applied to the Yangtze estuary under a wide range of river discharge conditions where the influence of both tidal amplitude and fresh water discharge on the longitudinal variation of the mean tidal water level is explored. Analytical model results show that in the tide-dominated region the mean water level is mainly controlled by the tide–river interaction, while it is primarily determined by the river flow in the river-dominated region, which is in agreement with previous studies. Interestingly, we demonstrate that the effect of the tide alone is most important in the transitional zone, where the ratio of velocity amplitude to river flow velocity approaches unity. This has to do with the fact that the contribution of tidal flow, river flow, and tide–river interaction to the residual water level slope are all proportional to the square of the velocity scale. Finally, we show that, in combination with extreme-value theory (e.g. generalized extreme-value theory), the method may be used to obtain a first-order estimation of the frequency of extreme water levels relevant for water management and flood control. By presenting these analytical relations, we provide direct insight into the interaction between tide and river flow, which will be useful for the study of other estuaries that experience substantial river discharge in a tidal region.

Refined seaward boundary conditions for modelling shallow water waves in semi-enclosed water bodies

Based on the theory of characteristics, this research elaborates on the numerical treatment of two types of seaward boundary conditions for modelling long-wave dynamics in truncated estuarine and coastal domains. These seaward boundary conditions are devised for the solution of the fully non-linear shallow water equations in the time domain. The first type is the clamped boundary, at which the water level variation is given and the velocity is computed along the characteristic line going out of the domain. The second type is the non-reflecting boundary, where the incident wave information is introduced and the reflected waves from inside the computational domain are allowed to escape at the same time. The essence of its numerical implementation is to distinguish the inward and outward characteristics and to disconnect the incoming characteristic relation from the actual flow inside the domain. Compared with previous techniques, the present method includes extra terms in the derivation to account for the effects of the uneven bed, bottom friction and shape of the characteristic lines. A shock-capturing finite difference method is used to solve the shallow water equations in the deviatoric format, but the seaward boundary algorithms constructed herein are generic and applicable to other solvers. The necessity of these refinements is highlighted by simulating the tidal oscillation in the Persian/Arabian Gulf, periodic wave runup on the coastline and the wave resonance in a narrow harbour. It is found that neglecting the bed slope at the boundary may result in biased mean water levels in the prediction.

Effects of lateral connectivity on zooplankton community structure in floodplain lakes

Hydrological conditions are responsible for the changes in lateral connectivity between the main river channel and the floodplain lakes, a factor controlling zooplankton abundance and diversity. We tested the hypothesis that the degree of connectivity between the aquatic habitats and the river channel governs the zooplankton densities and community structure. Abundances, community composition and species diversity of zooplankton were analysed against the gradient of lakes’ connectivity and the water quality parameters under a natural flood pulse in the Biebrza River (North-Eastern Poland). Our findings revealed that the water level fluctuations directly affect the availability of nutrients, aeration, what in turn controls the densities and biovolumes of zooplankton communities. Along with the increase in the lake isolation, the taxonomic diversity of zooplankton decreased, while the eudomination of taxa indicative of advanced trophy (Rotifera) was observed. Qualitative parameters, as number of species, diversity and richness, were significantly higher at mean water levels, which supports the intermediate disturbance hypothesis. The sensitivity of the zooplankton community to variable hydrological conditions and lateral connectivity gradient demonstrates its potential as an unexploited indicator of any habitat changes in the aquatic ecosystems.

The overgrowth of Zizania latifolia in a subtropical floodplain lake: Changes in its distribution and possible water level control measures

Wuchang Lake is a shallow lake located in the middle reaches of the Yangtze River. Historically, it was freely connected with the Yangtze mainstem and dominated by submerged macrophytes. For fishing and flood control, the river–lake connection was blocked by the Wanhe sluice in 1959; this led to large changes in water level and ultimately the overgrowth of the emergent macrophyte Zizania latifolia in Wuchang Lake. In order to devise a way to control the expansion of Z. latifolia, remote sensing and a geographic information system (GIS) were used in this study to analyze the changes in distribution of Z. latifolia. The results showed that the distribution area of Z. latifolia linearly increased from 1981 to 2009, and the annual average growth rate was about 1.8km2/year. In 2009, the plant covered about 53% of the whole lake, and the lake was presenting a serious tendency for terrestrialization. Analyses of the hydrological data indicated that the significantly decreased water level during the plant's germination period (February–March) may be the main cause for the overgrowth of Z. latifolia. By assessing the February–March mean water level before the building of sluice, and the plant's germination threshold for water depth, we recommend increasing the February–March mean water levels from 11.0m at present to 12.0m; this would reduce the potential distribution area of Z. latifolia from 56.8% at present to 8.9%. The results of this study should be useful for the control of aquatic macrophytes in other lakes subject to similar water level regulations.

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.

沿岸におけるうねり性波浪による水位変動の検証

Wave disasters caused by large swells have occurred on the coast of Toyama Wan, and these swells have been studied by many researchers. As a result of the studies, it is known that water level fluctuations with periods of several minutes occur off the coast of the Toyama Wan, when large swells invade the Toyama Wan. But generation mechanism of the water level fluctuations does not become clear. And so, the generation mechanism was investigated by wave observations off the coast of Toyama Wan and wave generating experiments using water tank. As the results, they became clear that the water level fluctuations with periods of several minutes are proper oscillations in horizontal shallow water off coast, and mean water level in a horizontal shallow water jutting out from a coast rises several meters due to swells invading the horizontal shallow water.

Comparative numerical and experimental study of two combined wind and wave energy concepts

With a successful and rapid development of offshore wind industry and increased research activities on wave energy conversion in recent years, there is an interest in investigating the technological and economic feasibility of combining offshore wind turbines (WTs) with wave energy converters (WECs). In the EU FP7 MARINA Platform project, three floating combined concepts, namely the spar torus combination (STC), the semi-submersible flap combination (SFC) and the oscillating water column (OWC) array with a wind turbine, were selected and studied in detail by numerical and experimental methods. This paper summarizes the numerical modeling and analysis of the two concepts: STC and SFC, the model tests at a 1:50 scale under simultaneous wave and wind excitation, as well as the comparison between the numerical and experimental results. Both operational and survival wind and wave conditions were considered. The numerical analysis was based on a time-domain global model using potential flow theory for hydrodynamics and blade element momentum theory (for SFC) or simplified thrust force model (for STC) for aerodynamics. Different techniques for model testing of combined wind and wave concepts were discussed with focus on modeling of wind turbines by disk or redesigned small-scale rotor and modeling of power take-off (PTO) system for wave energy conversion by pneumatic damper or hydraulic rotary damper. In order to reduce the uncertainty due to scaling, the numerical analysis was performed at model scale and both the numerical and experimental results were then up-scaled to full scale for comparison. The comparison shows that the current numerical model can well predict the responses (motions, PTO forces, power production) of the combined concepts for most of the cases. However, the linear hydrodynamic model is not adequate for the STC concept in extreme wave conditions with the torus fixed to the spar at the mean water level for which the wave slamming on the torus occurs and this requires further investigation. Moreover, based on a preliminary comparison of the displacement, the PTO system as well as the wind and wave power production, the STC concept will have a lower cost of energy as compared to the SFC concept. However, the cost of energy of either the STC or the SFC concept is higher than that of a pure floating wind turbine with the same floater.

Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan

Abstract. Fields of dislodged boulders and blocks record catastrophic coastal flooding during strong storms or tsunamis and play a pivotal role in coastal hazard assessment. Along the rocky carbonate coast of Eastern Samar (Philippines) we documented longshore transport of a block of 180 t and boulders (up to 23.5 t) shifted upslope to elevations of up to 10 m above mean lower low water level during Supertyphoon Haiyan on 8 November 2013. Initiation-of-motion approaches indicate that boulder dislocation occurred with flow velocities of 8.9–9.6 m s−1, which significantly exceeds depth-averaged flow velocities of a local coupled hydrodynamic and wave model (Delft3D) of the typhoon with a maximum < 1.5 m s−1. These results, in combination with recently published phase-resolving wave models, support the hypothesis that infragravity waves induced by the typhoon were responsible for the remarkable flooding pattern in Eastern Samar, which are not resolved in phase-averaged storm surge models. Our findings show that tsunamis and hydrodynamic conditions induced by tropical cyclones may shift boulders of similar size and, therefore, demand a careful re-evaluation of storm-related transport where it, based on the boulder's sheer size, has previously been ascribed to tsunamis.

Hydrology and water budget analysis of the East Joyce wetlands: Past history and prospects for the future

The East Joyce Wetlands (EJW) bordering northwest Lake Pontchartrain have a long history of human induced changes, such as leveeing of the Mississippi River that eliminated almost all riverine input to the area and segmentation of the east and west Joyce wetlands by the construction of a railroad, U.S. highway 51, and Interstate 55. Dredged drainage canals and associated spoil banks channel upland runoff around the wetlands. The deep canal associated with I-55 causes both rapid short-circuiting of freshwater runoff to Lake Maurepas and saltwater intrusion from Lake Pontchartrain. Increasing soil salinity has caused wide-spread loss of forested wetlands in the areas. Recently, the discharge of secondarily treated municipal effluent into the northeastern EJW as part of the Hammond wetland assimilation project has focused attention on the area (i.e., Bodker et al., 2015). In response, we carried out a number of studies at the Hammond Assimilation Wetlands (HAW) detailed in Shaffer et al. (2015), as well as a series of hydrological measurements and modeling detailed here. These data show that drainage under the railroad was minimal and most flow through the wetlands was to the southeast. Water levels in the HAW were highly variable prior to the beginning of effluent discharge in 2006, with relatively high mean water levels that did not increase substantially from 2007 through summer 2009 despite the addition of municipal effluent. Following effluent addition, surface water levels lacked the variability of the pre-discharge period and mean water levels were about 20cm higher from late 2009 until 2014 due to high rainfall in 2009, 2012, and 2013 and high effluent inflow due to significant infiltration into the city collection system. Historical net watershed inputs averaged 2.69cmyr−1 if this volume of water were spread over the 4km2 area immediately south of the effluent distribution system, compared to 0.38cmyr−1 for the effluent and 0.13cmyr−1 for direct precipitation. Salinity records from five sites in the EJW showed a gradient of increasing salinity from north to south and strong seasonality, averaging 1.9–2.1 PSU near the lake to 0.4–0.6 PSU in the northwestern EJW. Peak salinities were 4.6–5.1 PSU near the lake and 1.8 PSU in northwestern EJW. There was also a significant decrease in salinity over time. Salinity was lower beginning in 2010 coinciding with the closure of the Mississippi River Gulf Outlet, high precipitation in the fall and winter of 2009, and in 2012 and 2013, and continuing operation of the assimilation system. Proposed plans to alternate effluent discharge between east and west Joyce wetlands should increase surface water depth variability as seen prior to effluent discharge and minimize salinity intrusion in both areas.

Comparative experimental study of the survivability of a combined wind and wave energy converter in two testing facilities

Combining the offshore wind and wave energy on an integrated structure or in a farm configuration is beneficial for cost reduction since they could share the infrastructure and the ocean space. The spar torus combination (STC) is a combined wind and wave energy converter concept, which is composed of a spar floating wind turbine and a torus-shape heaving-body wave energy converter (WEC). Numerical simulations have shown a positive synergy between the WEC and the spar floating wind turbine under operational conditions. However, in extreme sea states, it is challenging to maintain structural integrity due to severe wave loads. Three survival modes have been proposed to study the survivability of the STC, and two of them are selected for further study by model testing. Two model tests were performed to investigate the performance of the STC under the two survival modes in extreme conditions: one test is carried out in the CNR-INSEAN towing tank, and the other one is performed in the MARINTEK towing tank. The two survival modes considered are when the torus is fixed to the spar at the mean water level (the MWL mode) and when the torus is fixed to the spar at a submerged position (the SUB mode). In this paper, the two model tests are described and then the measured responses in the two model tests are compared for each survival mode. In addition, the performance of the STC in the two survival modes for each model test is also compared. The differences in the two model tests are explained in detail. The focus of the model tests was wave-induced loads and responses, and wind was also included to model the mean wind thrust on the wind turbine rotor. In the model tests, the rigid body motions in six degree of freedom, the mooring line tensions, and the forces between the spar and torus in three directions (X, Y and Z) were measured. Finally, validation of a numerical model against the model test measurements is also briefly discussed.

Hydraulic performance of different non-overtopped breakwater types under 2D wave attack

The objective of this research was to develop a method to calculate the hydraulic performance resulting from the interaction of perpendicularly impinging water waves on various types of breakwater. Our study was based on data obtained from physical tests in a wave flume with irregular waves. Based on this information, it was possible to derive the complex wave reflection and transmission coefficients in terms of non-dimensional parameters representing the breakwater geometry, granular materials and incoming wave train. The overall dissipation rate caused by the structure was estimated by applying the energy conservation equation to a control volume, which included the breakwater section. The logistic sigmoid function was used to describe the variation in the modulus and phase of the reflection and transmission coefficients (as well as the energy dissipation rate). Remarkably, the sigmoid function was able to define the domain of the hydraulic performance of the most common breakwaters. It is shown that the sigmoid function depends primarily on a 2D scattering parameter Aeq/L2, where Aeq is the area of a porous medium under the mean water level and L is the wavelength, and on the relative grain size of the porous medium, Dk, where k is the wave number. The logistic sigmoid curves help to include the phase of the reflection coefficient when defining the wave regime in front of, inside, and leeward of the breakwater. Practical examples of how these results can be applied are also included.

Comparison of the Characteristics (Frequency and Timing) of Drought and Wetness Indices of Annual Mean Water Levels in the Five North American Great Lakes

In this study, we compared the frequency and timing of drought and wetness indices of annual mean water levels in the North American Great Lakes as they relate to teleconnection indices over the period from 1918 to 2012. In terms of timing, drought occurred in the Great Lakes watershed during the 1920, 1930 and 2000 decades, and was very intense in the East during the 1930’s and in the West during the 2000 decade. The main cause of extreme drought episodes in the 1920’s and 1930’s was a decrease in precipitation, while the 2000 decade drought is thought to be caused by increased water temperature (enhanced evaporation) due to a significant decrease in winter ice cover. The 1970 and 1980 decades were very wet over the whole watershed as a result of increased precipitation in the region. The succession of these dry and wet episodes did not have the same impacts on the stationarity of annual mean water levels in the five Great Lakes. Lake Superior shows an abrupt shift in mean in 1999, but a smoothed shift in variance since 1994, whereas Lake Erie shows four abrupt shifts in mean. Lake Ontario also shows the two first abrupt shift in mean and one abrupt change in variance. Extreme drought indices are negatively correlated with the North Atlantic Oscillation (NAO) for the two shallowest lakes (Ontario and Erie). In contrast, extreme wetness indices are positively correlated with PDO (positive correlation) and SOI (negative correlation) for Lake Superior only.

Shifting the waterlines of satellite images to the mean water shorelines considering wave runup, setup, and tidal variation

Shoreline evolution is a simple and common method to illustrate beach erosion or accretion in coastal engineering. Extracted waterlines on different satellite images are sometimes used for shoreline evolution. However, time-varying waterlines for tidal variation and wave runup are different from the shoreline at the mean water level. Waterline evolution may bring about misunderstanding of beach erosion or accretion. In a former study, the one-line shift method was proposed to determine the waterlines on a satellite image and to shift the waterlines to shorelines while only considering the tidal variation. The upward shift extension of the waterline due to wave runup and wave setup is considered. Some acceptable equations of wave runup were examined to correct the waterlines on three satellite images in one month when the foreshore beach slopes were specified. The suggested equation for wave runup includes both wave conditions and an average beachface slope at two points located shoreward and seaward away from mean water level by an equal distance of 62.5 m. When on-site shore bathymetrical measurement is sometimes unavailable, a method of minimizing the difference between the initially guessed and estimated foreshore beach slopes by the one-line shift method is proposed.

Long-term performance estimation of the Spar–Torus-Combination (STC) system with different survival modes

This work addresses a combined wind and wave energy concept called the Spar–Torus-Combination (STC) system. Two additional survival modes for the original STC system (in a survival mode with the tours locked to the spar) have been proposed, namely the submerged mode and the low power take-off (PTO) damping mode. The performance of the STC system with different modes has been investigated based on a coupled analysis of wind–wave-induced stochastic response using the SIMO-TDHMILL code in the time domain. The energy production, structural fatigue damage and extreme responses of the STC system have been estimated based on the long-term wind–wave joint distribution at two selected sites in European waters. The hydrodynamic loads on the spar and torus were estimated using potential theory by accounting for the hydrodynamic interactions. The aerodynamic loads of the rotor were calculated by a validated simplified thrust model (TDHMILL). The annual wind and wave power production have been obtained using the hourly mean output power of each short-term condition and its occurrence probability. The annual fatigue damage of both mooring lines and the tower has been calculated based on the S–N curve approach and the Palmgren–Miner׳s linear damage hypothesis. The extreme responses of the STC system have been investigated by assuming that the largest values in each short-term case follow the Gumbel distribution. The results show that the new proposed survival strategy can significantly reduce the long-term fatigue damage and extreme responses of the original STC system without sacrificing power production by avoiding the possible heave resonance effect when the torus is locked to the spar at the mean water level (MWL). Furthermore, considering the fact that long-term analysis with all sea states is time consuming, the contour line method has also been applied to effectively estimate the extreme responses of the original STC system and has been validated by a comparison with the full long-term analysis method.