Application of third generation shallow water wave models in a tidal environment

Wave modeling was performed in the German Bight of the North Sea during November 2002, using the spectral wave models, namely the K-model and Simulating WAves Nearshore (SWAN), both developed for applications in environments of shallow water depths. These models mainly differ with respect to their dissipation source term expressions and in exclusion or inclusion of nonlinear wave–wave interactions. The K-model uses nonlinear dissipation and bottom dissipation, and neglects quadruplet wave–wave interaction whereas, SWAN includes, besides bottom dissipation, dissipation by white-capping and depth induced wave breaking and triad wave–wave interaction. The boundary spectra were extracted from the WAM model results of a North Sea hindcast of the HIPOCAS project, wind fields, tidal current and water level variations from the results of models used in the Belawatt project. The purpose of this study was to test the performance of both wave models to see whether they were able to predict near-shore wave conditions accurately. The runs were performed with and without tidal current and level variations to determine their effect on the waves. Comparisons of model results with buoy measurements show that taking into account tides and currents improve the spectral shape especially in areas of high current speeds. Whereas SWAN performed better in terms of spectral shape, especially in case of two peaked spectra, the K-model showed better results in terms of integrated parameters.Responsible Editor: Hans Burchard     

Estimation of surface roughness (z0) over a stabilizing coastal dune field based on vegetation and topography

Accurate knowledge of the surface roughness and the resultant wind speed are important for many applications, such as climatic models, wind power meteorology, agriculture and erosion hazards, especially on sand dunes in arid and semi‐arid environments, where vegetation cover is scarce. In this study we aimed at quantifying the effects of vegetation cover and topography on surface roughness over a stabilizing dune field on the southern coast of Israel. Forty‐six wind measurements were made at various distances from the coastline, ranging from 10 to 2800 m, and z₀ values were calculated from the wind measurements based on the ratio between the wind gust and the average wind speed. We estimated vegetation cover using the soil adjusted vegetation index (SAVI) from Landsat satellite images for the upwind sector at various lengths, ranging from 15 to 400 m, and based on digital elevation models and differential GPS field measurements we calculated the topographic variable of the relative heights of the stations. z₀ values were positively correlated with the winter SAVI values (r = 0·87 at an upwind length of 200 m) and negatively correlated with the relative height (r = ?0·68 at an upwind length of 200–400 m for the inland dune stations). Using these variables we were able to create a map of estimated z₀ values having an accuracy of over 64%. Such maps provide a better understanding of the spatial variability in both wind speed and sand movement over coastal dune areas. Copyright © 2007 John Wiley & Sons, Ltd.     

Calibration of a coupled biological-physical model for prediction in a coastal inlet

We describe a numerical forecast system designed for prediction of physical and biological dynamics of a coastal inlet. It is based on a coastal ocean observatory that was located at Lunenburg Bay, Nova Scotia, Canada. Biological, chemical, optical, and physical measurements were collected from instrumented moorings, weekly sampling and detailed surveys from 2002 through 2007. Here we present a framework for calibration and evaluation of an ecosystem model using data from the summer of 2007. A three-dimensional hydrodynamic model was coupled to a simple biological (Nutrients-Phytoplankton-Detritus) model; a simple model was used so results could be compared directly to observed biological and chemical variables using skill scores as a first step toward data-assimilation modeling. As a complement to this analysis, variability of model output, e.g., the nutrient limitation term, was examined to understand the modeled biological response to the simulated physical environment. Skill scores based on variances in observed and simulated time-series of biological components were also investigated. Coastal upwelling/downwelling simulated through this model has been found to increase modeled biological activity in the bay. Also model skill in reproducing the observed patterns in nutrients and phytoplankton has been increased due to the restoring conditions for biology set up at the open ocean boundaries of the bay.     

Spatial variation of the vegetative roughness in Mediterranean torrential streams affected by check dams

This study focuses on the spatial variations in vegetative roughness associated with morphological channel adjustments due to the presence of check dams in Mediterranean torrential streams. Manning’s n values were estimated using methods established by previous studies of submerged and non-submerged vegetation in laboratory flume experiments and field work. The results showed a linear decrease in shrub density and rate of variation of the roughness coefficient versus degree of submergence with increasing distance upstream from the check dam, while downstream, the filling of the check dam and the bed incision had the most influence. A regression analysis was applied by fitting the data to different models: relationships between Manning’s n and the flow velocity were found to be of the power type for shrubs in all upstream sections, while relationships of flow velocity versus hydraulic radius in the sections closest to check dams showed a good fit to second-order polynomial equations.     

Development of a benthic index to assess sediment quality in the Tampa Bay Estuary

The identification, remedial treatment, and monitoring of contaminated sediments are among the priorities for managers of the Tampa Bay Estuary. Tampa Bay, as an urbanized estuary, is subject to the input of watershed sources of chemical contaminants, including metals, pesticides, and organic chemicals. Although the use of biological indicators and their incorporation into multi-metric indices is not new, the refinement and applications of such techniques for determining environmental condition still require further development and exploration. We present a single Tampa Bay Benthic Index (TBBI) that was developed specifically for Tampa Bay. Stepwise discriminant analysis was applied to a comprehensive list of potential benthic metrics. Results from the stepwise procedure identified the metrics that best discriminated between "healthy" and "degraded" conditions, as defined by sediment contaminant effect levels and dissolved oxygen. Discriminant analysis was then applied to the resultant three variables to determine the linear combination for the index.     

Weathering of coastal defensive structures in southwest England: a 500 year stone durability trial

Historic structures can be viewed as exposure trials of the stone of which they are constructed. As such, they represent a geomorphological weathering experiment. Several structures of Henrician (sixteenth century) and greater age on the coast of southwest England have been exposed to coastal salt weathering for 500–600 years. Long‐term weathering rates on five different rock groups are derived from careful study of weathering depths and forms. There is significant variation in weathering rate between five major rock groups. Rank ordering of weathering rate values reveals a durability order of these rock groups, which is confirmed by local juxtapositions. Controls on rock durability in the coastal weathering environment include both mechanical and mineralogical characteristics. Specific density, and combined quartz and muscovite content, are positively related to durability; high feldspar and chlorite content are associated with low durability. Copyright © 2000 John Wiley & Sons, Ltd.     

Reflection and transmission ofSH waves in an initially stressed medium consisting of a sandy layer lying over a fluid-saturated porous solid

Biot's theory is employed to study the reflection and transmission ofSH waves in a sandy layer lying over a fluid-saturated porous solid half-space. The entire medium is considered under constant initial stress. Effects of sandiness, initial stress, anelasticity and viscosity of the interstitial fluid on the partitioning of energy are studied. In the presence of initial stress the incident wave starts attenuating when incider beyond a certain angle (depending upon the amount of initial stress), even if the medium is perfectly clastic. Anelasticity of the solid layer results in the dissipation of energy during transmission. The direction of attenuation vector of incident wave affects the dissipation energy to a large extent. Effect on partitioning of energy reverse at incidence after the critical angle. A complete account of energy returmed back to the underlying half-space and that which is dissipated in the overlying layer has been discussed analytically as well as numerically.     

A preliminary scanning electron microscope study of honeycomb weathering of sandstone in a coastal environment

Honeycomb weathering has been observed in a Carboniferous sandstone at a coastal location near Ballycastle on the north coast of Northern Ireland. Specimens of this sandstone have been analysed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry. Results reveal that calcium sulphate (gypsum) is the only salt present and is found only at and immediately below the rock surface. SEM observations suggest that crystallization of salts in pores could easily dislodge quartz grains to promote granular disintegration, whilst etching of quartz grain surfaces attests to chemical weathering activity within the rock However, the reason for the development of the honeycomb pattern is not known.     

CO2 absorption of sandy soil induced by rainfall pulses in a desert ecosystem

Soil CO₂ flux is strongly influenced by precipitation in many ecosystem types, yet knowledge of the effects of precipitation on soil CO₂ flux in semi‐arid desert ecosystems remains insufficient, particularly for sandy soils. To address this, we investigated the response of sandy soil CO₂ flux to rainfall pulses in a desert ecosystem in northern China during August–September 2011. Significant changes (P < 0.05) were found in diel patterns of soil CO₂ flux induced by small (2.1 mm), moderate (12.4 mm) and large (19.7 mm) precipitation events. Further analysis indicated that rainfall pulses modified the response of soil CO₂ flux to soil temperature, including hysteresis between soil CO₂ flux and soil temperature, with F_s higher when T_s was increasing than when T_s was decreasing, and the linear relationship between them. Moreover, our results showed that rainfall could result in absorption of atmospheric CO₂ by soil, possibly owing to mass flow of CO₂ induced by a gradient of gas pressure between atmosphere and soil. After each precipitation event, soil CO₂ flux recovered exponentially to pre‐rainfall levels with time, with the recovery times exhibiting a positive correlation with precipitation amount. On the basis of the amounts of precipitation that occurred at our site during the measurement period (August–September), the accumulated rain‐induced carbon absorption evaluated for rainy days was 1.068 g C m^?2; this corresponds approximately to 0.5–2.1% of the net primary production of a typical desert ecosystem. Thus, our results suggest that rainfall pulses can strongly influence carbon fluxes in desert ecosystems. Copyright © 2014 John Wiley & Sons, Ltd.     

Variation of overtides by wave enhanced bottom drag in a North Florida tidal inlet

Month-long observations of waves and tidal currents at Ponce de Leon Inlet, North Florida are used to investigate the importance of wave-induced bottom drag as a mechanism for overtide generation in estuaries. While bottom drag can in theory lead to overtide generation, in practice, resolving unambiguously this effect is difficult as it tends to be overshadowed by the stronger effect of diurnal–semidiurnal tidal variance. Bottom boundary layer numerical simulations based on observational data suggest that waves can cause the bottom drag experienced by currents to increase by a factor of 1.7, compared with relatively calm conditions. Despite the relatively short duration and limited scope of the experiment, the analysis suggests that overtide modulations (East–West velocity components of the 5th and 6th diurnal constituents) are correlated with wave-enhanced drag trends. Therefore, wave-enhanced bottom drags may be enhancing generation of overtides. Further work is necessary to understand the scope and the strength of this mechanism, in relation to the characteristics (e.g., flow direction) of individual overtides.     

The influence of nearshore sand bank dynamics on shoreline evolution in a macrotidal coastal environment,Calais, northern France

Analyses of shoreline and bathymetry change near Calais, northern coast of France, showed that shoreline evolution during the 20th century was strongly related with shoreface and nearshore bathymetry variations. Coastal erosion generally corresponds to areas of nearshore seabed lowering while shoreline progradation is essentially associated with areas of seafloor aggradation, notably east of Calais where an extensive sand flat experienced seaward shoreline displacement up to more than 300 m between 1949 and 2000. Mapping of bathymetry changes since 1911 revealed that significant variation in nearshore morphology was caused by the onshore and alongshore migration of a prominent tidal sand bank that eventually welded to the shore. Comparison of bathymetry data showed that the volume of the bank increased by about 10×10⁷ m³ during the 20th century, indicating that the bank was acting as a sediment sink for some of the sand transiting alongshore in the coastal zone. Several lines of evidence show that the bank also represented a major sediment source for the prograding tidal flat, supplying significant amounts of sand to the accreting upper beach. Simulation of wave propagation using the SWAN wave model (Booij et al., 1999) suggests that the onshore movement of the sand bank resulted in a decrease of wave energy in the nearshore zone, leading to more dissipative conditions. Such conditions would have increased nearshore sediment supply, favoring aeolian dune development on the upper beach and shoreline progradation. Our results suggest that the onshore migration of nearshore sand banks may represent one of the most important, and possibly the primary mechanism responsible for supplying marine sand to beaches and coastal dunes in this macrotidal coastal environment.     

The formation of an anabranching planform in a sandy floodplain by increased flows and sediment load

Anabranching rivers evolve in various geomorphic settings and various river planforms are present within these multi‐channel systems. In some cases, anabranches develop meandering patterns. Such river courses existed in Europe prior to intensive hydro‐technical works carried out during the last 250 years. Proglacial stream valleys, inherited from the last glaciation, provided a suitable environment for the development of anabranching rivers (wide valleys floors with abundant sand deposits). The main objective of the present study is to reconstruct the formation of an anabranching river planform characterized by meandering anabranches. Based on geophysical and geological data obtained from field research and a reconstruction of palaeodischarges, a model of the evolution of an anabranching river formed in a sandy floodplain is proposed. It is demonstrated that such a river system evolves from a meandering to an anabranching planform in periods of high flows that contribute to the formation of crevasse splays. The splay channels evolve then into new meandering flow paths that form ‘second‐order’ crevasses, avulsions and cutoffs. The efficiency of the flow is maintained by the formation of cutoffs and avulsions preventing the development of high sinuosity channels, and redirecting the flow to newly formed channels during maximum flow events. A comparison with other anabranching systems revealed that increased discharges and sediment loads are capable of forming anabranching planforms both in dryland and temperate climate zones. The sediment type available for transport, often inherited from older sedimentary environments, is an important variable determining whether the channel planform is anabranching, with actively migrating channels, or anastomosing, with stable, straight or sinuous branches. Copyright © 2017 John Wiley & Sons, Ltd.     

Oblique wave diffraction by a flexible floating structure in the presence of a submerged flexible structure

Expansion formulae associated with the interaction of oblique surface gravity waves with a floating flexible plate in the presence of a submerged horizontal flexible structure are derived using Green’s integral theorem in water of finite and infinite water depths. The associated Green’s functions are derived using the fundamental solution associated with the reduced wave equation. The integral forms of the Green’s functions and the velocity potentials are advantageous over the eigenfunction expansion method in situation when the roots of the dispersion relation coalesce. As an application of the expansion formulae, diffraction of oblique waves by a finite floating elastic plate in the presence of a submerged horizontal flexible membrane is investigated in water of finite depth. The accuracy of the numerical computation is demonstrated by analysing the convergence of the complex amplitude of the reflected waves and the energy relation. Effect of the submerged membrane on the diffraction of surface waves is studied by analysing the reflection and transmission coefficients for various parametric values. Further, the derivation of long wave equation under shallow water approximation is derived in a direct manner in the appendix. The concept and methodology can be easily extended to deal with acoustic wave interaction with flexible structures and related problems of mathematical physics and engineering.     

Influence of hydro-sedimentary factors on mollusc death assemblages in a temperate mixed tide-and-wave dominated coastal environment: Implications for the fossil record

Mollusc death assemblages were recovered in 98 subtidal sampling stations on the seafloor of the shallow Pertuis Charentais Sea (Atlantic coast of France). Taxonomic composition and spatial distribution of death assemblages were investigated, as well as their response to sediment grain size (field data), bottom shear stress (coupled tide and wave hydrodynamic modelling), and sediment budget (bathymetric difference map). Results showed that molluscs are likely to be reliable paleoenvironmental indicators since death assemblages were able to acquire ecological changes within years (decadal-scale taphonomic inertia), and live–dead agreement inferred from existing data on living benthic communities was high, except close to river mouths and intertidal mudflats that provide terrestrial and intertidal species to subtidal death assemblages, respectively. Taxonomic composition of these within-habitat death assemblages strongly depended on sediment grain size and bottom shear stress, similarly to living subtidal communities. Post-mortem dispersal of shells, owing to relatively low bottom shear stress in the area, was only of a few 10s to 100s of meters, which shows that death assemblages preserved environmental gradients even at a fine spatial scale. Sediment budget had also a significant influence on death assemblages. Thick-shelled epifaunal species were correlated with erosion areas on one side, and thin-shelled infaunal species with deposition on the other, showing that mollusc fossil assemblages could be used as indicators of paleo-sedimentation rate. This new proxy was successfully tested on a previously published Holocene mollusc fossil record from the same area. It was possible to refine the paleoenvironmental interpretation already proposed, in accordance with existing stratigraphic and sedimentological data.     

Monitoring of the sediment dynamics along a sandy shoreline by means of airborne hyperspectral remote sensing and LIDAR: a case study in Belgium

Airborne hyperspectral data and airborne laserscan or LIDAR data were applied to analyse the sediment transport and the beach morphodynamics along the Belgian shoreline. Between 2000 and 2004, four airborne acquisitions were performed with both types of sensor. The hyperspectral data were classified into seven sand type classes following a supervised classification approach, in which feature selection served to reduce the number of bands in the hyperspectral data. The seven classes allowed us to analyse the spatial dynamics of specific sediment volumes. The technique made it possible to distinguish the sand used for berm replenishment works or for beach nourishments from the sand naturally found on the backshore and the foreshore. Subtracting sequential DTMs (digital terrain models) resulted in height difference maps indicating the erosion and accretion zones. The combination of both data types, hyperspectral data and LIDAR data, provides a powerful tool, suited to analyse the dynamics of sandy shorelines. The technique was demonstrated on three sites along the Belgian shoreline: Koksijde, located on the West Coast and characterized by wide accretional beaches, influenced by dry berm replenishment works and the construction of groins; Zeebrugge, on the Middle Coast, where a beach nourishment was executed one year before the acquisitions started and where the dams of the harbour of Zeebrugge are responsible for the formation of a large accretional beach, and Knokke‐Heist, located on the East Coast and characterized by narrow, locally reflective, beaches, heavily influenced by nourishment activities. The methodology applied allowed retrieval of the main sediment transport directions as well as the amount of sediment transported. It proved to be specifically suited to follow up the redistribution and the re‐sorting of the fill in beach nourishment areas. Copyright © 2007 John Wiley & Sons, Ltd.     

Short‐term changes in the plan shape of a sandy beach in response to sheltering by a nearshore mud bank,Cayenne, French Guiana

Montjoly is a headland‐bound embayed sandy beach in Cayenne, French Guiana, that shows long‐term plan shape equilibrium in spite of periodic changes in accretion and erosion that alternately affect either end of the beach. These changes are caused by mud banks that move alongshore from the Amazon. The mechanisms involved in changes in the plan shape of the beach in response to the passage of one of these mud banks were monitored between 1997 and 2000 from airborne video imagery and field work. The beach longshore drift to the northwest, driven by the incident easterly to northeasterly swell usually affecting this coast, became temporarily reversed as the mud bank, migrating from east to west, initially sheltered the southeastern end of the beach. The difference in exposure to waves engendered a negative wave height gradient alongshore towards the southeast, resulting in the setting up of a cell circulation and counter‐active longshore drift from the exposed northwestern sector to the southeast. Sand eroded from the exposed sector accumulated first in the southeastern, and then the central sectors of the beach. The effect of increasing beach sheltering by the mudbank moving west is highlighted on the videographs by an ‘arrested’ pattern of beach shoreline development. The videographs show hardly any changes in beach plan shape since January 1999, due to sheltering of the beach from wave attack by the mud bank. It is expected that the eroded sector will recover in the future as the mud bank passes, leading to re‐establishment of the northwesterly sand drift. This temporally phased bi‐directional drift within the confines of the bounding headlands results in a rare example of mud‐bank‐induced beach rotation, and probably explains the long‐term equilibrium plan shape of Montjoly beach. Copyright © 2002 John Wiley & Sons, Ltd.     

Relative role of subinertial and superinertial modes in the coastal long wave response forced by the landfall of a tropical cyclone

A set of numerical experiments has been performed in order to analyze the long-wave response of the coastal ocean to a translating mesoscale atmospheric cyclone approaching the coastline at a normal angle. An idealized two-slope shelf topography is chosen. The model is forced by a radially symmetric atmospheric pressure perturbation with a corresponding gradient wind field. The cyclone's translation speed, radius, and the continental shelf width are considered as parameters whose impact on the long wave period, modal structure, and amplitude is studied. Subinertial continental shelf waves (CSW) dominate the response under typical forcing conditions and on the narrower shelves. They propagate in the downstream (in the sense of Kelvin wave propagation) direction. Superinertial edge wave modes have higher free surface amplitudes and faster phase speeds than the CSW modes. While potentially more dangerous, edge waves are not as common as subinertial shelf waves because their generation requires a wide, gently sloping shelf and a storm system translating at a relatively high (∼10 m s⁻¹ or faster) speed. A relatively smaller size of an atmospheric cyclone also favors edge wave generation. Edge waves with the highest amplitude (up to 60% of the forced storm surge) propagate upstream. They are produced by a storm system with an Eulerian time scale equal to the period of a zero-mode edge wave with the wavelength of the storm spatial scale. Large amplitude edge waves were generated during Hurricane Wilma's landfall (2005) on the West Florida shelf with particularly severe flooding occurring upstream of the landfall site.     

Dissolved major ions,Sr and 87Sr/86Sr of coastal lakes from Larsemann hills,East Antarctica: Solute sources and chemical weathering in a polar environment

Dissolved major ions, Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios of 10 coastal lakes from the Larsemann Hills, East Antarctica have been studied to constrain their solute sources, transport and glacial weathering patterns in their catchments. In absence of perennial river/streams, lakes serve as only reliable archive to study land surface processes in these low-temperature regions. The lake water chemistry is mostly Na-Cl type and it does not show any significant depth variations. Sr isotope compositions of these lakes vary from 0.7110 to 0.7211 with an average value of 0.7145, which is higher than modern seawater value. In addition to oceanic sources, major ions and Sr isotopic data show appreciable amount of solute supply from chemical weathering of silicate rocks in lake catchments and dissolution of Ca-Mg rich salts produced during the freezing of seawaters. The role of sulphide oxidation and carbonate weathering are found to be minimal on lake hydro-chemistry in this part of Antarctica. Inverse model calculations using this chemical dataset provide first-order estimates of dissolved cations and Sr; they are mostly derived from oceanic (seawater + snow) sources (cations approximately 76%) and (Sr approximately 92%) with minimal supplies from weathering of silicates (cations approximately 15%); (Sr approximately 2%) and Ca-rich minerals (cations approximately 9%); (Sr approximately 7%). The silicate weathering rate and its corresponding atmospheric CO₂ consumption rate estimates for Scandrett lake catchment (3.6 ± 0.3 tons/km²/year and 0.5 × 10⁵ moles/km²/year), are lower than that of reported values for the average global river basins (5.4 tons/km²/year and 0.9 × 10⁵ tons/km²/year) respectively. The present study provides a comprehensive report of chemical weathering intensity and its role in atmospheric CO₂ consumption in low-temperature pristine environment of Antarctica. These estimates underscore the importance of Antarctica weathering on atmospheric CO₂ budget, particularly during the past warmer periods when the large area was exposed and available for intense chemical weathering.