Diurnal Inequality Research Articles

River plastic transport affected by tidal dynamics

Abstract. Plastic is an emerging pollutant, and the quantities in rivers and oceans are expected to increase. Rivers are assumed to transport land-based plastic into the ocean, and the fluvial and marine transport processes have been relatively well studied to date. However, the processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. As a consequence, flow velocity direction and magnitude can change diurnally. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bi-directional flows and varying water levels, can lead to an even higher likelihood of long-term retention. Here, we provide a first observation-based estimate of net plastic transport on a daily timescale in tidal rivers. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of plastic transport and discharge during full tidal cycles. We applied our method to the highly polluted Saigon River, Vietnam, throughout six full tidal cycles in May 2022. We show that the net plastic transport is about 20 %–33 % of the total plastic transport. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's R2 = 0.76). The net transport of plastic is higher than the net discharge (20 %–33 % and 16 %, respectively), suggesting that plastic transport is governed by factors other than water flow. Such factors include wind, varying plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The plastic net transport rates alternate between positive (seaward) net transport and negative (landward) net transport as a result of the diurnal inequality in the tidal cycles. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (10 %–16 % vs. 30 %–38 %), suggesting that transport dynamics strongly depend on item characteristics. Our results demonstrate the crucial role of tidal dynamics and bi-directional flows in plastic transport dynamics. With this paper we emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean.

Tidal characteristics and a numerical model for the m2 tide at the Estuarine Complex of the Bay of Paranaguá, Paraná, Brazil

The tides at the estuarine complex of the Bay of Paranaguá (ECBP) were investigated using stochastic analysis and numerical modelling. Sea level records from six tidal locations were analysed using harmonic analysis, and showed that M2 is the dominant tide. A numerical model was implemented based on the dominance of the M2 component. The model uses finite differences to represent the hydrodynamic equations, with an explicit resolution scheme and non-slip contour conditions. The tidal regime is semidiurnal with pronounced diurnal inequalities. The overtides, the even, and the shallow water constituents are important, indicating strong non-linear interactions, which allow the formation of up to six high and low tides per day during neap cycles. This higher frequency oscillation is ascribed to double high and double low water effects. Tidal ranges increase from the mouth to the end of the complex, by 160%. Tidal phases and ranges indicate that the tidal wave propagates in a mixed form. The model confirms the mixed tidal wave regime.

On nodal modulations of tidal-stream energy resource in north-western Europe

Estimation of tidal-stream turbine loading and energy yield requires a thorough understanding of the hydrodynamic processes that influence tidal currents over a wide range of timescales. In this study, we focus on the long-term variability of the tidal-stream energy resource associated with the 18.6-year lunar cycle. Three sites in north-western Europe, with strong potential for tidal array development, are considered; the Alderney Race (English Channel), the Fromveur Strait (western Brittany) and the Ramsey Sound (Irish Sea). The investigation relies on harmonic analysis and associated predictions of depth-averaged tidal currents. Results show that the variability in predicted annual power densities is comparable at the three measurement locations. This variability reaches +/- 10% over the 18.6-year lunar cycle, and is mainly associated with M2 nodal modulations which dominate the tidal signal in north-western European shelf seas. Maximum and minimum power density occur in 2015 and 2024, respectively. 2015 is characterized by (i) reduced spring-neap variabilities and diurnal inequalities of tidal currents, and (ii) increased tidal asymmetries (between peak ebb and flood), whereas 2024 shows reverse effects. This cycle also influences the annual maximum/mean current magnitudes. The differences in maximum speeds between 2015 and 2024 are of 0.16–0.17 m/s at the three sites, which represents 4-5% of the peak speeds. The simple approach considered here may be applied in broader locations as a preliminary assessment of the long-term variability of the available tidal stream energy resource. The modulations predicted from harmonic analysis are finally expected to be on the high side of the forecasting range. Indeed, the results of an 18-year numerical simulation performed on the Alderney Race suggested a reduced effect of the 18.6-year lunar cycle on the resource.

Identification, extraction and interpretation of multi-period variations of coastal suspended sediment concentration based on unevenly spaced observations

Suspended sediment concentration (SSC) is an important indicator for coastal morphodynamics and health, exhibiting significant periodic variations at multiple timescales in response to various cyclic forcing factors. Identification of key periods and the extraction of corresponding amplitudes and phases can greatly aid effort to characterize the representative portrayal of SSC, which contributes to further physical interpretation. Remote sensing missions are intended to satisfy the retrieval of long-term SSC variations. However, the unevenly spaced characteristic and low effective sampling rates of the retrieved time series would hamper the extraction and interpretation of the feature changes. Here, we retrieved a 9-year SSC time series in a coastal tidal basin of Jiangsu coast, China, based on hourly GOCI satellite images from 2011 to 2019, and applied the Lomb-Scargle periodogram with the phase-folded method to extract their periodic information. On the basis of the unevenly spaced data with effective rate of 6.9%, the semidiurnal, diurnal, fortnightly, and seasonal cycles were successfully identified as the main periods, with amplitudes of 33.1, 23.4, 54.1, and 71.4 mg/L, respectively. In accordance with investigations into phase lags between SSC and forcing factors, we found that the unimodal SSC pattern at the semidiurnal timescale was induced by flood dominance, while the diurnal inequality of water level and wave intensity accounts for the SSC variation at the diurnal timescale. Tidal ranges vary consistently with the SSC in the fortnightly scale while background wave dominates the SSC variation in the seasonal timescale, the latter resulting in the seasonal difference in semidiurnal and fortnightly SSC cycles. The annual semidiurnal SSC variations experience a regime shift from 2014 to 2016, which is in consistency with the sequential reclamation. The research presents a typical application of long-term unevenly spaced observations into coastal SSC, and the proposed methodology can be extended to similar studies aiming at uncovering the mechanism of SSC variations at multiple timescales.

Methane Dynamics Associated with Tidal Processes in the Lower Columbia River

Tidally varying methane (CH4) concentrations in estuaries may arise from physical advection and by chemical effects tied to varying exposure to salinity. An investigation of spatial and temporal variability in water-column CH4 was conducted in the lower Columbia River using shipboard surveys and time series data from fixed stations. Peaks in CH4 coincided with ebb tides at multiple sites located along the flank of the estuary adjacent to tidal flats and wetlands. High-resolution measurements taken at the outflow of a shallow lateral bay revealed that these CH4 peaks were positively related to tidal amplitude when the lateral bay was exposed exclusively to freshwater over the tide cycle; in contrast, this relationship was inversed when brackish waters were involved. A positive relationship between tidal amplitude and CH4 is consistent with a mechanism of tidal pumping from bottom sediments in the bay. In the presence of saltwater, however, a higher-than-expected flux of CH4 could occur via suppression of removal processes such as biological oxidation. We present a conceptual model of tidal pumping modified by diurnal inequality in tidal amplitude and effects of salinity on sediment CH4 oxidation to explain CH4 variability on tidal to seasonal time-scales. The combined influences of tides and salinity likely affect CH4 emissions in estuaries worldwide, making sea level rise and estuarine geomorphological change relevant factors for consideration when accounting for estuarine contributions to global methane budgets.

Predicting Deep Water Intrusions to Puget Sound, WA (USA), and the Seasonal Modulation of Dissolved Oxygen

Observations and predictions regarding oceanic intrusions at the entrance to Puget Sound, WA (USA), are presented. Four years of seabed observations at Admiralty Inlet show episodic periods high salinity, coincident with landward residual currents near the seabed. The observed residual currents are consistent with an estuarine exchange flow during minimal tidal mixing, coincident with neap currents and maximum diurnal inequalities. These subtidal intrusions can carry dense water into Puget Sound and, thereby, renew the bottom water that is trapped landward of the sill. The oxygen concentration levels of these intrusions are often low, though not hypoxic, and may influence the oxygen levels in Puget Sound. The water mass properties of these intrusions are influenced strongly by regional dynamics, because the entrance to Puget Sound is connected to the open ocean via the Strait of Juan de Fuca. Coastal upwelling and discharge from the Fraser River discharge control the exchange flow in the Strait of Juan de Fuca and thus control the availability of dense water at the entrance to Puget Sound. The net effect of the tidal and regional dynamics is for intrusions with low oxygen levels to prevail in the late summer months. To predict intrusions in future years, an empirical method is developed and validated for daily application. The prediction method is based on publicly available operational data products and does not require in situ observations. In verification, 98% of intrusion events with dissolved oxygen less than 4.0 mg/L are successfully identified in a hindcast prediction for the 4-year observational record.

Residual fluxes of water and nutrient transport through the main inlet of a tropical estuary, Cochin estuary, West Coast, India.

Determining robust values for estuarine material fluxes has been a complex task and an interdisciplinary research challenge. With the advent of acoustic Doppler profilers (ADPs) having bottom-track capability and which provides three-dimensional current velocity profiles, more accurate estimation of cross-sectional fluxes is far accomplished in unsteady and bidirectional flow conditions of estuaries. This paper reports for the first time the discharge measurements conducted across Cochin inlet using ADP to examine the spring-neap variability in residual fluxes of water and nutrients during dry season. Cross-sectional current velocity profiles and salinity profiles were captured using ADP and conductivity temperature depth (CTD) instrumentation. Samples of surface and bottom water were also collected at 3-h intervals. The results indicated that there is a distinct transition from the neap to spring tides related to flow and salinity structure. The neap tide was partially mixed with large diurnal inequalities whereas the spring tide was well-mixed with symmetric tides. During ebb, an increase in the concentrations of nitrate, nitrite, phosphate, and silicate was noticed indicating upstream sources for their inputs. In contrast, elevated levels of ammonia were found in the estuary throughout the period of observation. There was net residual outflow during both tides, and the computed residual water fluxes of neap doubled that of spring. The strong ebb currents and the increased nutrient concentrations during ebb resulted in the export of all nutrients (except ammonia during spring) into the sea. The findings of this study highlight the consequences of anthropogenic interventions in the estuary and their effects on the fluxes of ecologically relevant substances.

Record of Permian Tethyan transgression in eastern India: A reappraisal of the Barren Measures Formation, West Bokaro Coalfield

The Barren Measures Formation (middle Permian, ∼271–260 Ma), the non-coaliferous lithounit sandwitched between two major coal-bearing formations in the Indian Lower Gondwana succession, was earlier interpreted as continental fluvio-lacustrine deposits. The present contribution documents sedimentary attributes of marginal marine tide–wave interference from the Barren Measures Formation of eastern peninsular India, exposed along the Bokaro River section, West Bokaro Coalfield, Jharkhand. Tidalites, including various tidal bundle sequences with frequent reactivation surfaces (velocity asymmetry) and single/double mud drapes (pause planes), systematic arrangement of tidal beddings (flaser, wavy, lenticular) and tidal rhythmites with alternate sand-dominated and mud-dominated plane-laminated units, manifest sedimentation by open marine spring–neap–spring tidal cycles under diurnal inequalities in a semi-diurnal tidal system. The architecture of tidalites attests to sedimentation in shallow subtidal to intertidal flat facies, affected by intermittent strong to week reworking by open marine waves/storms. Wave reworking is manifested by wave ripples, combined-flow ripples and wave-generated tidal bundles. Their coexistence and gradation with tidalites indicate low-energy wave/storm interference with tidal currents in a sheltered, tide-dominated estuary, especially on tidal flats along estuary channel banks. Overall fining-up facies sequence, upward increase of prodeltaic mud over tide–wave led sediments, signify a sustained transgressive phase onlaping the estuary system in eastern peninsular India during middle Permian (Guadalupian).

Characterising the spatial and temporal variability of the tidal-stream energy resource over the northwest European shelf seas

As devices move from full-scale prototype to commercial installations, it is important that developers have detailed knowledge of the tidal energy resource. Therefore, the spatial distribution of the tidal currents over the northwest European shelf seas has been examined to improve understanding of the tidal-stream energy resource. Using a three-dimensional hydrodynamic model (ROMS) at ∼1 km spatial resolution, and applying device characteristics of the Seagen-S turbine, we show that the ratio of the amplitudes of the M2 and S2 tidal currents can lead to significant variability in annual practical power generation – variability that is not accounted for when considering only the mean peak spring tidal velocities, as is generally the case in resource feasibility studies. In addition, we show that diurnal inequalities (governed by K1 and O1 tidal constituents) and tidal asymmetries (governed by the relationship between M2 and its compound tide M4) over the northwest European shelf seas can further affect power generation at potential high-energy sites. Based on these variabilities, the spatial distribution of the tidal-stream ‘capacity factor’ has been calculated. We find that mean peak spring tidal velocities can under-estimate the resource by up to 25%, and that annual practical power generation can vary by ∼15% for regions experiencing similar mean peak spring tidal velocities, due to the influence of other tidal constituents. Therefore, even preliminary resource assessments should be based on annual average power density, rather than peak spring tidal velocity.

Extraction of Tide Constituents by Harmonic Analysis Using Altimetry Satellite Data in the Brazilian Coast

AbstractThis paper analyzes the sea surface height dataset from the TOPEX, Jason-1, and Jason-2 satellites of a 19-yr time series in order to extract the tide harmonic constituents for the region limited by latitude 5°N–35°S and longitude 55°–20°W. The harmonic analysis results implemented here were compared with the tidal constituents estimated by three classical tidal models [i.e., TOPEX/Poseidon Global Inverse Solution 7.2 (TPXO7.2), Global Ocean Tide 4.7 (GOT4.7), and Finite Element Solution 2102 (FES2102)] and also with those extracted from in situ measurements. The Courtier criterion was used to define the tide regimes and regionally they are classified as semidiurnal between the latitude range from approximately 5°N to 22°S, semidiurnal with diurnal inequality from 22° to about 29°S, and mixed southward of latitude 22°S. The comparison results among all tide approaches were done by analyzing the root-sum-square misfit (RSSmisfit) value. Generally, the RSSmisfit difference values are not higher than 12 cm among them in deep-water regions. On the other hand, in shallow water, all models have presented quite similar performance, and the RSSmisfit values have presented higher variance than the previous region, as expected. The major discrepancy results were particularly noted for two tide gauges located in the latitude range from 5°N to 2°S. The latter was investigated and conclusions have mainly pointed to the influence of the mouth of the Amazon River and the considerable distance between tide measurements and the satellite reference point, which make it quite hard to compare those results. In summary, the results have showed that all models presently generate quite reliable results for deep water; however, further study should done in order to improve them in shallow-water regions too.

Hydrological and Sediment Characteristics of Sand Ridge Field in Jiangsu Coastal Area of China

The sand ridges field in Jiangsu coastal area has been the key region of tidal flat reclamation, waterway regulation and port construction. Adopting the hydrological and sediment data in winter, the tidal patterns were revealed. In addition, the depth-averaged velocity, sediment grain size and sediment concentration in each station were analyzed and compared in detail. The result shows that the tidal in study area is regular semidiurnal and diurnal inequality. The velocity has a decreasing tendency with the Tiaozini water area being centered towards the surrounding areas. The grain size gradually decreases and then increases from north to south, while the sediment concentration gradually increases and then decreases from north to south.

Tidal characteristics near the Chinese Zhongshan Station in Prydz Bay, East Antarctica

Abstract: A permanent tidal station was installed at the Chinese Zhongshan Station in Feb. 2010. Harmonic constants of 170 tidal constituents were obtained from harmonic analysis of the first year’s data. The results of the eight main constituents showed good agreement with those of two tidal models. Tidal characteristics, such as tide type, diurnal inequality, tidal range, and water levels were also analyzed.

Processes controlling stratification on the northern Skagit Bay tidal flats

Estuarine systems range from highly stratified to well mixed, often passing through several states during a single tidal cycle. Here, the processes driving temporal and spatial variations of stratification are investigated on the broad, shallow, periodically inundated tidal flats (shoals) between the north and south forks of the Skagit River. Over a two-week period, observation-based estimates of the straining, advection, and mixing are in balance with the temporal changes in the stratification-induced potential energy anomaly at a mid-flat location. The water on the tidal flats does not completely de-stratify during the strong ebb, and thus the initial tongue of water crossing the flats on strong floods often is partly stratified, but becomes increasingly well mixed as the flood progresses. For nearly semi-diurnal tides, the maximum stratification occurs during mid-ebb tide. Although cross- and alongshore flows have similar magnitudes, the changes in stratification during semi-diurnal tides result primarily from cross-shore processes, similar to observations of narrow, strongly-forced salt-wedge estuaries. Stratification is stronger during tides with a large diurnal inequality (the elevations of the two daily low tides differ by more than 33% of the tidal range) than during nearly semi-diurnal tides (with low tide elevations that differ by less than 25% of the tidal range), and is a maximum in the middle of the weak flood that follows the small low. In contrast to prior observations in narrow estuaries, alongshore (roughly parallel to isopycnals) advection of stratified water is significant and contributes to the increased stratification during these tides. Furthermore, during the small low and weak flood, the strongly sheared (density-driven) flows, which are offshore-directed at the surface and onshore-directed at the bed, also contribute to the high levels of stratification. Temporal changes in stratification are similar across and along the flats. However, maximum stratification increases offshore and alongshore towards the north fork (the distributary channel closest to the instruments).

Analysis of internal tidal characteristics in the layer above 450 m from acoustic Doppler current profiler observations in the Luzon Strait

The characteristics of internal tides in the upper layer of the Luzon Strait are investigated on the basis of direct-observation current data recorded on April 25 and September 26, 2008 by an acoustic Doppler current profiler. Spectral analysis and energy estimation show that the diurnals and semidiurnals carry most of the energy of internal tides. Values of the depth-integrated total energy E for the three frequency bands of diurnal, semidiurnal, and high frequencies are 31, 6.9, and 3.4 kJ·m, respectively. Near-inertial peaks are only present in the baroclinic component. The behavior of typical tidal frequencies (i.e., O1, K1, M2, MK3, and M4) and the near-inertial frequency is basically consistent with linear internal wave theory, which predicts E+(ω)/E−(ω)=(ω−f)2/(ω+f)2 at depths above 66 m, while not all prominent tidal components coincide well with the relation of the linear internal wave field at other depths. Examinations of depth structures of the baroclinic tides and temporal variations show that the surface tides and internal tides are both of mixed type, having diurnal inequality and spring-neap fortnight periods. The K1 and O1 tides have comparable cross- and along-shelf components, while the M2 and S2 tides propagate toward the shelf in the northern South China Sea as wave beams. The amplitude and phase of internal tides vary with time, but M2 and S2 tides appear to have structures dominated by the first mode, while the K1 and O1 tides resemble second-mode structures. The minor to major axis ratios are close to expected values of f/ω in the thermocline.

MARÉS ASTRONÔMICAS NA BAÍA DA BABITONGA, SC

Tidal harmonic constituents for Praia da Enseada (# PE) and Port of Sao Francisco do Sul (# PSFS) were assessed to characterize the local astronomical tide, as well to evaluate the shallow water effects on the tidal propagation to the interior of the Babitonga Bay. The sea level at # PE were monitored with a pressure tide gauge during June to December, 1996. From this data set, 48 tidal harmonic constituents was determined using the harmonic method for tidal analysis. The 32 harmonic constituents for the # PSFS were obtained with the Diretoria de Hidrografia e Navegacao. Considering these information, the tide at the Babitonga Bay is classified as mixed, with semidiurnal dominance and diurnal inequalities, with Form Number of 0.32 for # PE and 0,29 for # PSFS. Nonlinear effects results in the amplification of the astronomical constituents towards the interior of the bay, showing hypersynchronous behavior. Some harmonic constituents increased about 2 to 7 times, and all presented around of 40 minute phase shift. Despite of the astronomical constituents amplification, there are also an asymmetry on the wave propagation, with flood dominate in bay. The role of the channel constriction effects over the bottom friction was clearly observed by the harmonic constituents speed increase towards the bay. The Babitonga Bay presents a natural oscillation period about 3,6 hours resonance with some of shallow water components. This effect can be a significant processes in the increasing of the astronomical components.

Characterizing the nonlinear internal wave climate in the northeastern South China Sea

Abstract. Four oceanographic moorings were deployed in the South China Sea from April 2005 to June 2006 along a transect extending from the Batanes Province, Philippines in the Luzon Strait to just north of Dong-Sha Island on the Chinese continental slope. The purpose of the array was to observe and track large-amplitude nonlinear internal waves (NIWs) from generation to shoaling over the course of one full year. The basin and slope moorings observed velocity, temperature (T) and salinity (S) at 1–3 min intervals to observe the waves without aliasing. The Luzon mooring observed velocity at 15 min and T and S at 3 min, primarily to resolve the tidal forcing in the strait. The observed waves travelled WNW towards 282–288 degrees with little variation. They were predominantly mode-1 waves with orbital velocities exceeding 100 cm s−1 and thermal displacements exceeding 100 m. Consistent with earlier authors, two types of waves were observed: the a-waves arrived diurnally and had a rank-ordered packet structure. The b-waves arrived in between, about an hour later each day similar to the pattern of the semi-diurnal tide. The b-waves were weaker than the a-waves, usually consisted of just one large wave, and were often absent in the deep basin, appearing as NIW only upon reaching the continental slope. The propagation speed of both types of waves was 323±31 cm s−1 in the deep basin and 222±18 cm s−1 over the continental slope. These speeds were 11–20% faster than the theoretical mode-1 wave speeds for the observed stratification, roughly consistent with the additional contribution from the nonlinear wave amplitude. The observed waves were clustered around the time of the spring tide at the presumed generation site in the Luzon Strait, and no waves were observed at neap tide. A remarkable feature was the distinct lack of waves during the winter months, December 2005 through February 2006. Most of the features of the wave arrivals can be explained by the tidal variability in the Luzon Strait. The near-bottom tidal currents in the Luzon Strait were characterized by a large fortnightly envelope, large diurnal inequality, and stronger ebb (towards the Pacific) than flood tides. Within about ±4 days of spring tide, when currents exceeded 71 cm s−1, the ebb tides generated high-frequency motions immediately that evolved into well-developed NIWs by the time they reached mooring B1 in the deep basin. These waves formed diurnally and correspond to the a-waves described by previous authors. Also near spring tide, the weaker flood tides formed NIWs which took longer/further to form, usually not until they reached mooring S7 on the upper continental slope. These waves tracked the semidiurnal tide and correspond to the b-waves described by previous authors. These patterns were consistent from March to November. During December–February, the structure of the barotropic tide was unchanged, so the lack of waves during this time is attributed to the deep surface mixed layer and weaker stratification along the propagation path in winter.

Rhythmic Climbing-Ripple Cross-Lamination in Inclined Heterolithic Stratification (IHS) of a Macrotidal Estuarine Channel, Gomso Bay, West Coast of Korea

Abstract Well-developed rhythmic climbing-ripple cross-lamination (RCRL) was described from estuarine tidal channels in Gomso Bay, west coast of Korea. Associated with upper intertidal point bars of closely spaced meandering channels, RCRL occurs between mean sea level and mean neap high-water level. RCRL is typically less than 40 cm thick, and constitutes the upper part of fining-upward channel-fill successions that are capped by intensely bioturbated mud. RCRL consists of mud-draped climbing-ripple cross-laminae that are continuous along strike direction of the channel for 10–20 m. Climbing patterns are dominantly supercritical and less commonly subcritical. Flood-oriented RCRL is developed in the landward part of the meander bend, while ebb-oriented RCRL is developed in the seaward part. RCRL demonstrates rhythmic change in cross-lamination thickness, which resembles various hierarchical tidal cycles, such as diurnal inequality, synodic neap–spring tidal cycle, and anomalistic tidal cycle. Truncated tidal rhythmicities registered in RCRL and fine-grained textures are consistent with an upper intertidal origin. Common links of RCRL with inclined heterolithic stratification (IHS) and tight meander bends as well as high sedimentation rate is suggestive that RCRL can serve as a diagnostic indicator of active channel migration in the fluvio-estuarine transition. This study highlights the significance of RCRL bearing well-preserved tidal rhythmicities in the reconstruction of paleodepositional environment, paleoelevation in terms of tidal frame, and paleogeography.

A numerical study on the generation of a distinct type of nonlinear internal wave packet in the South China Sea

A distinct type of nonlinear internal-wave packet, with the largest internal solitary wave in the middle of the packet, was regularly observed in the South China Sea during the Asian Seas International Acoustics Experiment in 2001. Data analysis shows that the occurrence of the distinct internal wave packet is closely related with the occurrence of lower-high internal tides; the internal tides are mixed in the experimental area and, thus, there is diurnal inequality between the heights of two neighboring internal tides. Modeling of internal tides and internal solitary waves in a shoaling situation suggests that this type of wave packet can be generated in the South China Sea by the large shoaling of internal solitary waves and internal tides. Both the internal solitary waves and the internal tides come from the direction of Luzon Strait. The initial large internal solitary waves contribute to the occurrence of the largest internal solitary wave in the middle of the packet and the waves behind the largest internal solitary wave, while the shoaling internal tides bring about the nonlinear internal waves in front of the largest internal solitary wave via interaction with the local shelf topography.

The Intermittently Exposed, Coarse-Grained Gravel Beaches of Prince William Sound, Alaska: Comparison with Open-Ocean Gravel Beaches

Abstract Long-term studies (1989–2007) of the gravel beaches in Prince William Sound, Alaska, conducted as part of the Exxon Valdez oil spill monitoring programs, provide the basis for understanding the geomorphology and sediment dynamics of these intermittently exposed, coarse-grained gravel beaches. There are key differences between these beaches and open-ocean gravel beaches, including bermlike ridges that occur at different levels on the beach as a result of the diurnal inequality of the tides. The 1964 earthquake, with up to 3.5 m of uplift, provided information on sediment transport patterns and rates. In less than 25 years, a stable cobble/boulder armor formed on the upper and lower platforms, and the beaches reached a steady-state or graded condition. Eighteen years after the spill, oiled sediments were still present at depths greater than about 25 cm on beaches with a stable armor in the middle and lower intertidal zones. Cleanup efforts to remove the deep oil penetration into the porous gravel o...

Time‐spatial variability observed in velocity of propagation of the internal bore in the Strait of Gibraltar

Some aspects of the time‐spatial variability of the phase speed of the internal bore generated almost every tidal cycle in Camarinal Sill, are revised using a set of high resolution experimental data collected in two different positions of the Strait during May 2003. This variability is mainly driven by the intense tidal currents, comparable with the intrinsic propagation velocity of the first mode baroclinic bore. It is shown that the importance of the diurnal tide in the Strait of Gibraltar induces a considerable diurnal inequality on the bore velocity, with an observed maximum difference of 0.7 ms−1 between the speed of two consecutives bores propagating along the eastern part of the Strait. A regularly spatial pattern has been also found: the internal bore reaches its maximum velocity in Tarifa Narrows. A theoretical estimation predicts an extreme phase speed of 2.6 ms−1 during our period of study.