Tidal Prism Research Articles

Morfodinâmica da desembocadura da Lagoa de Tramandaí (RS, Brasil)

A comunicação de corpos lagunares com o oceano ocorre através de canais de maré (tidal inlets), os quais constituem-se em importantes feições na troca de sedimentos. Algumas vari- áveis (prisma de maré, geometria do canal, etc.) controlam sua morfodinâmica e podem modificar sua posição geográfica. O conhecimento dos fatores que comandam sua estabilidade é de extrema relevância para gerenciar a costa e conservar o ambiente lagunar. Neste trabalho foi analisada a estabilidade da desembocadura da Lagoa de Tramandaí, com base em dados hidrodinâmicos, sedimentológicos e morfológicos integrados em uma análise espaço-temporal. A desembocadura desta laguna está localizada entre as praias de Tramandaí e Imbé (litoral norte do Rio Grande do Sul) e encontra-se fixada com apenas uma estrutura tipo guia-corrente ao longo da margem esquerda. A metodologia utilizada consistiu de levantamento batimétrico, técnicas de geoprocessamento e cálculos das relações empíricas de estabilidade. Os dados obtidos permitiram observar que anteriormente à fixação da margem esquerda do canal, a migração da desembocadura acompanhava a direção preferencial da deriva litorânea, para NE. A análise das relações de estabilidade física permitiu concluir que o canal apresentava forte instabilidade antes da obra de fixação da desembocadura. Após a obra do guia-corrente, a seção transversal mínima foi fixada e não existem registros de assoreamento.

Morfodinâmica da desembocadura da Lagoa do Peixe, litoral sul do Brasil

A morfologia de canais é resultado do ambiente em que se encontram, assim como de todas variáveis físicas atuantes. Se a desembocadura está situada em zonas costeiras, a tendência à instabilidade é amplificada pelo próprio meio. Neste artigo são apresentados os resultados do estudo morfodinâmico do canal de comunicação entre a Lagoa do Peixe e o Oceano Atlântico, localizado no Parque Nacional da Lagoa do Peixe, litoral médio do Rio Grande do Sul, Brasil. Também é medida a variabilidade da linha de praia adjacente ao parque. Com o objetivo de determinar a classifica- ção do canal e sua dinâmica natural, foram utilizados uma série de 47 anos de imagens Landsat (1973 – 2009), dados de precipitação da Agencia Nacional de Águas e equações. A interpretação da série temporal e os cálculos das médias de precipitação permitiram descrever o canal da Lagoa do Peixe como um canal de maré, com mecanismo de assoreamento morfodinâmico influenciado pelo regime de precipitação pluviométrica. Os resultados das equações indicaram como fatores determinantes à sua oclusão o prisma de maré e transporte e deposição sedimentar em frente a desembocadura, associados à deriva litorânea de padrão bidirecional da área de estudo. Sua desembocadura possui caráter intermitente e é instável quanto ao seu posicionamento e geometria. A mobilidade da linha de praia adjacente ao Parque Nacional da Lagoa do Peixe foi verificada pelo método de pontos extremos, pixel a pixel, entre duas imagens Landsat de mesmo mês, uma de 1987 e outra de 2009, sendo constatada erosão ao sul da desembocadura do canal da Lagoa do Peixe e acreção ao norte da mesma.

Changing tidal hydrodynamics during different stages of eco-geomorphological development of a tidal marsh: A numerical modeling study

The eco-geomorphological development of tidal marshes, from initially low-elevated bare tidal flats up to a high-elevated marsh and its typical network of channels and creeks, induces long-term changes in tidal hydrodynamics in a marsh, which will have feedback effects on the marsh development. We use a two-dimensional hydrodynamic model of the Saeftinghe marsh (Netherlands) to study tidal hydrodynamics, and tidal asymmetry in particular, for model scenarios with different input bathymetries and vegetation coverages that represent different stages of eco-geomorphological marsh development, from a low elevation stage with low vegetation coverage to a high and fully vegetated marsh platform. Tidal asymmetry is quantified along a 4 km marsh channel by (1) the difference in peak flood and peak ebb velocities, (2) the ratio between duration of the rising tide and the falling tide and (3) the time-integrated dimensionless bed shear stress during flood and ebb. Although spatial variations in tidal asymmetry are large and the different indicators for tidal asymmetry do not always respond similarly to eco-geomorphological changes, some general trends can be obtained. Flood-dominance prevails during the initial bare stage of a low-lying tidal flat. Vegetation establishment and platform expansion lead to marsh-scale flow concentration to the bare channels, causing an increase in tidal prism in the channels along with a less flood-dominant asymmetry of the horizontal tide. The decrease in flood-dominance continues as the platform grows vertically and the sediment-demand of the platform decreases. However, when the platform elevation gets sufficiently high in the tidal frame and part of the spring-neap cycle is confined to the channels, the discharge in the channels decreases and tidal asymmetry becomes more flood-dominant again, indicating an infilling of the marsh channels. Furthermore, model results suggest that hydro-morphodynamic feedbacks based on tidal prism to channel cross-sectional area relationships keep the marsh channels from filling in completely by enhancing ebb-dominance as long as the tidal volume and flow velocities remain sufficiently high. Overall, this study increases insight into the hydro-morphodynamic interactions between tidal flow and marsh geomorphology during various stages of eco-geomorphological development of marshes and marsh channels in particular.

Past and Future Ecosystem Change in the Coastal Zone

The coastal zone is in a constant state of flux. Long term records of change attest to high amplitude sea level changes. Relative stability though the Late Holocene has allowed for the evolution of barrier dune systems, estuaries and coastal lakes with associated plant and faunal associations. This evolution has been interspersed with changes in the balance between climate driven changes in outflow from catchments. These interactions have been considerably disturbed through the impacts of industrialised people who have diverted and consumed water and invested in infrastructure that has impacted on river flows and the tidal prism in estuaries. This has impacted their provisioning services to humans. It has also impacted their regulating services in that development along the coastline has impacted on the resilience of the littoral zone to absorb natural climate extremes. Looking from the past we can see the pathway to the future and more easily recognise the steps needed to avoid further coastal degradation. This will increasingly need to accommodate the impacts of future climate trends, increased climate extremes and rising seas. Coastal societies would do well to identify their long term pathway to adaptation to the challenges that lie ahead and plan to invest accordingly.

Morphological process of a restored estuary downstream of a tidal barrier

Hydrodynamic and morphological processes become more complex with large spatial variations after tidal barrier building and estuary restoration. Using a process-based model, responses in the estuary to a two-phase restoration measure in the Shuanglong Estuary are investigated. Numerical modeling indicates that tide-asymmetry in the barraged estuary is the primary factor that determines responses of morphology. After partial absence of tidal flats (first-phase restoration), flood tide-asymmetry is switched to ebb tide-asymmetry in the lower unrestored reach. This induces lower estuary erosion via increased tidal prism, though it is enhanced in the upper widened-deepened restored reach leading to severe siltation. After full restoration (second-phase restoration), current velocities are significantly increased with reduced flood tide-asymmetry. A quantitative comparison of the impact of the cross-sectional profile to net sediment transport shows that a switch from flood-dominated estuary to ebb-dominated estuary can be caused by reshaping the tidal flat which may reduce flood tide-asymmetry. The sediment flushing efficiency after the estuary restoration is further investigated in different sluice discharges with regards to two floodgate operations. Low sluice discharge during ebb tides increases the sediment flushing efficiency, but continuous high sluice discharge contributes to significant improvement of sediment flushing capacity and reduction of flushing duration. We conclude to prevent the flushed sediment from re-deposition into the estuary, a larger flushing capacity discharge released in a short period is required after the second-phase restoration in the Shuanglong Estuary.

Widespread infilling of tidal channels and navigable waterways in the human-modified tidal deltaplain of southwest Bangladesh

Since the 1960s, ~5000 km2 of tidal deltaplain in southwest Bangladesh has been embanked and converted to densely inhabited, agricultural islands (i.e., polders). This landscape is juxtaposed to the adjacent Sundarbans, a pristine mangrove forest, both well connected by a dense network of tidal channels that effectively convey water and sediment throughout the region. The extensive embanking in poldered areas, however, has greatly reduced the tidal prism (i.e., volume of water) transported through local channels. We reveal that >600 km of these major waterways have infilled in recent decades, converting to land through enhanced sedimentation and the direct blocking of waterways by embankments and sluice gates. Nearly all of the observed closures (~98%) have occurred along the embanked polder systems, with no comparable changes occurring in channels of the Sundarbans (<2% change). We attribute most of the channel infilling to the local reduction of tidal prism in poldered areas and the associated decline in current velocities. The infilled channels account for ~90 km2 of new land in the last 40–50 years, the rate of which, ~2 km2/yr, offsets the 4 km2/yr that is eroded at the coast, and is equivalent to ~20% of the new land produced naturally at the Ganges-Brahmaputra tidal rivermouth. Most of this new land, called ‘khas’ in Bengali, has been reclaimed for agriculture or aquaculture, contributing to the local economy. However, benefits are tempered by the loss of navigable waterways for commerce, transportation, and fishing, as well as the forced rerouting of tidal waters and sediments necessary to sustain this low-lying landscape against rising sea level. A more sustainable delta will require detailed knowledge of the consequences of these hydrodynamic changes to support more scientifically-grounded management of water, sediment, and tidal energy distribution.

Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA

Coupling between barrier islands and their associated backbarrier environments (salt marsh, tidal flats) leads to complex ecogeomorphic feedbacks that are proposed to control the response of barrier island systems to relative sea-level rise. This study tests the applicability of these still-theoretical concepts through investigation of the Virginia barrier islands (eastern United States), which are located in an area of accelerated sea-level rise. Using historical maps and photographs from A.D. 1851 to 2010, we determine that rapid landward island migration (1–6 m yr –1 ) is leading to backbarrier area reduction and large-scale salt marsh loss (63 km 2 or 19%) at a rate of 0.45 km 2 yr –1 . Landward barrier island migration far outpaces upland marsh migration and is responsible for 51% of marsh loss; the remainder is due to backbarrier processes (e.g., edge erosion). In direct contrast to proposed ecogeomorphic feedbacks linking barrier island and backbarrier environments, shoreline retreat rates were not related to changes in backbarrier marsh, open-water areas, or tidal prism. Rather, these results indicate that, for barrier island systems already undergoing migration, the primary barrier-backbarrier coupling is the loss of marsh and tidal-flat area because of barrier island migration.

Aggradation and lateral migration shaping geometry of a tidal point bar: An example from salt marshes of the Northern Venice Lagoon (Italy)

Although meanders are ubiquitous features of the tidal landscape, the architectural geometries of tidal point bar deposits are relatively unexplored and commonly investigated on the basis of facies models developed for their fluvial counterparts. The present study aims at improving current understanding of tidal point bar deposits developed in salt marsh settings, through a high-resolution investigation of an abandoned intertidal meander loop, located in the northern part of the Venice Lagoon (Italy). The study channel is 6m wide and was active until the 1950s, when it was deactivated as consequence of a neck cut-off. A total of 150 cores was recovered from the associated point bar. The bar erosionally overlies a subtidal platform consisting of sand and mud and is covered by both channel fill and salt marsh mud. The bar, floored by a shell-rich sandy lag, consists of stratified fine sand, grading upward into sandy mud. The outer bank of the bend is characterized by well-developed, sand-rich levee deposits and absence of crevasse splays, which represent a distinctive feature of alluvial sedimentation. Sediment grain size distributions suggesting that seaward and landward sides of the point bar experienced comparable changes of bed shear stress due to alternation between flood and ebb currents, highlighting a remarkable difference with the classical downstream-fining characterizing fluvial point bars. Spatial interpolation between key stratal surfaces shows an overall thickening of the bar from 1.2 to 1.7m in the direction of channel migration, associated with both lowering of the bar base and rising of its brink, which occurs in parallel with an increase in channel cross-sectional area, to progressively accommodate the increasing tidal prism. The bar top surface is characterized by a spoon-shaped geometry stemming out from a combination between lateral migration (8–10cm/yr) and vertical aggradation (2.5–3.0mm/yr) of the inner bank. In salt marsh settings, vertical aggradation plays, therefore, a major effect on point bar sedimentation, generating peculiar bar top geometries that are not common in fluvial meanders, where the high rate of lateral migration causes the cutoff to be reached before substantially thick deposits are accumulated on the bar top.

Modeling the growth and migration of sandy shoals on ebb‐tidal deltas

AbstractCoherent sandy shoals that migrate toward the downdrift coast are observed on many ebb‐tidal deltas. In this study, processes that cause the growth and migration of shoals on ebb‐tidal deltas are identified. Moreover, the effect of the incident wave energy and the tidal prism of an inlet on the migration speed of these shoals is investigated. For this, a numerical morphodynamic model with an idealized geometric setup is employed. The model computes the bed level evolution due to local erosion and deposition of sand driven by tides and waves. Analysis of model results shows that shoals grow when there is a local imbalance between the bathymetry and the wave conditions, which in this study was imposed by manually breaching the ebb‐tidal delta or by adding storms to the wave forcing. There are thresholds for shoal formation that depend on the distribution of the sand and the incident wave energy. Wave refraction over the shoals leads to focusing of wave energy and increased wave energy dissipation around the location of the local minimum water depth. This generates residual currents over the shoal and increased skin friction toward the local minimum water depth, which together create a sand transport pattern that induces the growth and migration of the shoal. Sand transport due to asymmetric waves contributes to keeping the shoal a coherent structure. The shoal migration speed increases with increasing incident wave energy and decreasing tidal prism; this is because tidal residual currents oppose the wave‐driven residual currents that cause shoal migration.

A numerical model for the entire Wadden Sea: Skill assessment and analysis of hydrodynamics

AbstractA baroclinic three‐dimensional numerical model for the entire Wadden Sea of the German Bight in the southern North Sea is first assessed by comparison to field data for surface elevation, current velocity, temperature, and salinity at selected stations and then used to calculate fluxes of volume and salt inside the Wadden Sea and the exchange between the Wadden Sea and the adjacent North Sea through the major tidal inlets. The model is simulating the reference years 2009–2011. An overview of tidal prisms and residual volume fluxes of the main inlets and their variability is given. In addition, data from an intensive observational campaign in a tidal channel south of the island of Spiekeroog as well as satellite images and observations of sea surface properties from a ship of opportunity are used for the skill assessment. Finally, the intensity of estuarine overturning circulation and its variability in the tidal gullies are quantified and analyzed as function of gravitational and wind straining using various estimates including Total Exchange Flow (TEF). Regional differences between the gullies are assessed and drivers of the estuarine circulation are identified. For some inlets, the longitudinal buoyancy gradient dominates the exchange flow, for some others wind straining is more important. Also the intensity of tidal straining (scaled covariance of eddy viscosity and vertical shear) depends on buoyancy gradient and wind forcing in different ways, depending on local topography, orientation toward the main wind direction, and influence by freshwater run off inside or outside the tidal basin.

Estimation of Net Flux of Water Mass and Tidal Prism at a Tidal Entrance through Bottom Tracking with ADCP

본 연구에서는 조석 ìœ ìž êµ¬ì—ì„œì˜ 단면통과 ìœ ëŸ‰ 및 조량 ì‚°ì •ì„ ëª©ì ìœ¼ë¡œ 목포해역 내 조석 ìœ ìž êµ¬ì—ì„œ ADCP를 이용한 단면 ìœ ì†ê´€ì¸¡ì´ 수행되었다. ìš°ì„ , 원시 관측자료는 ì§€ë°°ì ì¸ ìœ ì†ì„±ë¶„ 도출을 위하여 단면의 íšŒì „ì •ë„ë¥¼ ê³ ë ¤í•œ ì¢Œí‘œíšŒì „ì´ 수행되었으며, ì •ê·œí™”ë¥¼ 통하여 수평/수직방향 Sigma 좌표로 변환되었다. 또한, 관측이 이루어지지 않는 ì €ë©´ê³¼ 수면 근처의 Blank zone 데이터는 von-Karman ìœ ì†ë¶„í¬ì‹ì„ 이용하여 보간되었다. ë³´ì •ëœ 자료를 이용하여 단면통과 ìœ ëŸ‰ì´ ì •ëŸ‰ì ìœ¼ë¡œ ì‚°ì •ë˜ì—ˆëŠ”ë°, 조석에 따른 단면통과 ìœ ëŸ‰ì˜ 변화양상 뿐만 아니라 목포해역의 ë‚™ì¡°ìš°ì„¸ì  조석특성이 잘 나타나는 것으로 확인되었다. 또한, 과거 연구들에서 ì œì‹œë˜ëŠ” 조량 ì‚°ì •ìžë£Œì˜ 조석조건을 보완하여 현장 관측자료에 기반한 ì¡°ëŸ‰ì‚°ì • 방법의 조건이 ìž¬ì •ë¦½ë˜ì—ˆìœ¼ë©°, 새로운 ì •ì˜ì— 근거하여 조석 ìœ ìž êµ¬ì—ì„œì˜ 단면 ìœ ì†ê´€ì¸¡ 자료를 이용한 조량의 ì •ëŸ‰ì  ì‚°ì •ì´ 국내 최초로 수행되었다. 조량 ì‚°ì •ê²°ê³¼ëŠ” 과거 연구들로부터 도출된 조량과 조석 ìœ ìž êµ¬ 흐름 ë‹¨ë©´ì ì˜ 상관관계 결과와 비교·분석되었으며, ì‚°ì •ëœ 결과는 과거 연구결과들과 잘 일치하는 것으로 나타났다. 핵심용어: ADCP, 단면 ìœ ì†ê´€ì¸¡, 조석 ìœ ìž êµ¬, 단면통과 ìœ ëŸ‰, 조량

Assessment of three dredging plans for a wave-dominated inlet

Many small wave-dominated inlets are naturally unstable and require regular dredging. To mitigate the costs of these operations, the dredged channels should be designed to bring the inlet close to equilibrium and minimise flood dominance. However, it is often unclear how to optimise the configuration of the channels. This study focuses on a small lagoon on the western Portuguese coast that has been subject to frequent and diverse interventions. A process-based morphodynamic model is applied to compare the dredging plan that has been followed for the last 15 years with two new alternatives. These alternative dredging plans increase the tidal prism and reduce flood dominance, mainly due to the higher channel cross-section. Secondary transverse channels do not affect the morphodynamics significantly. For future dredging operations, it is suggested that the bottom depth of the main natural channel be dredged to chart datum and a secondary channel be added to the previous channel with the same bottom depth and half of the channel width.

Linking the infilling of the North Branch in the Changjiang (Yangtze) estuary to anthropogenic activities from 1958 to 2013

Many tidally-dominated estuaries of the world are experiencing variations in bottom topography due to changes in natural forcings and intensive human activities. Here we focus on the morphological evolution of the North Branch (NB), a tidally-dominated distributary of the Changjiang estuary. Our analysis is based on long-term bathymetric and hydrological data collected between 1950 and 2010. The results show that mean water depth, channel volume below 0m, and channel volume below −5m have respectively decreased by 43%, 53% and 92% in the last 50years. A reduction of the whole estuarine surface with aggradation in elongated tidal sand bars and erosion at the mouth are the main morphological variations of the NB, while a decrease in channel volume below −5m due to infilling is the second mode of morphological change. While the drastic decrease in sediment load from upstream is likely unrelated to the silting of the NB, local land reclamation along the banks is directly responsible for the reduction of estuarine surface area and related tidal prism. Between 1958 and 2013, enhanced flood-tide currents resulted in a large import of sediments from offshore into the NB, triggering a sustained decrease in channel volume below 0m. It is argued that the recovery of the funnel-shaped configuration of the estuary by restoring mud flats over 0m, dredging the southern part of the estuary bend and forbidding land reclamation could prevent the silting of the NB, otherwise the NB will likely vanish in few decades.

Controls on the stratigraphic framework and paleoenvironmental change within a Holocene estuarine system: Pamlico Sound, North Carolina, USA

This study examines the Holocene stratigraphic record and paleoenvironmental evolution of a large estuarine system, at high temporal and spatial resolution, in the context of changing climate, sea level, and hydrodynamics. New data are used to examine two time periods of increased marine influence within Pamlico Sound in northeastern North Carolina interpreted to be the result of extensive barrier island segmentation synchronous with periods of rapid climate change during the late Holocene. The study reveals the controls on the stratigraphic development and the highly dynamic character of the coastal system in response to climate, and sea-level change as it interacts with paleotopography. These findings can be used to inform projections of future environmental changes.Estuarine waters occupied paleovalleys in the Pamlico Sound region as early as ca. 7500calBP and Pleistocene interfluves separated the paleovalleys from the Atlantic Ocean until ca. 5500calBP, when they were flooded by rising sealevel forming the broad, shallow Pamlico Sound. Initial barrier islands and shoals likely formed sometime between 5500 and 5000calBP, providing the geomorphic setting for continued estuarine conditions behind the barriers. At ca. 4000calBP, an increase in marine influence and sand content is detected in multiple cores, and is correlated to seismic data within the Pamlico Sound basin. This change is interpreted to be the result of increased segmentation of barrier islands (thus a greater number of inlets, or wider inlets), which is consistent with a rapid transgression seen in other areas of the U.S. east coast. The segmentation may have been partially facilitated by a rapid increase in lagoonal area and tidal prism by overtopping of interfluves. Greater barrier island continuity is evident from 3500 to 1200calBP, as indicated by muddy deposits with low brackish estuarine foraminiferal assemblages. At ca. 1200calBP, seismic, sedimentological and micropaleontological data suggest an increase in wave and current energy and marine influence throughout southeastern Pamlico Sound. These changes are interpreted to represent extensive segmentation of the barrier islands during the Medieval Climate Anomaly (MCA). Since ca. 500calBP, the Pamlico Sound system has returned to a more restricted state, as inlets have closed. Currently only three major inlets segment the Outer Banks, the barrier island system fronting Pamlico Sound, but warming climate and increasing rates of sea-level rise suggest that another episode of barrier island segmentation began about a half century ago and appears that it may extend into the near future.

Tidal hydrodynamics under future sea level rise and coastal morphology in the Northern Gulf of Mexico

AbstractThis study examines the integrated influence of sea level rise (SLR) and future morphology on tidal hydrodynamics along the Northern Gulf of Mexico (NGOM) coast including seven embayments and three ecologically and economically significant estuaries. A large‐domain hydrodynamic model was used to simulate astronomic tides for present and future conditions (circa 2050 and 2100). Future conditions were simulated by imposing four SLR scenarios to alter hydrodynamic boundary conditions and updating shoreline position and dune heights using a probabilistic model that is coupled to SLR. Under the highest SLR scenario, tidal amplitudes within the bays increased as much as 67% (10.0 cm) because of increases in the inlet cross‐sectional area. Changes in harmonic constituent phases indicated that tidal propagation was faster in the future scenarios within most of the bays. Maximum tidal velocities increased in all of the bays, especially in Grand Bay where velocities doubled under the highest SLR scenario. In addition, the ratio of the maximum flood to maximum ebb velocity decreased in the future scenarios (i.e., currents became more ebb dominant) by as much as 26% and 39% in Weeks Bay and Apalachicola, respectively. In Grand Bay, the flood‐ebb ratio increased (i.e., currents became more flood dominant) by 25% under the lower SLR scenarios, but decreased by 16% under the higher SLR as a result of the offshore barrier islands being overtopped, which altered the tidal prism. Results from this study can inform future storm surge and ecological assessments of SLR, and improve monitoring and management decisions within the NGOM.

Variability of tidal signals in the Brent Delta Front: New observations on the Rannoch Formation, northern North Sea

Detailed observations on the Rannoch Formation in several deep Viking Graben wells indicate that the ‘classical’ wave-dominated Brent delta-front shows coupled storm–tide processes. The tidal signals are of three types: I): alternations of thick cross-laminated sandstone and thin mud-draped sandstone, whereby double mud drapes are prominent but discretely distributed, II): a few tidal bundles within bottomsets and foresets of up to 10cm-thick sets cross-strata, and III): dm-thick heterolithic lamination showing multiple, well-organized sand–mud couplets.During progradation of the Brent Delta, the Rannoch shoreline system passed upward from 1) a succession dominated by clean-water, storm-event sets and cosets frequently and preferentially interbedded with type I tidal beds, and occasional types II and III tidal deposits, toward 2) very clean storm-event beds less frequently separated by types II and III tidal beds, and then into 3) a thin interval showing muddier storm-event beds mainly alternating with type II tidal beds.It is likely that those variations in preservation bias of storm and tidal beds in each facies succession result from combined effects of 1) the frequency and duration of storms; 2) river discharge; and 3) the absolute and relative strength of tides. Tidal deposits are interpreted as inter-storm, fair-weather deposits, occurred preferentially in longer intermittent fair-weather condition and periods of lower river discharge, and well-pronounced in the distal-reach of delta-front. The formation and preservation of tidal signals between storm beds, indicate that the studied Rannoch Formation was most likely a storm-dominated, tide-influenced delta front 1) near the mouth of a large Brent river, where a significant tidal prism and high tidal range might be expected, and 2) in a setting where there were relatively high sedimentation rates associated with high local subsidence rates, so that the storm waves did not completely rework the inter-storm deposits. The documentation of the unconventional Rannoch Formation contributes to our understanding of mixed-energy coastal systems.

The effect of tidal exchange on residence time in a coastal embayment

Numerical simulations of an idealized lagoon that is connected to the ocean via a tidal inlet show that the mean residence time is inversely proportional to tidal exchange. In the Delft3D model the tidal exchange is controlled by varying the inlet length, width and depth. These changes in the inlet geometry affect the tidal prism and the ebb/flood flow structure, which are shown to control the exchange of lagoon water with seawater. To map residence time within the lagoon, a new method that implements dye tracer is developed and shows that the tidally averaged residence time exhibits significant spatial variability. For inlet systems in which, as a first approximation, the lagoon can be described by a uniformly fluctuating water level, a simple transport model is developed to elucidate the specific processes that control tidal exchange and their effect on residence time. In this transport model tidal exchange is decomposed into two fractions, an ocean exchange fraction and a lagoon exchange fraction. It is shown that both fractions need to be included to better describe tidal exchange. Specifically, inclusion of a lagoon exchange fraction improves previous tidal prism models that assume complete mixing in the lagoon. The assumption of complete mixing results in an under-prediction of residence time. Relating the spatially averaged residence time results to the exchange fractions for each inlet geometry show that the residence time is inversely proportional to the product of the tidal exchange fractions. For these single inlet systems, Keulegan's 0-D hydrodynamic model shows good agreement with Delft3D in predicting the tidal prism, maximum flow velocity, and exchange fractions. With these parameters, estimates of the mean residence time can be reached through a relationship derived from the simple transport model.

Estimation of annual energy output from BCM tidal barrage and the corresponding marine environmental impact

Based on the finite-volume coastal ocean model (FVCOM), a three-dimensional numerical model FVCOM was built to simulate the ocean dynamics in pre-dam and post-dam conditions in Bachimen (BCM). The domain decomposition method, which is effective in describing the conservation of volume and non-conservation of mechanical energy in the utilization of tidal energy, was employed to estimate the theoretical tidal energy resources and developable energy resources, and to analyze the hydrodynamic effect of the tidal power station. This innovative approach has the advantage of linking physical oceanography with engineering problems. The results indicate that the theoretical annual tidal energy resources is about 2×108 kWh under the influence of tidal power station; Optimized power installation is confirmed according to power generation curve from numerical analysis; the developable resources is about 38.2% of theoretical tidal energy resources with the employment of one-way electricity generation. The electricity generation time and power are 3479 hours and 2.55×104 KW, respectively. The power station has no effect on the tide pattern which is semi-diurnal tide in both two conditions, but the amplitudes of main constituents apparently decrease in the area near the dam, with the M2 decreasing the most, about 62.92 cm. The tidal prism shrinks to 2.28×107 m3, but can still meet the flow requirement for tidal power generation. The existence of station increases the flow rate along the waterway and enhances the residual current. There are two opposite vortexes formed on the east side beside the dam of the station, which leads to pollutants gathering.

Grain‐size distributions and coastal morphodynamics along the southern Maryland and Virginia barrier islands

AbstractThis study examined grain‐size distributions to address questions regarding geological and oceanographic controls on island morphodynamics along one of the longest undeveloped, mixed‐energy barrier island systems in the world. In particular, statistical analyses (i.e. analysis of variance, Tukey honest significant difference multiple comparison tests, nonparametric statistics and linear regression analysis) of 230 barrier island samples from Ocean City Inlet, Maryland, to the mouth of the Chesapeake Bay and 134 nearshore samples (d ≤ 10 m) identified grain‐size trends related to the morphodynamic characteristics of these systems. In general, the Virginia barrier islands north of Wachapreague Inlet and Assateague Island form a statistically different subset of grain sizes (medium‐grained to coarse‐grained sand) from the islands south of Wachapreague Inlet (fine‐grained sand). These textural trends corroborate the Pleistocene headlands of the Delmarva coastal compartment as the sediment source and indicate that some of the coarse‐grained to medium‐grained sediment bypasses the large sinks in the net southward longshore sediment transport system (i.e. Fishing Point and Chincoteague Inlet). This research also demonstrates that the preferential accumulation of coarse‐grained to medium‐grained sand on the ebb‐tidal delta at Wachapreague Inlet probably controls the erosional morphodynamics of the islands located downdrift (south) of the inlet. These results suggest that an increase in tidal prism, set up by sea‐level rise and/or a shift in wave climate/refraction patterns, may lead to barrier island fragmentation and a runaway transgression of this predominantly natural barrier island system. Consequently, a grain analysis of major coastal compartments, across multiple driving forces, can be used to assess coastal morphodynamics and the potential impact of climate change on coastal systems.

Water exchanges between a multi-inlet lagoon and the ocean: the role of forcing mechanisms

Understanding the influence of the main drivers controlling the circulation and the transport in coastal lagoons is a necessary step towards the description of the dynamics of their ecosystems. Thus, the influence of the main physical drivers on the water exchanges in a multi-inlet barrier island (western sector of the Ria Formosa coastal lagoon, Portugal) was investigated. Several scenarios of tide, wind, bathymetry and point source discharges were simulated using a three-dimensional circulation model and a particle-tracking model. The circulation is adequately reproduced by the hydrodynamic model, with root mean square errors of about 5–8 cm for the water levels and 5–10 cm/s for the cross-sectional averaged velocities in the western inlets and main channels. Wind has a negligible effect on the modelled water levels and cross-sectional velocities, with differences smaller than 1 % between the simulations with and without wind. However, results show that wind influences significantly the transport by affecting the residual circulation, with distinct effects depending on the wind direction. Upwelling favourable winds (with west component) increase the landward transport through the Faro-Olhao inlet and promote a larger dispersion of the water-borne material inside the lagoon. The residual circulation between the three inlets of the western sector of the lagoon is also significantly affected by the bathymetry. Results show a decrease of about 50 % of the tidal prism of the Ancao inlet between 2002 and 2011, which reduces its capacity to export waterborne material to the adjacent coastal area and increases the residence times in some areas of the lagoon, with potentially adverse effects on the ecosystems’ health. The dispersion of potential contaminants from point sources inside the lagoon to the coastal area depends on the combined effect of the tidal phase and amplitude and the location of the source. Mean residence times for the discharges from the wastewater treatment plants of the western sector ranged from 7 to 18 days. These findings improve the understanding of the influence of the physical forcings in the circulation and transport dynamics of multi-inlet coastal systems.