Tidal Residual Currents Research Articles

Effects of tides on freshwater and volume transports in the Changjiang River plume

Effects of tides on the Changjiang River plume are investigated using a numerical model of the East China and Yellow Seas. In the absence of tides, the buoyancy‐driven coastal current downstream of the river mouth is a surface‐trapped plume attached to the coast. In the presence of tides, however, the plume shifts away from the coast and features a bottom boundary layer. The freshwater transport by the coastal current accounts for only 35% of the Changjiang River discharge without tides but increases to 80% with tides. Tidal currents stabilize the plume bulge near the river mouth and force the plume water to move in the direction of tidal currents, thus increasing the downstream freshwater transport. In contrast, tidal residual currents are weak and make little contributions to the total freshwater transport. Tidal mixing reduces the plume's buoyancy but increases its thickness so that the freshwater transport is insensitive to the strength of turbulent mixing. Volume transport in the tidal‐affected buoyancy coastal current exceeds the river discharge by 4 to 12 times, as opposed to 2–3 fold amplification in the plume without tides. Tidal mixing between the plume and ambient shelf waters produces higher salinity classes in the plume so that the volume flux in the downstream coastal current increases to discharge the freshwater released from the Changjiang River.

A numerical study of the Magellan Plume

In this modeling study we investigate the dynamical mechanisms controlling the spreading of the Magellan Plume, which is a low‐salinity tongue that extends along the Patagonian Shelf. Our results indicate that the overall characteristics of the plume (width, depth, spreading rate, etc.) are primarily influenced by tidal forcing, which manifests through tidal mixing and tidal residual currents. Tidal forcing produces a homogenization of the plume's waters and an offshore displacement of its salinity front. The interaction between tidal and wind‐forcing reinforces the downstream and upstream buoyancy transports of the plume. The influence of the Malvinas Current on the Magellan Plume is more dominant north of 50°S, where it increases the along‐shelf velocities and generates intrusions of saltier waters from the outer shelf, thus causing a reduction of the downstream buoyancy transport. Our experiments also indicate that the northern limit of the Magellan Plume is set by a high salinity discharge from the San Matias Gulf. Sensitivity experiments show that increments of the wind stress cause a decrease of the downstream buoyancy transport and an increase of the upstream buoyancy transport. Variations of the magnitude of the discharge produce substantial modifications in the downstream penetration of the plume and buoyancy transport. The Magellan discharge generates a northeastward current in the middle shelf, a recirculation gyre south of the inlet and a region of weak currents father north.

조석 주기별 오염물질 방출에 따른 마산만의 체류시간 및 해수교환율 변화

EFDC와 라그랑쥐 입자추적모델을 이용하여 조석주기별로 오염물질의 방출 시점이 다를 경우에 대한 마산만의 체류시간 및 해수교환율의 정량적 차이를 계산하였다. 체류시간은 만 전체에 대해 약 40일이었고 그 범위는 부도 남쪽 해역에서 약 20일 이하, 소모도 상부에 위치하는 마산만 내측에서는 약 100일 이상으로 나타났다. 이러한 체류시간의 공간적 차이는 주로 조석잔차류와 만 내측으로부터의 거리에 영향을 받는 것으로 밝혀졌다. 만 전체 면적에 대한 체류시간의 평균값은 대조기 및 소조기에 각각 약 36일과 42일로 나타났다. 해수교환율은 30%가 되기까지 걸리는 시간이 입자 방류 시점에 따라 최소 약 65일부터 최대 105일까지 약 40일 이상 차이가 나는 것으로 계산되었다.

Respuesta barotrópica de los golfos norpatagónicos argentinos forzados por mareas y vientos

This work presents the results of a high resolution three dimensional non-linear barotropic hydrodynamic model used to examine the response of three north Patagonian gulfs in Argentina (San Matias, SMG; Nuevo, NG; and San Jose, SJG) forced by tides and idealized patterns of surface winds. The structure of the circulation induced by the dominant winds (Westerly) in all three gulfs consists of two independent gyres: one anticyclonic gyre in the west with an intense southward coastal flow and weaker return flow in the middle of the gulfs and one cyclonic gyre in the east that, in the SMG's case, connects with the shelf. The other two gulfs (NG and SJG) have much more restricted connections with the exterior. The inclusion of tidal dissipation effects reduces the intensity of the gyres, but the general spatial pattern of the circulation remains. Increasing the wind magnitude mainly intensifies the anticyclonic gyre and the cross-shelf transport through the SMG mouth and deepens the surface Ekman layer. If the gulfs are forced with meridional winds, the circulation splits into two gyres whose sense of circulation depends on the wind direction: a northern cyclonic (anticyclonic) gyre and a southern anticyclonic (cyclonic) gyre for Southerly (Northerly) winds. When the gulfs are forced simultaneously by tides and winds, the residual tidal currents dominates the general circulation and contributes to a greater dynamical isolation of the gulfs from the exterior shelf.

Spatio-temporal variability of currents over a mobile dune field in the Dover Strait

A one month field campaign featuring two spring–neap tide cycles and three strong storms has been performed in a mobile dune area located in the central part of the Dover Strait. These dunes are known to move in a complex manner as their migration direction varies in space and time ( Le Bot and co-workers, 2000, 2001, 2004). In order to gain some insights into the dune motion processes we present an analysis of the spatio-temporal variability of currents in the area emphasizing the relative influence of tides and storms. A total of eight different hydro-meteorological regimes have been distinguished during the experiment duration. The analysis of the currents measurements at five locations in the area shows that the eight hydro-meteorological regimes induce very different current responses at the bottom. The residual tidal currents exhibit a significant spatial variability both in direction and in intensity. A numerical model of tidal currents over the Dover Strait confirms the strong spatio-temporal variability of the residual tidal currents featuring three singular points. Amongst them, a saddle point is located just south of the I-dune at the convergence of opposite direction residual tidal currents. The wind-induced currents are almost uniform in space, their intensity and direction however strongly depends on the wind regime and thus on time. The mean total current feature a spatial pattern which can be tidal of wind-induced currents dominated, or either in balance, depending on the regime considered. At the PERMOD campaign time scale, the total current is dominated by the residual tidal current. These results proved to give valuable insights to explain the complex dynamics of dune motion observed in this area by Le Bot and co-workers (2000, 2001, 2004) at short and long time scales.

Migration and internal architecture of marine dunes in the eastern English Channel over 14 and 56 year intervals: the influence of tides and decennial storms

AbstractSubmarine dune dynamics are controlled by tidal currents and wind forces. According to the relative influence of these forces and the nature of dune sediment, different bedform behaviors can be observed. The footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes. This paper is concerned with bedforms that compose the thick sediment wedge located in the eastern English Channel, off the Bay of Somme. This sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages. The dynamics of large submarine dunes, which are organized in fields, are studied thanks to bathymetric and seismic data over the periods 1937–1993 and 1993–2007.Dune morphology presents low lee and stoss side slopes (on average 8° and 3°, respectively) and dune migration rate is not very high. Dune movements are in the direction of residual tidal currents, i.e. toward the east, with mean migration rates around 0·8 to 5 ± 0·25 m yr−1 and up to 6·6 ± 0·7 m yr−1, respectively, at multi‐decennial and decennial time scales. The dune internal architecture is complex with superimposed eastward prograding units, displaying locally opposite progradation. Second‐order discontinuities (dip of 0·5°–4° perpendicular to dune crests) constitute dune master bedding. By counting the number of second‐order reflectors between 1937–1993 and 1993–2007, the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years. These time intervals coincide with the long‐term tidal cyclicities and also with the inter‐annual to decennial variability of storm activity in northern Europe. Two theories were made to interpret the dune internal structures: the second‐order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods (weak tidal and storm action), or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast. Copyright © 2010 John Wiley & Sons, Ltd.

Patrón de transporte de sedimento en Bahía Magdalena, Baja California Sur, México, inferido del análisis de tendencias granulométricas

The grain-size trend analysis (grain size, sorting asymmetry) was determined at 58 stations in order to infer the residual vectors of sediment transport in Magdalena Bay, Baja California Sur, Mexico. Grain-size trend analysis and principal components analysis were applied to textural sediment parameters. The results indicated that well-sorted sediment is associated with fine to very fine sand. The residual vector transport suggests the presence of two gyres in the dispersion of clastic material: (1) acyclone-ic gyre in the deeper, central part and (2) an anticyclone-ic gyre in the southeast part of the bay. The sediment transport pattern is highly correlated with the residual tidal currents in Magdalena Bay.

Highly Skewed Tidal Circulation Pattern and Water Quality in Puerto Galera Bay, Mindoro Island, Philippines

Coastal waters in and around Puerto Galera Bay, Mindoro, Philippines, is drastically contaminated generally due to poorly constructed sanitation and household facilities and runoff from the hillsides associated with the tourism development. In this study, a field survey on tidal currents and water quality in and around Puerto Galera Bay was conducted with hydrodynamic numerical simulations of tidal currents, in which a nesting technique was employed for properly incorporating the effects of complicated tidal fluctuation outside the bay. The results reveal the existence of a highly asymmetrical pattern of tidal flow fluctuation having strong tidal residual flow and the vulnerability of the flow field to water quality deterioration within the semi-enclosed innermost bay area. The highly asymmetrical tidal flow fluctuation is attributed to the asymmetrical generation of detached eddying motion around Puerto Galera as a highly nonlinear phenomenon producing strong tidal residual currents. These results are useful in understanding the physical conditions of the bay, which are needed for consideration in the local government's coastal management plan.

Residual Exchange Flows in Subtropical Estuaries

Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves. Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides. This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual flows are mainly driven by tides.

Morphodynamic behaviour of a nearshore sandbank system: The Great Yarmouth Sandbanks, U.K.

Offshore sandbanks provide a major source of sediment and may have a significant impact on the evolution of the nearby shoreline. The prediction of sandbank evolution is extremely important for coastal engineers and managers. However, it is evident from the existing literature that additional investigations into the evolution of offshore sandbanks are needed. The present paper describes qualitative and quantitative investigations of the evolution of the Great Yarmouth sandbanks. A depth-integrated tidal model has been used to calculate the residual currents arising from the sequence of sandbank configurations observed from historic charts over a 150 year period. Tidal simulations were performed with a nested model covering the Southern North Sea and Great Yarmouth zone (east coast of the UK). Simulations were carried out for a period corresponding to several spring-neap cycles to calculate residual currents and were compared directly with field measurements. Model calibration and validation was performed against published tidal elevation charts based on many observations, tidal predictions and field measurements. These detailed comparisons have been used to determine the errors in the model predictions. Overall, the performance of the model is considered good. The model output has been analysed to investigate qualitative links between the sandbank configuration and the residual flow statistics. The results demonstrate that the long-term changes in sandbank morphology can be explained from the pattern of tidal residual currents.

Circulation on the Armorican shelf (Bay of Biscay) in autumn

Three drifters drogued at 65 m were launched on a transect on the Armorican shelf of the Bay of Biscay for 4 years. The experiments were conducted in autumn. They revealed a north-westward, poleward current over the 100 m isobath and a very weak eastward current over depths comprised between 120 and 150 m. A model was used to assess the role of residual tidal currents and wind-induced circulation. The results show that the former are quite weak and the latter do not explain the average velocity of over 10 cm s − 1 . It is thought that this current is mainly driven by the density gradient induced by the breakdown of stratification. Hydrological data and satellite images from the period are discussed, in the light of this hypothesis.

Distribution and retention of Sabellaria alveolata larvae (Polychaeta: Sabellariidae) in the Bay of Mont-Saint-Michel, France

The gregarious polychaete Sabellaria alveolata is an important foundation species whose reef structures add topographic complexity and high levels of biodiversity in sandflats. In the Bay of Mont-Saint-Michel, France, S. alveolata reefs are considered the largest reef formations in Europe and are increasingly endangered by anthropogenic pressure. In order to understand the role of larval transport on the resilience capacity of these biogenic reefs, multiple complementary approaches were carried out within the bay in 2002 to study horizontal and vertical distribution of S. alveolata larvae and assess larval dispersal processes. Plankton samples collected twice a month from March to October showed an extended period of larval occurrence, from the end of April to October, with 2 peaks of high abundance. The main spawning period occurred early in May, and a smaller peak occurred in September. Mean planktonic lifetime has been calculated to be between 4 and 10 wk. At large scale, larval distribution was patchy and the maximal abundance could reach about 28 000 larvae m -2 . Larval densities were strongly correlated with physico-chemical parameters so that larvae were more abundant in a warmer and less saline coastal water mass characterised by a higher concentration in chlorophyll a. The presence of a large anticyclonic gyre in the bay seemed to play a significant role in controlling larval dispersal by concentrating larvae within the bay. At 1 station in the bay, short-term variations of larval abundances in the water column appeared to be regulated by the tidal cycle, as larvae were more abundant at the end of ebb tide, confirming the very coastal distribution of larvae. Furthermore, S. alveolata larvae tended to migrate closer to the surface during flood and near the bottom during ebb, promoting a net landward transport of larvae. Despite a long larval lifespan, our results indicate that tidal residual currents, especially a coastal gyre, and tidal vertical migration may favour larval retention within the bay.

Surface current variability in the Keum River Estuary (South Korea) during summer 2002 as observed by high-frequency radar and coastal monitoring buoy

High-frequency (HF) radar observations of surface currents were conducted for 3 months during summer 2002 in the Keum River estuary. A comparison between HF radar-derived currents and directly measured ones form a buoy showed that the regression slope is close to 1 and the correlation coefficient greater than 0.86, with an RMS difference less than 13 cm/s which is less than 17% of the tidal current. This fairly good agreement allows us to use HF radar observation in investigating the surface flow and circulation in this tidal-current-dominant coastal-plume area. To examine the spatial variation in tidal current characteristics, as well as currents associated with non-tidal forcing, the HF radar-derived currents were separated into tidal and sub-tidal frequency currents. The overall pattern of M 2-current ellipse distribution in the study area showed a counterclockwise rotation, with the offshore maximum current direction to the northeast. Eccentricity, the direction of maximum current, and the phase of net motion of the ellipse changed near the estuary mouth and near the gap of the Saemangeum reclamation tide dyke due to the complex coastal geometry and the out-flowing jet during the ebb period. The Eulerian mean current field in July was similar to the circulation patterns suggested in the previous studies using the CTD data and numerical models, implying that the region of northwestward flow may have developed by the rivers discharges. The short-term variation in the surface sub-tidal flow field was governed primarily by that of wind forcing and was responsible for the changes in the hydrographic conditions in the surface layer off the Keum River estuary mouth. The spatially averaged current, which is calculated after extracting the Eulerian mean currents from the sub-tidal flow, flowed almost to the right of wind during 22 days for an observer looking down-wind-stream, with the mean angle of 33°, indicating predominant Ekman drift. The sub-tidal current observed at the buoy showed that there is an almost unidirectional, persistent current irrespective of the variable wind forcing, producing strong Eulerian mean jet from the northern dyke gap. This current could be formed due to firstly the residual tidal currents generated by the open edges of the dyke and secondly the offshore discharge of the diluted water through the gap. However, when the daily river discharge increased greatly, the sub-tidal current speed at the buoy increased as much as 21 cm/s under the very strong southerly wind condition. This event is interpreted by relaxation process of the trapped coastal water by wind and the large river discharge.

Sediment trend analysis of the hylebos waterway: Implications for liability allocations

Sediment trend analysis of the hylebos waterway: Implications for liability allocations

Mapping exchange and residence time in a model of Willapa Bay, Washington, a branching, macrotidal estuary

The numerical model GETM is used to examine transport pathways and residence time in Willapa Bay, Washington, a macrotidal estuary with a complex channel geometry. When the model is run with realistic forcing, it reproduces both tidal velocities and the decrease of the salt intrusion length with increasing river flow with errors of 5–20%. Furthermore, a more stringent test, when the model is run with tidal forcing only, it reproduces the along‐channel profile of the effective horizontal diffusivity K, a direct measure of the strength of subtidal dispersion, which is known from previous empirical estimates. A Lagrangian, particle‐tracking method is used to map subtidal transport pathways at the resolution of the model grid. This analysis reveals an interweaving of coherent lateral exchange flows with discontinuous, small‐scale dispersion as well as tidal residual currents that in some locations, sharpen rather than smooth gradients between water masses. Comparison between these Lagrangian results and an Eulerian salt flux decomposition suggests that along‐channel complexity (channel junctions and channel curvature) is at least as important as cross‐sectional depth variation in shaping the subtidal circulation. Finally, a nonconservative tracer method is used to produce high‐resolution, three‐dimensional maps of residence time. This analysis shows that consistent with previous observational work in Willapa, at all except the highest winter‐storm‐level river flows, river‐ and ocean‐density‐driven exchanges are discernable but secondary to tidal stirring. In all seasons, despite the fact that half the volume of the bay enters and leaves with every tide, average retention times in the upper third of the estuary are 3–5 weeks.

Modelling tide-driven currents and residual eddies in the Gulf of Kachchh and their seasonal variability: A marine environmental planning perspective

Tide-driven currents in the Gulf of Kachchh (GoK) have been studied using MIKE21 hydrodynamic model. The results are validated with measured currents for three different periods characterized by different wind fields. Comparison of model results showed very good agreement with the measured currents. The study suggests that though the currents of GoK are predominantly tide-driven, they respond significantly to the seasonally changing wind system. Strong southwesterly winds enhance the flood tidal currents by about 20% but reduce the ebb tidal currents by about 20% during June–July. The currents intensify during NE monsoon period as well as SW monsoon period when the winds become stronger. However, the currents are comparatively weak during pre-monsoon transition. Irregular topography of the western gulf slows down the tidal wave propagation and induces a phase shift. Tidal residual currents computed from the model results exhibit the presence of three eddies in the western gulf. The tide–topography interaction could be responsible for generating eddies in the residual flow field. It is possible that these eddies could effectively decrease the flushing rate of materials discharged in the upstream gulf, and subsequently increase their residence time. The hydrodynamic modelling setup forms a base for future ecological modelling endeavours planned for the gulf in order to reduce stress on the ecosystem especially in the sensitive areas such as the Marine National Park and Marine Sanctuary.

Modulation of shallow water tides in an inlet‐basin system with a mixed tidal regime

During the autumn of 1999 a 56‐day‐long observational study of sea surface elevation and currents was made in Yavaros Bay, a shallow inlet‐basin system with a mixed tidal regime in the Gulf of California. During the late spring of 1998 a similar experiment was performed at the same coastal lagoon as well, although it was shorter. The measurements are used to investigate the modulation of the shallow water tides. The complex demodulation approach revealed that not only the Moon's path cycle modified the generation of shallow water tides but also the lunar declination and the solar declination. The sixth‐diurnal tides showed a spring‐neap modulation. The fifth‐diurnal tides were above the noise level only close to the summer solstice. The fourth‐diurnal tides were modulated by the spring‐neap cycle at the basin, but this modulation was changed by friction at the inlet. At this site the fourth‐diurnal band of the tidal current had a maximum in amplitude during neap tides. The third‐diurnal tides were irregularly modulated. The advection and the nonlinear continuity terms were correlated with the third‐ and fourth‐diurnal bands of the tidal current at the inlet. The quadratic friction terms were correlated with the sixth‐, fourth‐, and third‐diurnal bands of the tidal current. The semidiurnal tides mainly provided the energy of the tidal residual current during the autumnal equinox of 1999. Near the summer solstice of 1998 the semidiurnal band induced a residual current seaward oriented, while the diurnal band induced a residual current landward oriented.

Sediment distribution study in the Gulf of Kachchh, India, from 3D hydrodynamic model simulation and satellite data

A fully 3D finite difference baroclinic hydrodynamic model ‘COSMOS’ is applied to the Gulf of Kachchh (GoK) to study tidal variation, ocean currents, residual tidal current, sea surface temperature distribution, etc. The ‘COSMOS’ is based on the hydrostatic and Boussinesq approximations and uses a vertical double sigma coordinate with a step-like grid. In addition to the momentum and continuity equations, the model solves two-transport equations for salinity and temperature and an equation of state to include baroclinic effects. The other objectives are quantitative assessment of suspended sediments by analyzing SeaWiFS data and understand the relation between residual tidal currents and erosion, movement and deposition of sediments. The model results showed that the dominant current system within GoK is controlled by the simulated tidal and residual tidal currents variation. The residual tidal current velocity reduces from the surface towards the bottom. The residual surface current velocity distribution displays cyclonic eddy continuity regions within the GoK. It is concluded that bottom topography variation plays an important role in determining the distribution of residual current velocity and thereby sediment distribution. The residual current velocity distribution map showed conformity with the gross geomorphic subdivisions and the sediment distribution map of Gulf of Kachchh. The suspended sediment plume pattern followed the current pattern obtained from modeling. The model results of the sea surface temperature of the December month showed good agreement with the temperature structure and the pattern obtained form NOAA/AVHRR data.

Fast hydrodynamic model for medium- and long-term dispersion in seawater in the English Channel and southern North Sea, qualitative and quantitative validation by radionuclide tracers

The database for medium- and long-term model validation using 125Sb released by the La Hague reprocessing plant includes 1400 measurements performed between 1987 and 1994 in the English Channel and the North Sea and data for each release since 1982. Antimony-125 has a conservative behaviour in water masses over a period of several years. These data can be used qualitatively and quantitatively to compare the measured concentrations with the calculated ones and quantities of tracers. Tritium measurements are also available for model calibration. A two-dimensional hydrodynamic model has been developed to allow repetitive long-term simulations. This model uses a database of residual tidal currents calculated using the Lagrangian barycentric method [Salomon, J.C., Guéguéniat, P., Orbi, A., Baron, Y., 1988. A Lagrangian model for long-term tidally induced transport and mixing. Verification by artificial radionuclide concentrations. In: Guary, J.C., Guéguéniat, P., Pentreath, R.J. (Eds.), Radionuclides: A Tool for Oceanography, Cherbourg 1–5 June, 1987. Elsevier Applied Science Publishers, London, New York, pp. 384–394]. The area covered by the model includes the English Channel, the southern North Sea and the Irish Sea with a mesh size of 1 km. The main adjustment parameters of this model are the sources of wind data used and the calculation method for evaluating wind stress at the sea surface. With these parameters, the fluxes of radionuclides and water masses in the English Channel and the North Sea were balanced for the whole period of field measurements (1987–1994). The correlation factor between individual measurements in seawater and calculation results is 0.88 with an average error of ±54%, the error attributable to the measurement process being 15% on average. The mean flux through the Dover Strait is 126,000 m 3 s −1, close from the one obtained from previous studies [Salomon, J.C., Breton, M., Guéguéniat, P. 1993. Computed residual flow through the Dover Strait. Oceanologica Acta 16 (5–6) (1993) 449–455; Bailly du Bois, P., Salomon, J.C., Gandon, R., Guéguéniat, P. 1995. A quantitative estimate of English Channel water fluxes into the North Sea from 1987 to 1992 based on radiotracer distribution. Journal of Marine Systems 6 (5–6) (1995) 457–481; Prandle, D., Ballard, G., Flatt, D., Harrison, A.J., Jones, S.E. Knight, P.J., Loch, S. McManus, J. Player, R. Tappin, A., 1996. Combining modelling and monitoring to determine fluxes of water, dissolved and particulate metals through the Dover Strait. Continental Shelf Research 16 (2) (1996) 237–257]. This model appears as a robust tool for modelling long-term dispersion at a low computation cost (it takes less than 1 h to simulate one year on a personal computer). It could forecast consequences of controlled or accidental releases of soluble substances in seawater under realistic conditions or calculate water masses fluxes and transit times.

Simulation of water exchange in Jiaozhou Bay by average residence time approach

A dispersion model coupled with the Princeton Ocean Model was used to estimate the average residence time of the water in Jiaozhou Bay. The tidal simulation agreed quite well with drift experiments and water elevation observations at the Dagang tide station in the east coast of the bay. In particular, in situ measurements of 228Ra and salinity were carried out to calibrate the dispersion model. The modelled average residence time was about 52 days, ranging from less than 20 days in the deep part near the bay channel, the only passage connecting the bay to the Yellow Sea, to over 100 days in the shallow area in the northwest. The spatial difference of average residence time was controlled by tidal residual currents and the distance to the bay channel. The modelled tidal exchange rate was uneven in the bay, and consistent with 228Ra observations. The temporal evolution of the passive tracer accords with the evolution of the rain fraction after the rainstorm in August 2001.