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The hydrodynamics of the Bons Sinais Estuary: The value of simple hydrodynamic tidal models in understanding circulation in estuaries of central Mozambique
... Fonte: (Kjerfve B. , 1990 (Fairbridge, 1980, retirado de Miranda et al., 2002 ........ 7 Os processos estuarinos são complexos e uma análise simples reprova ao tentar explicar processos de forma isolada (Miguel, Nehama, & Castro, 2017). A hidrodinâmica de um estuário, compreendendo marés, correntes de maré e circulação estuarina, desempenha um papel importante na formação e manutenção dos ecossistemas dentro do estuário e nas águas costeiras adjacentes (Hoguane, et al., 2020 areia pesada e dentre outros (Pereira, et al., 2010). ...
... As figuras (24) Onde, segundo (Hoguane, et al., 2020) (Kurniawan, et al., 2014). Quanto a assimetria encontrada nesse trabalho, corrobora com encontrada por (Mazzilli, 2015;Hoguane, et al., 2020) ...
... As figuras (24) Onde, segundo (Hoguane, et al., 2020) (Kurniawan, et al., 2014). Quanto a assimetria encontrada nesse trabalho, corrobora com encontrada por (Mazzilli, 2015;Hoguane, et al., 2020) ...
Estuaries are unique environments, located in the transition zone between the open ocean and the river, in which ocean waters are measurably diluted by freshwater from continental drainage. According to their characteristics, these ecosystems are extremely sensitive and contain very important habitats for biodiversity, biomass production and the preservation of numerous oceanic and terrestrial species. However, estuarine hydrodynamics are basically governed by three factors, river discharge, tidal currents and wind tension, subject to modifications by the Earth's rotation, bathymetry and estuary geometry. The aim of the work is to study the Hydrodynamics and Mechanisms of Salt Transport in the Bons Sinais Estuary under different tidal conditions. The campaign to acquire hydro-oceanographic data was carried out under spring tide conditions and two tidal conditions (spring and neap), over a period of approximately 14 days. The estimate of the average rainfall input presented a maximum value in January with 4.7 m^3⁄s and a minimum in October, with an intensity equal to -3.4 m^3⁄s, presenting the maximum freshwater input in the month of January. The total freshwater discharge (Q_f) due to the discharge of freshwater from the drainage basin and average rainfall input through the free surface of the estuarine channel, presented a maximum value in the month of February with an intensity equal to 46.10 m^3⁄s and -3.30 m^3⁄s representing the minimum in October, with a variation of 15.79 m^3⁄s. The temperature, salinity and density in the water column showed homogeneous behavior in both tidal conditions, although stratified at times in the syzygy and quadrature conditions. The main salt transport mechanism was the portion of river discharge with a participation of 90.06% in the spring tide and 69.68% in the neap tide condition in terms of total salt transport. Thus, in the syzygy condition, EBS is characterized as an importer and in quadrature as an exporter of salt. In both tidal conditions, the estuary presented dimensionless Richardson number values greater than 0.25, showing the dominance of stratification over turbulence. According to the diagram proposed by Hansen & Rattray (1966), the estuary was characterized as well mixed and weakly stratified type 1a in both tidal conditions. In order to characterize the movement regime in the Bons Sinais estuary, the dimensionless Reynolds number was calculated under spring and neap tide conditions. And therefore, EBS is characterized by a fully turbulent regime with intensified mixing.
Keywords: Bons Sinais Estuary. Estuarine hydrodynamics. Salt transport mechanisms. Estuarine
Indices. Physical forcing.
... Still is known that Bons Sinais Estuary has a high hydrodynamic pattern (Hoguane et al. 2020). However, the sedimentary dynamics, bottom sediment characteristics and its relation to the pollutant sorption were never explored. ...
... The Bons Sinais Estuary is located in Central Mozambique, to the north of Zambezi Delta (17°54'S, 36°49'E; Fig. 1) adjacent to Sofala Bank, is the largest and the most productive of the eastern African coast. An updated and more complete description of the Bons Sinais Estuary is given by Hoguane et al. (2020). It's a long (28 km) and shallow (10 m) estuary, with an extensive floodplain. ...
... Davis Jr. and Fitzgerald 2004;Corrêa 2005;Oliveira 2015). This aspect is reinforced by Hoguane et al. (2020) when they state that Bons Sinais Estuary has the highest tide amplitude (4 m) of the Western Indian Ocean, being therefore classified as meso to macro tide. According to Pinet (2009), most of the sediments in well-mixed estuaries are marine sediments. ...
This article addresses the Bons Sinais Estuary sedimentary dynamics and bottom sediment characterization and its implication to pollution, based on bottom sediment grain size and statistical parameters. There were collected 15 bottom sediment samples with a Van veen sampler, which were further analysed by sieve and pipette methods. Sand was the most dominant textural class (51%), followed by silt (25%) and clay (24%). The sedimentary dynamics have varied from moderate to high. Both size of sediments and Pejrup diagram suggests that the hydrodynamics of this Estuary is high. Although the percentage of coarse sediments is higher, the estuary is susceptible to pollution, as the amount of thinner sediments is considerable regarding the values of skewness. These results may be useful to environmental management actions including pollutants tracking.
... The geographical setting, history of settlement, ecosystems and socio-ecological importance of the estuary were summarized by Groeneveld et al. (2021). Hoguane et al. (2020; described estuarine circulation patterns, and Furaca et al. (2021) used remote sensing maps to infer changes in land use and land cover adjacent to the estuary over the past 27 years, including the growth of Quelimane city and its intrusion into the estuarine functional zone. ...
... Beach seines are commonly used in different littoral environments in the WIO(Wilson, 2012) making their properties a convenient benchmark for evaluating the selectivity of alternative gears, by using a relative selectivity measure as demonstrated here.Landings comprised mainly of small pelagic fishes and penaeid prawns, confirming the findings of previous studies on small-scale fisheries in coastal Mozambique(Gjosaeter and Sousa, 1983;Mualeque and Santos, 2011;Wilson, 2012;Cardinale et al., 2014;Hoguane and Armando, 2015). Similar taxa were caught by beach seine and chicocota nets at the estuary mouth and upper estuary (~ 30 km upstream) consistent with a well-mixed marine dominated system(Hoguane et al., 2020;. The marine dominance stems from reduced river runoff following the construction of theKariba-(1955) and Cahora Bassa dams(1974) in the upper Zambezi River(Beilfuss and dos Santos, 2001). ...
Small-scale fisheries in estuaries of the Western Indian Ocean form a key component of socio-ecological systems (SES) and food security, yet they remain poorly understood. This study describes the estuarine fisheries of the Bons Sinais Estuary in Mozambique based on fisheries and biological data collected by monitors between 2007 and 2016. Multiple gears were used, including beach seines (78 % of sampled fish), chicocota nets (12 %), gillnets (5 %), and hook and line (4 %). Landings were dominated by small pelagic fishes (Engraulidae 29 %, Pristigasteridae 16 %, Clupeidae 11 %) and prawns (Penaeidae 29 %). Monthly averaged fishing effort, landings and catch per unit effort of beach seine and chicocota nets peaked during the dry season, in April and July to October. The proportion of penaeid prawns in landings increased moderately at the end of the wet season, between January and March. The species and size composition selected by beach seine and chicocota nets overlapped, with chicocota nets also retaining marginally smaller individuals. Both gears exploited low trophic level species near the base of the food web, with low trophic values of 2.8 for chicocota and 3.0 for beach seine nets. The small-scale fishery in the Bons Sinais Estuary followed a ‘balanced harvest’ approach in which unselective fishing gear is used to exploit a mix of species and sizes proportional to natural productivity and relative abundance.
... Estuaries and coasts are natural systems that are useful for civilization, making them one of the most threatened ecosystems globally [1,2]. They act as ecological links between land and marine ecosystems and are influenced by the hydrodynamics of estuaries, including the phenomena of tides, currents [3,4], and periodic circulations from upstream through river systems (which results in lower salinity levels) [5]. Sometimes, bio-geomorphic phenomena also play active roles in building these ecosystems [6]. ...
This research aims to investigate the morphodynamics of the Wulan River estuary in Demak Regency using the integration of multispectral remote sensing images and numerical modeling. In this study, PlanetScope for a manualvisual analysis of estuary morphodynamics and Sentinel-2 MSI Level 2A to obtain periodic total suspended solids (TSS) information for the east and west monsoon seasons. MIKE by DHI software used to develop hydrodynamic numerical modeling in order to characterize the current circulation and sediment transport model. Based on a marine cartography aspect, the obtained results illustrated that the climatological phenomenon of seasonal forcing plays a role in the development of the current circulation and indirectly influences the sediment transport. During the west season, the morphodynamics in the Wulan River estuary are much more massive and significant as compared to the east season. A projection of the deposition that results from the sediment transport is described in the bed-thickness change; this occurs in the western part of the Wulan River estuary during the east season, while the bed-thickness change occurs predominantly in the northern part during the west season (where there was previously a beach sandbar phenomenon). This was verified through multi-temporal satellite imagery that the deposition that occurs in the northern part of the Wulan River estuary during the west season is increasingly progressive and massive.
... In contrast, an example of a permanently open estuary is the Bons Sinais system in central Mozambique ( Fig. 1) that has a semi-diurnal tidal regime of approximately 4 m during spring tides. This estuary receives a highly seasonal river flow, is 28 km in length, and has channel depths ranging from less than 2 m to more than 20 m in places (Hoguane et al., 2020). The Morrumbene Estuary (Fig. 1) is a similar type of system entering Inhambane Bay. ...
This review documents the occurrence of fish species in a range of estuaries from Mozambique. Altogether 217 fish species were recorded, belonging to 77 families, and dominated in terms of species richness by Gobiidae, Carangidae and Mugilidae. A guild analysis was conducted to compare the occurrence and degree of estuary-association by the various species in tropical Mozambique, with that recorded from nearby predominantly subtropical and warm-temperate estuaries in South Africa. The major difference in guild occurrence between the two countries centered on the higher representation of marine stragglers in Mozambique estuaries, a probable result of the wide mouths and macrotidal exchange of some of the larger systems along this coast. Estuarine lakes and lagoons within the Mozambique coastal region showed increasing reduction in marine connectivity with channel distance from the sea, and a concom-itant increase in freshwater fish species domination of these incipient coastal lakes and lagoons. Once these systems lose all connectivity with the sea, they become freshwater coastal lakes and lagoons, and all estuary-associated marine fish species disappear.
... In contrast, an example of a permanently open estuary is the Bons Sinais system in central Mozambique ( Fig. 1) that has a semi-diurnal tidal regime of approximately 4 m during spring tides. This estuary receives a highly seasonal river flow, is 28 km in length, and has channel depths ranging from less than 2 m to more than 20 m in places (Hoguane et al., 2020). The Morrumbene Estuary (Fig. 1) is a similar type of system entering Inhambane Bay. ...
This review documents the occurrence of fish species in a range of estuaries from Mozambique. Altogether 217 fish species were recorded, belonging to 77 families, and dominated in terms of species richness by Gobiidae, Carangidae and Mugilidae. A guild analysis was conducted to compare the occurrence and degree of estuary-association by the various species in tropical Mozambique, with that recorded from nearby predominantly subtropical and warm-temper- ate estuaries in South Africa. The major difference in guild occurrence between the two coun- tries centered on the higher representation of marine stragglers in Mozambique estuaries, a probable result of the wide mouths and macrotidal exchange of some of the larger systems along this coast. Estuarine lakes and lagoons within the Mozambique coastal region showed increasing reduction in marine connectivity with channel distance from the sea, and a concom- itant increase in freshwater fish species domination of these incipient coastal lakes and lagoons. Once these systems lose all connectivity with the sea, they become freshwater coastal lakes and lagoons, and all estuary-associated marine fish species disappear.
... The salinity is determined by S, the distance along the channel from the mouth elevation, the water level in the water column measured from the surface, and the water density (kg m −3 ) of water, and the water density (kg m −3 ) is determined by negative down (kg m −3 ). Pressure (N m −2 ) is calculated by the hydrostatic equation, k is the eddy diffusion coefficient (constant at 0.006 m 2 s −1 ; [14]), velocity (m s −1 ), u(m s −1 ) is velocity along the channel longitudinal axis, and eddy viscosity is determined by A. The vertical profile of the subtidal velocity along the channel, without wind effects, was obtained by averaging all variables tidally and width-wise and assuming constant longitudinal salinity gradients and constant tide-averaged eddy viscosities [15]: ...
The residual circulation in the Bouregreg Estuary (Morocco) was modeled using a simple model to reduce degrees of freedom and modeling effort. The Moroccan Estuary’s residual circulation profile has never been studied to our knowledge. Direct current measurements were used to determine residual currents using Hansen and Rattray’s analytical solution. According to MacCreary and Geyer, Reynolds-averaged equations in hydrostatic form and Boussinesq approximations are used to determine water salinity and momentum along channels. Besides momentum advection and Coriolis forces, only gradients along the channel were considered. An analytical solution combining Hansen and Rattray’s classical theory with Geyer and MacCready’s analysis of residual flow was used to model a fixed station in a river using residue flow, temperature, and salinity flow. The residual flow of Bouregreg Estuary was well described by a simplified model approach that was in agreement with observed data. Observation data was used to calibrate Hansen and Rattray flows. The residual circulation model can be used to predict chemical pollution in estuaries.KeywordsBouregreg estuaryGravitational circulationEstuarine circulationNet flow
... These driving forces are subject to natural variations (e.g. Hoguane et al., 2020;Schettini et al., 2013); however, anthropic activity is rapidly changing the way they interact, and consequently the estuary functioning (e.g. Reid et al., 2022;Schettini et al., 2017). ...
The São Francisco River is the fourth largest river in Latin America, and particularly important since it crosses the Brazilian semi-arid region. During the 1960s-90s, a series of dams for hydropower were built in cascade, changing drastically the flow regime. To evaluate the influence of changes in the flow on hydrodynamic processes and saline intrusion in the estuary, a three-dimensional hydrodynamic model was applied to the system. The model was calibrated and validated from hydrodynamic data collected in the estuary. The simulated scenarios of river discharges were Q min , Q 10 , Q 80 and Q max (corresponding to 600, 5000, 1200, and 12,000 m 3 s − 1 , respectively), determined from historical river discharge observations. There was an increase in the mixing and a decrease in the river contribution after the installation of dams. The average salinity in the estuarine domain ranged from 0.7 g kg − 1 for intermediate pre-dam discharge to 15.6 g kg − 1 in the lowest discharges after the dam installations. The results of the simulations were used to establish an exponential relationship between fluvial discharge and saline intrusion, which can be used by management agencies to optimize the water quality in catchments for human consumption.
... Zambezi sediments have built through time the widest continental shelf along the Indian Ocean coast of Africa (Walford et al. 2005;Ponte et al. 2019), reaching more than 100 km in width and contributing to the highest tidal range in the western Indian Ocean (up to 5 m; Sete et al. 2002;Hoguane et al. 2020). Large sediment volumes, however, are not deposited in front of the Zambezi mouth but are transported northeastward by longshore currents (Schulz et al. 2011;van der Lubbe et al. 2014), forming wide beaches as far as Quelimane and beyond (Fig. 1). ...
We here present the first comprehensive provenance study of the Zambezi deep-sea fan, based on integrated petrographic, heavy-mineral, elemental-geochemistry, isotope-geochemistry, and detrital-zircon-geochronology analyses of middle Pleistocene to Holocene turbidites. The Zambezi Valley and Fan represent the submarine part of an ∼ 5000-km-long sediment-routing system, extending from the heart of the South African Plateau to the abyssal depths of the Indian Ocean. Sediment is derived not only from the African side, but also from Madagascar Island mostly via the Tsiribihina Valley. Being shed by two dissected rifted margins, detritus supplied from opposite sides of the Mozambique Channel shares similar feldspar-rich feldspatho-quartzose composition, although with significant differences in heavy-mineral and geochemical signatures. The εNd values of Madagascar sand are markedly more negative and TNd model ages notably older. Zircon grains yield mostly Irumide (late Stenian) U-Pb ages in African-derived sand and mostly Pan-African (Ediacaran–Cryogenian) U-Pb ages in Madagascar-derived sand, which also yields a few grains as old as Paleoarchean and many discordant ages reflecting Pan-African reworking of Archean cratonic rocks. Lower Valley and Lower Fan deposits have intermediate fingerprints, indicating that sediment supply from Madagascar is not much less than from Africa despite a much smaller catchment area, which can be explained by deposition of a conspicuous part of Africa-derived sediment in the Intermediate Basin confined between the Zambezi Shelf, the Beira High, and the Îles Éparses.
By assuming that compositional differences between Quaternary submarine deposits and modern Zambezi River sands primarily resulted from sediment impoundment by large dams, we could evaluate the anthropogenic impact on natural sediment fluxes. Quaternary turbidites are somewhat higher in quartz and poorer in heavy minerals with higher relative amounts of durable ZTR species, and yield more Ediacaran, Neoarchean, and Carboniferous detrital-zircon ages than modern river sands. The Orosirian peak characterizing the Intermediate Basin sample points to prominent supply from the middle and upper parts of the Zambezi catchment in the middle Pleistocene. Rough calculations suggest that pre-dam Zambezi sediments were generated ≤ 10% in the upper catchment, ∼ 60% in the middle catchment, and only ≥ 30% in the lower catchment that provides the totality of sediment reaching the Indian Ocean today.
... SLR causes a change in the hydraulic parameter, leading to changes in the hydrodynamics and geomorphology of rivers, estuaries, and coastal areas (Khojasteh et al. 2021; Van et al. 2020). Tides play an essential role in governing the hydrodynamic regime and transport of sediment in rivers, estuaries, and coastal areas; furthermore, tides are even more important for areas with dominant tides (Hoguane et al. 2020;McLachlan et al. 2020;Rtimi et al. 2021). ...
The regional dynamics of the coastal area in Ho Chi Minh City affected by the river and sea hydrodynamic factors are incredibly complex. These factors such as tides, waves, and river flow strongly influence the process of accretion-erosion in both estuary and coastal areas. In this study, the characteristic changes in four main tidal constituents (K1, O1, M2, and S2) influenced by sea-level rise are investigated using a numerical model in the curvilinear coordinate system. The 2050 and 2100 sea-level rise scenarios are considered to evaluate the change in tidal harmonic constants and tidal ellipses. The results show that the amplitude of the four constituents significantly increases; however, there is a declining trend in the tidal phase for both the 2020 and 2100 scenarios. As the highest value in amplitude, the M2 constituent indicates that the tidal regime is the dominant semidiurnal tide in the study area. Despite a smaller amplitude than the M2 constituent, K1 has a higher value than both the O1 and S2 constituents. Tidal ellipses in the study area indicate the orientation of the major axes parallel to the shoreline, and the offshore axes that move clockwise are more circular than those in the river mouth. The strongest semidiurnal M2 velocity of ellipses in the Dong Tranh Gulf ranges from 0.6 to 0.7 m/s in 2020 and records an increase of 0.046 m/s in 2050 and 0.082 m/s in 2100. Most tidal ellipse velocities gradually increase under the influence of sea-level rise in 2050 and 2100.
... The estuary has been popular in the recent literature, including accompanying studies in this Special Issue on estuarine hydrodynamics (Hoguane et al., 2020;, small-scale fisheries (Costa et al., 2020;Mugabe et al., 2021) and adaptation of associated livelihoods in rural and urban space (Blythe, 2014;Blythe et al., 2014;Francisco et al., 2021). The geographic setting, ecosystems and socio-ecological importance of the estuary have been summarised by Groeneveld et al., 2021). ...
Estuaries supply direct and indirect multi-sectoral opportunities including for transport, natural resource use and climate protection. These provisions support livelihoods and contribute to social and economic development. The Bons Sinais Estuary in Zambézia Province, central Mozambique, is adjacent to the provincial administrative capital Quelimane, some 25 km from the coast. The rapid growth of Quelimane has increased the demand for natural resources from the estuary, including space, food, fuelwood, transport and raw materials for construction and economic activities. Expansion of the built environment has extended into low-lying lands, mostly within the critical estuarine functional zone with inevitable consequences, such as damage to natural habitats and flooding of occupied areas during rainy seasons. The aim of this study was to analyse three decades of change (1991 – 2018) in land use and land cover (LU/LC) in the Bons Sinais Estuary, focussing on the growth of Quelimane city and the transformation of estuarine and surrounding habitats. The method relied on open-access satellite images and a LU/LC change analysis to quantify the spatio-temporal changes brought about by economic development and related human activities. A combination of low-intensity fieldwork and satellite-derived data (Landsat-5, sensor: Thematic Mapper and Landsat-8; sensors:
Operational Land Imager, Thermal Infra-Red Scanner) was used to generate LU/LC information classified according to the features: mangrove trees; wetlands; estuary intertidal areas; built-up area; cultivated trees; and cultivated land. From 1991 onwards, there was an overall increase in cultivated crops (66 %), development (79 %) (including rural human settlements) and intertidal mudflats (12 %) with a concomitant decline in critical wetlands (16 %) and mangroves (12 %). The study predicts a worsening of the impacts on the estuarine ecosystem with further growth of Quelimane city. To reverse the negative trend on estuary health, the recommendation is for management interventions that promote sustainable LU, and urban development plans that consider ecosystem conservation and active restoration.
The prediction for tidal level of the tidal river is affected by the upstream runoff and the tidal wave in the outer sea. The aim of this study is to overcome the harmonic constant of tidal level prediction cannot remain changed for tidal reaches. A new idea named harmonic analysis combined with the least square method, Fourier-based corrected interpolation and ridge regression analysis method was proposed. The key step of the idea is as follows: firstly, the new harmonic constant can be determined via harmonic analysis of water level station data over the corresponding period. Secondly, the Fourier-based correction method is used to interpolate the short-term tidal level data and make the data interval equal. Thirdly, the application of the ridge regression analysis method is used to deal with the multicollinearity problem which may occur in the short-term harmonic analysis. Besides, the selection rules of tidal data from data length, tide type and starting point were considered. Finally, based on an exponential function which can describe relationship between the new harmonic constant and the upstream runoff, the tidal level prediction model of Baimao station in the Yangtze River tidal reaches is constructed. The result shows that the average error of model in the water level was smaller than 0.20 m, and all the time points with a total water level error smaller than 0.25 m exceeded 69% of all points. The mean error in the tide time was smaller than 34 min, and the maximum error was smaller than 60 min. The experimental results conformed to the trend of the observed water level changes, which verified the feasibility of the established model.
In this work, effects on the circulation of the Paranaguá Estuarine Complex, Brazil, caused by wind, freshwater flow and sea-level rise due to climate change are evaluated. A hydrodynamic model was calibrated for a baseline scenario and three representative scenarios of climate change adopted by the Inter-Governmental Panel for Climate Change (IPCC) were analyzed for year 2050. The Weather Research and Forecasting model (WRF) was applied to evaluate wind speed, direction, and precipitation for the IPCC scenarios. Comparing the different scenarios, results showed that the changes in water level are relatively small. A harmonic analysis of the main components (M2, S2, O1 and K1) showed that the diurnal harmonics are amplified for 2050 scenarios. The semidiurnal constituents were amplified or damped, depending on the station analyzed. The form number showed that for the Laranjeiras bay, there was a change from semidiurnal tide in the baseline scenario to mixed tide for the 2050 scenarios. It was also analyzed the amplification factor, defined as the relation between the amplitude of the simulated harmonic constant and the amplitude of the harmonic constant in the open boundary condition. Also, the results show a dominance of the principal lunar semidiurnal constituent (M2), principal solar semidiurnal constituent (S2) and the lunar terdiurnal (M3), that together are responsible for approximately 50% of the tidal amplitude in all the stations and scenarios. Another scenario was performed considering sea level rise, and the results showed the higher increase occurring in the stations located in the internal regions of the Paranaguá Estuarine Complex.
The rainfall mean state and variability of the tropical Southern Hemisphere are strongly affected by the Tropical Convergence Zones. Unlike the South Pacific and South Atlantic, the topography of Madagascar prevents, through the Mozambique Channel Trough, the direct transport of moisture from the Indian Ocean towards southern Africa
that feeds the South Indian Ocean Convergence Zone (SICZ). Numerical experiments using regional climate model
s reveal that a flatter than actual topography over Madagascar results in a strengthening of the SICZ through anomalously high easterly moisture fluxes transported from the Indian
Ocean and the Mozambique Channel. These in turn trigger a significant increase in precipitation over southern Africa extending from Mozambique to Angola and a decrease in rainfall over Madagascar. These results have important implications for the improvement of the representation of African rainfall mean state and variability, which has been
identified as a persisting issue in different generations of state-of-the-art climate models.
Application of the Dynamic Estuary Model (DYNHYD5) in a tropical tidal river is limited. The successfully calibrated and validated hydrodynamic model is valuable in subsequent water quality simulation for environmental management. Hence, a hydrodynamic modeling approach using the DYNHYD5 was conducted in a tropical tidal river in Malaysia. Samplings were conducted in the Sibu Laut River to collect the hydrology data for model simulation. The model was calibrated and validated by comparing the simulated flow and mean depth with the field data at different simulation periods of time. The results showed that the model DYNHYD5 was successfully calibrated with channel flows and mean depths and then reproduced with good agreement in validation. The observed and simulated data were linearly correlated ( R² > 0.8) with values of slope γ ranging from 0.891 to 1.204 in both calibration and validation. The Nash–Sutcliffe coefficient of efficiency (NSE) of more than 0.7 in both calibration and validation also indicated satisfactory comparison between the observed and simulated data. The result indicated that the application of the DYNHYD5 is feasible in a tropical tidal river in Malaysia.
The Sofala Bank, a wide shelf located along the central coast of Mozambique, hosts tides with high amplitudes. The Regional Ocean Modelling System (ROMS) was used to analyse the tidal currents on the bank and to investigate their effects on the stratification and generation of tidal fronts. During spring tides, barotropic tidal currents with maximum values ranging from 40 cm s–1 to 70 cm s–1 are found on the central bank. The major axis of the tidal ellipses for M2 and S2 follow a cross-shelf direction with mainly anticlockwise rotation. Similar to observations, three distinct regimes occur: (i) a warm well-mixed region on the inner shelf where the depths are <30 m; (ii) a wellmixed colder region above the shelf edge; and (iii) a stratified region offshore. The model shows that the tides lead to cooling where two criteria are satisfied: the Simpson and Hunter parameter log10(h/U3) <3.2 and the depth h >30 m. The shelf edge of the bank is important for internal tide generation. Two frontal structures result, one offshore between cooler mixed waters and warmer stratified waters and the other in shallow inshore waters, between cooler mixed waters and solar heated mixed waters.
Three years (2008–2011) of current measurements outside Pemba (∼13°S), Mozambique, are presented. Oil and gas reserves are found in the region, and knowledge of the ocean current climate is important for safe offshore operations. The full-depth current profile at Pemba was estimated by comparing with moorings further south. The moored record was also compared with geostrophic velocities obtained from sea surface height (SSH) fields, current profiles from ship-mounted Acoustic Doppler Current Profiler and surface currents from seven drifters passing the mooring. Velocities up to 60 cm/s were observed at 850 m depth. Altimetry-derived currents, while strong (often > 100 cm/s), are probably underestimates due to coarse resolution. Direct observations show surface currents up to 170 cm/s. Four strong current events were studied in detail. Three of the four events appeared to be associated with anticyclonic eddies nearby. The time series of SSH-derived geostrophic current always showed southward velocities, never below 20 cm/s. These findings indicate that currents off Pemba may display characteristics of a western boundary current rather than being completely dominated by eddies. Strong current events are probably caused by anticyclones interacting with the boundary current and speeds on the order of 200 cm/s can be expected.
The Incomati Estuary, with a length of about 30 km, a width of about 1.5 km, and an average depth of 10 m, is located in the Southeastern part of Africa, north of Maputo Bay, between the Latitudes 24o 00’S and 26o 30’S and Longitudes 29°30′ E and 33° 15′ E. The Incomati River basin as a whole has an area of 46,426 km2, of which 28,745 km2 is located in South Africa, 2,786 km2 in Swaziland and 14,856 km2 in Mozambique. There are several dams upstream, mainly used to supply irrigation water. The river discharge is 200–400 m3 s−1, corresponding to a river discharge of about 700–1000 Mm3, with peak and lower flows during the wet season (November to March) and dry season or winter (June-August), respectively. The morphology of the estuary is characterized by meanders, islands and sand banks, fringed by mangroves. The tides are semidiurnal, with amplitudes varying from 0.5 to 3.5 m during the spring and neap tides, respectively. The predominant winds are trade winds, with average speed of 4 m s−1. The estuary plays an important ecological role and sustains important socioeconomic activities. It is a nursery ground for important fisheries, a tourism attraction, and hosts significant agriculture and livestock activities. The sustainability of the natural resources and the activities in the estuary is threatened by the effects of the dams. The three basin countries agreed in 1991, during the “Piggs Peak Agreement,” to the maintenance of a minimum environmental flow of 2 m3 s−1, which has been violated several times by South Africa. The present work investigates how the hydrodynamics of the estuary, which controls most of the ecological processes, is influenced by the river discharges. Tides, currents and water masses in the estuary are described, based on historical hydrodynamic data. A simple 1-D, depth and width integrated, tidal hydrodynamic model forced by the observed tides at the mouth, was used to reproduce the hydrodynamics of the estuary. In addition, an analytical solution of the advection–diffusion of salt is applied to reproduce the salt intrusion. The model was then used to determine the extent of salt intrusion as a function of the river discharge, and so used to estimate the minimum environmental flow required for a healthy estuarine ecosystem in Incomati. The results indicated the estuary shifts, from a stratified condition in the wet season to vertically-mixed in the dry season. The tidal wave takes about 30 minutes to propagate from the mouth to the head of the estuary. The current varied between 0 and 1.2 m s−1, mostly driven by tides. The density driven circulation is weak, with a density gradient of about 0.10 kg m−3 per km, and a density-driven velocity of about 0.04 m s−1. The hydrodynamic model applied explains 87% of the current observed in the estuary, meaning the currents were mostly tidal driven, with the remaining 13% attributed to other factors such as density gradient and winds. The minimum recommended river flow required to prevent the intrusion of salt water to a distance 20 km upstream is 20 m3 s−1, against the 2 m3 s−1 value in the Piggs Peak Agreement. The results of this study may contribute to a better understanding of the dynamics of the ecosystems in the Incomati Estuary, and in the management of the river for the health of the downstream ecosystems.
Different types of tidal asymmetry (see review of de Swart and Zimmerman Annu Rev Fluid Mech 41: 203–229, 2009) are examined in this study. We distinguish three types of tidal asymmetry: duration and magnitude differences between flood and ebb tidal flow, duration difference between the rising and falling tides. For waterborne substance transport, the first two asymmetries are important while the last one is not. In this study, we take the Huangmaohai Estuary (HE), Pearl River Delta, China as an example to examine the spatio-temporal variations of the tidal asymmetry in a mixed semidiurnal tidal regime and to explain them by investigating the associated mechanisms. The methodology defining the tidal duration asymmetry and velocity skewness, proposed by Nidzieko (J Geophys Res 115: C08006. doi: 10. 1029/ 2009JC005864, 2010) and synthesized by Song et al. (J Geophys Res 116: C12007. doi: 10. 1029/ 2011JC007270, 2011), is utilized here and referred to as tidal duration asymmetry (TDA) and flow velocity asymmetry (FVA), respectively. The methodology is further used to quantify the flow duration asymmetry (FDA). A positive asymmetry means a shorter duration of low water slack for FDA, a shorter duration of the rising tide for TDA, and a flood dominance for FVA and vice versa. The Regional Ocean Modeling System (ROMS) model is used to provide relatively long-term water elevation and velocity data and to conduct diagnostic experiments. In the HE, the main tidal constituents are diurnal tides K
1, O
1 and semidiurnal tides M
2 and S
2. The interaction among the diurnal and semidiurnal tides generates a negative tidal asymmetry, while the interactions among semidiurnal tides and their overtides or compound tides result in a positive tidal asymmetry. The competition among the above interactions determines the FDA and TDA, whereas for the FVA, aside from the interaction among different tidal constituents, an extra component, the residual flow, plays an important role. The results show that the FDA exhibits a predominant tendency of shorter duration of low water slack, favoring the landward transport of fine sediment. The FVA demonstrates prevailing ebb dominance in the study period, favoring the seaward transport of coarse sediment. This ebb dominance is primarily induced by the interaction among the residual flow and the tidal constituents. The external TDA in the ocean experiences distinct cyclic variations with positive asymmetry when semidiurnal tides dominate and negative asymmetry during the periods when diurnal tides dominate. The funnel shape of the HE is advantageous for the development of positive tidal asymmetry as the semidiurnal tides are more amplified than the diurnal tides. The effect of river flow can enhance the ebb dominance, while the baroclinic effect is more complex. The existence of channel and shoals favors the development of residual pattern with seaward flow (ebb dominance) in the channel and landward flow (flood dominance) at the shoal when the tides are strong (semidiurnal tides dominate) and the residual pattern with landward flow (flood dominance) in the channel and seaward flow (ebb dominance) at the shoal when the baroclinic effect is dominant (diurnal tides dominate).
Background information about: Nile delta (Egypt), Incomati delta (Mozambique), Ganges-Brahmaputra-Meghna (Bangladesh), Yangtze (China), Ciliwung (Indonesia), Mekong (Vietnam), Rhine-Meuse (The Netherlands), Danube (Romania), California Bay-Delta, Mississippi River Delta (USA)
Long-term hydrodynamic and salinity transport modeling of the Sacramento-San Joaquin Delta is needed to evaluate the future Delta in terms of the California co-equal goals of ecosystem health and reliable water supply. While 2-D and 3-D hydrodynamic and water quality models are by definition better suited to modeling a complex network of tidally influenced flows under future conditions, a 1-D model is more computationally efficient in narrowing the large variety of multiple-year simulations required into a more manageable task. Still, a 1-D model of sea level rise in an estuary must account for the three-dimensional effects where increased depths will affect density driven (baroclinic) circulation and tidal dispersion of salt. In this paper, we use a simplified Delta network model with a tidally averaged computational approach to quickly perform multi-year simulations for sea level rise. The 1-D model uses tidal dispersion coefficients developed from 3-D hydrodynamic models. The resulting model is capable of performing very fast simulations over a wide range of conditions, providing guidance on what should be explored in depth with more detailed, but slower models. Comparisons of unimpaired Delta inflow with the historical case show that the south Delta and San Joaquin River would be much fresher without exports, while the Sacramento River would be fresher in spring and more saline in the fall. Sea level rise will increase salinity throughout the Delta over time. With peripheral conveyance of export, water salinity will intrude upstream in the Sacramento River, be slightly lower up the San Joaquin River and increase in the south Delta. With sea level rise, peripheral conveyance will have similar trends to changes to the historical case, but export salinity will be improved by the peripheral conveyance component. A larger peripheral conveyance can benefit both the ecosystem and exports if managed properly.
Recent importance has been placed on the ecological and socio-economic aspects of mangroves for adjacent coastal populations, in terms of flood defense, food resources, employment and generation of income. Anthropogenic stressors, such as direct clearance, hydrological alterations, climatic change effects or chemical pollution contribute to mangrove ecosystems degradation. While the relative impact is not well understood, the hydrodynamics specific to mangroves (intertidal, land-marine interface) are undoubtedly influencing those effects. In the present study, a computer-based model was built to understand the hydrodynamic flow characteristic in a mangrove ecosystem, the Sungei Buloh Wetland Reserve in Singapore, with the wider intent to better understand the transport of chemical substances in mangroves. Field surveys in the mangrove and the preliminary development of a two-dimensional hydrodynamic (2DH) model have been carried out. Higher bottom roughness was considered in the vegetated part of the model domain to account for the effect of mangrove roots. Spatial and temporal distributions, as well as minor mean differences between simulated and observed results, suggest that the developed model capture satisfactorily the tidal dynamics within the river, the wetland area covered with mangroves and in the strait. These results indicate that the hydrodynamics are properly understood within the Sungei Buloh mangrove ecosystem and can be used for modeling the fate of chemicals.
The River dos Bons Sinais Estuary is one of the most important estuaries in the central region of the Mozambican coast. It is situated between the confluence of Cuácua and Licuári rivers and the Mozambican Channel in the Indian Ocean. The climate is subtropical with the rain season generally from November to April. The domestic wastes from the city of Quelimane are discharged directly into the Bons Sinais Estuary and across from the city there is industrial aquaculture activity. The estuary is also the way of entrance to the port of Quelimane, 20 km away from the ocean.
The aim of this study was to analyze the dissolved oxygen and the biochemical oxygen demand in the port of Quelimane, where the discharge of municipal sewage is entering the estuary. BOD indicates the amount of organic matter present in the water. Therefore, a low BOD is an indicator of good quality water, while a high BOD indicates polluted water.
The results found during the measuring period indicate that during August dissolved oxygen values were more than critical the value of 2 mg L-1 whereas during September and October dissolved oxygen had values lower than this critical value. The values of biochemical oxygen demand obtained are characteristic of unpolluted waters which suggest that the municipal effluents discharged into the estuary are negligible or are flushed away by the tides.
It is often assumed that, in short, shallow estuaries, at leading order
velocity and elevation are exactly out of phase, so that duration
asymmetries in the rise and fall of the tide should be manifest as
skewed velocities. We observed alternating ebb- and flood-dominant
velocity skew in response to the spring-neap modulation of incident
asymmetry generated by the mixed, mainly semidiurnal astronomical tides
within the macrotidal Skagit River delta in Puget Sound, Washington. We
describe three factors that may contribute to local asymmetries: (1) ebb
dominance caused by phase lags between the surface gradient and local
depth, (2) ebb dominance due to fluvial discharge, and (3) near-bed
flood dominance due to baroclinicity. Large spring tides led to greater
frictionally generated phase lags and resulted in ebb-dominant velocity
skew. This ebb dominance caused by tidal drainage was reinforced by
fluvial discharge across the tidal flat at lower-low water. The
baroclinic component of this discharge, however, produced flood-dominant
near-bed velocity skew that countered the ebb dominance of the
frictional effects. The balance of these processes depends strongly on
the spring-neap cycle, magnitude of river discharge, and position within
the tidal flat and channel system. Our observations are notable in the
context of previous studies describing these processes because our
analyses indicate that these mechanisms are relevant over very short
spatial scales of just a few kilometers and in very shallow systems.
This study represents a detailed examination of intratidal variations in boundary-layer structure in a relatively narrow, straight section of the Hudson river estuary during high river discharge. The presence of active mixing is inferred from estimates of the Richardson number. The maximum height above bed to which mixing occurs is comparable during flood and ebb; however, the regions of mixing differ in density structure. During the flood, active mixing is confined to a well-mixed near-bed layer, whereas during the ebb, active mixing occurs throughout regions of significant stratification. Tidal straining maintains stratification during the ebb, while it promotes the growth of a uniform bottom, mixed layer during the flood. In general, maximum and minimum near-bed stratification occur during late ebb and flood, respectively, reflecting the dominant role played by tidal straining in determining intratidal variation. Vertical stretching was also found to be an important term in the stratification balance during the ebb.
[1] Intricate networks of tidal channels such as are found in fjordic, barrier island and branching estuarine systems are often at risk from contaminant inputs and can be important as spawning grounds or migration pathways for marine organisms. These intricate systems are rarely spatially resolved in regional-scale numerical tidal models, and the setting up of a specific detailed numerical model for the purpose of rapidly assessing the likely tidal behavior of such geometrically complex systems carries a high overhead. As a first step we describe a simple, not published before, algorithm, which permits rapid assessment of the linear tidal dynamics in a single tidal channel. The method will be extended to channel networks in a companion article. The method needs only a minimum of input data, namely, (1) the forcing tidal elevation amplitude and phase at the entrances of the channel directly exposed to the open sea and (2) the length and depth of the channel. The performance of the method is tested against observations in the Menai Straits, UK, by applying the case of a single channel forced at either end with friction.
Measurements of hydrographic conditions in the Ponta Rasa tidal mangrove swamp, Inhaca Island, Mozambique were made in August–October 1994 during the winter dry season. The Ponta Rasa swamp/creek is tidally choked on account of the narrow channel that connects it to Maputo Bay and at neap tides, a sill prevents bay water entering the creek system altogether. Temperature variation in the swamp (15–25°C) was predominantly diurnal with an additional signal due to the tidal advection of bay waters. There is no river discharge into Ponta Rasa and during the observation period, there was no significant rainfall. The mean salinity in the swamp (c. 38) was controlled by evaporation and transpiration by mangroves and an overall evapotranspiration rate of 0·5cmday−1was estimated from a steady salt balance. Salinity variation (c. 2) was predominantly due to semi-diurnal tidal advection of lower salinity Maputo Bay water into the swamp/creek. A model which incorporates tidal dynamics coupled to heat and salt balance equations reproduces many of the observed features of the system.
The generation of residual circulation in a tidally energetic estuary with constant longitudinal salinity gradient and parabolic cross section is examined by means of a two-dimensional cross-sectional numerical model, neglecting river runoff and Stokes drift. It is shown how the longitudinal and lateral residual circulation can be decomposed into contributions from various processes such as tidal straining circulation, gravitational circulation, advectively driven circulation, and horizontal mixing circulation. The sensitivity of the residual circulation and its components from various processes to changes in forcing is investigated by varying the Simpson number (nondimensional longitudinal buoyancy gradient) and the unsteadiness parameter (nondimensional tidal frequency), as well as the bed roughness and the width of the estuary. For relatively weak salinity gradient forcing, the tidal straining circulation dominates the residual exchange circulation in support of classical estuarine circulation (up-estuary flow near the bed and down-estuary flow near the surface). The strength of the longitudinal estuarine circulation clearly increases with increased salinity gradient forcing. However, when the Simpson number exceeds 0.15, the relative contributions of both gravitational circulation and advectively driven circulation to estuarine circulation increase substantially. Lateral residual circulation is relatively weak for small Simpson numbers and becomes flood oriented (divergent flow near the bed and convergent flow near the surface) for larger Simpson numbers because of increasing contributions from gravitational and advectively driven circulation. Increasing the unsteadiness number leads to decreased longitudinal and lateral residual circulation. Although changes in bed roughness result in relatively small changes in residual circulation, results are sensitive to the width of the estuary, mainly because of changes in residual exchange circulation driven by tidal straining.
Both the drag force and the horizontal eddy viscosity play a dominant role in the tidal-scale hydrodynamics in mangrove wetlands.
Using field observations and basic fluid mechanics laws, the drag coefficient and the coefficient of dynamic eddy viscosity
are found to be predictable as a function of the Reynolds Number based on the characteristic length scale of the vegetation.
The characteristic length scale of the vegetation varies greatly with vegetation species, vegetation density and tidal elevation.
Both these coefficients decrease with increasing values of the Reynolds Number. At the low range of the Reynolds Number both
these coefficients reach much higher values than those typical of vegetation-poor estuaries and rivers. Consequently, the
tidal flow within mangrove areas depends to a large degree upon the submerged vegetation density that varies with the tidal
stage. These findings may be applied also in other vegetated tidal wetlands, including salt marshes.
Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters. Predicting the development
and breakdown of such stratification is an inherently more difficult problem than that involved in the analogous case of stratification
induced by surface heating because the buoyancy input originates at the lateral boundaries. In the approach adopted here,
we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing
effect of fresh water and the vertical mixing brought about by tidal and wind stirring. Freshwater input induces horizontal
gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid
moving landward below. This contribution to stratification is expected to vary in time as the level of turbulence varies over
the tidal cycle. The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic
input to stratification which is positive on the ebb phase of the tide. Comparison of this input with the available stirring
energy leads to a simple criterion for the existence of strain-induced stratification. Observations in a region of Liverpool
Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical
mixing apparent around high water except at neap tides when more permanent stratification may develop. A simulation of the
monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with
the main features of the observations.
The Atlantic coast of the North and South American continents, including the Gulf of Mexico and the Caribbean, measures 37,000 km on a map of scale 1:5,000,000 (Figure 1). Portions of this coast are characterized by extensive regional mud deposits, especially where one or more rivers discharge significant quantities of suspended sediments to deltas and adjacent marginal estuaries, lagoons, chenier plains, and tidal flats. The delivery of terrigenous sediments by rivers is the main factor responsible for coastal mud deposits. Secondary factors favoring the development of muddy coastal deposits are a broad gently sloping coastal plain and adjacent shelf topography, macrotidal conditions, and the absence of energetic oceanic wind waves. Muddy coasts can equally well develop where relative sea level is rising or falling. Whereas large portions of the Atlantic and Gulf of Mexico coasts of the USA and Mexico are subject to rapidly rising relative sea level, the Atlantic coast of South America experiences falling relative sea level. We describe five major muddy coasts along the American Atlantic in terms of their morphodynamics. These systems include (i) the mesotidal lagoon-salt marsh coast of the South Atlantic Bight (southeastern USA), (ii) the microtidal Mississippi River delta and chenier plains (USA), (iii) the macrotidal Amazon chenier plain coast of the Guyanas, (iv) the macrotidal mangrove coast of Pará and Maranhão (Brazil); and (v) the microtidal la Plata estuarine system (Argentina and Uruguay).
Mozambique lies on the east coast of Africa between latitudes 10°20’ to 26°50’ South (Fig. 4.1) and possesses the third longest coastline in the Western Indian Ocean at 2700 km. The coastline is characterized by a variety of habitats including; delta estuaries, sand beaches, mangroves, island archipelagos and coral reefs. The continental shelf (down to a depth of 200 m) is very narrow along much of the coastline, typically less than 20 km wide, although this widens in the central part of the country to 130 km.
A high-resolution, two-way nested Regional Ocean Modeling System, forced with monthly climatologies, has been set up for the Sofala Bank and adjacent deeper ocean of the Mozambique Channel to investigate the role of offshore mesoscale eddies on the shelf circulation, hydrographic structures and river plumes. The model is shown in comparison with available observations and published studies. Most known oceanographic features are reproduced by our model. We applied Self-Organizing Maps and showed that offshore passing eddies, depending on their strength and proximity to the shelf, modulate the shelf circulation and river plume direction and spread. The presence of a strong cyclonic eddy close to the shelf induces northward surface shelf currents. In contrast, the presence of a strong anticyclonic eddy close to the shelf induces a strong southward current over most of the shelf, except off Beira. Our analyses confirm that the plume of the Zambezi River is bi-directional. The southward-directed plume patterns, opposite to the dominant northwards, occur in response to nearby offshore anticyclonic eddies (26% of occurrence). This behavior could have an influence on water dispersal, shelf ecosystems and important fisheries. Therefore, offshore mesoscale eddies should be taken into account when studying the ocean dynamics of the Sofala Bank.
The roles of straining and dissipation in controlling stratification are derived analytically using a vertical salinity variance method. Stratification is produced by converting horizontal variance to vertical variance via straining, that is, differential advection of horizontal salinity gradients, and stratification is destroyed by the dissipation of vertical variance through turbulent mixing. A numerical model is applied to the Changjiang estuary in order to demonstrate the salinity variance balance and how it reveals the factors controlling stratification. The variance analysis reveals that dissipation reaches its maximum during spring tide in the Changjiang estuary, leading to the lowest stratification. Stratification increases from spring tide to neap tide because of the increasing excess of straining over dissipation. Throughout the spring-neap tidal cycle, straining is almost always larger than dissipation, indicating a net excess of production of vertical variance relative to dissipation. This excess is balanced on average by advection, which exports vertical variance out of the estuarine region into the plume. During neap tide, tidal straining shows a general tendency of destratification during the flood tide and restratification during ebb, consistent with the one-dimensional theory of tidal straining. During spring tide, however, positive straining occurs during flood because of the strong baroclinicity induced by the intensified horizontal salinity gradient. These results indicate that the salinity variance method provides a valuable approach for examining the spatial and temporal variability of stratification in estuaries and coastal environments.
Tidal asymmetry plays a pivotal role in the transport of sediment and morphological change in shallow inlet/estuarine systems, particularly those with extensive tidal flats and channels. This study examined tidal circulation patterns of a shallow and well-mixed estuary on the central coast of New South Wales, Australia. A numerical modelling study was carried out by applying a depth-averaged flow model (MIKE 21 HD). The model was calibrated and successfully validated against recently acquired hydrodynamic data. Model results reveal the presence of a number of eddies within the estuary, which may have substantial influence on net sediment transport when combined with wave effects. Tidal phase duration and magnitude asymmetries indicate that, in contrast to typical flood/ebb dominance, double ebb-dominance is in deep channels and the entrance cross-section, whereas the flood ramp area shows double flood-dominance. The implications of tidal asymmetry on net bed-load sediment transport were inferred from the duration and magnitude asymmetries of the above threshold velocity. The ebb-directed net transport occurs at main tidal channels and may result in further deepening channels, whereas a flood-directed net transport in the flood ramp area may cause an accumulation of sediment on the delta or a landward progradation of the tidal flat. The present-day estuary is characterised as ebb-dominated, tends to flush bed-load sediment seaward more effectively and may represent more stable geometries.
The dispersion or spread of a dissolved or suspended substance in an estuarine system occurs mainly due to the non-uniformity of velocity distribution, including turbulent fluctuations, shear stress at the boundary and surface stress caused by winds. The mixing and dispersion phenomena in rivers and estuaries are extremely important in water quality management and control. The development of a dispersion model in harmony with the nature of the flow field in a river or estuary is necessary in the estimation and correlation of dispersion parameters, called dispersion coefficients, which may, in general, be anisotropic in a multidimensional transport process. The earlier one-dimensional models have gradually given way to higher dimensional models for better description of the phenomena as well as for more accurate estimation of parameters. Field studies of dispersion of tracers have been the most important method of generating data for parameter estimation. A number of correlations for mixing and dispersion coefficients in terms of flow rates and other fundamental system parameters are available. The present study incorporates the analysis, assessment and applications of various dispersion and mixing models available. Also, a critical appraisal of the validity, inherent degree of uncertainty and the range of applications of different correlations has been incorporated.
A model for the Zambezi River plume, the largest on the Indian Ocean coast of Africa, is presented and the results of experiments with different discharges and wind forcings are analysed. Although the river plays an important role in the southern African economy through power generation on large dams, artisanal fisheries, and frequent flooding events that impact greatly on local populations, the plume has not been well studied. Observations during the period 2004–2007, when the winds were mainly easterly or south-easterly, indicated that the plume waters can extend both downstream (equatorwards) and upstream (polewards) of the Zambezi Delta with a recirculating bulge near the river mouth. The model is constructed using the Regional Ocean Modeling System (ROMS), with a 40-km long, 3-km wide river discharging into a rectangular coastal ocean with a linearly sloping bottom. When the model is forced only by a constant river discharge of 1 000 m3 s−1 (typical of observed discharge amounts in summer), the Kelvin and Froude numbers for the resulting plume imply a ‘large-scale’ buoyant discharge with a coastal current that is close to being in geostrophic balance with the across-shore pressure gradient and a recirculating ageostrophic bulge near the mouth. The distributions of the bulge and plume waters are found to be relatively insensitive to the discharge amount. Under constant wind forcing, the plume distribution changes dramatically. Northerly and easterly winds produce the largest changes with the latter able to deflect the plume up to 180° due to Ekman drift. When sea breeze-like winds are imposed, accumulation of water in the bulge occurs with substantial spreading upstream. Stronger sea breezes lead to less downstream spreading of the plume than gentle winds. When the winds are mainly across-shore, Ekman drift dominates, but the dynamics become almost geostrophic when the winds are roughly aligned to the coast. These experiments suggest that the Zambezi River plume is sensitive to the winds on diurnal to synoptic time-scales.
The complex seabed contours and geometry of basin lead to a unique tidal pattern along
India’s Gulf of Khambhat (GoK). Estimation of tidal levels and associated currents in
this basin is challenging due to the amplification of tides and intense current magnitudes.
In this study, the finite difference based numerical model, Delft3D-FLOW has been set
up. Spatially varying bottom roughness parameters have been used to estimate the tidal
levels and currents for the entire basin and the results are compared with an earlier work
of Sathish Kumar and Balaji (2014 and 2015) based on an alternative finite element
based numerical model, Telemac2D. An inter-comparison of model results shows good
agreement with each other and with the field measurements. The tidal levels are observed
to vary from 2m to more than 10m in the narrowing portion of the gulf while the
maximum current velocity is seen as 3m/s.
Preface 1. What is phytoplankton? 2. Mechanisms of suspension 3. Spatial and temporal distribution of phytoplankton 4. Photosynthetic activity of phytoplankton 5. Nutrients 6. Growth and survival 7. Loss process 8. Periodicity and change in phytoplankton composition Glossary and symbols References Index to lakes and rivers Index to genera/species General index.
Many scholars at home and abroad have studied tidal velocity profile in estuarine and coastal waters. The results show that velocity profile of tidal current often deviated obviously from the traditional logarithmic profile, due to the flow including wind-driven current, wave-driven current, salinity and so on. In estuarine waters, runoff should be included as a component. In this paper, two patters of vertical velocity profile of tidal current are firstly divided, that are monotone increasing I-velocity profile (during flood tide and ebb tide stage) and non-monotone increasing II -velocity profile (during slack tide stage). Then, by use of variable fractal dimension to obtain the law of II-velocity profile, the results show that: (1)The II -velocity vertical profile does express a first-order accumulated variable-dimensional fractal phenomenon. (2) Through of the fractal dimension, the various formula fitting II-velocity profile were compared, and the results showed that, in practical applications of engineering, cubic function formula is the best fit as the measured data have been verified. Finally, the cubic function formula is applied to flow separation of II -velocity measured in Yangtze River. By the easily separation of the measured flow, we can not only get a reasonable surface flow velocity, such as wind-driven current, runoff, salinity, as well as their driving force weight in the hydrodynamic of estuarine and coastal waters, but also gain the data of bed roughness length and sea surface wind speed, providing reasonable border conditions of the seabed and sea surface for the hydrodynamic numerical simulation in estuarine and coastal waters.
Recent research in estuaries challenges the long-standing paradigm of the gravitationally driven estuarine circulation. In estuaries with relatively strong tidal forcing and modest buoyancy forcing, the tidal variation in stratification leads to a tidal straining circulation driven by tidal variation in vertical mixing, with a magnitude that may significantly exceed the gravitational circulation. For weakly stratified estuaries, vertical and lateral advection are also important contributors to the tidally driven residual circulation. The apparent contradiction with the conventional paradigm is resolved when the estuarine parameter space is mapped with respect to a mixing parameter M that is based on the ratio of the tidal timescale to the vertical mixing timescale. Estuaries with high M values exhibit strong tidal nonlinearity, and those with small M values show conventional estuarine dynamics. Estuaries with intermediate mixing rates show marked transitions between these regimes at timescales of the spring-neap cycle.
Observations of turbulent stresses and mean velocities over an entire spring-neap cycle are used to evaluate the dynamics of tidally averaged flows in a partially stratified estuarine channel. In a depth-averaged sense, the net flow in this channel is up estuary due to interaction of tidal forcing with the geometry of the larger basin. The depth-variable tidally averaged flow has the form of an estuarine exchange flow (downstream at the surface, upstream at depth) and varies in response to the neap-spring transition. The weakening of the tidally averaged exchange during the spring tides appears to be a result of decreased stratification on the tidal time scale rather than changes in bed stress. The dynamics of the estuarine exchange flow are defined by a balance between the vertical divergence of the tidally averaged turbulent stress and the tidally averaged pressure gradient in the lower water column. In the upper water column, tidal stresses are important contributors, particularly during the neap tides. The usefulness of an effective eddy viscosity in the tidally averaged momentum equation is explored, and it is seen that the effective eddy viscosity on the subtidal time scale would need to be negative to close the momentum balance. This is due to the dominant contribution of tidally varying turbulent momentum fluxes, which have no specific relation to the subtidal circulation. Using a water column model, the validity of an effective eddy viscosity is explored; for periodically stratified water columns, a negative effective viscosity is required.
Measurements of velocity, density, and pressure gradient in the lower Hudson River estuary were used to quantify the dominant terms in the momentum equation and to characterize their variations at tidal and spring-neap timescales. The vertical momentum flux (assumed to be due mainly to turbulent shear stress) was estimated indirectly, based on the residual from the acceleration and pressure gradient terms. The indirect estimates of stress compared favorably to bottom stress estimates using a quadratic drag law, supporting the hypothesis that the tidal momentum equation involves a local balance between tidal acceleration, pressure gradient, and stress divergence. Estimates of eddy viscosity indicated that there was significant tidal asymmetry, with flood tide values exceeding ebb values by a factor of 2. As a consequence of the asymmetry, the vertical structure of the tidally averaged stress bore no resemblance to the tidally averaged shear. In spite of the asymmetry of vertical mixing, the tidally averaged, estuarine circulation was found to depend simply on the intensity of bottom turbulence, which could be parameterized by a Rayleigh drag formulation based on the tidal velocity magnitude and the tidally averaged near-bottom flow. This seemingly paradoxical result indicates that the estuarine circulation can be modeled without detailed knowledge of the effective eddy viscosity, only requiring an estimate of the bottom drag coefficient, the tidal forcing conditions, and the baroclinic pressure gradient. A notable characteristic of this solution is an inverse dependence of the estuarine circulation on the amplitude of the tides.
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The Dongjiang River Delta lies in the eastern part of the whole Pear River Delta, Guangdong Province, China. In this paper,
based on the 1-D numerical model and measured data, a study on the characteristics of tide, the suspended sediment transport
during the flood and drought of the Dongjiang River Delta is made. Based on the “Three Steps Methods” and “Junction Control
Method for suspended sediment”, the 1-D networks numerical model for simulating unsteady flow and suspended-sediment transport
in river networks was presented. The calculated results were calibrated with the hydrological data synchronously measured
in the Pearl River networks in Feb. 2001 and July 2003. A good agreement of the calculated results with the measured data
shows that the model could be used to describe the hydrodynamic and suspended sediment process of these regions. Based on
this numerical model, this paper provides a brief analysis of the characteristics of tide, the suspended sediment transport
during the flood and drought of the Dongjiang river networks. Result shows that during the dry and flood season, characteristics
of flow, sediment discharge tide are similar: the velocity, discharge and sediment concentration during flood tides is bigger
than that during ebb tides, while time of the ebb current is longer than that of the flood current.
Time series measurements of flow and pigment concentrations (Chl) in the Menai Strait have revealed that the strong residual flow in a tidal channel (∼ 500 m³ s− 1) transports phytoplankton from the open sea into the channel where much of it is consumed by suspension feeders, mainly in commercial beds of Mytilus edulis. The progressive depletion of phytoplankton along the channel results in a strong horizontal gradient of plankton and hence Chl. Tidal displacement of this gradient causes large (± 50% of mean) oscillations of Chl in the vicinity of the mussel beds. Vertical mixing by the strong tidal flows is sufficiently vigorous for most of the tidal cycle to ensure that downward diffusion can resupply the near-bed layer although there are indications of some transient depletion around slack water.
We present two models of one-dimensional flow in a tidal estuary, derived from the spatial discretization of the finite amplitude and the linearized shallow-water equations. Under the simplifying assumptions that the depth and width of the estuary are piecewise linear and that water level and mass flow are piecewise constant with distance downstream, we derive models in the form of sets of ordinary differential equations. We use parameters applicable to the Severn Estuary to test the validity of the models against Admiralty predictions and against two more complex numerical models. In the part of the estuary under consideration our results appear to differ from observation by no more than those of the more complex models; a gain in accuracy for unit cost appears therefore to have been made. Since the functional form of our model is fairly simple, it is of particular use in further studies involving optimal control of the barrage for maximum energy.
For a tidal channel with water of homogeneous density, an approximate analytical expression is derived for the tidally averaged transport of coarse sediment in terms of the amplitudes and phases of the tidal-current constituents. Transport is in the form of bed load and the rate of transport is proportional to some power of the local current speed. It is assumed that the tidal current is dominated by the M2 constituent. From the analytical expression it follows that the interactions of M2 and M0 (= Eulerian mean current) and of M2 and any of its even overtides, M4, M8etc., constitute the major contribution to the tidally averaged sediment transport. A combination of the M2 tidal current and a fundamental constituent in the diurnal, semidiurnal or subsequent period bands results in a tidally averaged transport that fluctuates with the corresponding beat frequency. Therefore, for the long-term mean bed-load transport only the contributions of M2 and M0 and of M2 and its even overtides are of interest. Application to the main channel of the Ems estuary showed good agreement with transport pathways derived from the grain-size distribution pattern. The tidally averaged sediment transport is largely the result of the interaction of the M2 and M0 tidal current constituents.
The changes in tidal regime and extreme levels in the River Thames which will result from the closure of the Woolwich Barrier have been estimated from the results of both hydraulic and numerical models of the estuary (GLC, (1969); (1970)). It was clearly important that the assumptions and methods used in these models be checked as thoroughly as possible; one obvious way of verifying the numerical model was to use the same techniques to investigate the effects of an existing structure, the half-tide barrier at Richmond, where the predictions could be checked against direct observations. A detailed, one-dimensional, numerical model of the Thames Estuary above Chelsea was developed to examine the dynamics of this complex region; a region which had been represented only very crudely in previous models of the whole estuary. The model was found to accurately describe the tidal regime in this area, correctly reproducing the effect of a mobile barrier closing across the tide. Opportunities for the detailed verification of numerical model predictions of the changes resulting from a major alteration in the geometry of the system are rare, usually only being possible in a follow-up study after a new structure has been completed. It is therefore encouraging that the present model successfully reproduces the dynamics of an existing system.
Temporal and vertical variations of zooplankton at a station on the north-western Portuguese shelf, close to the Ria de Aveiro inlet, were studied during two periods of 25 h each in the summer of 1994. Zooplankton sampling was carried out every 2 h, using horizontal plankton tows (mesh size, 500 μm). Sampling was carried out in two depths, 4 and 12 m. The vertical profiles of salinity and temperature were obtained immediately before the plankton sampling. Higher zooplankton concentrations were usually observed during the night and at 4 m depth. Most taxa also occurred inside the Ria de Aveiro, indicating an exchange of planktonic forms between the Ria and the shelf. A classification analysis of the most abundant taxa suggests some groups, which differ according to taxonomic features and their origin. This was attributed to the high variability of the forcing agents that affect zooplankton abundance.
Based on hydrographic data from 1977 to 1980 off the coast of Mozambique and historical data from the Mozambique Channel, the general circulation pattern of the area is described. The circulation pattern is characterized by the influence of three anticyclonic gyres covering the northern, the central, and the southern parts of the channel. Additionally, smaller cyclonic eddies are observed, of which some probably are topographically induced. The results strongly indicate that in the upper 1000 m the role of the Mozambique Current as one of the tributaries to the Agulhas Current is of minor significance and draw into question the concept of the Mozambique Current as a continuous one.
Salt intrusion in estuaries is important for ecological reasons as well as water extraction purposes. The distance salt intrudes upstream depends on a number of factors, including river discharge, tidal and wind mixing and gravitational circulation. In this paper, an analytical solution is presented for the salt intrusion in a well mixed, funnel-shaped estuary whose cross sectional area decreases exponentially (with decay coefficient β) with distance, x, inland, and in which longitudinal mixing is constant along the length of the estuary. The solution predicts that a graph of the logarithm of salinity against exp (βx) should be a straight line, with slope proportional to the mixing coefficient Kx. The solution is tested against observations from 15 surveys over a four-year period in the Incomati estuary. Good straight line fits, as predicted, are observed on all surveys, with a mean R2 = 0.97. The average value of Kx for all surveys is 38 m2 s−1. The solution is used to make predictions about the minimum river flow required to prevent salt intruding to an extent where it causes a detrimental effect on water extraction. The minimum recommended river flow required to prevent this is 35 m3 s−1. In recent years, flow has fallen below this level for several months each year.
The generation of tidal asymmetries is clarified via numerical integration of the one-dimensional equations for channel geometries characteristic of shallow estuaries. Channels without tidal flats develop a time asymmetry characterized by a longer falling than rising tide. This behavior is enhanced by strong friction and large channel cross-sectional area variability over a tidal cycle. Resulting tidal currents have a shorter, intense flood and a longer, weak ebb (flood-dominant). Addition of tidal flats to the channels can produce a longer rising tide and stronger ebb currents (ebb-dominant), if the area of tidal flats is large enough to overcome the effects of time-variable channel geometry. Weaker friction with flats can also produce this asymmetry.Despite the physical complexity of these systems, essential features of estuarine tidal response can be recovered from one-dimensional models. Shallow estuaries are shown to have a system response leading to stable, uniform senses of tidal asymmetry (either flood- or ebb-dominated, due to phase-locking of forced tidal constituents), with down-channel development in magnitude of asymmetry. These concepts are illustrated by modeling idealized representations of tidal channels at Nauset Inlet, MA, and Wachapreague Inlet, VA, which have flood- and ebb-dominance, respectively.
Estuarine morphology is to a large extent determined by the residual sediment transport pattern. However, the inverse statement is also true. Residual sediment transport depends on differences in magnitude and duration between ebb and flood tidal currents. Such differences (“tidal asymmetry”) are produced by the distortion of the tidal wave propagating on the coastal shelf and entering bays and estuaries. In this study the relationship between tidal asymmetry and estuarine morphology is investigated. Based on theoretical considerations some general principles are derived and compared with field observations.
A standard part of any oceanic pressure gauge or current meter analysis is the separation of tidal from non-tidal components of the signal. The tidal signal can either be discarded, or its characteristics described in some fashion useful for further analysis. Although tidal signals can be removed by standard high or bandpass filtering techniques, their relatively deterministic character and large amplitude make special techniques more effective. In classical harmonic analysis, the tidal signal is modelled as the sum of a finite set of sinusoids at specific frequencies related to astronomical parameters. A set of programs has been written in MATLAB to (a) perform classical harmonic analysis for periods of about 1 year or shorter, (b) account for (some) unresolved constituents using nodal corrections, and (c) compute confidence intervals for the analyzed components.