Estuary Hydrodynamics

Estuaria merupakan wilayah pesisir semi tertutup yang mempunyai hubungan bebas dengan laut terbuka dan menerima masukan air tawar dari daratan. Sebagian besar estuaria didominasi oleh substrat berlumpur yang merupakan endapan yang dibawa oleh air tawar dan air laut. Daerah perairan yang termasuk dalam estuaria ini adalah muara sungai, teluk dan rawa pasang surut. Estuaria daratan pesisir merupakan tipe estuaria yang paling umum dijumpai (Kamal, 2004)

Estuaries are key areas in between land and ocean which play a major role in the spreading of continental runoff drained by large watershed. This study focused on the Loire Estuary and its adjacent bays (i.e. Bourgneuf bay and Mor-Braz sea) all located in the north-east side of the bay of Biscay. It is influenced by the large tidal wave that propagates upstream the mouth on more than a 100 km, by highly mid-latitude meteorological forcing that may not only induced High variability in the circulation drivers but also on the river runoffs that may vary from 1 to 10 from early spring to late summer. This High variability is studied thanks to numerical simulation and tools dedicated to describe the circulation with synthetic index such as transit time and mean age of water. The approach lies on a numerical model discretized on a structure grid which constraints have been relaxed to better fit the fractal coastal line using non orthogonal grid cells. The optimal coordinate framework (co or contra-variant) have been discussed, and implemented within a pre-existing code (i.e. MARS-3D). This tools was validated with test cases and implemented on a domain with a particular complex geometry. The numerical simulations catch very accurately the dynamic of this large plume at least as it is described by available in situ observations. This numerical solution allowed to exhibit the main path of water masses through the area and from place to place and their variability according to the main forcings.

A crevasse is a geomorphological unit that consists of channel, bar, and splay deposits that form at a breach in the natural levee of a river, extending into the flood basin. They are common features in river delta and tidal landscapes where they form relatively highly elevated elements in otherwise low-lying flood basins. Hence, crevasses have an effect on sediment distribution within deltas and tidal landscapes.

So far, research on crevasses is generally limited to single crevasse case-studies in fluvial settings. Due to the focus on the effects of fluvial boundary conditions, there is a knowledge gap in the understanding of the effects of tidal boundary conditions. Therefore, the objective of this thesis is to assess the effect of tidal and fluvial boundary conditions, their interplay, and the initial flood basin conditions on the formation, evolution, and morphology of crevasses in the fluvial-tidal realm.

To address the objective, the Old Rhine system was used as a case study. The Old Rhine is a former main branch of the Rhine in the Western part of the Rhine-Meuse delta in the Netherlands. Multiple abandoned crevasses are well-preserved and its palaeogeographic development and changes in boundary conditions have been generally well reconstructed. The sedimentary build-up of four long crevasses was reconstructed based on borehole data of hand-augered corings. Lithogenetic cross-sections of each crevasse were constructed. Archaeological data were used in conjunction with the crevasse stratigraphy to get an indication of crevasse ages. Hence, three crevasse phases of the Old Rhine were identified. Their timing was compared to changes in boundary conditions and flood basin configuration of the Old Rhine to infer the effect of these changes on crevasse formation and evolution.

It was found that with regular tidal flushing of the crevasse throats (i.e. the initial natural levee breach) due to water level fluctuations induced by the tidal backwater effect, more crevasses can be formed by preventing crevasse healing. As such, regular tidal flushing facilitates crevasse evolution and progradation. When the tidal backwater effect decreased in the Old Rhine due to gradual closure of the estuary, the number of active crevasses decreased. This occurred because of the abandonment of existing crevasses on one hand and a decrease in the formation and establishment of new crevasses on the other as infilling of the crevasse throats was no longer or to an increasingly lesser extent prevented.

Flood events cause the initial natural levee breach which starts the crevasse formation. Subsequent flood events cause crevasse maturation and progradation, given that filling-in of the crevasse throat is prevented by regular flushing by the tidal backwater effect. Periods with a high frequency of flood events coincide with phases of increased crevasse formation and extensive progradation. Periods with a decreased frequency of flood events compared to a preceding high frequency period show a decrease in number of crevasses formed and a cease of crevasse progradation. Storm surge events probably have the same effect on crevasses as flood events, but the lack of a storm surge record for the Old Rhine estuary prevents any definitive statements about their effect.

Pre-existing channels, in the case of the Old Rhine peat drainage channels draining the raised bogs in the flood basins, can be intercepted by progradating crevasses. Hence, hybrid crevasse-peat drainage channels are formed. Such channels can still function as drainage channels of the raised bogs after a decrease or cease of morphodynamic crevasse activity due to a decrease in the tidal backwater effect, in the flood frequency or both. In the case of the Old Rhine, the three most upstream located crevasses in this study were formed as hybrid crevasse-peat drainage channels.

The outcomes presented in this thesis increase the understanding of the formation and maturation of crevasses in the fluvial-tidal realm of lowland river systems. They show that changes in both upstream as well as downstream boundary conditions affect the potential of crevasse formation and maturation. The presence of raised bogs in the flood basins and associated peat drainage channels can lead to the formation of hybrid crevasse-peat drainage channels.

These outcomes have implications for the archaeology of the Old Rhine area and future natural land-building projects in deltas. The three identified crevasse phases aid archaeologists to estimate the archaeological expectation of crevasse deposits in the Old Rhine area. Crevasses could be used in natural land-building projects to provide sediment to vulnerable flood basins to match (relative) sea level rise rates. Whether crevasse sedimentation rates would be high enough cannot be concluded based on the outcomes of this thesis. Implementation of crevasses for natural land-building projects would require that enough fluvial or marine sediment is available, sufficient vegetation cover is present to trap sediment efficiently, and flood events and storm surges are allowed to flood the flood basins.

Set in the western edge of the abandoned and eroding western Ganga–Brahmaputra delta, the Hugli is a flood dominated macrotidal estuary and a sediment sink. Comparative study of maps and images between 1851-55 and 1997 reveal that while progressive accretion or cyclic accretion and erosion characterise its non-reclaimed tidal islets, mainly erosion predominates in the reclaimed islands and estuary margins. Dredging and embankment repairing are regularly needed to sustain its navigational channels and to prevent bank erosion. For maintenance of morphological steady state, length of a resonant macrotidal estuary, like the Hugli, tends to equal 25 per cent of the wavelength of the tide entering into it. The tidal wavelength, in turn, depends critically upon the mean depth of the estuary—any alteration of which throws the estuary out of equilibrium. The hydrographic charts of 1904-05 and 1988-92 indicate that this equals 17 per cent in the Hugli, explaining the coastal erosion and in-channel siltation seen there. A rise in the future sea level will lead to an increase in the mean depth of the estuary and remove it even further from the steady state condition. In view of this, controlled retreat from its reclaimed islands and marginal areas and relocation of the resident population should be seriously considered for wetland restoration. These, however, are not easy tasks because of the high population density and high growth rate of population in the interior areas.

El estuario del río Hueque se localiza en la costa nororiental del estado Falcón (11º25’40” – 11º26’29” L N y 68º57’52” – 68º59’46” L O), éste representa la parte terminal de una cuenca de 2.109,4 Km2, que desemboca en el Mar Caribe. El mismo se caracteriza por ser un estuario con comportamiento similar a un caño de marea, cuyas aguas están mezcladas y sin estratificación durante todo el año. El río Hueque de 131 Km de longitud, mantiene actividad estuarina en los últimos 4 Km. Las aguas de este estuario poseen una fuerte tendencia a la hipersalinidad (38,5 ‰ promedio anual) aun en la época de precipitaciones (27,8 ‰), y con pH que varían de neutro a ligeramente básico (6,8 a 8,4). Esta característica se debe a que el estuario es controlado por los siguientes factores: (a) la energía del oleaje, (b) la persistencia de la corriente litoral proveniente del NE, (c) la elevada evaporación anual (2.166 mm), (d) la escasa energía de la acción fluvial, (e) la baja escorrentía local, (f) la escasa pendiente de la cuenca baja (1 %), y (g) los cerros aislados (Terciario – Mioceno) que generan acantilados bajos en la costa y ejercen control geomorfológico (efecto de sombra). Por lo que el propósito principal de este trabajo es establecer un modelo geomorfológico que sintetice la dinámica fluvio – litoral del área de estudio, considerando que se trata de una zona de gran valor ecológico, debido a que en el entorno de este río, se ha desarrollado un bosque de manglares de 5.000 Ha, y a su vez es un hábitat y lugar de reproducción de fauna acuática y avícola, así como sustento de una amplia diversidad biológica, reguladora del bioclima local y fuente de pesca artesanal. Sobre la base de los factores y los procesos implicados, las característicassedimentológicas de los ambientes depositacionales, y el desarrollo y evolución de la zona, se definió que la desembocadura del río Hueque, es un ejemplo de un sistema depositacional fluvio – marino de edad Reciente (Post-Flandriense), en el cual se diferencian dos complejos depositacionales bien definidos: complejo cordón litoral y complejo pantanoso costero.

Coastal gravel structures have been well documented worldwide and are formed dominantly by onshore wave transport of gravel, mainly during storm events. They are commonly observed along paraglacial coastlines where their origins are more ambiguous because of the effects of antecedent patterns of sediment supply, glacioisostatic sea-level change, and contemporary coastal processes. This paper describes the properties and polygenic origin of Connell’s Bank, a small paraglacial gravel structure on the Atlantic-facing coast of NW Ireland. This feature has been shown on historical maps, air photos and satellite imagery since c. 1850 but its outline has varied depending on seasonal migration, expansion and contraction of a sand veneer. It has also acted as a major control on tidal channel position within the estuary, and thus on sensitivity of the estuary system to ocean forcing. In detail, the bank surface is composed of cobbles sourced from outside of the immediate catchment. These surface cobbles are strongly winnowed, forming a lag deposit, whereas below the surface, cobbles exist within a granule and shell matrix. A significant proportion of surface cobbles shows evidence for recent ventifaction by blown sand at low tide. The paraglacial evolution of Connell’s Bank since the last glaciation comprises the following stages: (1) deposition of coarse glacigenic sediment as a moraine or proximal outwash fan during the late Pleistocene lowstand; (2) reworking of sediments onshore during early Holocene sea-level rise; (3) surface winnowing during mid to late Holocene tides and storms; and (4) surface cobble modification by contemporary wind abrasion.

A large area hypoxia has been already reported respectively by two interdisciplinary surveys off the Changjiang Estuary in August 1999 and 2006. The hypoxic zone shows distinct inter-annual variability. The observed oceanographic data are first analysized and reveal a big difference for the Changjiang Diluted Water (CDW) between these two periods. These great changes are related to the tremendous reduction of the freshwater discharge and variations of wind fields between these two years. It is also found that the monthly mean intrusion of Kuroshio and its branches has increased in the northern East China Sea (ECS), but decreased in the southern ECS in August of 2006 comparing with 1999 on the base of general circulation models. Then, the Regional Ocean Modelling Systems is applied to the ECS to evaluate the influences and their relative importance from driving forcing of the river discharge, wind forcing and open boundary data. Our simulations reproduce the phenomena that more fresh water extends northeastward in 2006 and forms a negative sea surface salinity (SSS) anomaly to the northeast of the river mouth comparing with 1999, which is consistent with observations. The five group numerical sensitivity tests suggest that the variation of CDW is primarily due to the wind forcing, followed by the Kuroshio and its branches, while the river runoff play the minor role. The study implies important roles played by hydrodynamic processes on the variability of hypoxic zone in the investigating areas.

Abstract
This paper outlines a numerical modelling study to predict the average residence time of a conservative tracer in a macro-tidal estuary, namely the Mersey Estuary, UK. An integrated hydrodynamic-dispersion model was used to predict the average residence time in the estuary for various tidal level and freshwater discharge conditions. The numerical model was verified against six sets of field measured hydrodynamic data, with the model-predicted water elevations and salinity levels generally agreeing well with the field measurements. The numerical model results show that in the Mersey Estuary both the tidal level and river discharge affect significantly the predicted average residence time. The value of the average residence time is also shown to be closely linked to the intensity of the residual tidal current. This is due to the fact that a large proportion of the Upper and Inner Estuary dries out during low tides, thus a significant amount of the tracer material is transported through the deep channels. An increase in the freshwater discharge causes a considerable increase in the intensity of the residual current along the main channels and thus a reduction in the average residence time. The predicted overall tracer residence time for the whole estuary is relatively short for a relatively large estuary, ranging from less than 1 day to 4 days for various tidal level and freshwater flow combinations. When the tidal range and freshwater discharge are both small, then the local tracer residence time in the upper part of the estuary can be significantly longer than the values predicted for the middle and lower reaches of the estuary.

Khur-e-Musa is a predominantly tide-driven marine creek located in the northwestern part of the Persian Gulf. The port city of Mahshahr and several important industrial enterprises are located in the vicinity of this marine creek. Therefore, marine pollution due to discharge from regional industries into this water body has been a matter of interest for environmental studies. In this paper, nutrient assimilative capacity of the Khur-e-Musa during the summer time was studied. In order to perform any engineering project or marine environment study related to Khur-e-Musa, the prediction of currents is an essential task. Therefore, MIKE 3-FM hydrodynamic and quality model was used to simulate nutrients and chlorophyll a concentrations. OECD open boundary index was used to determine the trophic status probabilities. Different scenarios were defined and simulated to find the minimum nutrient load that causes eutrophication in all parts of Khur-e-Musa simultaneously. The results showed assimilative capacity of 7,180 kg/day TN and 1,305 kg/day TP for Khur-e-Musa

Bangladesh is a low lying riverine country formed by a deltaic plain at the confluence of the three mighty rivers
Ganges, Brahmaputra, Meghna and their tributaries. Most of the flows of these major rivers are connected to
the Bay of Bengal through Meghna Esuary. The Lower Meghna River with a very dynamic estuarine  system
conveys the maximum flow through the estuary to the Bay of Bengal during dry and wet season (Rahman et al,
2013). The sediment load and flow volume in Lower Meghna is respectively the highest and the third highest
(after Amazon and Congo) in the world (Ahmed S. 1998). Owing to this tremendous water volume  and sediment
load delivered by the rivers, the estuary is an area of an active land erosion -accretion and dynamic water
circulation. The main physical processes in the estuary are mainly dependent on the estuarine morpho -dynamics
that causes erosion and accretion in the scale of hundred sq. kilometers per year and cause several thousands of
people to become landless and homeless every year (Biswajit et al., 2013). Chandpur district situated at Padma
and Upper Meghna confluence point in Meghna estuarine system is selected as the study area, since it is the
most vulnerable zone for erosion and accretion due to sediment load. In this study, the long term suffering due
to erosion, bank shifting characteristics and sand bar formation of the Meghna River has bee n unveiled. At first
a 1D model is set up for an average year to compute the yearly maximum and average discharge of Lower
Meghna River at Chadpur confluence. Then the erosion analysis has been done using high resolution of Landsat
satellite imagery (Band  4 to Band 7) from the year of 1973 to the year of 2010 and Google Earth image from
2000-2013. Net accretion, net erosion and the erosion trend are derived from the image processed using ILWIS
3.7.2 and ArcMAP 10. It is obvious that the erosion rate was in  decreasing order from the year 1973 to 1984,
3.4km2/yr to -8.4km2/yr. Analysis also showed that in the history of the last forty years erosion rate was higher
from the year 2003 to 2005,  -8.4km2/yr to 15.4km2/yr. Recent year analysis predicted that the deposition is
higher than the erosion. However, further research should be focused on future channel shifting pattern, erosion
rate and vulnerability of different locations as well as determining effective measures to decrease erosion
severity and its consequences.

El propósito principal de este trabajo es establecer un modelo geomorfológico que sintetice la dinámica del sistema estuarino de la desembocadura del río Hueque. Para el desarrollo del modelo se realizaron trabajos de campo en los que se recolectaron muestras de sedimentos de los ambientes depositacionales identificados, así como mediciones de las dimensiones de éstos, pendientes de playa, batimetría, oleaje y mareas. Se midieron parámetros físicos y químicos del agua como temperatura, pH, salinidad, conductividad eléctrica y transparencia. El trabajo de laboratorio se fundamentó en los análisis de textura, carbonatos, materia orgánica, minerales pesados y mineralogía por difracción de rayos X de las muestras de sedimentos. Se analizaron datos de clima, red de drenaje, mareas y oleaje y se realizó una fotointerpretación detallada del área. El estuario del río Hueque se comporta similar a un caño de marea, con aguas mezcladas y sin estratificación, fuerte tendencia a la hipersalinidad y pH entre neutro y ligeramente básico. Sobre la base de los factores y los procesos implicados, las características sedimentológicas de los ambientes depositacionales y el desarrollo y evolución del área, se concluyó que la desembocadura del río Hueque puede ser definida como un sistema estuarino de edad reciente.

Luis González* Resumen. En este trabajo se presentan los resultados de la caracterización geomorfológica y sedimentológica del complejo estuarino de la desembocadura de los ríos Hueque y Curarí, localizada en la costa nororiental del Estado Falcón (Venezuela). La investigación se desarrolló en tres fases. En la fase de campo se recolectaron muestras de sedimentos y se realizaron mediciones morfométricas de los ambientes depositacionales. En la fase de laboratorio se ejecutaron análisis sedimentológicos de textura, carbonatos, materia orgánica, minerales pesados y mineralogía por difracción de rayos X. La fase de gabinete se basó en la interpretación y análisis espacial de los ambientes depositacionales con apoyo en fotografías aéreas y mapas topográficos a escala 1:25 000; en la confección de los mapas y perfiles del área, y en la elaboración del modelo sedimentológico. La desembocadura de los ríos Hueque y Curarí se encuentra en un borde costero de clima semi-árido, con una cuenca de baja densidad de drenaje y dinámica litoral de rango micromareal, marea mixta, con oleaje de alta energía y corriente litoral favorable. Se identificaron dos grandes conjuntos sedimentológicos: Complejo Cordón Litoral (infraplaya, mesoplaya, supraplaya, dunas playeras, contrabarrera y barra), de litofacies arenosas y el Complejo Pantanoso (cauces de los ríos Hueque y Curarí, canal de marea y caño de marea), de litofacies arcillo-limosa. Los canales fluviales se comportan como estuarios hipersalinos.

1] Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline-normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5-15 cm s À1 ) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay-RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary-shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay.

Luis González* Resumen. En este trabajo se presentan los resultados de la caracterización geomorfológica y sedimentológica del complejo estuarino de la desembocadura de los ríos Hueque y Curarí, localizada en la costa nororiental del Estado Falcón (Venezuela). La investigación se desarrolló en tres fases. En la fase de campo se recolectaron muestras de sedimentos y se realizaron mediciones morfométricas de los ambientes depositacionales. En la fase de laboratorio se ejecutaron análisis sedimentológicos de textura, carbonatos, materia orgánica, minerales pesados y mineralogía por difracción de rayos X. La fase de gabinete se basó en la interpretación y análisis espacial de los ambientes depositacionales con apoyo en fotografías aéreas y mapas topográficos a escala 1:25 000; en la confección de los mapas y perfiles del área, y en la elaboración del modelo sedimentológico. La desembocadura de los ríos Hueque y Curarí se encuentra en un borde costero de clima semi-árido, con una cuenca de baja densidad de drenaje y dinámica litoral de rango micromareal, marea mixta, con oleaje de alta energía y corriente litoral favorable. Se identificaron dos grandes conjuntos sedimentológicos: Complejo Cordón Litoral (infraplaya, mesoplaya, supraplaya, dunas playeras, contrabarrera y barra), de litofacies arenosas y el Complejo Pantanoso (cauces de los ríos Hueque y Curarí, canal de marea y caño de marea), de litofacies arcillo-limosa. Los canales fluviales se comportan como estuarios hipersalinos.