IP13a

A Stommel model of the Role of the Air-sea-land Interaction in the Ocean Flushing of Shallow Estuaries During Drought Periods and Some Human Influences.

Clifford J. Hearn

Oceanography, ADFA Campus of University of New South Wales, Canberra, ACT, 2600, AUSTRALIA.

During periods of drought, shallow estuaries may become inverse, ie have densities which increase with distance from the ocean. Inverse shallow estuaries tend to have very long flushing times and this makes them liable to become eutrophic if subjected to excessive nutrient input. Examples of such estuaries are those which experience the long dry summers of a mediterranean climate. Estuaries may also be subjected to drought conditions during other seasons or over annual time scales. There are numerous examples of such estuaries in both the northern and southern hemispheres (on the coasts of Chile, Brazil, Argentina, Australia, and South Africa). Estuaries which experience drought and high evaporation may show some degree of hypersalinity whilst drought conditions during winter may produce an estuary which is colder than the ocean. Both of these effects tend to make the estuary inverse. The morphology of this class of estuary often includes a bar across the entrance, which forms during the long drought periods, through which cuts a tidal channel connecting the ocean to an inner estuarine basin. The dynamics of the estuary during drought are controlled by the length and depth of this basin.

This paper presents a simple Stommel (1961) model of the buoyancy dynamics of inner basin during drought periods. The model describes the steady state balance between the surface buoyancy flux (due to the air-sea interaction, ie heating, cooling, and evaporation) and the buoyancy loss by ocean exchange. It shows that there are several possible types of estuarine steady state associated with Stommel (1961) attractors in the density/buoyancy phase plane. They include the classical state (which occurs if the estuary is short or deep), and inverse estuaries (for longer shallow basins). There is also an estuarine state, called 'quasi-neutral', in which the density difference between the basin and ocean is almost neutral but just slightly inverse ie the surface buoyancy flux is small and negative.

A laboratory simulation is used to illustrate the relationship of these estuarine states to the length of the estuarine basin. Results of a detailed numerical simulation provide the dependence of the flushing time, and other estuarine parameters, on this length. The type of estuarine state which occurs in a narrow basin is also highly dependent on its depth. As the basin becomes longer and shallower it tends to become more inverse and its flushing time increases. Harvey Estuary in southwest Australia and Shark Bay, in the arid region to its north, are examples of such inverse estuaries.

The estuarine state is also sensitive to changes in the basin which arise from other interactions with the land margin. These include alterations in basin geometry which may result from human influences such as increased riverine sediment deposition (during floods) and the reclamation of bordering wetlands. Another important factor is the change in the buoyancy balance of some of these estuaries during drought due to freshwater inflow from reservoir storage systems.

Stommel H., 1961, Thermohaline convection with two stable regimes of flow. Tellus, 13 (2), 224-230.

IPI3B

THE INVESTIGATION OF INTERACTION of RIVER WATERS WITH SEAWATERS BY a METHOD OF PHYSICO-CHEMICAL MODELING FOR the SOLUTION of SOME ECOLOGICAL PROBLEMS

Lev M. Gramm-Osipov, Valentina N. Gramm-Osipova and Svetlana Belkovskaja

Pacific Oceanological Institute, Far Eastern Branch
Russian Academy of Sciences, Vladivostok, RUSSIA,
Far Eastern State University, Vladivostok 690000 RUSSIA

The determination of forms, migration and transformation of chemical polluting elements in natural and polluted waters is a very important ecological problem. To determine the behaviour of these elements under mixing of different types of waters and to find out how these elements will behave themselves under changing physico-chemical conditions is no less important. Both problems are good tasks for geochemists and chemical modeling by computer (method of minimization of Gibbs energy). Several examples of modeling are presented in the report: the changing physico-chemical conditions in mixing river waters with sea waters, and the behaviour of some trace metals (Cd, A1, Zn etc.) under these processes. Modelling enables us to establish the forms of major ions of the general salt composition of natural waters. It was found that a geochemical barrier was formed by mixing river waters with sea waters. The above examples show that physico-chemical modeling by computer is a powerful instrument for description and prediction of the behaviour of chemical elements in the natural environment when different types of natural waters mix.

IPI3C

physico-chemical interactions in highly stratified southeastern australian estuaries

John Sherwood

School of Aquatic Science and Natural Resources Management, Deakin University, Warrnambool Campus, AUSTRALIA

Southeast Australia is characterised is characterised by a micro-tidal coastline and a Mediterranean climate. Maximum river discharges in Winter may flush all saltwater from estuaries for several weeks at a time. As discharge drops in late Spring-early Summer salt water advances up the estuary as a salt wedge in most estuaries (however, the Barwon estuary is well-mixed due to a greater tidal range for example). During Summer and Autumn river discharge drops substantially and the mouths of the estuaries become blocked (sometimes completely) so that seawater exchange is greatly reduced. Under these conditions deep water becomes anoxic. Such conditions may persist for 3 - 6 months. Sulphate reduction in these deep waters results in relatively high concentrations of nutrients and sulphide accumulating in them over 1 - 2 months. The process is fueled by inputs of organic-rich detritus from the highly productive oxic surface waters. Important physical processes in these estuaries are those which encourage vertical movement of water. Wind-generated turbulence brings nutrient-rich deep water to the surface layer while storm events may introduce fresh seawater to the estuaries - displacing older water vertically. Both these processes benefit the predominantly aerobic communities of the estuaries. Estuaries with a relative large surface area to depth ratio may become hypersaline under extreme conditions because of greatly reduced water inputs during the region’s dry autumn. Prediction of adequate (environmental) river flows for these estuaries would be greatly helped by models for these systems.

IP13d

Transport Processes in Tropical Estuaries - Significance of Circulation and Formation of Turbidity Maximum

K. Rasheed, T.K. Anujee, J.K. Ajith and A.N. Balchand

Department of Physical Oceanography, School of Marine Sciences, Cochin, India

Transport processes in estuaries are examined with emphasis on formation of high turbidity zones (turbidity maximum) in relation to physical processes such as circulation, sorption, mixing and diffusion. The lithological heterogeneity of the particulate matter constituting turbidity maximum, with varied physico-chemical features, bring about asymmetric distribution of material transport in most estuarine waterways. The physical influence regulating the distribution of suspended load in tropical estuaries by way of development of turbidity maximum zones (also observed in many subtropical waterways) give rise to complex sorption processes. The influence of salinity - a parameter of prime concern in estuaries - on the solubility of particular associated materials appears to play a dominant role in controlling and redistributing the particulate fraction. This study in a tropics well exemplifies the behaviour of metals on estuarine transport. The investigation helps to recognize the transformation process from particulate to dissolved form in the low salinity regions of upper mid-estuary and with increasing amounts of suspended solids in the downstream regions, the increasing salinities accelerate the removal of the dissolved form. In contrast with temperate estuaries, where the development of turbidity maxima caused the removal of metals in the low salinity regions, the estuaries of tropics modify the fluvial transport by way of redistribution between the dissolved and particulate fractions in the intermediate salinities.

IPI3e

Major Variations in Environmental Conditions During a Decadal Time Scale in Chilka Lake, East Coast of India

Pravakar Mishra, Takashige Sugimoto, Supriya Pal, Pratap Kumar Mohanty, S Satyanarayana Murty and K.V. Rama Rao

Ocean Research Institute, University of Tokyo, Japan
Department of Marine Sciences, Berhampur University, India
Fresh Water Biological Station, ZSI, Hyderabad, India

Chilka lake (19ƒ 28’ - 19ƒ54’N; 85ƒ06’ - 85ƒ 35’E) with an area of 1000 km² along the east coast of India is one of the largest brackish water body in Asia. It is connected to the Bay of Bengal by a 35 km long intricate channel. The lagoon receives drainage in the north from deltaic branches of a major river. Temporal variations in physico-chemical, meteorological and biological conditions over the lake were monitored from 1985-1995 during different months. The present paper highlights some of the major features of the environment conditions during the last decade. The physico-chemical and meteorological parameters are observed to be the dependent of the physiographical conditions of the lake. The southern and cental sector of the lake is deeper (~2m) than northern portion of the lake (~1m) due to heavy siltation by the rivers. The waters of the southern sector is warmer than that in the other regions of the lagoon due to surrounding hills and insignificant exchange of water with other sectors. Comparatively high saline water prevail in the central sector and decreases both north and southwards, can be attributed to the combined effect of evaporation and tidal influx of high saline water. An important feature of the salinity distribution, a tounge-like intrusion of the central lagoon water into the northern part, which acts as a boundary between high saline and low saline water. This boundary begins to develop after the cessation of monsoon floods and persists till the next floods. The surface-bottom differences is more prounced in the northern sector. The reason may be attributed to piling up of the surface water due to larger fetch area and greater wind stress. The lagoon waters are well oxygenated, however extreme values are recorded in some areas due to thick vegetation and poor mixing. The fresh water with huge suspended sediments from the rivers flow along the eastern bank of the lake, which is examined from the satellite data, moves towards mouth of the lake resulting in gradual silting up of the lake in the northern portion. Influx of river water and the siltation gave rise to growth of dense vegetation in the northern portion of the lake and it is estimated from IRS data that the spread of vegetation is 25 km²/year. It is concluded that vegetated and sedimented zones are likely to transform into swamps and lands, leading to further shrinkage of the lake over time.

IP13F

MIXING, CIRCULATION AND ESTUARINE/SHELF EXCHANGES IN PATOS LAGOON FROM A 3D MODEL

Osmar M–ller Jr. and Pascal Lazure

Lab. Oceanografia FÌsica, Depto. FÌsica, Universidade do Rio Grande, BRAZIL
Lab. Hydrodynamique et SÈdimentologie, Dept. Environement Littoral, IFREMER Centre Brest, FRANCE

Temporal and longitudinal salinity variations in Patos Lagoon are studied through a 3D mathematical model forced with wind, river discharge and coastal water levels obtained for the entire year of 1988.The main objective is to distinguish the relative importance of these factors on the longitudinal and vertical salinity distribution of this area. The series of measurements demonstrated that salinity variations are mainly related with the barotropic oscillations forced by the wind action. However, the intensity of salt water intrusion is conditioned by river discharge that can influence the water circulation in two ways: a) through a barotropic response, associated with water level differences between the lagoon and the coast; b) through a baroclinic mode that induces a gravitational circulation which plays an important role during periods of light winds. The vertical salinity profile depends on the relative intensity of wind and river flow. Most of the time the estuarine area tends to be stratified, however well mixed conditions are forced in two cases: a) during strong SW wind events by the advection of large amounts of sea water; b) by flushing during high river contributions.

IP13g

Sediment Movement and Silting in the Approach Channel Area of Mangalore Port - India

K.V.S.R. Prasad and G. Venugopal

Department of Meteorology and Oceanography,
Andhra University, Visakhapatnam, India

Sediment is considered to be any detritus, from gravel to clay particles, which moves into and is deposited in harbours and their approach channels by natural means such as stream flow, tidal flow or wave action. Most of the harbours in India are located in tidal estuaries. In many harbours in tidal estuaries a sizeable tidal prism moves through the harbour as a result of the tide filling in the tidal water area further up the estuary. In some of these harbour areas the velocities of tidal flow particularly when reinforced by the river flood flows - are competent to move sand and the problem of sand shoaling is of concern. Sometimes silting or shoaling of the harbour areas may also be due to silt and clay. Whenever the process of sedimentation reduces the navigational or approach channel depth of an harbour, or tends as secondary effect to shift channel locations, the problem becomes serious. The importance of this problem is emphasised by the tremendous cost of maintenance dredging. In order to reduce the scale of this public loss. It is essential to understand the mode, rate and extent of siltation in the approach channel areas of the harbours.

The Mangalore port which is situated at the confluence of Gurpur and Netrawati rivers (Lat. 12ƒ51’N; Long. 74ƒ51’E) is one of the important ports along the west coast of India. One of the navigational hazards in this port is the large bar in the sea just outside the entrance over which the depth varies from 2 to 3 m below low water. The more or less crescent shaped bar is about 200m wide east - west and begins from about 600m due west from the entrance.

The results of studies carried out on various factors involved in the sediment movement and silting in the approach channel area of the Mangalore port during different seasons are summarised. The currents near the Netrawati-Gurupur estuarine mouth are found to be mainly controlled by river discharge during the SW Monsoon months. The rest of the months of the year the currents are observed to be mainly influenced by the tides. Surface currents at 12m depth in the sea are directed towards South from May to September. Suspended sediment load values near bottom are found to have wide fluctuations both in the estuarine mouth and in the nearshore waters during different months. Seasonal variations of beach levels on the Ulial and Begre beaches adjacent to the port entrance are observed to be primarily influenced by waves and littoral currents.

IP13h

aspects of air-sea interactions in the fresh water discharge zone of the rio de la plata estuary

D.N. Severov and V.A. Severova

Facultad de Ciencias. Univercidad de la Republica Oriental del URUGUAY

The variability of the synoptic processes that are more noticeable in the transitory seasons of the year produces a significant influence on the mesoscale dynamics of the shelf water between the Rio de la Plata, and the Brazil-Falkland (Malvinas) Confluence. The stable transition from the zonal atmospheric circulation to the meridional atmospheric circulation has been more evident in the transitory period between autumn and winter. In particular, this transition manifests itself by the sharp change of the winds from the stable Northern direction to the stable Southern direction when the polar atmospheric front passes over the research area. The influence of this process on the formation of the mesoscale cyclonic eddy in the Falkland (Malvinas) Current on part of the confluence is determined using daily satellite data (IR). The possibility that this type of process influences the mechanism of freshwater discharge from the estuary to the shelf and from the shelf waters to the open ocean, is discussed.

IP13i

Survey Design Strategies for Interdisciplinary Estuarine Studies

E.C.V. Butler, S.I. Blackburn, D.A. Griffin, J.R. Hunter, P.P. Morgan, J.S. Parslow and J.K. Volkman,

CSIRO Division of Marine Research, Hobart, AUSTRALIA

Effective design of sampling procedures for interdisciplinary studies of estuaries is essential for resolution of the key time and space scales of events and processes in the water column and in the sediments. The approaches described in this presentation are the result of experience gained in surveys of estuaries and other coastal water bodies of southern Australia. A current project active in the Huon River estuary in southern Tasmania serves as the model. It is not practicable to address all temporal and spatial variability in one survey procedure, or even in several procedures; nor is it usually appropriate to combine surveys of water column and sediments. We have chosen to address seasonal changes in physical, chemical and biological parameters by doing quarterly samplings of the water column at a grid of stations that encompasses the full estuary from a freshwater end-member to a designated marine boundary. More rapid events are captured by monitoring (both automated instrument systems and manual sampling) at hourly to weekly intervals at a small representative subset of the estuarine stations. Underway measurements from the research vessel can map surface and near-surface features, and also enable swift response to special or extreme episodes such as floods, algal blooms or storm surges. Sediments are long-term integrators of processes in the water column. It is pragmatic to do an intensive spatial survey of the sediments; characterise all samples for a set of primary characteristics (grain size, colour, loss on ignition, etc.); and then take a representative subset of samples for further chemical and biological measurements. Sediment-water exchange processes can be investigated in other targeted surveys with specialist field equipment. Other details such as the selection of a suite of physical, chemical and biological measurements, laboratory experiments and emulations, and feedback from data analysis and environmental modelling will be discussed.

IP13j

Physical characteristics of the inner shelf offshore BahÌa Blanca Estuary (Argentina)

D.G. Cuadrado, M.C. PÌccolo and G.M.E. Perillo

Instituto Argentino de OceanografÌa. Avda Alem 53. 8000 BahÌa Blanca. ARGENTINA
Departamento de GeografÌa. San Juan y 12 de Octubre. 8000. BahÌa Blanca. ARGENTINA
Departamento de GeologÌa. San Juan 670. 8000. BahÌa Blanca. ARGENTINA

Over a section of the Argentinian continental shelf, southward the BahÌa Blanca Estuary, an oceanographic study was made. The objective was to know the thermohaline circulation and the suspended sediment transport in the zone. Temperature, salinity and suspended sediment data along the depth was obtained on 35 sample stations. Current measurements, tidal range and meteorological parameters were also determined.

The suspended sediment concentration on the shelf was uniform with values of 21.6 mg/l. Near the coast the concentration reached values over 100 mg/l. Two plumes of suspended sediment was observed. They would be associated with the RÌo Colorado, which flows out to the south of the estuary mouth, and the BahÌa Blanca estuary, respectively. The former has a SW-NE direction and the latter a NW-SE direction. Otherwise a high concentration zone was detected south of the RÌo Colorado, at a place named BahÌa Anegada.

The salinity spatial distribution presents the same behaviour as the suspended sediment. In the same place that the suspended sediment plumes were found, strong salinity gradients were present. At the estuary mouth the variation was 1 psu in 25 km long.

The meteorological conditions explain much of the fluctuations in the circulation and dynamics of the zone. A spectral current analysis revealed that the currents are principally tidal.

Martos and PÌccolo (1988) had studied a piece of Argentinian continental shelf that includes the present study zone. They presented the climatology and hydrography under different seasons over a climatological study that covered 23 years.They determined that the isothermal temperature distributions suggest a warm water flow towards the southwest along the outer shelf from spring to autumn. They did not find any horizontal or vertical gradients on the surficial temperature, nor the salinity.

This paper concluded that working with a different time scale, a difference with the climatological data existed. The water temperature, salinity and suspended sediment showed a vertical and horizontal difference, detecting freshwater with high sediment concentration flowing from RÌo Colorado towards the mouth of the BahÌa Blanca Estuary.

ipi3k

Salinity and dissolved oxygen in the stratified estuary of the Yarra River, Melbourne, Australia

J. David Smith

Marine Chemistry Laboratory, School of Chemistry,
University of Melbourne, Parkville, Australia

The Yarra River rises on the southern side of the Great Dividing Range and flows in a generally westerly direction. In its lower reaches the river descends some short rapids at Dights Falls. From the base of the rapids the course of the river has been substantially altered to improve drainage. This part of the river, passing through the city of Melbourne, is estuarine. Between the city and the outlet into Port Phillip Bay the river passes through the Port of Melbourne in a channel constrained between wharves and dredged to a constant depth. Port Phillip Bay is a large shallow marine bay that connects through a narrow and shallow passage with the ocean of Bass Strait. The tidal range at the mouth is about one metre. The Yarra River has an average flow of about one cubic kilometre per year but in common with most Australian rivers the flow has large seasonal variation. Minimum flow in summer is maintained at not less than 245 megalitres per day by releases from water storage reservoirs in the upper part of the river. The conditions result in strong stratification along the entire length of the estuary. This stratification exerts a controlling influence on the salinity and dissolved oxygen regimes in the estuary. The estuarine behaviour of the Yarra River is described in the paper.

IP13l

Methodology to estimate residual fluxes in estuarine cross-sections

G.M.E. Perillo and M.C. Piccolo

Instituto Argentino de Oceanografia, Bahia Blanca, ARGENTINA,
Dept de Geografia, Universidad Nacional del Sur, Bahia Blanca, ARGENTINA

Although seldom explained in the methodology section of most papers, measurements of most physical variables in estuarine cross-sections are made sequentially through the water column at individual stations. Commonly in developing countries but also true in many advanced ones, studies of estuarine cross-sections for residual fluxes are made only with one set of instruments and one boat. Therefore, the purpose of the present paper is to describe a general field and data reduction procedure for residual fluxes analysis on a estuarine cross-section. The characteristic grid and the necessary equations for estimation of residual fluxes are also presented.

Field methodology includes a detailed bathymetric chart of the estuarine reach containing the cross-section to be studied. The data should include the mapping of the lateral tidal flats. The number of stations to be sampled is a function of the: 1) geometry of the cross-section, 2) the desired error in the calculation of the fluxes, and 3) the operation time schedule.

The field measuring scheme followed with one single boat results in data obtained at different times and positions (vertical and across the section). The proposed data reduction procedure allows non-synoptic vertical profiling to be converted into pseudosynoptic space distributed information. The method is based on the transformation of the corresponding original values to standard heights above the bed and at standard reference times by means of an interpolating method developed by the authors. With the procedure described for each variable a data matrix is obtained in which rows correspond to standard depths and columns to standard times. When the method is applied to all measured variables at all stations, a final h x t x N_s matrix for each variable is formed, where h is the number of standard levels, t is the number of standard times and N_s is the number of stations.

Evaluations of the available grid schemes shows that the only one that fulfills all statistical criteria has both proportional columns and files. Therefore, the resulting cell areas are all equal and do not bias the estimations. Once the data is transferred to area-unbiased matrix, estimation of residual fluxes can be readily computed for each individual cross-section.

ip13m

The Interaction Between Sea, Land and Air of the Estuarine Systems of the Two Rivers SaIgon and Mekong

Le Huy Ba and Nguyen Duc An

Ho Chi Minh City’s National University, Vietnam

Results of many years of research reveal that there is a close relationship between sea, land and air in the coastal areas of the estuaries of the two rivers Saigon and Mekong. These estuaries are among the biggest ports of Vietnam on one side, and are located, on the other side, not very far from all ports and centers of exploitation.

Due to the activities of sea waves, currents and winds, some areas are eroded at the annual rate of between 40 and 100 metres per year. The process of erosion takes place so strongly the application of mechanical and physical protection methods in some places appears to be useless. Salt condition and the increasingly heavy pollution of the tropical environment combine to have a great impact on port constructions, including those that do not touch water, ie. those that are surrounded by land and air. The estuaries with mangrove forests are greatly destroyed; the sulfate acidification of soil processes very fast and strongly; in the last 20 years, the average temperature has increased by 0.5ƒ C while the rainfall has decreased by 100mm. The strong and rapid rate of the deposition of alluvial soil causes the sedimentation of estuaries or the shift of currents. Sedimental estuaries are polluted with heavy metals, especially Cd and Hg. Oil pollution in muddy soil, in water and even in air makes the biodiversity of estuarine areas go down to the least level. So far, there have been many evidences of the interaction between sea waves, winds and the sedimentation of soil, as well as between organic pollution and oil pollution in sedimental soil, in air and in water.

Transporting activities also contribute a great part to the change of the relationship between sea, land and air.