Investigations into the hydrodynamics of North West Bay

Despite the fact that modern oceanography has been established since at least the nineteenth century, bays and estuaries appear to have been singled out for particular study only within the past two decades. Yet their ecological importance, within the marine complex, is completely disproportionate to their area. They are generally highly productive zones and thus a major source of nutrients for coastal communities; they serve as refuges for many freshwater and marine species; and most importantly they are frequently important nursery grounds for the infantile stages of a variety of marine species, many of which spawn and spend much of their adult life at sea, but return seasonally to the estuary. Apart from their importance in the marine complex, bays and estuaries are undoubtedly amongst man's most valuable natural resources, being significant to human welfare in a number of diverse, and often conflicting, roles. Being semi-enclosed they provide natural harbours; they connect the oceans and the inland rivers so they are natural transportation centres; they commonly support important fisheries; they offer scope for a wide range of recreational pursuits; and they are often convenient repositories for waste disposal. It is in this latter role that man's impact on estuaries has been the most significant, for many of the world's major estuaries have become little more than a septic tank for the urban industrial complex. The extent of man's pressure on these waters is well illustrated by the fact that, of the ten largest metropolitan districts in the world, seven border estuarine areas (New York, Tokyo, London, Shanghai, Buenos Aires, Osaka and Los Angeles). These cities contain over sixty million people and foster enormous industrial activity. Ironically, recent concern with environmental degradation has often only increased the pressure on waterways. For example, to satisfy clean air regulations many industries have introduced 'scrubbers', (the Electrolytic Zinc Company plant at Risdon, Tasmania, is an example), the effect of which is not to eliminate pollution but to transfer the pollution load from the atmosphere to the waterways. Estuaries are particularly vulnerable to human influences for, being a confluent for land drainage, they receive the impact of many human activities throughout an entire watershed. Dams, diversions, irrigations, agriculture, forestry, changes in run-off due to urbanisation, and a multitude of other activities all have an ultimate impact on the complex physical, chemical, and biological interactions that occur within an estuarine eco-system. The physical processes that occur within estuaries have a strong influence on the development and continued viability of their ecosystems. In general, the most important of these physical factors are, the circulation, that is the patterns of mass transport and associated mixing processes such as turbulent diffusion and entrainment, and the resultant distribution of salinity. To a very large extent they control the dispersion and the rate of removal of contaminants; patterns of erosion and sedimentation; exogenous renewal of nutrients, oxygen, and larvae; and the distribution of estuarine biota. Thus a detailed knowledge of the hydrodynamic characteristics of bays and estuaries is not only of extreme importance but also has many practical applications. To the ecologist and the marine scientist it is part of the key to understanding the distribution of estuarine flora and fauna. To the industrialist and the environmental engineer it is one of the essential elements in evaluating the capacity of any estuarine or marine environment to receive industrial or domestic wastes. It should also be an important parameter considered in the planning and construction of offshore, nearshore, and shoreline structures, and the design of effluent disposal systems. Yet the importance of the complex role of estuarine circulation is often not appreciated. This appears to be particularly true of the Tasmanian situation where data on the hydrology and hydrodynamics of estuarine systems is sparse. The present lack of such information generated interest in carrying out a study into the circulation of a bay or estuary that is, or is liable to become, an area of environmental concern. Due to signs of increasing pressure and development, North West Bay, an area in close proximity to Hobart, was chosen as the study area. Brief investigations of its environs were carried out by students of the Environmental Studies Centre in the third term of 1976. From these pilot studies it was obvious that little was known about the marine environment of North West Bay, a situation common to most Tasmanian estuaries. This stimulated the investigations that are documented in this report. This report on the study carried out between February and November 1977 has been submitted as part requirements for the degree of Master of Environmental Studies and represents the culmination of two terms' work. As with any research project to be completed within an extremely short time, the design of the field program for the study area was strongly influenced by weather conditions, availability of equipment, and limited manpower. Hence within these constraints the broad aim of the study were to provide baseline hydrological and hydrodynamic data and guidelines for future studies in North West Bay and other estuarine environments in the region.