Numerical modeling of Gazi Creek (South Coast, Kenya): (I) Hydrodynamics; (II) Descriptive ecological model
Onyango, H. B. A. (1989). Numerical modeling of Gazi Creek (South Coast, Kenya): (I) Hydrodynamics; (II) Descriptive ecological model. MSc Thesis. Vrije Universiteit Brussel: Brussel. 137 pp.
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Document type: Dissertation
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| Keywords |
Physics > Mechanics > Fluid mechanics > Hydrodynamics
ISW, Kenya, Gazi Creek
Marine/Coastal |
| Abstract |
The activities going on along the Kenyan coast are typical of any tropical coast: recreation, fisheries, waste dumping from urban centers and industries, pollution from sea traffic and natural inputs from rivers ( inputs which have changed in composition with time due to human related activities upstream ). These, and others, have given rise to the need for creating preservation measures to protect marine life and enhance their continued profitable exploitation. The management of such a system is attended with intricate decision-making problems. Furthermore, the cost in terms of time and resources becomes prohibitively high for a developing country like Kenya, which may have more pressing problems other than marine life preservation. The need therefore, for the development and extensive use of computer-aided models for management and prediction of trends of particular parameters or the overall system cannot be over-emphasized. This work is an attempt to contribute to improved management of resources along the Kenyan coastal waters (fig 1). Of course one realizes the increasingly important role "ecology" is playing in all facets of human activities; the "ecosystem" has indeed become a very important unit. Indeed, for top management and general policy development, ecology is a keyword today, and thus ecological data must be shaped into easy-to-understand indices that aggregate data into understandable forms. In this work, the ecosystem is a coastal marine one, a creek ecosystem. The complexity of an ecosystem comes to light for instance when all the interacting parameters in the ecosystem are considered, more so in a coastal ecosystem. The zooplankton and filter-feeders graze on phytoplankton and bacteria, which in turn need nutrients and light etc. Zooplankton and filter feeders contribute by way of excretion and death, to the total nutrient budget. Such complexities are "trimmed" down to manageable levels, and it is often at these levels that models are built. Here, the modeling of the residual circulation is first attempted, and then followed by an essentially conceptual ecological one. In coastal sea environments, the currents associated with tides and storms have rather high velocities, of the order of up to several meters per second. The period of the dominant tide is however only about half a day, and the characteristic life-time of a synoptic weather pattern is of the order of a few days. The very strong currents produced by the tides and atmospheric forcing are thus relatively transitory, and for time scales of ecological interest, they change and reverse so many times that they more or less cancel out, leaving only a small residual water circulation (Nihoul, 1981). This yields a 'steady-state' condition, which gives a simplified but convenient first approximation of the ecosystem's residual circulation pattern suitable for following the evolution of ecological parameters. The residual circulation isoclines are often used to delimit zones assumed homogeneous in biological or chemical parameters. Tidal currents are however important especially in coastal areas where the resulting tidal pumping is often the most important means of export of nutrients and sediments from the inter-tidal zone to the sea. This is particularly important in a mangrove biotope which is submerged at high tide and emerged at low tide. Thus it is necessary to include in the model the tidal influence, to give a picture of this mangrove-to-sea nutrient and sediment transport. The models will be largely theoretical in nature, using literature data and some little available data for guidance. On return to Kenya, the validation will be done in the actual creek in cooperation with other scientists involved in a comprehensive research project at Gazi. |
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