Hydrology
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Browsing Hydrology by Subject "Hydrology--Kwa-zulu natal--Lake st lucia"
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- ItemA geohydrological assessment of the behaviour and response of the Zululand coastal plain to both environmental influences and human activity(1991) Rawlins, Brian Kenneth; Kelbe, B.The Zululand coastal plain on the eastern seaboard of South Africa contains large quantities of readily available fresh water. Lake St Lucia, located within the plain, is a wetland habitat internationally recognised for- its environmental importance. Yet over the past few decades extensive forest developments have taken place within the lake catchment. The considerably higher transpiration rate of the fast growing trees planted there over those of grassland environments they replaced is well established. The effects on the hydrological balance of the lake and its environs of the greater water loss resulting from this land use modification has however been the subject of much speculation. This study analyses the hydrological regime of the eastern shores plain over the 17 years from 1973 to 1990. During this period both wet and dry climatic conditions were experienced, and in 1981 a plantation covering 1100 ha was established. In order to clarify the magnitude and variability of hydrological parameters under natural and under altered conditions, comparisons were made between wet and dry periods, and between grasslands and forested areas. The extent to which plantations have modified the hydrological balance could thus be determined and placed in regional perspective. The study began with reviews of the physiography of the eastern shores catchment area and of the existing data base (chapter 2). Following a substantial upgrading of the hydrometric network (chapter 3), specific data collection took place. The components of the hydrological cycle were then assessed in order to identify differences in hydrological response both to climatic variability and to land use. The results of these assessments are presented as individual sections on meteorology (chapter 4), surface hydrology (chapter 5), and geohydrology (chapter 6). It is the conclusion of the study that in this sub-tropical environment, with shallow depths to groundwater, commercial forests are responsible for an additional consumptive use of water of between 150 and 175 mm/year. In the regional perspective of the water balance of Lake St Lucia, the total forested area of 25 000 ha will thus be seen to lower average inflow to the lake by between 10% and 12%. During extreme dry periods this figure increases to approximately 30%. Although they are greatest during dry climatic cycles, the effects of such a reduction in water yield are at all times significant. In the first place, the water balance of the lake is seriously affected. In the second, with.a reduction in its fresh water input, the salinity balance of the lake cannot but be influenced. In the third, changes in this salinity balance will have a direct impact on the flora and fauna of the lake and its environs. It is to be hoped that following the proposed future removal of the plantations from the eastern shores, the situation will improve. However, the remaining 20 000 ha under commercial forestry on the western shores will continue to have a significant effect upon the hydrology of Lake St Lucia.
- ItemGeohydrological studies of the Western Shores of lake St Lucia(1998) Nomquphu, Vaward wandile; Kelbe, B.ELake St Lucia is a sensitive estuarine system of international importance which is threatened with over-development within its catchment area by afforestation, overgrazing, proposed dredge mining on the eastern shores and other exploitations. These disturbances and proposed developments have invoked considerable public concern for the sustainability of the Lake St Lucia system. This study looks at the contribution of the groundwater from the western shores area to the lake system in order to evaluate its importance to sustaining the ecology of the system. Under severe drought conditions, the lake experiences considerable lowering of water levels which are accompanied by high rises in salinity levels because of the low freshwater recharge. While the western shores catchment of Lake St Lucia has a number of rivers that contribute freshwater to regulate the salinity of St Lucia, most of these rivers are non-perennial and have been exploited by developments within their catchments. In this study the aquifer system of the southern portion of the western shores area of Lake St Lucia is delineated with the specific intention of defining the coastal geohydrology of the western shores in order to determine its contribution to the water balance of Lake St Lucia. The geological succession of the Zululand coastal plain is described using geophysical information and borehole data which have been compared with field observations and findings in adjacent areas. The general stratigraphic succession, constructed from a number of electrical resistivity soundings and a few boreholes drilled around the western shores area of Lake St Lucia, is described as the first step in defining a conceptual model of the present system for numerical simulation studies. The resistivity soundings have shown that the permeable succession is restricted to post-Cretaceous sediments. The low permeable Cretaceous siltstones, the Lebombo volcanics and the Karoo sediments were all selected to coincide with the base of the numerical model. The overlying, semi-permeable to permeable, upward-fining calcareous and clayey sands of Pleistocene age, known as the Port Durnford Formation, were generally designated as the second layer (LAYER 2) of the model. The electrical resistivity soundings reveal that, on the western shores, the Port Durnford Formation has a thickness of 0 to 30 metres. Overlying the Port Durnford Formation is a relatively thin cover of unconsolidated aeolian, alluvial and estuarine sands of Holocene age which were generally selected as the top layer (LAYER 1) of the model. The upper surface of these coversands coincide with the surface topography, and they range in thickness from 0 to 8 metres and play a major role in the hydraulics of the shallow coastal aquifer. A three-dimensional finite difference model (INTERSAT), was developed from the conceptual geological model. This model was used to determine piezometric surfaces and subsurface flow into Lake St Lucia. Geohydrological and other hydrological parameters assigned to the model were estimated from existing literature and published reports because there were no observation data available. The model parameters for effective rainfall and evapotranspiration were derived from consideration of landuse features to account for surface-groundwater interactions. After the steady state condition had been achieved, the model was validated by comparing the simulated piezometric surfaces (heads) and simulated stream recharge against the observed water table elevations and estimated outflow from rivers, respectively. The simulation results have shown that the groundwater contribution from the western shores to the lake is approximately 40x106m3/year. This component of the water budget may be sufficient to have a buffering effect on the salinity of the lake along the edges of the shores if it is released in times of surface flow droughts. To quantitatively evaluate the groundwater contribution from this area and other sections of the lake system, the presently existing network of electrical resistivity surveys needs to be supplemented with the boreholes or observation wells that should be scattered over the area. From these records a good groundwater database could be built. It is recommended that simulation studies should be extended to the northern shores in order to evaluate its contribution to the water budget of Lake St Lucia.