Publication Date:
2021-05-19
Description:
The waters of Lake Victoria are set into motion by energy exchange processes across its surface. Winds blowing over the surface can set near surface waters into motion and cause horizontal circulation (currents) as well as generating wave-related turbulence that can mix waters more deeply into the depths of the lake. Where rivers enter, both horizontal transport and vertical mixing can be set in motion by the momentum and physical characteristics of the entering rivers The Kagera river is exceptional in the dominance of open lake processes affecting the behaviour and mixing of its plumes as it enters into the lake directly. Most of the other rivers entering Lake Victoria enter behind islands or into protected gulfs and embayments where the hydrodynamics of river mixing are governed by local coastal process and the dynamics of the bays. The water motions that result from the net affect of all these energetic inputs determine the distribution of the dissolved and suspended materials in the water as well as affecting even the free swimming biota. Thus, it is necessary not only to determine the vertical transport of nutrients but also the horizontal transport of matter throughout the lake. The strength of horizontal circulation will also determine how widespread events such as pollution spills will be. Coordinated observations overall of Lake Victoria have confirmed phases 2 and 3 of the annual thermal and stratification cycle as defined by Talling (1966) for the northeastern part of the lake. Phase 2 is the development of the deep (40 m) thermocline in the period February to May, and phase 3 is the total vertical mixing that occurs in July-August. Phase 1 (September-December) is less obvious, i.e. the gradual warming of the water column is weak, and almost total mixing occurs in December-January at some stations. All three phases are less developed on the western side of the lake. The western part of Lake Victoria is much more influenced by the wind forces, and therefore experiences more mixing and cooling patterns. The eastern part of the Lake is much more influenced by persistent thermal stratification, and therefore vertical mixing is mainly by seasonal temperature dependent density currents. The implications are that the potential for nutrient transfer, sediment resuspension is higher in the western part of the lake, which maintains well oxygenated conditions favourable for fish species requiring high oxygen concentrations such as the Nile perch. Main processes determining the fate and transport of pollutant that are mixing (dispersion) and flow (advection) were also studied.. These were modelled/simulated using the hydrodynamic module which is based on the generic DELFT3D-FLOW simulation package. The only wind data which could be used in the model was the global winds for 1998 which were delivered together with the framework model. The results from the model agreed well with the measured currents during the month of September.
Description:
Published
Keywords:
Hydraulics
;
Hydrodynamics
;
Current forces
;
Water circulation
;
Pollutants
;
Water motion
;
Water currents
;
Transport processes
;
Water mixing
Repository Name:
AquaDocs
Type:
Report Section
,
Not Known
Format:
pp.42-61
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