Lake Hydrodynamics and Pollution Transport under Climate Change The Case of Lake Victoria

Abstract

A very small part of the total earth’s water is freshwater (only 2.5 %). Unfortunately, due to climate change and pervasive manmade activities, surface freshwater quality in many places of the world has become degraded. This is manifested in the Rift Valley lakes, a series of lakes in Eastern Africa that runs from Ethiopia in the north to Malawi in the south. Water quality degradation in the Rift Valley lakes is driven by various factors, including water quantity and scarcity, pollution and contamination, nutrients loading, and general water use by industry and society. In particular, Lake Victoria, the world’s second-largest freshwater body and the largest tropical lake, has seriously polluted near lakeshore areas, which is a great regional development problem causing misfortune for millions of people. This dissertation contributes new insights into lake hydrodynamic processes and pollution transport in shallow lakes through developing more accurate models to understand the complex processes of water quality degradation. Based on empirical data this thesis developed systematic methods to consider lake bathymetry, lake flow, water level verification, water balance, hydro-climatological processes, transport and dispersion of pollutants and nutrient particles. The data-driven hydrological model of Lake Victoria that is developed in the thesis considers hydro-meteorological and climatological data, river discharges and outflow, wind speed and direction, atmospheric deposition, nutrient loading, concentration of pollutants and nutrients, and remote sensing satellite data. The thesis illustrates the power of numerical and hydrodynamic methods that uses one- and two-dimensional mathematical equations (1D and 2D) to model the three-dimensional (3D) behaviour of shallow lakes over time. The results indicate that the lake hydrodynamics of Lake Victoria are heavily influenced by lake bathymetry and regional weather patterns and are thus connected to increasing climate variation. The hydro-meteorological processes, verified by empirical data on precipitation, lake flow and lake water levels, show that extreme weather events are responsible for changing the characteristics of lake water balance, changing seasonal variations, and exhibiting strong correlations among water level and hydro-meteorological data. The model of the movement of pollutants and nutrient particles shows how pollutants and nutrients travel within Lake Victoria and where they concentrate in the lake and its sediments. The wind hydrodynamic modelling shows that the wind, along with hydrodynamic stability, plays an important role in pollution flow patterns and that pollutants can be transported from shallow parts, when they leave rivers and shorelines, to deeper lake areas. The hydro-climatological model demonstrates the crucial interdependence between hydrodynamic processes and climatological factors at the catchment scale of Lake Victoria. The numerical models and calculation methods that have been developed in this dissertation represent additional contributions to hydrodynamic research and can be used to investigate hydrodynamic processes in other lakes. The thesis contributes to UN Sustainable Development Goals related to water security, drinking water, food, and health. A potential area of application lies in supporting analysis and mitigation of pollution and climate change effects and more generally aid in the natural resource governance of this vital African lake.