Hydrological and hydrogeological consequences of rapid and large-scale urbanization

Sammanfattning: Urbanization is a process where rural population move to urban places for better job opportunities and facilities. This is a natural process and more or less unavoidable. But when this process takes place rapidly and at large -scale, the urban cities will undergo various changes that will impact the environment. Rapid and large-scale urbanization is mainly observed in developing countries of Asia and Africa. These changes affect the hydrological and hydrogeological processes of the cities, which in turn affects the local people. In order to meet the demands of the growing cities, the land use is changed, for example the lakes are encroached, and the impervious area is increased. This thesis aims to study the hydrological and hydrogeological consequences due to rapid and large-scale urbanization through a literature review of three cities. The catchment area of one of the three cities is further studied by computing surface runoff and amount of infiltration through the ground surface for two different years, using software such as extension tools of ArcGIS and HEC-HMS. Shanghai, Hanoi and Bengaluru cities were chosen for the literature review. These cities were chosen because they face different consequences due to rapid and large-scale urbanization. One of the catchment areas of Bengaluru city, namely Vrishabhavathi valley, was used to estimate the surface runoff and amount of infiltration through the ground surface by numerical simulations. In the case of Shanghai, the city mainly faces land subsidence due to over exploitation of groundwater. In the case of Hanoi, the city faces water quality degradation. Even though groundwater is exploited there is hardly any changes in the groundwater levels as the city´s aquifer is laterally recharged by the Red river that flows adjacent to the city. Flooding during monsoon, was one of the major effects faced by the Bengaluru city, as the city´s lake areas have been encroached. The lakes once acted as flood controllers. The estimation of surface runoff and amount of infiltration through the ground surface was carried out on Vrishabhavathi catchment area in Bengaluru city. The years 1975 and 2017 were selected for past and present scenarios respectively. According to the computed simulation, the surface runoff was more important for the year 2017 whereas the amount of infiltration through the ground surface was large for the year 1975. This is due to the adverse change in the land use of Bengaluru city. Before the large-scale urbanization the lakes of this city were connected to each other and these lakes reduced the peak discharge and hence controlled the floods. Generally, rapid and large – scale urbanization does have effects on the hydrological and hydrogeological processes. As urbanization is more or less unavoidable, care should be taken before the city is expanded. For example, measures such as artificial recharge, rainwater harvesting and flood management should be taken in order to save the city from further negative hydrological and hydrogeological effects.