Stormwater management in Gaborone - A study of the quality and quantity of water in Segoditshane River

Abstract: This master thesis was partly performed as a field study in Gaborone, the capital city of Botswana, located in southern Africa. The semi-arid climate in Botswana together with a rapid financial development in the country during the last decades has increased the need for water. The last years the water situation has been strained in Gaborone, and Gaborone dam which used to be one of the city’s drinking water sources has come down to a level of only a few per cent of its total capacity. The water scarcity results in a need for the country to transport water from northern Botswana as well as from South Africa to meet the need for water in Gaborone. The severe water situation results in need for alternative water sources for Gaborone. Reuse of urban stormwater is one alternative water source worth to investigate. A large amount of the stormwater from Gaborone discharges into Segoditshane River, a watercourse which flows through Gaborone. The aim with this project was to investigate the quality and quantity of water from Segoditshane River and the possibilities of collection and reuse of the water. Water samples were collected from Segoditshane River at three times and at two locations each time. One sample of stormwater was also collected from a parking lot at University of Botswana and another water sample was collected from a roof at Motheo Apartments, both located in Gaborone. Quality parameters for the water samples were measured at the sample sites and in laboratories at University of Botswana. The water contained several metals. Most outstanding were arsenic, lead, cadmium and aluminium. Hydrocarbons connected to traffic pollution were also noticed in the water. When water from Segoditshane River was tested for pollution of microorganisms no E. Coli was found but other enteric bacteria were found and hence possible inflow of sewage water in Segoditshane River cannot be disregarded. However, the enteric bacteria may also origin from faeces from animals. Depending on the planned reuse of the water, different water treatment would be needed. Drinking water needs more treatment than irrigation water. Biological treatment methods along Segoditshane River may improve the quality of the water. A stormwater dam downstream can store the water and prevent flooding as well as further improve the water quality through settling. Rainfall data for the years 2001-2010 was given from Department of Meteorological Services in Botswana and were processed in order to estimate the water volume possible to collect in the urban part of Segoditshane River catchment. In the quantity calculations the evaporation is not included and the annual water volumes originating from stormwater will therefore be lower than the volumes presented below. Three scenarios were set up to describe potential situations for the stormwater in the catchment. Scenario 1 includes the stormwater runoff from larger roads and gravel areas and according to this scenario a water volume of less than 4.8 Mm3 is possible to collect. This corresponds to 15% of the total annual water usage in Gaborone. Scenario 2 includes the stormwater runoff from all house roofs in the city and in this scenario a water volume of less than 3.6 Mm3 is possible to collect, 10% of the total water usage in Gaborone. Scenario 3 describes the amount of stormwater runoff from large roads, gravel areas and house roofs ending up in Segoditshane River and in this scenario a water volume of less than 7.2 Mm3, 23% of the total water usage in Gaborone, can be collected. There is a potential worth considering of collecting stormwater from Segoditshane River and rainwater from house roofs in Gaborone in order to reuse the water and hence improve the stressed water situation in Gaborone.