Urban water, generally, comprises of water supply and water supply sources, stormwater and wastewater. Until integrated approach was recognized recently, they were managed separately as stormwater and wastewater were seen as wastes. Since major urban areas (e.g. Singapore) could not meet their water demand only from conventional water supply sources, they are looking for alternative sources such as rainwater harvesting (RWH) and wastewater recycling. Addis Ababa is undergoing fast urbanization, creating increased impervious surfaces contributing to the generation of significant stormwater. Although the Addis Ababa city administration has put considerable effort to provide potable water to the city, the mismatch between the existing water supply and water demand is continuing. One of the reasons is due to the fact that stormwater and wastewater are not integrated into the water supply system. Anthropogenic impacts coupled with population growth and climate change have put substantial impacts on the urban water management of Addis Ababa. The above mentioned problems are the rationale for initiating this PhD study. Therefore, the aim of this PhD research was to study the existing urban water management, and to assess and develop the potential for landscape based stormwater management (LSM) measures and strategies for improved management of stormwater and water resources in Addis Ababa. The specific objectives were to: (i) examine the hydraulic capacity of existing stormwater drains and stormwater management challenges, (ii) investigate the existing water supply and water supply sources management, (iii) investigate the impact of stormwater on the quality of three rivers in the Little Akaki river watershed, and (iv) develop the potential of LSM measures and strategies to link it with integrated urban water management (IUWM).
The study was conducted in Addis Ababa (and two representative case sites in the older and new part of Addis Ababa to evaluate the hydraulic capacity of the existing drains). Detail field survey and personal observation, questionnaires assisted by stakeholder focus group discussion and literature survey and on-site physicochemical and laboratory measurements were conducted. 28 informants representing five organizations working in urban water, greening, planning and environment were contacted by purposive sampling on the basis of questionnaires and interview survey. This was done to explore some data related to causes of flush flooding, water shortage, and challenges of stormwater and water management in Addis Ababa. The surveys were conducted as the stakeholders are key partners from problem identification to implementation of the outputs of this PhD study. The ArcGIS and PCSWMM along with a spreadsheet were employed in analyzing the collected data. The results of the analysis of the hydraulic capacity of the existing drains at the two case sites showed that from the total surveyed length of existing drains 30% from the older and 15% from the new part of the city were found undersized. This revealed that the drains' hydraulic capacity were inadequate to convey safely the stormwater generated from the corresponding contributing watersheds. Correspondingly 70% and 85% of the hydraulic capacity of the existing drains were found oversized. Thus, the present study assumed that the major causes of overtopping that leads to flush flooding are clogging of drains by wastes, as 25% of the solid and 55% of the liquid wastes are dumped anywhere with possible destiny to existing drains. This study further revealed that stormwater is directly discharged into rivers which degrades the quality of river water and most of the drains have no outfalls, causing river bank erosion.
Therefore, the city administration should provide appropriate stormwater filtration system including the provision of buffer zones. For example, implementing the low-cost stormwater filtration system developed and tested for its performance in this study (presented in Annex 3) may have significant contribution to improve the quality of stormwater The results of water supply sources showed that they are endangered due anthropogenic impacts. The study also investigated the contribution of groundwater supply which has raised from nil in 1994 to 52% in 2016. However, the efforts to recharge the groundwater were minimal. According to this study, the water supply gap of Addis Ababa was estimated 41% due to population growth, urbanization and the city's dependence only on conventional water supply sources. In connection with this, to supplement the water supply and reduce its backlog the potential of rooftop RWH from large public institutions was assessed. The findings revealed that up to a maximum of 2.3% of the city's actual water supply can be provided. Alternatively, a maximum of 649% of an individual institution's actual water supply can be supplemented from RWH during the rainy months. This implies that installing large sized tanks for the large public institutions will enable to store the excess rainwater and supply the demand for the dry months. The state of river water pollution on three major rivers draining into Little Akaki river showed that the concentration of DO, some nutrients (NO2-N, NH4-N, PO4-P) and some heavy metals (Cr(VI), Cu) exceeded international environmental standards, and some parameters exceeded Ethiopian wastewater discharge emission limits, indicating a generally poor water quality of all three rivers in both seasons. Some pollutants were higher in dry season only (NO3-N, Mn and Zn).
Although stormwater improved DO, EC, PO4-P, Cr(VI) and Zn due to dilution, the levels were still critical, pointing to construction sites, agriculture and pit latrines to contribute contaminants in wet season. The relation between urbanization gradients and river pollution was weak as the impact of urbanization was offset by the presence of point sources (e.g. industries, latrines) from upstream to downstream. Based on the investigations of the existing stormwater and water supply and water supply sources management, and existing condition of river water pollution LSM planning measures and recommended strategies have been developed at household, institution, and city level which would fit to the context of the local conditions to link LSM with IUWM. To demonstrate the achievability of the LSM planning measures, the potential for rooftop rainwater harvesting (Annex 1) and low-cost stormwater filtration system (Annex 2) were examined in Addis Ababa. They showed potential contributions to minimize the water supply deficit and improve the stormwater quality. The implementation of these LSM planning measures and recommended strategies are expected to reduce the current stormwater runoff from impermeable surfaces by 23% and the city wide stormwater runoff by 12%.