Browsing by Author "Dejene Tesema Bulti"

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    Analyzing the Impacts of Urbanization and Climate Change on Urban Flood and Planning of Resilience-Based Flood Hazard Management: A Case of Adama City, Ethiopia
    (Addis Ababa University, 2021-06-01) Dejene Tesema Bulti; Birhanu Girma (PhD)
    Urban flooding, which occurs when rainfall exceeds the capacity of urban drainage systems, has become a major concern in many cities across the world. Due to urbanization-driven increases in impermeable surfaces and climate change-induced increases in extreme precipitation, urban flood is anticipated to rising in frequency and intensity in the future. The majority of Ethiopian cities are susceptible to urban floods, although there is little research on the subject. Understanding of the contributions of main drivers at appropriate spatial and temporal scales, features of potential floods under current and future conditions and various flood frequencies, as well as flood adaptation measures at smaller spatial scales can aid efforts to effectively respond to the current problem of urban flooding, as well as consideration of its potential future increase. This study aims to analyze the impacts of urbanization and climate change on urban flood in Adama City and to devise resilience-based flood hazard management strategies. By mapping LULC of the City at about 5-year interval from 1995 to 2019 and computing the runoff depth at respective years using SCS-CN method, the dynamics of the City’s hydrologic characteristics attributable to urbanization-induced spatio-temporal changes of LULC was analyzed. Statistical downscaling model (SDSM) and extreme precipitation indices were, respectively used for downscaling daily precipitation from the projections of two Global Circulation Models (CanESM2 and HadCM3) and for analyzing the impacts of climate change on the historical and future extreme precipitation events. Further, the potential changes in the relationship between intensity-duration-frequency (IDF) of extreme precipitation in present-day and future periods were compared and contrasted. IDF curves and their functions were deduced using Gumbel Type I probability distribution and power-regression model, respectively. Flood inundation model was developed with coupled 1D-2D flood modeling method using PCSWMM, and used for simulating potential floods for a range of return periods and possible combinations of existing and future LULC and climate scenarios. Flood hazard levels were determined based on flow depth-velocity approach, for each scenario. The theory of urban resilience to floods was adopted for assessing the flood resilience level of the study area and for planning resilience-based flood hazard management. Flood-prone area was selected from the 100-yr flood scenarios and under the combined future LULC and climate. Localized flood adaption strategies were identified and their suitability for the selected prone area was assessed.The findings show that the built-up area undergone 7.9% expansion rate from 1995 to 2019. Likewise, the runoff depth is increased by 9.5 % in the City administration and 12.9 % and 6.9 % within the two sub-watersheds. At all spatial scales, the temporal change of runoff depth is linearly associated with the rise of imperviousness ratio. Moreover, statistically significant trends were obtained for the majority of extreme precipitation indices computed for historical daily rainfall records of 1967-2016, indicating that climate change has had an impact on historical precipitation. Moreover, extreme precipitation is expected to rise in the future up to 2080. The findings also reveal that extreme precipitation intensity over the years 2021-2070 in Adama City would increase up to 49.5% or decrease up to 106.2%, depending on GCM, storm duration and return period considered. Furthermore, the study area is flooded under both existing and future land cover and climate conditions, with increasing in the water depth, flow velocity and inundation extent as the return period increases. Under historical climate and existing land-use scenario, 123.7 (5-yr)-204.3 ha (100-yr) is prone to flood whereas the extent varies from 178.2 to 396.8 ha, under the combined effect of future land use and climate changes. Moreover, the study area is associated with lower level flood resilience. Finally, elevated configuration, dry-proofing, wet-proofing, temporary measures and site and landscape interventions are proposed as effective strategies for building flood resilience of the prone community. In line with sustainable flood risk management in the City, it is suggested that the stakeholders recognize the level of potential associated risk and improve the awareness of the prone community. Future developments should be guided with impervious surface based land-use regulation in order to better control the hydrological effects of urbanization. Further, the standards and guidelines presently employed by the City for the planning and design of stormwater management infrastructure should be revised in such a way that they reflect global climate change impacts at local level. Designing and updating local development plans on flood-prone areas should also aim to ingrate localized flood adoption strategies to build flood resilience of the prone community. Finally, urban planning policies should aim to promote urban flood modeling as a base for urban flood hazard management operations, and personal responsibility in flood safety. Keywords: urban flood, climate change, flood modeling, flood hazard, statistical downscaling, flood resilience, extreme precipitation, IDF
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    Environmental Monitoring and Assessment Climate change-induced variations in future extreme precipitation intensity-durationfrequency in flood-prone city of Adama, central Ethiopia
    (Springer Nature, 2021-11-01) Dejene Tesema Bulti; Birhanu Girma Abebe; Zelalem Biru
    The influences of climate change on the features of extreme rainfall events have become unprecedented that needs improved understanding at all levels for planning effective management strategies of the potential risks. This study aims to assess the potential influences of climate change on extreme rainfall characteristics in floodvulnerable city of Adama. For this, daily precipitation records of 1967-2016 and projection of global circulation models (GCMs): CanESM2 and HadCM3 for 2021-2070 were disaggregated into shorter time resolutions using the Hyetos model. Gumbel Type I probability distribution and power-regression model ( [[EQUATION]] were used for deducing intensity-duration-frequency (IDF) curves and for describing their functions, respectively. The extreme rainfall intensity of the historical and future periods for a range of storm durations and return periods were compared and contrasted. A close agreement is obtained between the observed and the modeled rainfall intensity with high values of coefficient of determination (> 0.996) and Nash-Sutcliffe efficiency (> 0.850). Besides, statistically significant (p<0.05) direct linear relationship is found between the return periods and the coefficient parameter of the IDF models. Moreover, the intensity of extreme precipitation over 2021-2070 in Adama city would increase up to 49.5%, depending on storm duration and return period considered. This could have consequences of the way the city’s drainage infrastructures are designed, operated and sustained. Hence, flood-prone areas should be recognized in order to formulate effective strategies for mitigation and adaption of potential impacts. The standards for designing future drainage infrastructures should also be updated aiming to reflect the effects of climatic change The standards for designing future drainage infrastructures should also be updated aiming to reflect the effects of climatic change.