Due to global warming and climate change, frequent heavy precipitation and urban flooding are becoming serious problems in many countries all over the world [1
]. Recent studies confirmed that there is a high probability of extreme rainfall occurrence in the future due to climate change [3
]. Other researchers agreed on future climate impacts on precipitation variables and corresponding causes [4
]. In recent years, many urban areas in Ethiopia have experienced various levels of flooding events as common tragedies [9
]. The flash and high-volume floods on streets disrupting transportation facilities are becoming critical problems demanding solutions. Addis Ababa City is currently one of the urban areas vulnerable to flooding as a result of rapid urban sprawl, poor or no implementation of urban flood management strategies in addition to global common climate change impact. Furthermore, an increase in summer season precipitation is expected with a possible risk of flooding in urban areas [10
]. The city is susceptible to both riverine and flash floods due to extreme rainfall events and upper catchment modifications (increased paved areas through deforestation).
As with most global urban areas, the vulnerability to flooding in Addis Ababa is caused the low level drainage standards, poor drainage capacity, inappropriate drainage alignment, aged drainage facilities and complicated drainage pipe networks with unreasonable settings, rapid housing development along river banks, and interruption of drainage facilities with other urban utility lines. It is then difficult to drain logged water out of the system in time before it causes severe flooding. Taking Arada and Addis Ketema Sub cities as an example, the drainage pipe network infrastructures in some areas are more than 100 years old, lacking the capacity to meet the current storm runoff volume. Existing drainage networks are fully or partially blocked by solid waste, the areas are fully occupied by impermeable pavements and buildings which are increasing the expansion of urban built-up areas, decreased or no infiltration into the underground, thus causing flooding on the Earth’s surface everywhere in every summer season. In addition, natural depression storages, ponds and wetlands, which are functioning as natural water storage facilities to delay and store flood events, are gradually filled or built-up artificially for other purposes. Decadal growth rate of both population and built-up areas is above 30% within the city which indicates that there is significant sprawl of urban or impervious areas [13
]. Following rapid urban spread, there is an estimated annual peak runoff increase with a growth rate beyond 2.5% and more than half of the city’s population live in slum areas which are susceptible to riverine and flash flooding as a result of unplanned and informal settlements [10
According to a report done by the Addis Ababa Fire and Emergency Prevention and Rescue Authority (AAFEPRA), 121,000 houses in the city, including 1000 governmental and private institutions, are in locations vulnerable to flooding and 143 areas have been identified as having a high risk of flooding. In 2017, for instance, there were 76 flood events in the city, damaging houses and properties worth 20 million birr. The events and their corresponding damages are constantly increasing within all sub cities. As a result, the city has started developing a city-wide drainage master plan study through Addis Ababa City Roads Authority (AACRA) to overcome existing urban flood and drainage problems.
Many researchers agreed to build a ‘sponge city’ which means creating optimized urban environment through maximizing infiltration, storage, water quality and minimizing runoff magnitudes [1
]. Low Impact Development (LID), an equivalent storm water management technique has also been developed by researchers of United States since 1990s [15
]. LID aims to control the runoff and pollution by means of decentralized, small-scale source control, making the development area as close to the natural hydrological cycle pattern as possible. It is a kind of environmental technique which easily realizes urban rainwater collection and utilization. Its facilities mainly include rain gardens, green roofs, permeable pavements, vegetative swales, bio-retention cells and rain storage through rain barrels. Low Impact Development is the basis of storm water management with the goal of simulating a site’s pre settlement hydrology by using design techniques that infiltrate, filter, store, evaporate, and detain runoff close to its source. Since LID uses a variety of useful methods for controlling runoff, designs can be customized according to local regulatory and resource protection requirements, as well as site constraints [16
]. Even though many developed countries are implementing the strategy for their urban environments, Germany was the first country to adopt governmental control systems of urban runoff, and runoff management laws and regulations, technology guidance, and economic incentive policies on LID were also established [18
Although the present control effect and performance of LID measures on runoff has been investigated and agreed among many scholars, there are still limited studies and implementation strategies that have focused on the control effects of the indicator and combined LID measures on small urban and semi-urban catchments under climate change-induced variable rainfall characteristics. Impacts of LID on runoff reduction with rainfall durations greater than 1 h (relatively for longer storm events) are done by [1
]. However, urban runoff problems frequently occur as a result of intensive short rainfall durations causing flash floods [21
], which needs independent analysis to elude the problem. Due their efficiency, sustainability and climate adaptability, currently there are high global demands to use LID to manage urban storm water [22
]. Hence, the main objective of this study is to reduce urban flood magnitudes using various LID-based catchment optimization techniques. The research will deliver an intuition on the performance of selected LID scenarios to overcome increasing challenge of urban flood management under climate uncertainty.
In this research, four selected LID scenarios (1) no application of LID practice, (2) infiltration-based LID practice, (3) an LID method based on water harvesting and (4) an LID technique based on the combination of infiltration and water harvesting are designated to analyze runoff characteristics in various land uses and rainfall characteristics. This study focused on (1) Storm Water Management Model (SWMM) model construction and validation using the Runoff Curve Number method; (2) the selection and design of LID scenarios; (3) evaluation of four LID scenarios in peak runoff reduction; (4) the impact of land use changes on LID applications; and (5) runoff characteristics of various LID scenarios under different rainfall patterns. The results of this study will introduce decentralized urban flood management strategies and offer technical inputs for the design, planning and construction of environmentally adaptive urban drainage systems.
5. Conclusions and Recommendations
This study explored the effect of LID on urban runoff peaks and magnitude reduction under various rainfall patterns as a result of climate change and urbanization on a selected urban area in the city of Addis Ababa, Ethiopia. The analysis was performed using the SWMM model for selected LID scenarios. Three LID scenarios (storage, infiltration and combined) were applied with three rainfall patterns and two climate change scenarios (base line and RCP 4.5). Intensive and short rainfall durations from 10 min to 1 h are used to analyze the impacts.
The study showed that both LID storage and infiltration methods have significant runoff reduction roles to reduce flash flood impacts. If there are adequate urban spaces to provide LID measures, urban runoff can be managed, and the cost of drainage structures will be minimized by using combined LID techniques. The rainfall pattern on dry surfaces (Antecedent Moisture Condition, AMC I) has a more significant impact on peak runoff magnitude than those for moderate and wet surface conditions. The study showed that short rainfall durations are more sensitive than long rainfall durations to rainfall pattern changes and highly affect runoff peak magnitudes. However, the results showed that there is no significant change in runoff volumes as a result of rainfall pattern changes.
The results of the study agree with various previous findings on the influence of climate change and urbanization on increasing peak runoff magnitudes. The study indicates that consideration of the rainfall pattern changes as a result of climate change in design and analysis of drainage structures on small and hilly urban and semi-urban watersheds. Climate change impacts will alter future rainfall patterns and runoff magnitudes significantly unless proper climate adaptive urban storm water management practices like LID techniques are implemented properly.
This research concluded that LID techniques are effective for urban runoff management based on available space, soil type and land use/land cover conditions of the area. As the proposed LID methods have shown the significant advantages of peak and volume runoff reductions regardless of other multiple advantages such as ground water recharge, aesthetic values, cost reduction of urban drainage structures and creation of a climate resilient urban environment to be studied in the future, the study highly recommends considering and implementing the methods for sustainable and climate adaptive urban runoff management through various policies and strategies of urban planning, design and implementation.