Application of Curb Extension for Sustainable Storm Water Management of Local Streets in Water Logging Lands:The Case of Residential Areas in Addis Ababa:
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Date
2021-02
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Addis Ababa University
Abstract
The application of sustainable stormwater management system in waterlogging areas of urbanized
catchments is the most effective method that helps to control surface stormwater runoff caused by
recurrent and extreme rain events. Applying a bio-retention system for flood reduction through
infiltration and evapotranspiration processes will help to create an attractive urban street with better
environmental performance. Since cities are becoming more impervious, the stormwater runoff
increases and will affects the downstream land and water bodies by flooding. In Addis Ababa, applying
a sustainable stormwater management system is a timely act due to the ongoing rapid urbanization and
increment in impervious surfaces that result in stormwater runoff that causes flooding and surface
water pollution. In this regard, innovative and flat terrain considerate design responses are needed in
low-lying plains with difficulties to apply conventional pipe-based drainage systems. The design
intervention will reduce the waterlogging effect occur due to extreme rain events. The objective of the
research is to apply curb-extensions as a bio-retention system for sustainable stormwater management
of waterlogging local streets in the low-lying residential areas of Addis Ababa. Specifically, the study
identified areas affected by peak stormwater flow that creates a waterlogging problem over local streets
of the case study area. A sustainable design solution is proposed by considering the infiltration,
retention, and evapotranspiration capacity of curb-extensions. The study identified flooding and
waterlogging areas in the city, at the river catchment, and in the study, site using hydrological and
spatial data. Using ArcGIS-based hydrologic analyst tools, blue spots located along the flow direction
of the streams identified as depressions usually experienced an overflow of stormwater and
waterlogging. To minimize the risk of waterlogging, a sustainable design solution as a curb extension
system designed by estimating the water balance from the actual water holding capacity of natural
depression (blue-spot) and the stormwater runoff generated from the catchment area that naturally
drained into the specific blue-spot. The curb-extension system was designed as a bio-retention and
streetscape element by modifying the native soil, gravel fill, water intake plants on the top of the system
for infiltration and retention purpose. In this regard, the designed curb extension will substantially
manage the highest intensity of precipitation (40 mm/hr) of 10 years of rain event from IDF curve of
Addis Ababa. The design reduces the volume of stormwater runoff from 22.895 m3 to 8.477 m3, which
is 62 % of runoff. The study concludes that if the municipality of Addis Ababa develops a curb extension system over the local streets of the entire neighborhoods affected by waterlogging, the
flooding problem will be reduced substantially with more greening & environmental benefits. The
system will also contribute to the reduction of downstream flooding by minimizing stormwater runoff
flowing into the Akaki river system. As a multifunctional drainage design facility, the system has the
potential to create a green street that helps to improve the standard of living and boost the
environmental quality of the area. Finally, the study recommended further study on the upscaling of
proposed curb-extension by developing more empirical and experimental settings as a pilot project.
The pilot projects will help the city to consider the curb-extension system for the greening, drainage
design, and urban planning practices. Moreover, the tests and demonstration works will justify its
competency for improving the livability and livelihood of the local people within urban settings that
were frequently affected by waterlogging.
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Keywords
Blue spots,, peak runoff, IDF curve, bio-retention,, infiltration