Nature-based Design for Stream Rehabilitation Affected by Imperviousness in Addis Ababa: The Case of Kaba River
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Date
2020-10
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Addis Ababa University
Abstract
Urbanization of watersheds substantially changes the health of a stream by increasing
impervious surfaces that deliver pollutants directly to waterways. The major environmental
problem that Addis Ababa facing is the increasing impervious surface. Although improving
the riparian zone has been attempted to restore urban stream health in Addis Ababa, that
does not significantly improve it, since it is not equivalent to the scale of degradation of
streams based on recent studies. The main objective of the study is to introduce an
appropriate type of watershed-level nature-based design measures for the rehabilitation of
urban streams affected by the higher level of imperviousness in Addis Ababa. Stream
restoration can be achieved in this case by a cross-sectional study of the specific case
watershed to address the issue of urban streams and river system rehabilitation at the
watershed level by introducing appropriate nature-based design solutions and by
upscaling the solution from the case site into the entire city river system. The study site is
selected based on the density by comparing the built-up of upper catchments. The study
estimates the existing level of effective impervious area for ‘Kaba’ river watershed and
stream health improvement solution. The total impervious cover of the city and effective
impervious cover of the study area are 52 and 27.8 percent respectively. The current
condition of the stream will not support its intended hydro-ecological functions. Different
design scenarios are proposed in the watershed to reduce the effective impervious areas
to a level at which measurable improvement in the ecological indicators. Finally, the study
concludes that the proportion of the effective impervious areas is a good predictor of the
ecological condition of streams and it should be reduced to a level of recommended
proportion to maintain stream health.
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Keywords
Hydro-ecological function, Runoff volume, Green infrastructure(GI), Retrofitting measures