Low Impact Development (LID) Design Retrofit to Reduce the Impact of Flooding and Diffused Pollutants from Affecting the Urban Lakes, The Case of Hawassa City, Ethiopia
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Date
2022-06-01
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Addis Ababa University
Abstract
Rapid urbanization has become a prevalent trend in the twenty-first century, putting development
pressures on existing urban areas as a result of socio-economic changes, which has an impact on
the natural system, particularly the hydrological system, by increasing flooding and degrading
water quality. The recent expansion and densification of Hawassa city contribute greatly to the
generation of more pollutants, which are transported to the city's lake through stormwater runoff
from built-up areas. The objectives of the study is to see how fast urbanization impacts the city's
natural environment, particularly the lake ecology, while also addressing sustainable stormwater
management to safeguard the city's lake using the Low Impact Development (LID) site design
approaches. The city's natural and man-made land cover were investigated to determine the
source of Lake Hawassa's ecological problem, which was connected to an increase in
impermeable surfaces. In this regard, the study used a data set that included a Digital Elevation
Model (DEM), a high-resolution satellite image, an existing land use map, precipitation data, and
water quality samples. Using spatial analysis tools in QGIS 3.6.3 and a simulation model in
SWMM 5.1, the quantity and quality of stormwater runoff impacting the lake environment were
evaluated. The imperviousness level of 14 sub-catchments that directly flow into the lake, as well
as the pollution load from impacted catchments, have all been investigated to determine which
sub-watersheds contribute the most pollutants to the lake. The study revealed that between 2013
and 2018, the imperviousness level in catchments that directly drain to the lake increased from
12.20 % to 20.32 %. That shows an increment of 43.4 % of runoff from impervious covers within
five years period. COD, TSS, TN, and TP levels in water samples tested from the degraded
catchments' lake intake streams were 2307.5, 510.5, 2.09, and 10.4 mg/l, respectively, which are
all above the permitted range. The SWMM model's simulation results also show that surface water
quality is substantially degraded and exceeds acceptable limits. The study identified various types
of LID design features as well as potential retrofitting open spaces in the study catchments. After
incorporating the proposed LID retrofits, simulated results reveal a reduction of 16.0 % and 17.4
% in stormwater runoff and peak flows for catchment 10 (C10) and a drop of 66.3 % and 5.0 %
for catchment 11 (C11). Regarding to water quality, the COD, TSS, TN, and TP pollutant loads
were reduced by 16.5 %, 16.2 %, 16.3 % and 16.3 %, and for C10 and 26.7 %, 13.1 %, 26.6
percent, and 26.2 %, for C11 respectively. Finally, flooding and water quality impairment shows
the disruptions in the urban hydrologic cycle resulting from the construction of more impermeable
surfaces have contributed to the degradation of Hawassa Lake's water quality. Incorporating
effective LID solutions into the city watershed management systems and limiting a significant
increase in impervious surfaces in urbanized catchments has the potential to reduce urban
flooding, nonpoint source pollution into the lake, and the oncoming environmental crisis.
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Keywords
Imperviousness, Urban Hydrology, Urbanization