Urbanization and Its Effect on Surface Runoff (A Case Study on Great Akaki River, Addis Ababa, Ethiopia
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Date
2016-06
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Addis Ababa University
Abstract
On this study the effect of high urbanization rate of Addis Ababa City in general the study area
on the Great Akaki River is assessed. The surface runoff generated from the catchment is
estimated based on the rainfall intensity and major characteristics of the catchment area which
are the major factors for designing urban storm water drainage facilities and structures.
Satellite image for 1989, 2000 and 2010 of the catchment area is taken based on the quality of
data and the available resolution. ArcGIS and GIS extension tools are used to extract
hydrological characteristics of the catchment; HEC-RAS for hydraulic modeling, and HEC –
HEC-HMS to simulate rainfall - runoff process on Great Akaki watershed which is the major
watershed located at the center of Addis Ababa.
The hydrological and hydraulic modelings are accomplished by dividing the watershed in to
different sub-catchments. To compute infiltration loss SCS CN method; converting excess
rainfall to runoff model SCS unit hydrograph, and channel flow routing accomplished by using
Muskingum routing method of HEC-HMS model. To evaluate the accuracy of the simulation
model calibration and validation was conducted.
The hydrological modeling classified in to two: the first simulation shows the effect of high
urbanization growth on the basin. Accordingly, the peak discharge for 1989, 2000 and 2010 at
the Bridge outlet along Addis Ababa Bishoftu Road is 131, 153.4 and 188.1m3/s respectively. To
avoid the effect of rain fall variation on the generated peak discharge similar hourly rainfall of 18
July 2010 was used for each respective year.
The second simulation uses frequency storm method for 10, 50 and 100 return periods and peak
discharge of 403.80, 546.50 and 634.70m3/s for each respective return period was obtained. The
result found from HEC-HMS frequency storm method used for hydraulic analysis and flood map
hazard generation. Flood inundation maps produced using ArcGIS to visualize flood depth and
extent for each return period. Accordingly, maximum flood depth of 7.86, 9.07 and 9.82m for
10, 50, and 100 year return periods respectively was found with flood extent of 82.34 for 10
return period and 100.15 for both 50 and 100 year return periods at the middle of the final reach
of the study area.
Keywords: DEM, TIN, flood modeling; HEC-HMS/RAS; HEC- GeoHMS/RAS
Description
Keywords
DEM, TIN, flood modeling, HEC-HMS/RAS, HEC- GeoHMS/RAS