Adama Flood Problem –Flood Risk Mitigation Option for Boku Shenen Area
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Date
2016-04
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Addis Ababa University
Abstract
Flooding is becoming a usual event that occurs in urban and rural areas. It is an interaction between mankind and nature. A city like Adama is nowadays becoming a flood prone area. A flood that comes from the adjacent watershed area seriously affects the low lying areas as seen at Adama – Boku shenen Kebele.
Boku Shenen kebele is one of a flood prone area of Adama city every year. Four different watershed catchments that produce flood exist around the kebele. Because of lack of integrated flood risk management system; loss of lives, displacement of people, and destruction of social infrastructures are becoming increasing from time to time.
Adama has been vulnerable to flash flooding from rainfall, in particular of the ungagged Kersa and Golba Tegene catchments which pass through South East of the city.
It has not been tried to estimate the amount of flood and not recommended mitigation options based on the natural situation of the area previously.
Hence, this thesis is to identify peak flood and recommend mitigation options based on the environmental and geological situation of the area that can be affected by extraordinary floods.
This thesis tries to consider more options and fills the gaps not covered by others adopting more than four application soft wares like Easy fit, ArcGIS, Global Mapper and AutoCAD. Specially, peak flood discharge estimation and recommend mitigation measures selection method believed to be the gap not properly covered in the previous flood risk mitigation works. The data usage for this thesis tried to make very intensive by considering different data options like gridded (DEM and contour), digitized (soil type, land use and 1:50,000 scale map) and rainfall data.
For precipitation modelling, ERA Intensity-frequency-duration curve was used for frequency storm and for the gage weights annual maximum daily rainfall for 24 hours and 6 hours duration storm are used since flood estimation requires a properly recorded data more than 30years from metrological stations. For the study area, I just used a rain fall data of 62years of record from Adama meteorological station.
30m x 30m resolution DEM for catchment was used to delineate the watershed area using GIS.
In this thesis, the flood magnitude estimated for a 100years return period was computed by using SCS excel model method for Kersa and Golba tegene catchments and flood mitigation option selected to be detention pond incorporating the designing criteria.
Adama Flood Problem-Flood Risk Mitigation Option for Boku Shenen Area 2016
Detention pond/dam was designed for both catchments. For Golba Tegene catchment, after analyzing the area of the reservoir/pond, having 8m of dam height 130962 m3 volume of flood can be absorbed/held which a 25 years design flood is. The spillway was designed for 100years return flood and can spill maximum of 5.586m3/s routed discharge. For Kersa catchment, a controlled local pond type of 3m height at 8584m2 and hence 26421m3 volume flood could be held which is also a 25years design flood. It also has spillway and controlled outlet gate that can discharge a maximum of 0.78m3/s. After the junction point of the two gullies a maximum of 6.36m3/s discharge could pass through the existing gully. However the existing gully was modified so as to accommodate safely the maximum 6.36m3/s discharge.
In doing so, this thesis will help and become an input in flood risk mitigation process. It will benefit the study area community directly where as any others can be benefited indirectly.
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Risk management system