DESIGN OF APPROPRIATE HYDRAULIC STRUCTURES FOR SUSTAINABLE FLOOD CONTROL IN THE NYABUGOGO CATCHMENT, WITH FOCUS ON THE MPAZI SUB CATCHMENT

Abstract

A flood is a great flowing or overflowing of water onto land that is not usually submerged. A flood happens when too much rain falls and cannot be absorbed by the soil. Most of countries in Africa are regularly affected by severe and often multi-year disasters including landslides and floods. Rwanda is among countries which face floods disasters because of its topography. The Nyabugogo area due to low altitude and its nature of convergence zone of drainage systems of Kigali city has been repeatedly subjected to floods. The Mpazi sub-catchment (8.37km2 ) is the major contributor to the flooding in the Nyabugogo area, due to degraded steep slopes in the water heads of Kigali Mountain, dense unplanned settlements in Kimisagara, Nyakabanda, Cyahafi and Muhima areas, clogged culverts and bridges and that impede the flow of water in the Mpazi channel, and then cause the flooding. The aim of this study was to design appropriate hydraulic structures for sustainable flood control in the Nyabugogo catchment with focus on the Mpazi Sub catchment. Daily rainfall data from January 2003 to June 2014 have been collected from two meteorological stations; the KANOMBE airport station and the Nyabugogo wetland station. The peak rainfall height determined by using the extreme value theorem has fallen on 23rd February, 2010. The peak runoff water discharge has been calculated by using the rational method and its prediction in 30 years has been calculated by using the linear model (Q= 118.9 m3 /s). The discharge capacity of the existing hydraulic structures has been calculated; by using the Manning equation for the open channel and the rectangular culverts design formula for the culverts. The existing hydraulic structures are not able of carrying the predicted peak or design discharge. For a long term sustainable flood control solution for the Mpazi sub catchment as well as in the Nyabugogo catchment, new hydraulic structures have been proposed. The proposed new design, once applied will contribute to the flood control observed in this sub catchment.