Design Of Hydraulic Structures For Sustainable Runoff Control In Kinamba Sub-Catchment, Rwanda

The process of urbanization is among the causes of the continuous increase of flood hazard events and the associated losses worldwide; mainly due to the increasing impervious surfaces and the exposure of people and their wealth. (Muis et al., 2015)  In Rwanda, mainly in Kigali City, heavy rainfall events cause rapid surges in the flow of rivers and drainage systems leading to floods in the downstream. It has been identified that inadequate drainage system in Kinamba sub-catchment located in Nyabugogo catchment is the major cause of flooding in the downstream of that area.The aim of this study was to design hydraulic structures for sustainable runoff control in Kinamba Sub-catchment. Daily rainfall data for 30 years have been collected from Gitega meteorological station. The peak runoff water discharge has been calculated by using the rational method and its prediction in 30 years by using the linear model. In this research ArcGis played a key role while analyzing spatial data.The results showed that the two existing culverts are capable of carrying water discharges at 91.168 m3/ sec while the estimated current peak runoff discharge is 108.73 m3/sec while considering daily maximum rainfall of 98mm. The population growth rate increase compelled to estimate the future area to be covered for facilitating human settlement. It was found that in next 30 years, all easily usable area will be covered by population leaving around 6.5% area. The remaining catchment area would be covered by houses and roads  with a predicted discharge rate of 118.41m3/sec during next 30 years.Based on the needs, a trapezoidal channel of designed 14.27 m2 cross section has been recommended along with a rectangular  box culvert with 6 boxes of 2.8m depth and 2m width with a discharge carrying capacity of 118.41 m3/sec. It is also recommended to have household level water harvesting and management of catchment area by keeping balance in housing and open area while respecting Rwanda housing policy

Effects of drainage intensity on water and nitrogen use efficiency and rice grain yield in a semi-arid marshland in Rwanda

Drainage management is important in intensification of irrigated paddy rice production. This study assessed the effects of drainage intensity on water and nitrogen use efficiency and rice grain yield in a field experiment conducted during three seasons in Rwanda. The experiment comprised 12 plots with four blocks and three treatments: DS0.6(0.6 m deep drain), DD1(1.2)(1.2 m deep drain, control structure open four times per week), and DD2(1.2)(1.2 m deep drain, control structure open two times per week). Outflow was calculated from water balance. Nitrogen (N) content in drainage water was determined weekly. Crop yield and N uptake were determined in grain and straw. In all seasons, grain yield was 61-131% higher, crop N uptake was 24-90% higher, harvest index (HI) was 24-65% higher and water use efficiency (WUE) was 50-150% higher in treatments DD1(1.2)and DD2(1.2)than in DS0.6. There was a decrease in soil carbon/nitrogen ratio at the end of Seasons 2 and 3. Recirculating straw to fields is thus necessary to replenish SOC for long-term soil fertility. A practical implication of the study is that managed deep drainage systems could enhance water use efficiency and rice grain yield in poorly drained paddy fields