Flooding Effects On Road Transportation And Subgrade Slope Stability In Kigali City,Rwanda

Flooding is considered to be the most frequent and damaging of all type of natural disasters.Every year in the rainy season,the road floods occur from time to time,which has become a persistent disease affecting the road safety and operation,and has brought high economic losses.The city of Kigali has been exposed to frequent flooding owing to lack of well-planned drainage infrastructure and increased urban development with high proportion of impermeable surfaces that prevent the infiltration of water into the soil.Floods disrupt traffic,limit the pedestrian movements,damage roads and give rise to accidents and congestion,which presents a great cost to the city’s residents.Based on this,this study carried out the following research in Kigali city,Rwanda:Firstly,the causes and effects of flooding on road transportation and road subgrade were investigated.The data were collected by administering 150 questionnaires to the respondents within the study area and by conducting a physical survey.The hypotheses were formulated and tested using the statistical method of the student’s t-test.Based on the findings of this study,the strategies to mitigate road flooding were recommended.Secondly,the rainfall Intensity-Duration-Frequency（IDF）empirical equations and curves for designing appropriate and sustainable hydraulic structures for mitigating road flooding in the study area were developed.In addition,in order to address the problem of inadequate drainage systems,a new drainage system with hydraulic discharge(Q_lics)greater than the predicted peak rate of runoff(predicted hydrological discharge(Q_logy))should be designed.The Gumbel’s Extreme Value Distribution Method（GEVDM）was applied to develop the IDF Curves and equations for various durations of rainfall in Kigali city with two meteorological stations namely Gitega and Kigali Aero.Finally,a trial has been made to develop the precipitation for the different return periods and different durations of‘n’such as10,20,30,60,120,180,360,720 and 1440 min adopting GEVDM for the generation of IDF relationships with 2 districts located in Kigali city.Model performance indicators such as correlation coefficient（R）was used to examine the performance of the established IDF relationships by GEVDM for the design of rainfall intensity for the areas under study.The results showed that:（1）Heavy rains,poor drainage systems,illegal disposal of waste in drains,and poor construction and maintenance of roads are the main causes of road flooding in the city of Kigali.The results also revealed that,during flooding,the subgrade will be affected in terms of its capacity to support the pavement and the subgrade deterioration will cause the failure of the road pavement.（2）It was observed that different parts of Kigali city reported traffic Jams and increased travel time,slow vehicular movement,damage to road structures and traffic accidents,and blockage of drainage channels at percentages of 26%,14%,48%and 12%respectively.（3）Estimated rainfall in mm and its intensities in mm/hr for all return periods and their corresponding durations were analyzed using the Gumbel technique.The results showed that the annual maximum daily rainfall（AMDR）of 144 mm has been observed at Gitega station.（4）Through the application of the Gumbel frequency factor method.The extreme rainfall（P_T）,the average and standard deviation for the duration（10,20,30,60,120,180,360,720 and1440 min）were estimated from the annual maximum 24h rainfall IMERF（Indian Meteorological Empirical Reduction Formula）.The results show clearly that for those short durations,the rainfall intensity increases,and also increases with increasing precipitation in the return period of 2,5,10 50,and 100 years.（5）From the calculated rainfall（P_T）values and intensities（I_T）,rainfall IDF curves were developed for the two selected meteorological stations in Kigali City.The curves show that rainfall intensity increases when rainfall duration decreases.In addition,referring to the results of this study,the highest intensities which are 185.3,and 177.43,mm/hour will occur at Nyarugenge District（Gitega Station）,and Kicukiro District（Kigali Aero.Station）respectively;this means that the highest rainfall will happen after 100 years.Therefore,Flood mitigation measures should be taken before and any hydraulic structure should be designed well with reference to these results.（6）The formulation of the new hydraulic structures was done with the consideration of the predicted peak rate of runoff for the whole study area.Using Gumbel distribution and Rational methods,the empirical IDF and the predicted peak rate of runoff(Q_logy)equations were derived.With these equations,the rainfall intensity and predicted peak rate of runoff were both determined for the return periods of 2,5,10,25,50,and 100 years.It was found that the predicted Q_logy（peak rate of runoff）was increased with the increase in the rainfall intensities.For the rehabilitation and reconstruction of damaged or clogged drainage systems that cause floods in the study area,it was found that the predicted peak rate of runoff(Q_logy)should be less than the hydraulic discharge(Q_lics)to overcome or manage flooding when Engineers are designing any hydraulic structure(Qlogy?Q_lics).In addition,the designed period equations in this study can be used from 2017 to 2117 which means,the range of the design is 100 years as a designed period.