Describe briefly the principle of the Rational method and thelimitation of this method for runoff estimation. A sub-catchment ofarea 0.1km2 is built on gently sloping land. The averageslope of the land is 0.001. The length of the longest natural flowpath L is 276.3m. The flow time tf in the drainagesystem is 3min. Determine the 50yr design discharge for thesubdivision using the rational method, given that the averagerunoff coefficient of the subdivision is C=0.7.
The above sub-catchment is drained by a pipe system which isalso used to drain the second sub-catchment. The secondsub-catchment is of area 0.2km2, average slope 0.002,L=300m, and tf=3min, C=0.5. If the pipe system isdesigned for the 50-yr rainfall event, what is the appropriateduration to be used? Explain briefly. Hence determine the 50yrdesign discharge of the drainage system.
The 50-yr rainfall intensity i can be calculated fromi(mm/hr)=a/(td+b)c, where a=687, b=4.2, c=0.42,td(min)=duration.
