A rational-method unit check
A fictional drainage screening exercise stipulates a runoff coefficient of zero point six, a rainfall intensity of thirty millimeters per hour and an area of two hectares. The analyst is instructed to use the simplified rational method with a unit conversion to cubic meters per second. A second analyst multiplies the three displayed numbers and labels the result as flow without converting area or time. The team must show the dimensional conversion and state why the simplified peak estimate is not a complete drainage-system approval. No return period, climate allowance, concentration time, inlet arrangement, downstream boundary, blockage allowance or actual municipal standard has been provided. All values are supplied exercise assumptions rather than prescribed national design coefficients.
1. Convert and calculate the modeled flow.
Read the worked answer
Intensity30mm/h is0.03/3600m/s and area2ha is20000m^2. ThusQ=0.6*(0.03/3600)*20000=0.1m^3/s under the stipulated rational model.
2. Which design questions remain open?
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The suitability of intensity duration and runoff assumptions, actual catchment response, storage, inlet and pipe performance, downstream conditions, failure scenarios and applicable design objectives remain open. The conditional peak estimate is only one engineering input.