Question
What are main boundaries of Rational Method in calculating runoff?
Answer
Calculation of runoff is compound matter that depends on a lot of factors similar to ground permeability, rainfall period, rainfall prototype, catchment area kinds etc. essentially, Rational way is a means to discover out most free suitable for designing purpose. In this process, it is supposed that rainfall duration is similar as time of attentiveness and return stage of rainfall intensity is the same as the peak runoff. Time of concentration refers to time required for mainly remote location of inside catchment to flow to outlet. As time of attentiveness is equivalent to rainfall stage, maximum discharge happens and rainfall collected in catchment comes to similar outlet point.
Rational means provides peak discharge only and it cannot create a hydrograph. If an additional thorough pattern of runoff is necessary, unit hydrograph or previous means have to be used. The correctness of rational process depends very much on our right selection of runoff coefficient and description of catchment area.
Rational process is a rather conservative way. One of essential assumptions of rational formula is that rainfall intensity has to be constant for interval at least like to time of attention. For long duration of rainfall, this assumption might not hold true.
Furthermore, runoff coefficient in rational process is difficult to be determined precisely and it depends on a lot of factors like moisture condition of soils, rainfall strength and duration, amount of soil compaction, vegetation. In adding, In Rational means runoff coefficient is free of rainfall intensity and does not reproduce actual situation.