Rational Method and Time of Concentration

[RJSH]

I am evaluating an existing roadside ditch capacity for approx.2,100 ft length. The drainage area contributing to the ditch is a narrow section of roadside ditch area, and half of the pavement, so there is not much flow added. With the rational method, as the Time of Concentration increases, the rainfall intensity decreases, which is resulting very small flow increase, in some sections constant, and in some sections decrease. Although, it doesn't sound realistic in the practical world that the flow is not increasing downstream, the less increase in the flow or decrease is due to the time it takes for the upstream flow to outfall. What is your thoughts on appropriateness of the rational method for capacity evaluation of the ditch in may case.
Ditch Length 2,100 ft approx., Tc= 23 minutes, Total Contributing Drainage Area upto Outfall (along 2,100 ft of ditch) = 7.13 acres (310,830 sft)

 

[pinkpig]

drainage area bigger, the ToC will be greater and the Intensity will be smaller. The assumption here is a larger area is hard to get a uniformly distributed rainfall at a higher intensity.