If I remember correctly, the Time of Concentration is the time it takes for a unit of runoff water to travel from the farthest reach in a drainage area to the outfall of the drainage area. So, as that time of concentration increases for particular drainage area and storm, the peak flow goes down. There is the same volume of runoff flowing slower through the drainage area, therefore the peak flow is lower.
Please refer to any hydrology text to confirm this.
If you examine the relationship between time and intensity for any given storm event, you will note that intensity decreases with time. Remember that the Rational Method assumes that peak flow occurs at the time of concentration when the entire watershed is contributing flow at the outlet. In order to determine peak flow, the intensity is determined by examining the intensity-duration-frequency graph. The rainfall intensity is read at the intersection of the time of concentration and the corresponding curve for the design frequency being examined. Since intensity is time dependent, most reviewing engineers will closely examine your calculation of time of concentration.
