ABSTRACT
This chapter provides an overview of the methods that could be utilized for estimating the amount of water that needs to be drained out of a pavement structure. The design frequency and return period or recurrence interval are used to estimate the likelihood or probability of a storm with a specific Intensity and duration of rainfall. A unit hydrograph shows a plot of time versus runoff caused by a 1 inch deep water that is uniformly distributed over the whole drainage area for a given storm and duration of rainfall. Subsurface flow occurs when the infiltrated rainfall encounters a layer of lower hydraulic conductivity and travels laterally over the boundary of that layer, reappearing as seep or spring. On the hydrograph, channel runoff appears at the start of the storm, remains throughout the storm and varies with rainfall intensity.
