Hydrology Essentials: Rational Method, Unit Hydrographs & Time of Concentration

In this lecture, we walk through three foundational hydrology problems that every civil and environmental engineering student needs to master. We start with the Rational Method, comparing pre- and post-development peak runoff for a shopping mall site in Pittsburgh to see firsthand how urbanization nearly doubles stormwater discharge. From there, we dive into Unit Hydrograph theory — deriving a 1-hour UHG from observed storm data, then applying convolution to predict runoff from both a uniform 3-hour storm and a non-uniform event with heavier front-loaded rainfall. We close with Time of Concentration, calculating travel times across sheet flow, shallow concentrated flow, and open channel flow to understand how watershed timing drives design decisions. By the end, you'll see how these three methods connect into a single workflow that underpins real-world stormwater design.