Core Concepts
Hydrologic Cycle
The continuous movement of water: evaporation → condensation → precipitation → infiltration → runoff → back to evaporation.
Rational Method
Q = CiA, where Q = peak runoff, C = runoff coefficient, i = rainfall intensity, A = drainage area.
Aquifer
An underground layer of water-bearing permeable rock. Confined aquifers have an impermeable layer above; unconfined do not.
Design Storm
A rainfall event used as a basis for hydraulic design, typically expressed as a return period (10-year, 50-year, 100-year).
Worked Examples & Applications
Rational Method Example: Drainage area A = 50 acres. Runoff coefficient C = 0.4 (suburban). Rainfall intensity i = 3 in/hr. Q = 0.4 × 3 × 50 = 60 cfs (cubic feet per second).
Groundwater Flow (Darcy's Law): q = -K × dh/dl, where q = specific discharge, K = hydraulic conductivity, dh/dl = hydraulic gradient. Example: K = 10 m/day, gradient = 0.01 → q = 10 × 0.01 = 0.1 m/day.
Practice Questions
Q: What is the Rational Method? A: Q = CiA — an empirical formula for estimating peak runoff from a drainage area.
Q: What is the difference between confined and unconfined aquifers? A: Confined aquifers have an impermeable layer (aquitard) above them, creating artesian pressure. Unconfined aquifers are directly connected to the surface water table.