Stormwater Fundamentals & Basic Hydrologic Calculations

Understanding How Rainfall Becomes Runoff, Why Development Changes Drainage, and How Stormwater Infrastructure Manages the Difference

Designing Resilient Communities Through Practical Stormwater Management

Every stormwater system begins with one simple question: What happens when it rains?

Module 4 introduces the fundamentals behind modern stormwater management, drainage design, and basic hydrologic calculations. The module explains how rainfall becomes runoff, why impervious surfaces change site hydrology, how engineers use rainfall data and design storms, and why drainage areas influence storm pipe sizing, detention storage, and water quality treatment.

Readers will learn how pervious and impervious surfaces affect runoff, how storm frequency and rainfall intensity influence design, how drainage areas are identified, and how the Rational Method helps explain the relationship between runoff coefficient, rainfall intensity, drainage area, and peak flow.

Using practical examples, Nashville rainfall data, and real-world development scenarios, the module connects stormwater theory to the infrastructure shown on construction plans: storm pipe, catch basins, drainage structures, detention systems, retention practices, water quality units, hydrodynamic separators, outlet controls, and emergency overflow routes.

The module also reinforces TerrainIQ’s practical stormwater review sequence: understand the site, understand the water, understand the runoff, understand the requirements, understand the infrastructure, and then understand the products.