How does the rate of runoff change after development?

• A. Runoff rate decreases
• B. Runoff rate remains the same
• C. Runoff rate increases post-development due to impervious surfaces
• D. Runoff rate becomes negligible

Answer: (C)

When land is developed, most of the surface becomes impervious—roofs, pavement, compacted soils—so water cannot infiltrate or be stored as readily. That means a larger portion of the rainfall turns into surface runoff rather than seeping into the ground. Because this runoff has less time to infiltrate and is funneled quickly toward channels and storm sewers, the peak discharge during storms rises. In short, development tends to push runoff rate upward due to the increased impervious surface and reduced infiltration and storage. This is why engineers talk about higher runoff coefficients and the need for detention or other controls to manage the bigger, faster flows.