Which of the following is NOT a part of an extended detention basin?

• A. Inflow
• B. Sediment forebay
• C. Armored spillway
• D. Vegetated Buffer Zone

Answer: (D)

Extended detention basins are designed to hold runoff for an extended period to allow time for sediment and pollutants to settle and for the flow to be released in a controlled way. Key features include structures that manage how water enters the basin, how sediment is captured, and how water is released safely.

The inflow is the channel or structure that brings runoff into the basin, which is essential for directing water into the treatment area. A sediment forebay sits at the front of the basin to trap coarse sediments before they reach the main storage, making maintenance easier and helping the overall treatment efficiency. An armored spillway provides a robust, erosion-controlled path for water exiting the basin during high flows, protecting the embankment and downstream channels.

A vegetated Buffer Zone, while valuable as a landscape treatment to filter and slow runoff around the site, is not a structural component of the basin itself. It functions outside the basin to improve water quality before or after it enters the basin, but it isn’t part of the basin’s detention and outlet system.