forebay

A forebay is a basin or pool of water located upstream of an intake structure, reservoir, or water-treatment facility. Its primary purpose is to regulate flow, provide a buffer against fluctuations, and give debris time to settle before water enters a pump, turbine, or treatment process.

In hydroelectric installations, the forebay sits immediately upstream of the powerhouse and turbine intakes. It stabilizes water level, reduces surge and vibration, and accommodates intake screens and trash racks that remove debris and entrained air before water flows into the penstock.

Common components include trash racks or screens, sluice channels or grit pits for sediment removal, baffles or energy-dissipating walls, and air-release vents. The forebay may be open to the atmosphere or partially enclosed, and its size is matched to the plant’s flow and sediment loads.

Forebays are also used at irrigation and municipal water intakes to temper demand, trap sediment, and improve pump efficiency. In water-treatment facilities, forebay basins help with initial sediment removal and provide space for pre-oxidation or chemical addition equipment.

Maintenance involves periodic dredging to remove accumulated sediment, cleaning of screens, and inspection of gates and vents. Design considerations include location relative to the intake, available storage volume, expected debris loads, risk of flooding, and protection of fish passage.