Spillway Discharge (Creager/Ogee)
Calculate the discharge over a Creager/ogee dam spillway, Q = C·L·H^1.5, from the discharge coefficient C (typically 2.0-2.2 in SI for well-designed ogee profiles), the crest length L (m) and the head over the crest H (m). The spillway is a dam's most critical safety structure: it releases floods safely, preventing overtopping — the leading cause of dam failure. The ogee profile follows the shape of the underside of a free nappe, maximizing discharge while keeping crest pressure near atmospheric (avoiding cavitation). The coefficient C absorbs gravity and approach effects, exceeding that of a sharp-crested weir. Spillway design starts from the design flood (often the 10,000-year flood or the PMF) and sets the required crest length. Enter the discharge coefficient, crest length and head.
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Spillway discharge (Creager/ogee)
The spillway is a dam's most important safety element: it is where floods are released in a controlled manner, preventing overtopping of the dam — the number-one cause of dam failure worldwide. The discharge over a Creager/ogee profile spillway is Q = C·L·H^1.5, where C is the discharge coefficient (2.0 to 2.2 in SI units), L the crest length, and H the head over the crest. The ogee profile matches the shape of the underside of a free-falling nappe: the water thus adheres to the crest without separating, maximizing the discharge and keeping the pressure close to atmospheric (which prevents cavitation, which would destroy the concrete). The coefficient C already embeds gravity and approach effects, which is why it exceeds that of a sharp-crested rectangular weir. Spillway sizing starts from the design flood — often the 10,000-year flood or the PMF (probable maximum flood, for large dams) — and computes the crest length needed to release that discharge without the reservoir level rising enough to overtop the dam crest. Erring on the low side is catastrophic; that is why spillways tend to be oversized. Enter the discharge coefficient, the crest length, and the hydraulic head.
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