A surge tank acts as an open hydraulic flywheel between a long low-pressure headrace tunnel and steep high-pressure penstocks. When turbines trip, it intercepts the moving water column, transforming high-speed destructive acoustic water hammer into slow, safe mass oscillations with a free water surface.

Under constant-power governor control, water level drawdowns induce governors to open turbine gates further, which can destabilize mass oscillations into continuous power hunting. The Thoma criterion defines the absolute minimum cross-sectional area (Athoma = [L * At] / [2 * g * Cf * Hnet]) required for oscillations to damp out naturally. Standards enforce a safety margin of 1.50 to 1.80.

Upsurge is calculated via Calame-Gaden and Jaeger formulations: Zmax = Z* * exp(-π * hf0 / [4 * Z*]) - hf0, where Z* = v0 * √(L * At / [g * As]) represents the theoretical undamped potential surge and hf0 accounts for tunnel friction dissipation.

When turbines ramp up rapidly from zero to full load, water is pulled from the surge tank faster than the headrace tunnel can accelerate. If the downsurge water level drops below the penstock intake portal, air is sucked into the penstock or column separation occurs, risking catastrophic inward vacuum buckling of the steel pipe.

A restricted orifice introduces a concentrated head loss throttle port at the base of the surge shaft. This creates an immediate counter-pressure head that decelerates the tunnel water column faster, reducing maximum upsurge and downsurge by 15% to 30% and significantly cutting construction shaft volume.