Flow regimes and turbulence downstream of broad-crested weirs

Broad-crested weirs are widely used, but the downstream flow regimes under varying headwater and tailwater submergence are still not well characterized. This study experimentally investigates the transition of flow regimes in the tailwater channel under the combined influence of upstream and downstream submergence. A comprehensive set of measurements was obtained using an acoustic Doppler velocimeter to capture instantaneous velocities and a propeller anemometer to measure near-surface streamwise velocity, together with point-gauge measurements of free surface elevation. Four distinct regimes: impinging jet (IJET), surface jump (SJMP), stationary nonbreaking wave (SNW), and surface jet (SJET) are identified. A key contribution of this work is the identification and characterization of bistability between IJET and the surface regimes. The flow transitions from SNW to SJET at downstream tailwater depth t ≈ 0.84h, where h is the upstream head; the weir height H does not influence the transition. At lower tailwater depths, the regime transitions and the boundaries of the bistable zone depend not only on t and h but also on H. The study further quantifies the strength of the jump in the SJMP regime, along with the different characteristics of waves in the SNW regime. The hysteresis effect weakens at increasing headwater submergence h/H. At the bistable zone boundaries, the inclination angle of the IJET relative to the horizontal decreases exponentially with h/H. Detailed distributions of velocity show the highest near-surface velocity near the jump toe in the SJMP regime, reaching up to 1.8 times the critical velocity on the weir crest.