Broad-Crested Weir Operation Upstream of a Steep Stepped Spillway

Gangfu Zhang, Hubert Chanson

Monday 29 june 2015

14:05 - 14:20h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 2F. Engineering - Instrumentation

Basic experiments were conducted on a large-size broad-crested weir with an upstream rounded corner located upstream of 1V:1H stepped spillway. Detailed free-surface, velocity and pressure measurements were performed for a range of flow conditions. In presence of a downstream sharp edge, flow deflection was observed for a range of discharges. The jet deflection was suppressed by installing a downstream rounded nose. The overflowing nappe adhered to the rounded nose because the convex wall curvature imposed a pressure field modification within the nappe inducing a suction pressure. The resulting Coanda effect acted on the rounding surface in a direction normal to the flow direction. On the broad-crest, the experimental data indicated the rapid flow distribution at both upstream and downstream ends of the weir. The data were analysed taking into account the non-uniform velocity and non-hydrostatic pressure distributions. Critical flow conditions were achieved above the weir crest for 0.1 < x/Lcrest < 0.9, even when the pressure distributions were not hydrostatic. On the steep stepped spillways, detailed air-water flow measurements showed the strong flow aeration. The void fraction profiles presented an inverted S-shape and the bubble count rate data indicated a maximum in bubble count rate increasing with increasing distance for the given flow rate. The interfacial velocity distributions followed closely a power law downstream of the inception point of free-surface aeration. Altogether the air-water flow properties were comparable to experimental measurements downstream of ogee crest spillway.