Overflow of Dual Parapet Walls on Dam Crests

To facilitate service vehicle access, dam and weir crests and spillways are often constructed with a flat crest. Parapet walls are often included on the upstream and downstream sides for safety. In such cases, it is sometimes assumed that the resulting structure performs like a broad-crested weir, as though the fluid between the parapet walls is stagnant and has the same effect on the overflow as a solid crest. Physical measurements were undertaken to test the effects of dual-parapet walls with three configurations: a reference flat weir without parapet wall and two configurations with dual-parapet walls. The experiments showed some markedly different flow patterns above the weir crest in presence of parapet walls. The water trapped between the parapet walls behaved as a recirculating pool, and this affected the flow over the weir and the head–discharge relationship. With increasing relative discharges, the flow pattern above the weir crest evolved from plunge flow over the first parapet wall to undular, then breaking, and later deflected, before achieving the flow characteristics of a thick-crested weir at the largest discharges. Quantitative measurements also highlighted some large energy dissipation. The discharge coefficient was smaller in presence of parapet walls, with a reduction of 7% to 12% in the discharge capacity compared with that of a broad-crested weir.