Role of structure submergence on scour evolution in gravel bed rivers: Application to slope-crested structures

This paper, written to celebrate the 60th anniversary of the Journal of Hydraulic Engineering, examines scour evolution around sloped-crest structures, known as barbs. The results extend the literature database of hydraulic structures and provide the first experimental investigation of gravel bed scour around a sloped-crest barb structure for flow and sediment conditions that are representative of gravel bed rivers in mountainous to submountainous topography. The postulate examined in this study is that the degree of barb submergence or overtopping ratio triggers different interactions between the primary vortex and overtopping flow regime, which in turn affects scour progression (i.e., fast and slow phases of scour) as well as the location of the maximum equilibrium scour depth around the barb tip. The results of this study provide a fundamental insight of the interaction of a plunging jet and the primary vortex through complementary laboratory and numerical investigation. As the overtopping flow becomes of similar influence to the primary vortex, traditional scour prediction formulas tend to break down due to the interaction of the plunging jet and primary vortex which induces an intermittent flow behavior, and, hence, scour equilibrium takes nearly twice longer to be attained. The results of this study also highlight the beneficial utility of barbs for river training applications. In this manner, the sloped-crest feature of the barb has beneficial effects on flow as it transfers kinetic energy from the main core and tip of the structure to turbulent energy generation and dissipation on the lee side of the structure. The net result provides a hydraulic structural design for the barb that reduces the need for excessive stability measures to withstand high flows because maximum fluid shear at the tip and, hence, maximum scour are reduced relative to unsubmerged structures.