From Developing to Developed Phase in the Scour Evolution Due to Vertical and Sub-Vertical Plunging Jets: New Experiments and Theory

Previous studies have analyzed the evolution of the scour process without furnishing clear insights on how the scour pothole proceeds before reaching the equilibrium configuration. Apparently, there is only one study that addressed the scour evolution for sub-vertical plunging jets, identifying two different phases: 1) the developing phase, occurring at the very first instants of the scour process, when the jet entering the water body impacts on the granular bed material and starts removing sediment and form the downstream dune; 2) the developed phase, when the scour hole and dune evolution are characterized by a proportional (homothetic) expansion of the main geometric lengths, and the scour origin does not shift significantly. During the developing phase, the flow structure is essentially three-dimensional and the role of the momentum horizontal component is very significant. During the developed phase, the flow structure within the scour hole can be either quasi-cylindrical or fully three-dimensional, according to the resulting scour pothole shape and channel geometry. This paper focuses on the analysis of the transition from developing to developed phase. The results of another study of the same authors were further analyzed, and new tests were developed by assessing the scour evolution due to vertical plunging jets. Experimental evidence showed that the jet inclination plays a prominent role on the transition between the two phases and provided further insights on the quantitative definition of the non-dimensional time in which the transition occurs. The work is completed with a theory which elegantly predicts the time evolution of the scour process.