Abstract: A series of lock-exchange experiments are carried out to study effects of vegetation density and height for short vegetation canopies on gravity current motions. A high-speed camera is applied to acquire the evolution process of the gravity currents, and a particle image velocimetry system is used to investigate the microstructures of gravity currents. Experimental results show that when the gravity current flows through short vegetation canopies, the gravity currents profile will be in the shape of classic ellipse or a well-defined triangle. Movement process is divided into slumping phase and self-similar phase. Furthermore, vegetation canopies can significantly promote the transition of gravity currents from slumping phase into self-similar phase, but cannot influence the current front velocity during the slumping phase. As the non-dimensional vegetation height (the ratio of vegetation height to water depth) is 0.21 with the vegetation density being 18.0%, the gravity current flows along the upper edge of vegetation and within vegetation at the same time. Since the current density on the top of vegetation is larger than that within vegetation, the Rayleigh-Taylor instability occurs. In addition, the entrainment coefficient of gravity current within vegetation decreases as the current moves further, and it is smaller than that without vegetation. When the density current moves into the submerged vegetation, some current climbs on the top of the vegetation where a new “bottom boundary” for gravity current is formed and negative vorticity is produced. In the upper boundary of the current, the current keeps mixing with the ambient fluid and induces the positive vorticity. The results also suggest that vegetation reduces the current speed as well as the strength of positive vorticity, and the vegetation density is proportional to the reduction degree of the positive vorticity.

Key words: gravity currents; short vegetation canopies; submerged rigid vegetation; interface instability; entrainment coefficient