Effects of varying pycnocline thickness on interfacial wave generation and propagation

Interfacial solitary waves (ISWs) can be observed in stratified ocean and laboratory flumes which generally comprise a pycnocline in finite thickness. This has led to the adoption of a two-layer or three-layer fluid system for studying the ISWs. However, this assumption has limited success for understanding the effects of variable pycnocline thicknesses on the wave generation and propagation. This paper discusses the broadening of pycnocline thickness on the generation of an ISW in a wave flume and its subsequent effects on wave amplitude and potential energy across a submerged trapezoidal obstacle under laboratory conditions. Three different definitions for the nominal pycnocline thickness are discussed, based on the measured fluid density profiles and the calculated buoyant frequency profiles. As the pycnocline thickness increased after repeated wave-makings, the incident ISW amplitude decreased as expected, as well as the wave speed across the obstacle. Moreover, as the thickness broadened, the fluid system appeared to contain two or multiple peaks rather than single peak when the pycnocline was thin and finite. This implies that the resultant wave might consist of several modes (at least mode-2) in the flume even at its generation stage. •ISW amplitude decreases at generation as the pycnocline thickness increases.•More upward spreading of heavier fluid is observed as pycnocline broadened.•A conservative approach for defining the nominal pycnocline thickness is suggested.•Multilayers and mode‐2 waves were detected in wave flume as pycnocline broadened.•Wave transmission over a trapezoidal obstacle is analyzed as pycnocline broadened.