Hydraulic Characteristics of Undular Hydraulic Jumps Over Different Bed Roughness

Abstract

AbstractOccurrence of undular hydraulic jumps has impact on sediment transport and bank erosion in estuarine areas. The hydraulic properties vary when it forms on a rough bed, leading to modification of turbulent mixing processes. This paper reports on a study of undular hydraulic jumps generated with different types of bed roughness including smooth, rough rubber-matted and grated beds. The experiment was conducted with a flow rate of 241 m3/h using acoustic displacement meters (ADMs) and an acoustic Doppler velocimeter (ADV). The obtained results reveal a discernible descending order in the undulations of the first three waves: smooth, grated, and rough rubber mat. In instances where roughness deviates to either extreme—being excessively small or large—the undulation of the first three waves intensifies. Through both free surface wave and turbulent flow field evolution experiments, it was determined that the roughness of the bed primarily influences the first wave, exerting the most pronounced impact on the crest of the initial wave. As the wave progresses, this influence gradually diminishes. Therefore, this article further posits that heightened bed roughness corresponds to an increased fluctuation in velocity at the bottom of the first peak, resulting in a weakened impact of toe jump oscillation on the flow field. Consequently, this diminishes the likelihood of negative velocity occurrences.