Experimental Investigation of Total Bottom Shear Stress for Oscillatory Flows Over Sand Ripples

Abstract

Shoaling waves often produce vortex ripples on a sandy seabed, which significantly increases the local bottom shear stress. To improve the quantitative understanding of this phenomenon, a full-scale experimental study is conducted using an oscillatory water tunnel. In the tests, sinusoidal oscillatory flows generate two-dimensional uniform sand ripples from a 9-m-long movable bed made of coarse sand. The total bottom shear stress is estimated from the change of water pressure for driving the oscillatory flow. Flow around individual ripples is also measured using particle image velocimetry. After correction for imperfect flow generation and facility vibration, the leading three odd harmonics of total bottom shear stress are kept in the final measurements. The first harmonic is the dominant one and is generally in phase with the free-stream velocity. Its amplitude decreases as ripples are washed-off by increasing flow intensity. The third and the fifth harmonics are about 10–20% of the first harmonic in amplitude. Their presence makes the intraperiod variation of total bottom shear stress quite wavy with three peaks within one half period. Flow measurements suggest that these peaks are closely related to coherent vortex motions. For two selected tests, total bottom shear stress is estimated from velocity measurements via a control volume analysis. The results are in good agreement with those from the pressure-based technique, which verifies both approaches. A new predictor for the equivalent sand grain roughness in Humbyrd's (2012, http://hdl.handle.net/1721.1/78234) formula for wave friction factor is calibrated based on the obtained measurements of maximum total bottom shear stress.