Bed shear stress under skewed and asymmetric oscillatory flows

We present a new formulation to predict the bed shear stress under skewed/asymmetric oscillatory flows (with or without a co-linear mean current), extending the work of Nielsen (1992). The nonlinearity of the oscillatory flow is incorporated through the use of two parameters: the index of skewness or nonlinearity, and the waveform parameter. The new formulation is tested against the bed shear stress estimated from the log-fit and momentum-integral methods, using oscillatory data from oscillating water tunnel experiments. The new formulation and the momentum-integral method agree well, but differ from those with the log-fit method, possibly because both methodologies lead to different results for the phase lead between the bed shear stress and the free-stream velocity. The new bed shear stress formulation is incorporated in a quasi-steady bedload formula, and accurately reproduces net transport rates under non-linear, nonbreaking waves with and without an opposing current.