Bottom friction effects in the combined flow field of random waves and currents

Abstract

Bottom friction effects are quantified for the combined flow field comprising random waves propagating in the presence of currents. From the available methods of analysis for the monochromatic wave case a suitable theoretical model is first chosen using as criteria ease of implementation, rigorous derivation, and satisfactory description of the physical mechanism of the interaction. This model is then extended in order to deal with random waves and currents. Suitable formulae for the bed shear stress are first developed for the extreme cases when the wave field is much stronger than the current and vice versa, and then a derivation procedure for the general case is described, including a suitable parameterization which significantly reduces the required computational load. The parameterisation for the general case is compared with results from the expressions for the two extreme cases and very good agreement is obtained. Comparisons are also made with results using simple representations of a random wave field in terms of average quantities like the significant or root mean square waveheight and it is shown that such representations which are based on monochromatic wave ideas overpredict the corresponding coefficients. Finally results from the full parameterized solution for the general case of random waves propagating in the presence of currents are compared with two sets of full scale data and good agreement is obtained. It is shown that the calculation formulae developed and presented in this work offer reliable means of treating the complex interaction taking place at the sea bed between random waves and currents.