SEDIMENT TRANSPORT AND RIPPLES DUB TO WAVES AND CURRENTS

Abstract

Water velocities and shear stresses have been determined for a laminar boundary layer of a progressive wave travelling over a regular series of ripples. The Lavrentiev variational method was used to transform conformally the water area with ripples into a strip with flat bottom, while the Lin approach permitted solution of the boundary layer equation. The theoretical prediction of the bed friction was verified experimentally with a new mechanical apparatus. By coupling the theoretical shear stress at the rippled bed with laboratory data for ripple parameters one can expose the friction conditions that control the growth and decay of ripples. If waves develop higher values of shear stress, the rippled bed becomes gradually washed out. Eor known shear stresses, basing on the Erijlink-Bijker formula one can compute sediment transport rates. In the respective diagram, a curve of s.t. rate versus bottom friction consists of two branches. The stages of the growth and decay of ripples are reflected in the lower and upper branches of the curve. For identical ripple height there are two values of s.t. rate , for two different wave intensities, likely to differ by as much as 25 per cent. Three-dimensional ripples have been analyzed with regard to bed friction and compared with two-dimensional conditions.