Micro-pumps were used to record suspended sediment concentration very near the seabed under irregular, unbroken waves at 4 sites with mixed sand/mud seabeds in Poverty Bay, New Zealand. The 4 sites were chosen to have successively higher mud content in depths of 4, 6, 8 and 10 m. Samples were taken over a 1-minute period using an array of 5 inline impeller pumps at elevations of 0.02, 0.04, 0.08, 0.16 and 0.32 m. RMS orbital velocities ranged from 0.21 to 0.54 ms(-1) while residual currents were 0.00 to 0.05 ms(-1). Surficial sediments have a single modal peak in the sand size fraction, with fines tailing off, but the suspended sediment concentrations display a bi-modal grain size distribution with fine sand and mud modal peaks. Both peaks become finer and more poorly sorted with elevation above the bed, and the 1st modal peak changes from fine sand size to mud at an elevation dependent on the strength of the near-bed orbital motion. The paper considers the grain size, concentration profile and eddy diffusivities / mixing lengths as a function of elevation. All suspended sediment concentration profiles display a distinct concave-up profile shape in log-linear space, both for the full grain size distribution and when each size class in the grain size population is considered individually. Piece-wise analysis of vertically consecutive suspended sediment concentrations show that the mixing length profile has a gradient close to the value of the von Kaman constant (kappa = 0.4). The ratio of the sediment diffusivity derived from the concentration profile to eddy viscosity derived from the wave-induced friction velocity (beta = epsilon(s)/epsilon(f)) is found to be less than 1.0, in keeping with prior studies. However, a new function is presented for beta where, beta = -0.95 - 0.40 psi/ln psi and psi = w(f)/U-*, while w(f) is the sediment fall velocity and U-* is the friction velocity. The least scatter occurred when U-* was derived from U-max; the maximum orbital velocity in the 1-minute measurement period.
