Tidal variations in suspended sediment concentration in the Ems estuary: origin and resulting sediment flux

Variations in suspended sediment concentration, having frequencies of M2 and higher, were examined using observations at four elevations and extending over a period of eight M2 tidal cycles. Most of the variance in the concentration records was at the M2, M4, M6, M8 and M10 frequencies. For the higher elevations the major contribution was at the M4 frequency. The energy at the M2 frequency was the result of the water columns passing the measurement station carrying larger volumes of sediment during ebb than during flood. Energy at the M4, M6, M8 and M10 frequencies was related to (1) corresponding energies in the time series of the vertical mixing strength (local current speed) and (2) nonlinear interactions between time dependent settling velocities and concentration and time dependent mixing strength and concentration. For periods smaller than 2 h, variations in concentration and mixing strength took on a stochastic nature. Disregarding the contribution of the mean concentration and velocity, at the lower elevations the major contribution to the net (over the eight tidal cycles) sediment flux was from the M2 variations in concentration and velocity. At the higher elevations, variations at the M2, M4, M6–M10 frequency contributed equally to the net sediment flux. High frequency (>0.5 h−1) fluctuations contributed little to the net flux.