Characteristics of flow fluctuations in a tide-dominated estuary: Application of triple decomposition technique

Abstract

Small tide dominated estuaries are affected by both large scale flow structures manifested as oscillatory residual currents and small scale bed generated turbulence. The present understanding of the effects and relative contributions of these factors are inadequate due to limited systematic observations. A field study was conducted in a small sub-tropical estuary in which high frequency (50 Hz) turbulence data were recorded continuously for about 48 h. A triple decomposition technique was introduced to isolate the contributions of tides, slow fluctuations and ‘true’ turbulence in the flow field. A striking feature of the data set was the slow fluctuations which exhibited large velocity amplitudes up to 50% of the tidal amplitude under neap tide conditions. The skewness of the fast ‘true’ turbulence were within the range of −4 and +2. The analysis of the fast fluctuations revealed that variation in the turbulence characteristics are linked with the tidal velocity suggesting that the dispersion and mixing are dependent on some large scale flow properties. The results also indicated enhanced mixing associated with the slow fluctuation in the flow field. The results provided useful sets of information about the properties of slow and fast fluctuating components and their variations through tidal cycles in a micro-tidal estuary.