Transverse Structure of Semi-diurnal Currents Over a Cross-section of the Merbok Estuary, Malaysia

Measurements are presented of the transverse distribution of longitudinal semi-diurnal currents over a 900 m-wide cross-section in the seaward reaches of the Merbok Estuary, Malaysia. Data from two experiments, one at four stations extending over 31 tidal cycles and the other at nine stations extending over one tidal cycle, showed significant and systematic variations in the speeds and phases of the depth-averaged semi-diurnal currents. A depth-averaged cross-estuary numerical tidal model was used to interpret these data. Larger frictional-drag coefficients led to somewhat greater damping of modelled currents in the shallower water. Smaller drag coefficients led to larger modelled phase variations across the estuary. Both modelled and measured longitudinal semi-diurnal current speeds were strongly dependent on local water depth. Current speeds in the deeper parts of the cross-section were significantly faster than those in the shallower parts. The modelled and measured longitudinal, semi-diurnal current phases were such that the times of maximum flood and ebb, and changes of flow direction, occurred earlier in the shallow waters near the shore, as predicted from theoretical considerations.