Abstract

Underway profiles of current velocity were combined with stationary profiles of temperature and salinity around a vertically mixed scour pit of the Seto Inland Sea throughout a semidiurnal tidal cycle. This was done with the purpose of determining (a) whether flood flow is asymmetric relative to ebb over a pit with weakly stratified conditions, and (b) whether there is a dynamic transition from frictionally dominated tidal flow to advectively dominated tidal flow over the pit. These questions arose from previous studies elsewhere under stratified water columns, in contrast to the unstratified conditions at the study site. Observations showed an acceleration of the flood tidal flow over the pit and a deceleration during ebb. The flow acceleration over the pit during flood and deceleration during ebb was attributed to asymmetric patterns of flow convergence/divergence. In turn, these divergence patterns were influenced by the direction and strength of the baroclinic pressure gradient force, which was 10 to 30% of the advective term, despite the relatively weak horizontal gradients of order 10−5 kg/m4. The non-negligible influence of the baroclinic pressure gradient was possible from the relatively large depths that exceeded 100 m at the deepest part of the pit, compared to the surrounding depths of 30 m. From depth-averaged dynamical terms derived for flood and ebb phases of the tidal cycle, it was found that the advective terms became more important than frictional terms over the deep parts of the pit. Advection became more prominent than friction where the bottom slope exceeded the value of the bottom drag coefficient (∼0.003). Otherwise, frictional effects dominated outside the pit.

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