Impacts of Currents and Waves on Bottom Drag Coefficient in the East China Shelf Seas.

High‐frequency measurements of waves, currents, and turbulence were made using the bottom‐mounted tripod equipped with the Nortek 6‐MHz acoustic Doppler velocimetry at eight mooring stations in the East China Shelf Seas. The observational data are analyzed to estimate the bottom drag coefficient and also the contributions from currents and waves. Variations of bottom drag coefficient caused by currents show no obvious relationship with water depth. Generally, the current‐induced drag coefficient decreases with the strengthening currents, and the wave‐induced drag coefficient increases with the enhancing waves. Synthesis analyses of the observational data reveal a scale relationship between the current‐induced drag coefficient and the turbulent Reynolds number and that between the wave‐induced drag coefficient and the bottom wave orbital velocity. The analyses show no significant impacts of waves on turbulent Reynolds stress, and the total bottom drag coefficient is the sum of that due to currents and waves. The empirical formula for bottom drag coefficient derived from this study may help to improve the simulation of tides, storm surges, and sediment transport in coastal seas. Plain Language Summary: Ocean floor exerts a frictional force on ocean currents, and this force is particularly important in coastal oceans. This force is difficult to measure directly and is usually parameterized using the near‐bottom currents and a bottom drag coefficient. In this study, we derive estimates of bottom drag coefficient from observed currents and turbulence at eight mooring stations in the East China Shelf Seas. The estimated bottom drag coefficient is not a constant. Empirical relationships are established to link the impacts of currents and waves on bottom drag coefficient to separate dynamic parameters, that is, the turbulent Reynolds number and the bottom wave orbital velocity. The total bottom drag coefficient can be taken as the sum of the two components. The empirical formula for bottom drag coefficient derived from this study may help to improve the simulation of tides, storm surges, and sediment transport in coastal seas. Key Points: Near‐bottom currents, waves, and turbulence were measured at eight mooring stations in the East China Shelf SeasThe estimated bottom drag coefficient shows significant variationsScale relationships are established to represent the impacts of currents and waves on bottom drag [ABSTRACT FROM AUTHOR]