The effect of discharge, tides, and wind on lift-off turbulence

Data from three deployments of a 1200 kHz moored Acoustic Doppler Current Profiler (ADCP) were used to study the factors affecting turbulent kinetic energy (TKE) production in the lift-off zone of a mid-sized river plume (Merrimack River, Newburyport, MA) during the spring freshets of 2007, 2010 and 2011. TKE production was estimated from the ADCP data, during periods of minimal wave activity, using the variance method, with significant variability in plume thickness and TKE production observed between ebbs. Correlations with this observed variability and the primary environmental variables, such as river flow, wind speed/direction, and tidal range, were noted. On the basis of these observations, we quantify the contribution of these forcing mechanisms to the observed TKE production using an empirical approach based on the marginal value of the discharge Froude number (which is scaled from the environmental variables) above a critical value of one. The resulting regression provides a means for estimating TKE production in the lift-off zone as a function of only the environmental variables, and produces results consistent with previous observations from other turbulence measurement techniques in the Merrimack plume. The regression also provides an indication of the relative importance of the various forcing mechanisms, and suggests that onshore (east) winds and river discharge are the most important factors in controlling TKE Production in the Merrimack plume, with tidal range of lesser significance.