The Impact Of River Discharge, Tidal Range, Wind On River Plume Spreading And Tidal Average Mixing Parameterization

A river plume is complicate water mass caused by mixing between fresh riverwater and saline ocean water. Because very different water masses are collidingwithin an estuary, river plume can often be characterized by strong gradients, ofwhich the most dynamically important are density gradient and vertical and horizontalshear. The discharge of fresh river water represents a significant source of nutrients,sediments, pollutants and other terrestrially derived material to the world’s oceans.Strong gradients of dissolved and suspended constituents such as nutrients, pollutants,and suspended sediment are also typically present, and it is the ultimate disposition ofthese components that provides much of the motivation for understanding the localphysics. River plume may influence the costal environment in two ways: throughenhanced mixing and through spreading. Terrigenous matter are translated into theambient water together with river plume and mixing play an important role ofexchanging nutrient between upper layer and down layer. Spreading and mixing arethe most important aspects of river plume dynamics interacting with each other.Modern numerical models of buoyant flow on the continental shelf are not able toadequately resolve processes that occur within the supercritical outflow region,because of the complicate environments of estuary. Spreading and mixing aresignificantly affected by external forcing such as variation of river discharge,flood-ebb and spring-neap tide and also wind. The study on river plume spreadingmechanism and mixing parameterization cannot only provide open boundarycondition to numerical modeling but also gives better understanding of river plume dynamics. This thesis utilizes field data from the Merrimack River facing to the east, ahighly stratified system located in Newburyport, MA, USA, to investigate the natureof river plume spreading and mixing processes.Surface salinity and temperature data were measured from buoy together withADCP, and drifters were deployed to tracking the path of river plume spreading. Windeffect on mid-field are ignored because previous study consider mid-field arecontrolled by buoyant jet, wind can only play an important role in far-field plume. Bycombining the measurement data and wind data, the effect of wind on mid-fieldplume spreading dynamics is studied. ADCP results indicate wind is the main force todrive the mid-field plume with a correlation coefficient between wind speed andsurface velocity as high as0.8. By using T-S diagram, water mass analyses indicatethat upwelling favorable wind is the major force which balance the Coriolis force anddrives the plume spreading to north.20to45degree angles between wind directionand plume spreading direction are detected from the statistic of all the drifterdeployments, indicating that shallow water Ekman effect is the major mechanism howwind drives mid-field plume. Because the definition of mid-field is not clear before,this work indicates that2times friction depth can be used as a criterion to distinguishmid-field and far-field.Velocity profiles were measured by1200kHz moored Acoustic Doppler CurrentProfilers (ADCP) in lift-off zone during the spring freshets of2007,2010and2011. Inthis area, high stratification and strong velocity shears create a very active turbulentfield, which results in rapid mixing of the discharging water with ambient oceanwaters. Turbulent kinetic energy Production (TKE production) were calculated using“Variance method”. Shear and stratification within the plume can be thought of asnearly balanced processes competing to determine the overall plume structure whichmakes the Froude number always remains O(1). Significant variations in plumethickness and TKE production were observed from ebb to ebb, and positivecorrelations were noted with environmental variables, such as river flow, wind speed/direction, and tidal range. River discharge can not only provide buoyancy fluxto stratification, but also enhanced the shear in mix layer. Enhanced shear overcome sthe stratification indicating river flow is one of the TKE source. A hypothesis isproposed to explain the wind-induced mixing mechanism. Surface water istransported upstream casing a wind-induced shear through barotropic effect based onvolume conservation.In order to quantify TKE production by using environmental variables only, twoassumptions are proposed：1. Reduced gravity(stratification) doesn’t depend on thevertical-mixing coefficients and varies only with the river flow;2. Vertical shear arecomposed by a linear combination between river discharge, tidal range and windcaused wind. On the basis of these observations, we quantify the contribution ofthese forcing mechanisms to the observed TKE production using an empiricalapproach based on the marginal value of the discharge Froude number (which isscaled from the environmental variables) above a critical value of one. The resultingregression provides a means for estimating TKE production in the lift-off zone as afunction of only the environmental variables, and produces results consistent withprevious observations from other turbulence measurement techniques. Theregression also provides an indication of the relative importance of the variousforcing mechanisms, and suggests that east (onshore) winds and river discharge arethe most important factors in controlling the variance associated with TKEProduction, with tidal range of lesser significance.The research results outlined above mainly revealed effects of river discharge,tidal range and wind on the spreading and mixing processes in a highly stratified riverplume which provide some theoretical evidence to understand the observations andphysically plausible inter-tidal mixing parameterizations.