Estuarine mixing

Abstract

This review paper presents, explains and discusses major aspects of estuarine mixing which is defined as the destruction of salinity variance. Due to the large amounts of brackish water in estuaries produced by mixing of fresh river discharge and salty ocean water, mixing is one major characteristic of what is an estuary. In this review, mixing is quantified locally as well as on estuary-wide scales. Diagnostics of integrated mixing are given for estuarine volumes bounded by transects as well as isohalines (surfaces of constant salinity) moving with the flow. It is shown how entrainment across a moving isohaline surface depends on gradients of turbulent salt flux and mixing per salinity class. Various relations are derived that link estuarine salt mixing to other estuarine properties such as the freshwater discharge and the bulk estuarine circulation. For estuaries bounded towards the ocean by a fixed transect, the Knudsen mixing law is explained, where estuarine mixing is the product of the Knudsen salinities of inflowing and outflowing water masses and the river discharge. When the estuarine volume is bounded by a moving isohaline surface of salinity S, mixing inside the estuary is simply the product of S2 and the river discharge. Major processes that drive estuarine mixing are presented on various time scales (tidal, fortnightly, weather and discharge time scales) and spatial scales (channel-shoal interaction, mixing fronts). As underlying methods for the quantification of mixing, observational concepts, as well as numerical modelling methods such as consistent turbulence closure modelling and numerical mixing analyses are presented. As an outlook, some future perspectives are sketched. Many of the concepts presented in this review are illustrated using simulation results from a numerical model setup of the Elbe River estuary.