Mixing, exchange, and residence times in estuaries with complex bathymetry

Abstract

Circulation in estuaries is complicated by irregular bathymetric features such as branching basins, constrictions, islands, river mouths distributed along the coastline, multiple entrances, and shallow sill areas. This dissertation examines the dynamics within complex fjord-type estuaries and their exchange with the coastal ocean. A combination of realistic and idealized numerical models is used to provide descriptive results and generalizable insights. In Chapter 2, output from a realistic numerical ocean model is used to study mixing in the Salish Sea. Salinity variance budgets are used to quantify the mixing in different regions within the estuary and to evaluate the importance of numerical mixing in the model. The timing of mixing in the Salish Sea is found to be tightly coupled to the river flow, and mixing hotspots are located at the river mouths. A simple approximation based on the river flow and the estuarine exchange flow salinities is shown to predict the total mixing in the Salish Sea and Puget Sound with good accuracy. Over a long time average, the exchange flow salt transport can also be used to estimate mixing in Puget Sound. In Chapters 3 and 4, an ensemble of idealized models is used to study the effects of varying sill length and its ratio to the tidal excursion. Five idealized estuary models are developed with geometry loosely based on Puget Sound and its entrance sill at Admiralty Inlet, with sill lengths ranging from 5 km to 80 km. Chapter 3 seeks to characterize the mechanisms driving the estuarine exchange flow in estuaries with long and short sills. Three different salt transport decompositions are applied at cross sections along the sill in the idealized estuary models. The behavior of salt transport over the spring-neap cycle is used to identify the dominant mechanism of exchange as either gravitational circulation or tidal pumping. At the ends of the sill, tidal pumping is found to be important regardless of sill length, whereas the middle of the sill develops a stronger gravitational circulation in the long sill models. A momentum balance analysis also supports these results. A transition from tidally-driven to gravitationally-driven exchange is shown to occur when the sill length exceeds the tidal excursion. Chapter 4 investigates the relationship between sill length and estuarine residence times. The idealized model ensemble is used to conduct particle tracking experiments and construct box models of different regions of the estuary. Both approaches are used to calculate transport time scales representing retention in the estuary, as well as reflux coefficients, which quantify subtidal recirculation on the sill. Longer sills are found to produce greater retention in the inner basin of the estuary due to increased reflux and reentry from the sill region. The fate of particles on the sill is shown to be linked to the tidal cycle and the ratio of the sill length to the tidal excursion.