Michael L. MacWilliams

Parameterization of Estuarine Mixing Processes in the San Francisco Estuary Based on Analysis of Three-Dimensional Hydrodynamic Simulations

The purpose of this analysis is to develop a dispersive mixing parameterization method and specific dispersion coefficients used to predict salinity in a one-dimensional tidally-averaged transport model. Longitudinal dispersive mixing is parameterized based on the analysis of three-dimensional simulations spanning a range of freshwater inflows in the San Francisco Estuary. The three-dimensional simulation results were analyzed at 28 cross-sections in the San Francisco Estuary to calculate salt fluxes, salinity gradients and other information required for the parameterization method. This method represents the spatial variability of dispersion and includes dependence on horizontal Richardson Number, Delta outflow and other environmental variability. These parameterizations are incorporated into a tidally-averaged one-dimensional model for predicting salinity in Central San Francisco Bay, San Pablo Bay, Suisun Bay and the Sacramento-San Joaquin Delta. This simplified model was developed for the Delta Risk Management Strategy (DRMS) project, funded by the California Department of Water Resources, in order to simulate thousands of levee failure scenarios. The dispersion coefficients are currently applied in the simplified model to represent all dispersive transport in most of the model domain. The resulting simplified model accurately predicts observed monthly-averaged salinity through the San Francisco Estuary for a 15 year simulation period.

An Overview of Multi-Dimensional Models of the Sacramento–San Joaquin Delta

Over the past 15 years, the development and application of multi-dimensional hydrodynamic models in San Francisco Bay and the Sacramento– San Joaquin Delta has transformed our ability to analyze and understand the underlying physics of the system. Initial applications of three-dimensional models focused primarily on salt intrusion, and provided a valuable resource for investigating how sea level rise and levee failures in the Delta could influence water quality in the Delta under future conditions. However, multi-dimensional models have also provided significant insights into some of the fundamental biological relationships that have shaped our thinking about the system by exploring the relationship among X2, flow, fish abundance, and the low salinity zone. Through the coupling of multi-dimensional models with wind wave and sediment transport models, it has been possible to move beyond salinity to understand how large-scale changes to the system are likely to affect sediment dynamics, and to assess the potential effects on species that rely on turbidity for habitat. Lastly, the coupling of multi-dimensional hydrodynamic models with particle tracking models has led to advances in our thinking about residence time, the retention of food organisms in the estuary, the effect of south Delta exports on larval entrainment, and the pathways and behaviors of salmonids that travel through the Delta. This paper provides an overview of these recent advances and how they have increased our understanding of the distribution and movement of fish and food organisms. The applications presented serve as a guide to the current state of the science of Delta modeling and provide examples of how we can use multi-dimensional models to predict how future Delta conditions will affect both fish and water supply.