Research Areas > Wetlands > San Gabriel Estuary Modeling

Project: San Gabriel Estuary Modeling

Background and Objectives

Most water quality modeling begins with assessment of pollutant loading from watersheds. The output from these models is typically pollutant concentration and load at the bottom (outflow or mouth) of the system being modeled. However, to link watershed-based pollutant sources with receiving water effects, managers need to understand the effect of coastal estuaries and embayments on the fate of pollutants, since they may be deposited, transformed, degraded or re-suspended. Coastal estuaries and embayments represent an intermediate zone after runoff exits the watershed, but before it enters the receiving waters of interest (typically the nearshore ocean). Thus, estuary modeling is the next logical step in the development of an integrated set of management tools to support water quality management decisions in coastal regions. Ultimately, estuary models will provide the missing link between watershed models and receiving water models.

This project was the first in a series of studies to develop estuary models, using the San Gabriel River Estuary as a pilot. The goal of this effort was to capture the physics (hydrodynamics) of the system under dry weather conditions and to provide a foundation for future development of estuarine water quality models. The San Gabriel watershed is an appropriate pilot watershed for development of an estuary model for several reasons. More than twenty miles of the San Gabriel River, including the San Gabriel River Estuary (SGRE), are identified as impaired for water quality with respect to their designated beneficial uses and, consequently, have been added to the U.S. Environmental Protection Agency’s 303(d) list. There are five publicly owned treatment works (POTWs) and other discharges regulated by NPDES permits within the watershed that discharge treated effluent to the river, as well as two power generating stations (PGSs) that discharge directly to the estuary.

Status

This project was completed in 2007.

Methods

The hydrodynamic model Environmental Fluid Dynamics Code (EFDC), which can be used to simulate water movement and associated water quality, was selected to model the SGRE. Its governing hydrodynamic equations are three-dimensional (i.e., it addresses water movement up and down stream, vertically in the water column, and horizontally across the channel). The model balances water pressure while allowing water density and water surface elevation (WSE) to change with turbulence-averaged equations. It is a three-dimensional sigma-coordinate model, meaning that there are a constant number of layers throughout the model domain each with a specified percentage of total depth and thus, the thickness of those layers changes with WSE.

Data were collected to characterize the model domain, inputs, and the conditions within the estuary. Data sources included as-built drawings, discharge measurements, atmospheric measurements, WSE, and in situ salinity and temperature measurements. Data detailing velocities throughout the water column at two locations were also taken by the USGS. The calibrated and validated model was then used to investigate the residence time of a conservative tracer in the estuary under three scenarios. Additional sampling and hydrodynamic modeling will be needed to characterize the SGRE under storm flows. The model can then be further expanded by incorporating pollutant transport to better understand water quality, assist in planning activities, and aid in the design and implementation of a watershed monitoring and management program.

Findings

The hydrodynamic model accurately simulated patterns of water movement in the SGRE. However, model performance at a given location varied based on the availability and quality of input data for calibration. In general, more comprehensive temperature, salinity, and flow data was available for the lower estuary (associated with the PGS discharges) than for the upper estuary (associated with watershed discharge). Consequently, the model predictions more closely simulated measured values in the lower portion of the estuary.

The velocity data collected by the USGS showed that water consistently flowed seaward downstream of the PGSs throughout the tidal cycle. While that data wasn’t sufficient for calibration/validation comparisons, the USGS estimated an average seaward flow of 22 cubic meters per second. The results from the residence time simulations show that a parcel of water entering the SGRE remains there for about 10 to 20 hours. Once the water mass reaches the location of the PGS discharges, it is advected out of the SGRE more quickly. Thus, dissolved pollutants that enter the estuary from the watershed generally would be flushed to sea within a day. Particulate pollutants were more likely to be deposited upstream of the PGSs because of the lower bi-directional velocities. 

Partners

Model calibration data was collected in partnership with the US Geological Survey (USGS), Menlo Park.