English

Karla Gleichauf,Nancy Monsen,Philip Wolfram,Stanford University ,Oliver Fringer,Stephen Monismith,Los Alamos National Laboratory

doi:10.15447/sfews.2014v12iss4art1

Abstract

doi: http://dx.doi.org/10.15447/sfews.2014v12iss4art1 In branching channel networks, such as in the Sacramento–San Joaquin River Delta, junction flow dynamics contribute to dispersion of ecologically important entities such as fish, pollutants, nutrients, salt, sediment, and phytoplankton. Flow transport through a junction largely arises from velocity phasing in the form of divergent flow between junction channels for a portion of the tidal cycle. Field observations in the Georgiana Slough junction, which is composed of the North and South Mokelumne rivers, Georgiana Slough, and the Mokelumne River, show that flow phasing differences between these rivers arise from operational, riverine, and tidal forcing. A combination of Acoustic Doppler Current Profile (ADCP) boat transecting and moored ADCPs over a spring–neap tidal cycle (May to June 2012) monitored the variability of spatial and temporal velocity, respectively. Two complementary drifter studies enabled assessment of local transport through the junction to identify small-scale intrajunction dynamics. We supplemented field results with numerical simulations using the SUNTANS model to demonstrate the importance of phasing offsets for junction transport and dispersion. Different phasing of inflows to the junction resulted in scalar patchiness that is characteristic of MacVean and Stacey’s (2011) advective tidal trapping. Furthermore, we observed small-scale junction flow features including a recirculation zone and shear layer, which play an important role in intra-junction mixing over time scales shorter than the tidal cycle (i.e., super-tidal time scales). The study period spanned open- and closed-gate operations at the Delta Cross Channel. Synthesis of field observations and modeling efforts suggest that management operations related to the Delta Cross Channel can strongly affect transport in the Delta by modifying the relative contributions of tidal and riverine flows, thereby changing the junction flow phasing.

Highlights

Hydrodynamic processes in the Sacramento–San Joaquin River Delta affect ecosystem function through the transport of salt, sediments, heat, contaminants including selenium and ammonium, and, SAN FRANCISCO ESTUARY & WATERSHED SCIENCE in more complex ways involving behavior, organisms such as larval fish
The stations with the largest velocity change, which were likely affected by the Delta Cross Channel (DCC) closing, were Georgiana Slough (GS) and North Mokelumne (NMK), which is consistent with the general circulation pattern of Sacramento-derived water in the central Delta (Figure 3)
Drifter data provides a Lagrangian view of the circulation patterns within Georgiana Slough junction (GSJ) and identifies key junction flow features needed to develop a conceptual model of how mixing occurs in this junction

Summary

Introduction

Hydrodynamic processes in the Sacramento–San Joaquin River Delta affect ecosystem function through the transport of salt, sediments, heat, contaminants including selenium and ammonium, and, SAN FRANCISCO ESTUARY & WATERSHED SCIENCE in more complex ways involving behavior, organisms such as larval fish. This suggests that if junction dynamics are not resolved, as is the case for the DSM2 Particle Tracking Model (Kimmerer and Nobriga 2008), modeled dispersion may differ from the actual mixing processes that occur at junctions. DSM2 Particle Tracking assumes that particles at a junction are instantaneously and completely mixed, distributing particles out of the junction based on the fractional volume outflow. DSM2 resolves the tides, whereas other transport models may use tidally averaged flows In both tidal and tidally averaged one-dimensional models, longitudinal dispersion coefficients are needed to account for tidal and super-tidal processes, i.e. time scales smaller than a tidal cycle. Better representation of longitudinal dispersion at Delta junctions that account for complex physics within junctions should improve the predictive ability of less resolved hydrodynamic models

Methods

Results

Discussion

Conclusion