Science activity

The research we are proposing here is focused on developing a thorough, mechanistic understanding of how rafting vegetation, such as hyacinths or egeria, is transported in the Sacramento-San Joaquin Delta. Our approach is to examine in detail the forces that act on rafts of vegetation, and the resulting raft accelerations, to establish a predictive model of raft pathlines. Our model development will be built around a series of field experiments that include measurements of raft movement using GPS-logging drifters integrated into rafts, tidal and wind-forcing using a boat mounted current profiler and an anemometer, and direct estimation of the water-induced shear stress using a point velocity meter incorporated into the actual rafts. These field observations will be used to critically evaluate a numerical model of both channel (tidal) flows and resulting raft movement. Our initial development will include a highly-resolved channel flow model, which will explicitly capture more lateral variability, including low velocity side “pockets”, than is typically resolved with Delta scale hydrodynamic models. Initially, this will allow us to carefully evaluate the quality of our raft tracking calculations. Once the approach is established to be accurate, however, these high-resolution flows will be used to numerically calculate the effective advection and dispersion of rafts in the Delta channel under consideration. This analysis will be focused on parameterizing the effects on raft transport of structures and processes that are unresolved in typical Delta hydrodynamics models. An example of a process that is likely to be important to parameterize is the trapping and retention of rafts along the perimeter of channels due to off-axis wind forcing, and the resulting along-channel dispersion of rafts. In order to examine the effective advection and dispersion of rafts in Delta channels, we propose to pursue this combination of field and numerical studies of raft transport in locations of increasing complexity: first in idealized, straight channels, then in a natural, sinuous channel and a channel junction, and finally throughout the entire Delta. Our research is strongly motivated by the desire to provide a predictive model of dispersion in the Delta for floating objects that respond to both wind and tidal forcing. Immediate applications involve the movement of hyacinth rafts and egeria to evaluate potential management strategies. Important future applications are likely to include consideration of other biological invasions, due to the potential for rafts to provide a transport pathway, and analysis of the movement of accidental or intentional releases of floating material in the Delta.