Dispersion in bar-built estuaries

Transport and dispersion in estuaries influence movement of contaminants, nutrients, and suspended sediments in estuaries. Dispersion studies have traditionally focused on large estuaries, but on coastlines with Mediterranean climates, small, bar-built estuaries are ubiquitous. These systems are important habitat for aquatic flora and fauna, and are vulnerable to contamination (e.g. by mining heavy metals in the case of Chilean estuaries). Characterized by narrow (and sometimes closed) inlets, tidal velocities in bar-built estuaries are small, though high velocities associated with nearshore infragravity motions have been observed. Limited vertical mixing allows the maintenance of strong salinity stratification. Using salinity as a conservative tracer, velocity and salinity measurements are used to calculate estuarine dispersion. Here, we present dispersion measurements from two estuaries: the Pescadero estuary in Central California (37.2N) and the Maipo estuary in Central Chile (33.6S). Measurements in the Pescadero estuary show longitudinal dispersion to vary with salinity, with low dispersion at high salinity, and much higher dispersion at low salinity. We attribute this variability to topographic trapping of saltwater and vertical mixing as trapped water is removed on ebb. Variability in the transition between the two values of K_x changes with the spring-neap cycle as higher salinity water is pushed further upstream on spring tides. These results will be compared with recent data collected in Chile to move toward a general understanding of the dominant processes controlling dispersion in bar-built estuaries.