Net drift in an atypical estuary, Long Island Sound
Long Island Sound is an estuary with two-layered flow at the eastern opening to the ocean and a salinity gradient of about 5 parts per thousand between the eastern and western extremes. Tidal currents, wind-driven circulation, and river inflow and fresh water influx are investigated as factors affecting the net drift. Current measurements in eastern Long Island Sound indicate that the tidal circulation features that predominate there do not normally affect the net drift at the New Haven dump site. Drifter returns and a numerical model indicate that seasonal prevailing winds seem to be the primary cause of the net drift in central Long Island Sound.
Circulation in central Long Island Sound
Current meter records from 28 stations are used to define the flow of water near the bottom of central Long Island Sound. Records were made at two of the stations for over 1 year and for 10 days or more at most of the others. Tidal and nontidal flow components are separated. Random fluctuations of up to 10 day's duration occur in the nontidal flow; they are not directly influenced by wind, rainfall, river runoff, or variations in sea level along the shore. Salinity observations show the presence of well-defined surface and bottom water layers. Mixing between these is confined to shore side zones where the water is less than 10 m deep and to shoals where strong turbulence is generated. The current meter data show the bottom water at depths greater than 20 m to be flowing upstream at a rate that decreases toward the head of the estuary. At depths less than 20 m there is a shoreward flow of bottom water toward the mixing zone. The salinity and current data are used to construct a circulation model for the sound. The large-scale flow is apparently due to gravitational convection associated with salinity differences. However, response to changes in the freshwater inflow is delayed by about 2 months because of the large volume of surface water relative to the freshwater supply.
Tools for Sediment Management in Rivers
Sediment management, including supply and continual redistribution of sediments for channel maintenance and channel forming, is critical for effective environmental water programs. Many tools have been developed to more effectively manage sediment in rivers for ecosystem maintenance and improvement. This chapter reviews some of these tools, including sediment mobilization theory, environmental water requirements for geomorphic maintenance, and complementary options. Sediment mobilization theory addresses entrainment, transport, and deposition of sediment particles. Environmental flows assessment approaches that support geomorphic processes include threshold-based approaches, field/laboratory methods, and holistic methods. Complementary options for sediment management include catchment-scale conceptual models and budgets for sediment, basin land management (e.g., reforestation, better agriculture practices, etc.), sediment management through dams, and sediment augmentation below dams. These complementary options often have cobenefits with other management objectives. The chapter emphasizes that sediment management is generally complex, such that continual research is needed, ranging from improved sediment mobilization models to large-scale river experiments.