Long Island Sound currents

Fig. iO. Velocity vectors recorded for successive 20-min time intervals by a current meter 2 m above the bottom near the geometrical center of Long Island Sound. The rotary character of the tide and the net drift of bottom water due to the estuarine circulation are shown. 

Le Lecheur, E. A., and Sammons, J. C. (1932). Tides and currents in Long Island Sound and Block Island Sound. U. S. Coast Geodetic Surv.. Spec. Publ. 174. 

We have previously reported that, although most storms do not alter the bottom-water resultant flow vectors, the probability of large water speeds over the bottom is increased (Bokuniewicz et ai, 1975b) when the wind stress is high. We now present further evidence on this effect. A periodogram calculated for a current meter record obtained in the western end of Long Island Sound is presented in Fig. 6. Three tidal peaks are... 

Fig 6. Periodogram calculated from a current meter record made in western Long Island Sound (7.4 km north of Eatons Neck) showing the dominant semidiurnal tide and two shallow water constituents (CPH, cycles/hr). 

Fio 9. Square of the wind speed (H histogram) and mean square fluctuating velocity (u ) measured 2 m above the bottom near the center of Long Island Sound. The wind-speed histogram is shifted to lead the current by 3 hr. 

Waves cause significant water velocities over the bottom of Long Island Sound within a wave-affected zone that is confined to a shoreside area where the water depth is less than 18 m. Wind-driven currents occur only in the upper third of the water column throughout the deep water of the Sound. The tide is the dominant source of power for bottom pro-... 

Since a layer of unbound particles is almost always present on the muddy bottom of the Sound, and the formation of the permanent sediment (i.e., the sediment not subject to excitation by water movements) occurs below this layer, it follows that the detailed mechanics and local patterns of mud transport by currents in Long Island Sound have no direct bearing on the sedimentation process. In dealing with sediment formation it suffices to know that mud-size minerals are everywhere supplied faster than they can be incorporated into the permanent bottom sediment. 

The specific dissipation due to wave power is strongly dependent on water depth and, therefore, will have sharply defined bounds in most estuaries. It Is determined by the depth, the available fetch, and the intensity of the winds having sufficient duration to raise a fully developed sea. For Long Island Sound the wave-dominated zone is that in water shallower than 18 m this constitutes 54% of the total area of the Sound. Within the wave-dominated zone the particle motion due to waves at the water surface is more effective in exciting sediment from the bottom than other causes of water movement. Large quantities of sediment may be set in motion by the waves and relatively small currents can then effect substantial transport of the material so excited. An example of an estuary in which wave-excited sediment is an important fraction of the total sediment available for estuarine processes is the Tay, where wave erosion followed by overland flow on bare mudflats exposed on the ebb of the tide results in large sediment concentrations in the water of the estuary (Buller et al., 1975). 

The net westward sand flux has been attributed to the superposition of an estuarine circulation on the strong tidal currents. The way in which this situation results in the net, one-way transport of sand may be made more clear with the aid of a simple representation of the transporting currents and the resulting sediment flux. The currents in Long Island Sound may be decomposed into three components, the long-term mean flow component, the periodic, tidal component, and random variations in the flow. Over short times the bottom currents may be approximated well as a one-dimensional flow in the east-west direction with the tidal component represented by a simple sinusoid so that... 

In deriving this expression, was assumed to be symmetrically distributed about zero and to be statistically independent of the phase of the tide. For the currents near the floor of Long Island Sound, these characteristics of u have been demonstrated with field data (Bokuniewicz et al., 1975). The first term of (4.12) dominates the sum, and the relative magnitudes of the respective terms are 10 1 1. As a result, the net sand flux is approximately proportional to 1.5 uul. The direction of the average sand flux is controlled by the sign of the mean velocity although the magnitude of the sand flux is dominated by the amplitude of the tidal velocity. 

Fig. I. Map of Long Island and Block Island Sounds showing the locationsof tide gauges,anemometers,and current meters. The tide gauges were at New London (NL), New Haven (NH), Bridgeport (Bpt), Port Jefferson (PJ), New Rochelle (NR), Montauk (M), and Sandy Hook (SH). Newport (Np) is 67 km east of NL and not shown on the map. Current meters were operated at locations J, D, S, X, and Y a water level recorder was also operated at J. Stratford Point is St. Anemometer locations are shown by open circles. Power calculations are done for the section between A and B. 

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