Turbidity maxima

The Hudson River Estuary appears to have two turbidity maxima formed by different mechanisms. Oneis associated with the landwardlimit of sea salt. Itis apparently formed by the estuarine circulation,

a AXIAL DISTANCE (Km)

b AXIAL DISTANCE (Km)

Figure 4.2. Near-synoptic, axial sections of salinity (a) and suspended sediment concentrations (b; 27 May, 1981).

b AXIAL DISTANCE (Km)

Figure 4.2. Near-synoptic, axial sections of salinity (a) and suspended sediment concentrations (b; 27 May, 1981).

although its location may be modified by bathy-metric influence in the deepest parts of the estuary (the gorge). The second is formed at mid-estuary by the tidally modulated and geomorphically controlled formation and migration of salt fronts into the estuary. Secondary, mid-estuary turbidity maxima are sometimes seen, but these may be residuals from the previous tide.

Evidence for a turbidity maximum in a relatively limited reach of the estuary along the Manhattan shore below the George Washington Bridge was documented in a series of vertical distributions of water temperature, salinity, and suspended sediment concentrations measured along the axis of the Hudson River Estuary nine times between

November 1980 and September 1981 (Hirschberg and Bokuniewicz, 1991). The average salinity section and the average section of suspended sediment concentration are shown in Figure 4.1. The observations did not extend to the limit of sea salt, but a strong turbidity maximum was found at the estuary floor between 79th Street and the Spuyten Duyvil (approximately at the position of Grant's Tomb at 122nd Street). Suspended sediment concentrations reached levels over 100 mg L-1 and the highest recorded value was 447 mg L-1. This turbidity maximum, however, was not present in all the individual transects. On 27 May 1981, a nearly synoptic section was done from a helicopter (Fig. 4.2). Although there was well-developed

Figure 4.3. Axial section of the suspended sediment concentration showing two, mid-estuary turbidity maxima (30 April, 1981).

salinity stratification, no strong turbidity maximum was found. At another time (30 April 1981; Fig. 4.3), two turbidity maxima were found. This region of elevated turbidity was not associated with a local permanent or quasi-permanent salt front as one might expect in light of the conventional wisdom concerning the formation of estuarine turbidity maxima.

The turbidity maximum (Fig. 4.1) was located in the vicinity of the average position of the strong salinity gradients. Observations of this turbidity maximum showed near-bottom suspended sediment concentrations of 100 to 200 mg L-1 in the summer of 1992 increasing to between 100 to 400 mg L-1 during high discharge in 1993 (Geyer, 1995), although maximum concentrations reached 800 mg L-1 (Geyer, 1995). In the turbidity maximum, the concentration of the finest grained particles (less than 4.65 ^min diameter) increases about 50 percent over ambient levels to where it comprises 55 percent to 60 percent of the suspended load (Menon et al., 1998).

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