Discussion Mechanism for a Mid Estuary Turbidity Maximum

The reach of the estuary in the vicinity of the George Washington Bridge is characterized by large fractional changes in channel cross section area, in maximum channel depth, and in width. The channel cross-sectional area varies from approximately

II,000 m2 to 177,000 m2 (R. Wilson, 1999 Marine Sciences Research Center, personal communication). Preliminary observation in 1992, using a 200 KHZ echo sounder to visualize the halo-cline, and an AMS CTD showed the existence of large, quasi-stationary undulations in the halo-cline during maximum ebb as well as bottom salinity fronts situated in the vicinity of chan-nelexpansions. Hydraulicallyinfluenced, intratidal

Figure 4.4. Sandwaves as shown by a multi-beam survey in the vicinity of Saugerties. Bedforms in the left-hand (west) side appear to be migrating up-estuary while those in the right hand (east) side are migrating down-estuary (courtesy of R. Flood and V. Ferrini, Marine Sciences Research Center, Stony Brook University, Stony Brook, NY).

Figure 4.4. Sandwaves as shown by a multi-beam survey in the vicinity of Saugerties. Bedforms in the left-hand (west) side appear to be migrating up-estuary while those in the right hand (east) side are migrating down-estuary (courtesy of R. Flood and V. Ferrini, Marine Sciences Research Center, Stony Brook University, Stony Brook, NY).

halocline behavior could lead to advection of bottom fronts, which would influence particles trapping the area.

Evidence suggests the following mechanism for the formation of the mid-estuarine turbidity maximum (Bokuniewicz and Ullman, 1995). During an ebbing tide, a salt front is found downstream of the George Washington Bridge as a result of the downstream expansion of the channel below the construction at the Bridge. This front is characterized by strong salinity gradients that intersect the bottom and a strong halocline. Suspended particles settling through the halocline become trapped in the lower water layer. As the ebb tide wanes and the flood begins, the salt wedge moves northward into the estuary gravitationally. Additional sediment is resuspended as it transgresses and this sediment is trapped behind the front under the halo-cline. The front's progress seems to be arrested on the bathymetry south of the George Washington Bridge even as the flood continues as evidenced by arise in the halocline. During the flood, suspended sediment apparently is also transported laterally to the west side of the river (Geyer, 1995). As the flood tide ends and the ebb begins, the salinity gradients become unstable and the front breaks down. This event apparently can strand turbid water near the northernmost position of penetration of the salt wedge while a newfront is generated further downstream to begin the process again. I would suggest that the second mid-estuary turbidity maximum, which is sometimes seen north of the first, may be turbid water formed on the previous tide and stranded as the next ebb began.

The occurrence of these turbidity maxima are influenced, as expected, by the freshwater discharge. The mid-estuary maximum tends to remain fixed in the vicinity of Grant's Tomb and seems to be found as long as the freshwater discharge allows saltwater penetration to that location. The maximum at the head of salt migrates down the estuary at times of high discharge and up-estuary at low. The freshwater discharge to the estuary averages 550 m3 s-1 (Olsen, 1979). Figure 4.5 shows the axial distribution of salt and suspended sediment in May 1994, when the discharge was 2,690 m3 s-1.

Salinity (o/oo), May 1994

Salinity (o/oo), May 1994

Axial Distance (km)

a * the interval used for contouring: 1 unit (i.e. o/oo)

Axial Distance (km)

a * the interval used for contouring: 1 unit (i.e. o/oo)

Suspended Sediment Concentration (mg/l), May 1994

Suspended Sediment Concentration (mg/l), May 1994

b Axial Distance (km)

Figure 4.5. Axial distribution of salt (a) and suspended sediment (b) under high discharge (May, 1994). The salinity contour interval is one part per thousand; the suspended sediment concentration contour interval is 10 mg L-1. The Battery is at kilometer 31 on this scale.

b Axial Distance (km)

Figure 4.5. Axial distribution of salt (a) and suspended sediment (b) under high discharge (May, 1994). The salinity contour interval is one part per thousand; the suspended sediment concentration contour interval is 10 mg L-1. The Battery is at kilometer 31 on this scale.

Three turbidity maxima were seen. One at the limit of sea salt, a second south of its expected position near Grant's Tomb and a third south of the Battery. By contrast, the summer of 1995 was a drought. The discharge in September 1995 was 255 m3 s-1 and the axial distribution of salinity and suspended sediment is shown in Figure4.6. The estuary is well mixed and two distinct turbidity maxima are seen; one at the head of salt, and one near Grant's Tomb.

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