Manhasset Bay

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Physical Characteristics of the Bay and Its Watershed


Physical Characteristics of the Bay

              The physical characteristics and hydrodynamics of the Bay have been investigated as part of various studies of the Sound and its bays (Koppelman, 1976), wastewater studies (Hydroscience, 1973), and as part of USEPA's Section 208 and Section 201 programs (Tetra-Tech, 1977 and 1981). The information from these studies reveals that at mean tide, Manhasset Bay has a surface area of approximately 2,725 acres with a volume of approximately 9 billion gallons. The Bay is approximately 4.5 miles long north to south and on average is approximately 1 mile wide. It is shallow at its narrow southern end (on average 2 to 3 feet depth) and its depth increases to the north by Long Island Sound to an average depth of approximately 15 to 20 feet, by Long Island Sound.

  Hydrodynamics of the Bay

              Based on a number of studies (Koppelman, 1976; LIRPB, 1978; Tetra-Tech, 1981) of the Bay, the movement of water within, into and out of it (i.e., its hydrodynamics) is heavily dominated and influenced by the Sound. The Bay's hydrodynamics, primarily influenced by tidal action which causes an average change in depth of 7.3 feet in a 12-hour period (one complete tidal cycle).

              At high tide, the tidal prism that enters the Bay (i.e., the additional water brought into the Bay from low tide to high tide) increases the Bay's mean low water volume by approximately 73 percent. The net effect of this is that every 12 hours approximately 50 percent of the Bay's 9 billion gallons of mean-tide volume is exchanged with the water in Long Island Sound. This corresponds to an average residence/detention time of approximately 12 hours. Therefore, every 24 hours, a volume equal to the Bay's 9 billion gallon mean tide volume is exchanged with the Sound.

Freshwater Discharges to the Bay

            On average, the volume of freshwater flows discharged to the Bay from its watershed is approximately 37 million gallons per day (mgd). This flow into the Bay is only 0.4 percent of the Bay's 9 billion gallon total volume at mean tide. The freshwater flow originates from four major sources:


(a)    dry and wet weather flow primarily caused by storm water runoff that discharges through ponds, streams and storm water outfalls around the Bay;

(b)   direct rainfall onto the Bay's surface;

(c)    groundwater underflow that is continually recharged by storm water that infiltrates into the ground in the watershed and which then seeps up through the Bay's bottom; and

(d)   the treated discharge (effluent) from three municipal wastewater treatment plants (one in Port Washington and two in Great Neck).


The estimated volumes of these four major freshwater flows are presented below.


Freshwater Sources

Estimated Flow


dry/wet weather runoff

10 mgd


groundwater underflow

11 mgd


direct rainfall

9 mgd


three wastewater treatment plants

7 mgd



37 mgd


mgd = million gallons per day



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Last updated: January 2006
Last modified:  August 2015

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