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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 the Long Island Sound to an average depth
of approximately 15 to 20 feet, by the 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 influence
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.
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Freshwater Sources
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Estimated Flow
|
Percent
|
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dry/wet
weather runoff
|
10
mgd
|
27
|
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groundwater
underflow
|
11
mgd
|
30
|
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direct
rainfall
|
9
mgd
|
24
|
|
three
wastewater treatment plants
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7
mgd
|
19
|
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37 mgd
|
100
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mgd
= million gallons per day
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