The Maine Department of Transportation
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HYDROGRAPHIC CHARACTERISTICS OF UPPER PENOBSCOT BAY
1.0 INTRODUCTION
Penobscot Bay is one of the largest embayments along the Maine
coast (Figure 1) extending from the western Gulf of Maine to the mouth
of the Penobscot River estuary near Verona Island. Sears Island is
located in the upper reach of the Bay and separated from the mouth of
the estuary by Cape Jellison. The island is connected to the mainland
at Kidder Point by a narrow causeway that is exposed for several hours
during low tide (Figure 2). On the east side, Sears Island is separated
from Cape Jellison by Stockton Harbor, and on the west side, from
Searsport by Long Cove.
Many hydrographic studies have been carried out in Penobscot
Bay by Normandeau Associates, Inc. (NAI, 1978), the National Ocean
Survey (NOAA—NOS, unpublished), and others. Characterizations of the
Penobscot River estuary have been reported by Haefner (1967). The
results of many studies have been summarized by TRIGOM—PARC (1974).
A year—long study of the Upper Penobscot Bay, labeled "Study
Area" in Figure 1, was carried out from August 1974 through early
November 1975 by NAI (1975, 1982)
Aquatech (1975) conducted a hydrographic study in Stockton Harbor in July 1975.
Results from hydrographic studies in the Upper Penobscot Bay have been
summarized by Central Maine Power Company (CLP, 1977).
An anchor station survey was also conducted by NAI on
T\the west side of Sears Island on 19-20 August 1982
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Figure 1 Reference Map Penobscot Bay
Sears Island Marine Dry Cargo Terminal, 1982
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Figure 2
Reference Map, Sears Island, Stockton Harbor, Long Cove, Searsport harbor
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2.0 TIDAL HEIGHTS
Tidal forces in Penobscot Bay and the Gulf of Maine are domi-
nated by the "M2" or main lunar semidiurnal tidal constituent (TRIGOM—
PARC, 1974) which has a period of 12.42 hours.
Since the lunar day is approximately 50 minutes longer than the solar day of 24 hours, times of
high and low water occur about 50 minutes later on each succeeding day.
Succeeding high and low tides are generally not equal in height; this
condition is referred to as the semidiurnal inequality. Tidal heights
also vary over the course of the lunar month (29.5 days), with the
largest ("spring tidal ranges at these times.
When the sun, moon and earth are arranged in a perpendicular order
during the first and third quarter phases, neap tides occur. Minimum high tides and maximum low tides occur to form the smallest tidal ranges at these times.
All of these features are present in the tidal height data collected at the Searsport town pier (Figure 3; NAI, 1975).
Tidal heights in Penobscot Bay range from about 2.8 m (9.3 ft)
out where the Bay meets the Gulf of Maine, to 4.0 m (13.1 ft) at Bangor on the Penobscot River. The spring tide ranges are about 15% higher than these average ranges. The average tidal range measured at Searsport is 3.0 m or 9.9 ft (NAI, 1975).
3.0 TIDAL CURRENTS
Tidal currents, which accompany the vertical rise and fall ofthe tides in shallow, restricted embayments such as Penobscot Bay, are reversing or bidirectional (Figure 4). Maximum tidal current velocities are approximately coincident with the mid—ebb and mid—flood tides, where as slack waters occur during high and low tides.
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FIGURE 4 Upper Penbscot Bay Current Roses. Oct 1974
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Tidal currents in the-vicinity of Sears Island have been
studied by MAI (1975, 1982) and Birge (1978). The NAI studies were con-
ducted on the southeast and southwest sides of Sears Island (Figure 4)
between August 1974 and early November 1975.
Birge (1978) investigated water mass flows on the west side of
Sears Island using a pair of drogues deployed 1.5 m (~5 ft) and 6.1 m (~20 ft) below the water surface,at five stations during August and September 1977..
On the flood tide,maximum velocities ranged between 31.4 and 41.8 cm/sec (0.6 to 0.8 kt) at -1.5 m, and between 11.6 and 21.6 cm/sec (0.23 to 0.42 kt) at -6.1 m.
On the ebb tide, the maximum speeds ranged between 31.1 and 52.4 cm/sec(0.6 to 1.0 kt) at -1.5 m, and between 7.9 and 18.6 cm/sec (0.15 to 0.36 kt) at -6.1 m.
Thus, currents at -6.1 m are were found to be approximately 50% of those at
1.5 m; also, mean ebb tide currents appeared to be only about 76% as strong as
Both ebb and flood currents tended to flow parallel to the bottom
contours. A two—layer flow was inferred with the deeper currents
flowing in the direction opposite to that of the nearfsurface currents.
4.0 ANCHOR STATION CURRENT RESULTS
On 19 and 20 August 1382, NAI conducted an anchor station sur—
vey in the vicinity of the proposed wharf in order to measure current
velocities, temperatures and conductivities with respect to depth.
Measurements were made every 2 m to the bottom beginning at 2 m below the water surface. Current velocities were measured using a Bendix Model Q—15 current meter and model 270 recording package.
The data were recorded on standard coding forms in the field. Each
survey day began at low slack water and three stations were occupied approximately once each hour over a 12-hour period. The first three stations were occupied on the 19th and the latter three on the 20th (Figure 5).
Current meter data from the anchor station survey appear in Appendix 1.
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Pg 8 Figure 5 Station locations w/ respect to proposed Wharf location for
the Anchor Station Survey of 19-20 August 1982.
- From: Sears island Marine Dry Cargo Terminal 1982
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Near-surface and near—bottom current speeds were plotted for a
complete tidal cycle at each station (Figure 6). Near—bottom currents
tend to be out of phase with the near—surface currents, and maximum
currents have unequal strengths between the ebb and flood tidal phases
at both the near—surface and near—bottom in agreement with earlier fin—
dings (Section 3.0). Station 3 had the strongest near-bottom current
speed, 18 cm/sec (0.35 kt) during flood tide, and Station 4 had the
strongest near—surface current speed, 23 cm/sec (0.45 kt) also during
flood tide.
Current velocity profiles with depth (Figure 7) demonstrate
existence of a well—defined counter—current in the bottom water at
Station 3 which was not, however, evident at Station 4. At Station 4,
the near—bottom current direction simply appears to be slightly out of
phase with the current directions higher up in the water column.
Data presented in Appendix 1 indicate that the deeper stations (1, 3 and 5),
which are near the dredged channel, all exhibit two—layer flow patterns,
that is, near—surface current directions are approximately opposite to
near—bottom current directions.
Shallower stations (2, 4 and 6), on the other hand, are all closer to the west shore of Sears Island, and exhibit single—layer flow patterns. A two—layer flow pattern in the deeper regions of an embayment is typical for such estuarine systems and reflects partial mixing of the water mass by turbulent energy dissipated due to bottom friction (Dyer, 1973)
The one—layer flow pattern at the margins of an estuarine channel typifies a system in which a nearly honngeneous, vertical density distribution is dominated by bottom fric- tion (Dyer, 1973).
Instead of fixed—frame or Eulerian measurements (current
meters), Birge (1978) used moving—frame or Lagrangian measurements
(drogues), hence the results of these two studies are not directly com-
parable. His study was conducted over several tidal cycles at each sta-
tion in the same general study area as NAI's single tidal cycle study.
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5.0 CIRCULATION
The circulation patterns within Penobscot Bay, previously
discussed by NAI (1975, 1978), show a net (non~tidal) movement of sur-
face water south out of Penobscot Bay into the western Gulf of Maine,
and a net (non—tidal) movement of bottom water north from the Gulf of
Maine into Penobscot Bay, a classic picture of estuarine circulation
(Pritchard, 1967).
The contribution of Sears Island to the circulation of the larger Penobscot Bay is not very important. Numerical modeling studies by NAI (1978) and Stone and Webster (1979) of the circulation in Penobscot Bay do not include the fine detail which Stockton Harbor and Long Cove represent.
Current studies by NAI (1975,1982), Birge (1978) and the anchor
station survey of 19-20 August 1982 all suggest that the circulation in
both Stockton Harbor and Long Cove are simply reversing or bidirec—.
tional.
The Maine Department of Transportation (MDOT, 1982) has esti-
mated the flow across the causeway between Stockton Harbor and Long Cove to be small, about 2—4% entering Long Cove on the ebb tide and less than 1% leaving Long Cove on the flood tide. Similar findings were given in an earlier study by Aquatech (1975).
For sedimentological reasons, flow across the causeway would have to
be of such small magnitude, because the cross—sectional area is small
and the causeway is covered with water only during the flood phase of the
tidal cycle.
Substantially larger flows across the bar would have produced a much
more eroded condition
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and far more coarse—grained materials would have been featured in the
adjacent mud (clam) flats.
6.0 TEMERATURE, SALINITY AND DENSITY
Temperature and salinity in the vicinity of Sears Island were
investigated by NAI (1975, 1982), CM (1977), Birge (1980), and the pro-
ject area revisited by NAI during August 1982. Information was
collected by NAI from several moored instruments off the southeast and
southwest sides of Sears Island in combination with field cruises of the
study area (Figure 1) from August 1974 through early November 1975. CWP
also conducted many field cruises in this area during the same time
period. Birge (1980) has described the weekly temperature statistics
obtained from near—surface and near—bottom thermal sensors located on
the east, west and south sides of Sears Island from March 1975 through
December 1979.
NAI (1975) reoccupied a station, designated LC—4, seventeen
times from August.1974 through July 1975. Station LC—4 is located at
the northern end of the dredge channel (Figure 5) opposite anchor sta-
tion 1. Temperatures and conductivities were measured with a Beckman
RS5—3 portable salinometer. A plot of the surface and bottom tem-
peratures at Station LC—4 is given (Figure 8) for August 1974 through
July 1975 along with the mean daily and mean monthly air temperatures
recorded at Portland by the U.S. Weather Bureau.
Several general trends are evident (Figure 8): The surface
water temperature peaks in late summer (16.9 C or 62.3 F) and has a
minimum value in mid—winter (0.5 C or 32.9 F), while the bottom tem—
perature lags behind with a maximum in early fall (11.6 C or 52.9 F),
and a minimum in late winter (1.3 C or 34.3 F). In October, February
and March the water column is thermally homogeneous. From April through
September surface temperatures exceed bottom temperatures as the air
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temperatures increase from the spring into the summer. From November
through January bottom temperatures exceed surface temperatures as the
air temperatures decrease from the fall into the winter. The surface
water temperature does not respond quickly to the mean daily air tem-
perature, but rather reflects the trends of the mean monthly air tem—
peratures.
Surface and bottom salinities at Station LC—4 have been plotted
(Figure 9) along with the mean daily and mean monthly discharge rates of
the Penobcot River measured at West Enfield approximately 100 km
(60 mi) from Searsport.
The surface salinity has a maximum in October (29.10 °/oo), a primary
minimum in June (14.15 O/oo) and a secondary minimum in Decembe
r (23.42 9/oo).
The bottom salinity has a maximum inOctober and November (32.25 to
32.76 O/oo), and a minimum in December (29.09 °/oo).
The surface salinity responds quickly to strong discharge
events represented by high mean daily discharge rates (Maine State
Planning Office, 1972).
Bottom salinity, on the other hand, responds
weakly to these strong discharge events. Since the December survey was
preceeded by several days of strong storm winds (NAI, 1975) and high
riverine discharge measured by the U.S. Geological Survey (USGS, 1976),
the minimum bottom salinity in December was presumed to be a storm-
related overturn event.
Vertical profiles of temperature at Station LC—4 at bimonthly
intervals (Figure 10), graphically illustrate the seasonal temperature
distribution seen in Figure 8. In February and October the vertical
temperature distribution is approximately isothermal. In April, June
and August, the temperature distribution decreased from surface to bot-
tom, whereas in December the profile was inverted, or increasing from
the surface to the bottom.
Regardless of season, salinity tended to increase from surface to bottom
(Figures 8 and 10). Whenever high discharge from the Pcnobscot River
freshens the surface layer, the vertical salinity profile is strongly affected
as seen in April, June andDecember.
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Temperature and conductivity measurements were also made at
each anchor station in August 1982, every 2 m from the surface to the
bottom, using a Beckman RSS3 portable salinometer. The temperature,salinity and density data from this survey appear in Appendix 2.
Profiles of temperature and salinity from Stations 3 and 4 are plotted
(Figure 11) for each of four tidal phases: mid—flood, high slack, mid-
ebb and low slack.
The August 1982 data show that not only do temperatures and
salinity varies with both tidal cycle and season. Irrespective of
depth or station, the temperature can vary as much as 2.5 C or 4.5 F,
and the salinity as much as 1.5 O/oo. Apparently, the cooler and more slaine bottom water at Station 3 during low slack and mid—flood, represents the intrusion of bottom water from the Gulf of Maine into the deeper waters of Penobscot Bay.
7.0 RIVERINE DlSCHARGE
The source of most of the fresh water entering Penobscot Bay is
the Penobscot River, which has an annual discharge rate of
332.2 m3/sec (11,730 cfs) averaged over 73 years, and a drainage areaof 17,280 kmz (6,670 miz) determined at the gauging station at West Enfield (USGS, 1976).
During the operation period of this gauge, a maximum discharge of 4,330 m3/sec (153,000 cfs) occurred on 1 May 1923, and a minimum of 46.2 m3/sec (1,630 cfs) on 29 October 1905 (USGS,1976).
Over the 1974-1975 study period, the maximum discharge of
1,345 m3/sec (47,500 cfs) occurred on 15 June 1975, and the minimum of 119 m3/sec) (4,190 cfs) on 2 September 1975 (NAI, 1975).
Ten—year average monthly discharge rates show a minimum in
August, a primary maximum in April during the spring freshet, and a
secondary maximum in November(Figure 12). Yearly variations in the
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weather can substantially affect the monthly discharge rates. For
example, discharge rates for November 1974 were less than 50% of normal
for that month, for April 1975 about one-third less than normal, and for
June 1975 over 50% greater than normal.
The high mean daily discharge rates in April and May 1975
(Figure 13) are typical of the spring freshet that occurs throughout the
northeast when rain and rising temperatures melt the snow cover and fro-
zen ground. Comparing monthly Local Climatological Data reports from
Portland, discharge events in June 1975 are coincident with 1.3 in-
(3.3 cm) and 1.9 in (4.7 cm) rainfalls on the 6th and 13th, respec-
tively.
The discharge event in December 1974 is coincident with a
0.5 in (1.3 cm) rainfall when the temperatures rose above freezing on
the 8th; these events followed a storm on the 2nd with subfreezing tem-
peratures, a condition analogous to the spring freshet.
8.0 WIND VELOCITIES
The wind patterns in Maine are dominated by the prevailing
westerlies, typical of weather throughout the north temperate U.S. In
the winter the winds tend to be strong and predominantly out of the
northwest, whereas in summer they are weaker and predominantly out of
the southeast (Figure 14).
From October 1974 through October 1975, NAI (1975, 1982) main—
tained a wind velocity recording unit at the Searsport town pier at a
height of 11.9 m (39 ft) above mean low water. Monthly summaries of
wind velocity distribution (Figure 15) show the wind coming predomi-
nantly from the northwest in winter and from the south in summer.
The lack of an easterly component in summer is probably due to the southerly configuration, orientation and topography of Penobscot Bay, features which also appear to enhance southerly blows during winter.
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As a first—order approximation, oceanographers often utilize
the "three—percent rule" which states that a wind~generated current is
approximately 3.5% of the wind speed and in the same direction as the
wind (NAI, 1978). Assuming a linear relationship between the wind-
generated current and the tidal current, the net current is simply the
vector sum of these two currents.
The typical storm wind speed for Searsport is 10 m/sec
(19.4 kts). The maximum wind speed recorded at Searsport during a
winter storm by NAI (1975) is 101 m/sec (52 kts) on 26 February 1975.
Using the "three percent rule" for the typical storm wind value, the
associated wind—generated current is about 35 cm/sec (0.7 kt), which is
almost twice as large as the maximum current speeds measured during the
anchor station survey of August 1982, and almost as large as the maximum
current speeds measured by Birge (1978) of 42 cm/sec (0.8 kt) on the
flood tide and of 52 cm/sec (1.0 kt) on the ebb tide.
These results suggest that wind—generated currents dominate the overall
circulation of this region during strong storm occurrences. However, such
effects would be transient, probably lasting no longer than one or two days
after the storm abated.
9.0 REFERENCES
Aquatech, 1975. Hydrographic study of Stockton Harbor, Searsport,
Maine, July 1975. Prepared for Central Maine Power Company and
Yankee Atomic Electric Company. 85 pp. plus appendices.
Birge, R. P., 1978. Water mass flow measurements from drogue obser-
vations in an area west of Sears Island, Maine. Environmental
Studies Department of Central Maine Power Company, Searsport, Maine.
Report SI~78—1. 13 pp.
Birge, 1980. Surface and bottom water temperatures, Upper
Penobscot Bay, Maine, March 1975—December 1979. Environmental
Studies Department of Central Maine Power Company, Searsport, Maine.
Report SI—8O—4. 37 pp.
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CMP, 1977. Scope and description of aquatic studies, Upper Penobscot
Bay for Sears Island Coal Unit No. 1. Central Maine Power Company,
Augusta, Maine. 35 pp.
Dyer, D. R., 1973. Estuaries: a physical introduction. Wiley, London
H and New York. 140 pp.
Haefner, P. A., Jr., 1967. Hydrography of the Penobscot River (Maine)
estuary. Journal of the Fisheries Review Board, Canada. Vol. 24,
No. 7, 1553-1571.
MDOT, 1982. Environmental accessment, Island Road, Town of Searsport,
Waldo County. Project Number RS—O252(1). Prepared in consultation
with the Federal Highway Administration of the U.S. Department of
Transportation by the Bureau of Project Development of the Maine
s~ Department of Transportation. 26 pp.
Maine State Planning Office, 1972. The Penobscot Bay Resource Plan
Appendix. 60 pp.
NAI, 1975. Environmental Survey of Upper Penobscot Bay, Maine, Annual
Report for August 1974 through July 1975. Prepared for Central
Maine Power Company, Augusta, by Normandeau Associates, Inc. 700
PP-
, 1978. An oil pollution prevention, abatement and management
study for Penobscot Bay, Maine. Vol. II, Chapters 6-7. Prepared
for the State of Maine Department of Environmental Protection by the
Center for Natural Areas, Normandeau Associates, Inc., and Seacoast
Ocean Services. 160 pp.
, 1982. 1974-1975 Upper Penobscot Bay Hydrographic Survey.
Draft Report. Prepared for Central Maine Power Company, Augusta,
Maine, by Normandeau Associates, Inc. 78 pp.
Pritchard, D. W., 1967. What is an estuary: physical viewpoint. IN:
Estuaries. G. H. Lauff (ed.). AAAS Publication No. 83, Washington,
D. C.
Stone and Webster Engineering Corporation, 1979. Report on calibration
of tidal circulation model and preliminary assessment of meroplank—
ton dispersion for design of 1979 sampling program. Sears Island
Coal Unit No. 1. Prepared for Central Maine Power Company, Augusta,
Maine. 31 pp. plus appendix.
TRIGOM—PARC, 1974. A socio—economic and environmental inventory of the
North Atlantic region: Sandy Hook to Bay of Fundy. Volume I:
Environmental Inventory, Book 2: Chapters 4 and 5. Prepared for
the Bureau of Land Management by The Research Institute of the Gulf
of Maine (TRIGOM) and the Public Affairs Research Center (PARC).
804 pp.
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USGS, 1975. Water Resources Data for Maine, 1974. U.S. Geological
Survey, Augusta, Maine.
USGS, 1976. Water Resources Data for Maine,1975. U.S.Geological
Survey, Augusta, Maine. 180 pp.
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