7.1 CLIMATOLOGY
For the purposes of this study, any tropical cyclone approaching within
180 nmi of New London is considered a threat. It is recognized that a few
tropical cyclones that did not approach within 180 nmi may have affected
New London in some way, so to some extent this criterion is arbitrary.
Considerable information on Atlantic tropical cyclones that passed near
New London is available as far back as 1886. Limited information is
available for prior years. Tables
VIII-1A and VIII-1B
contain a descriptive history of all tropical storms and hurricanes
passing with 180 nmi of New London during the 111-year period 1886-1996.
Unless otherwise indicated, all of the tropical cyclone statistics
utilized in this report for storms passing within 180 nmi of New London
are based on the data set used to compile
Tables VIII-1A and
VIII-1B.
Although tropical cyclones have occurred in the North Atlantic Ocean
during all months of the year, all but one of the 84 tropical cyclones
which have threatened New London occurred during the months of June
through November. The one exception occurred in February 1952, when an
unnamed tropical cyclone with 50 kt maximum winds was recorded.
Table VIII-2 shows that
the tropical cyclone threat to New London is primarily limited to the
months of June through November, with very few storms recorded during the
individual months of July and November. The main threat months are June
and August through October. September has the greatest frequency of
occurrence of tropical storms and hurricanes, with 39% (33 of 84)
occurring during the month.
Table VIII-2 also shows
the motion history of the 84 tropical storms and hurricanes that entered
New London's 180 nmi threat radius. The average movement at their closest
point of approach (CPA) for all 84 storms was 037° at 24 kt, with the
average speed of movement for individual months varying from 20 to 27 kt.
Because storm movement is determined at CPA, it is not obvious in the
table that eight of June's nine storms (89%) were, at some time before
entering New London's 180 nmi threat radius, over the Gulf of Mexico
before crossing the southeastern United States on a track toward New
England. Most early season and some late season storms follow this track.
Figure VIII-4 depicts
the annual distribution, in seven-day increments, of the 84 tropical
storms and/or hurricanes passing within 180 nmi of New London during the
111-year period 1886-1996. While storms have occurred outside the primary
threat period, the figure clearly shows that the main threat to New London
lies between the dates of July 30th and October 14th.
During the 111-year period 1886-1996, there were 84 tropical storms
and/or hurricanes that entered New London's 180 nmi threat radius, an
average of less than one per year. During that period, only 32 hurricanes
threatened New London, which represents an annual recurrence interval of
one storm every 3.5 years. Figure
VIII-5 depicts the distribution of storms for the 111 years of
record. Only four of the 111 years of record had more than two storms
threaten the harbor, while 38 years had only a single threat. It is
interesting to note the scarcity of storm activity during the period 1905
through 1931, a 27-year period during which only six storms passed within
180 nmi of New London.
Figure VIII-6 depicts
the octants from which the 84 tropical storms and/or hurricanes approached
New London. The length of each arrow is proportional to the number of
storms from that direction. It is obvious in the figure that the main
tropical cyclone threat at New London is from the southwest octant. As
Figure VIII-6 shows, over
76% of the storms entering New London's 180 nmi threat radius approached
from that direction. It should be noted that the approach direction is
determined at CPA and may not represent the initial approach direction of
the tropical cyclone toward New London.
The primary threat axes of 73 tropical storms and hurricanes passing
within 180 nmi of New London during the 111-year period 1886-1996 are
depicted in Figure VIII-7.
Eleven of the 84 storms that passed within 180 nmi of New London are not
included because they formed or were first detected north of the 72-hour
mean time lines depicted in the figure. Two tracks are shown. The
westernmost track is typical of June storms, and some of the storms
occurring during October and November. The easternmost track represents
the primary track taken by storms occurring from July through September.
The line intersecting each track represents the average position of storms
in the data set when they were 72 hours away from CPA to New London. The
two insets show the distribution of tune in hours for the tropical cyclone
centers to move from the solid lines intersecting the tracks to CPA. As
shown by the insets, there is considerable variation in the time it takes
tropical cyclones to travel from the solid line to CPA. The 72-hour
average time line is farther from New London for those storms emanating
from the Gulf of Mexico than the one for the easternmost track, indicating
faster storm speeds. There are two reasons for this: (1) the Gulf of
Mexico storms tend to occur in June and after October 1st, when the
steering winds are stronger than during the mid-season, and (2) storms
moving with a northeasterly component of motion have completed the
re-curvature process and are in phase with the westerlies.
The natural protection offered by the shape of the eastern coast of the
United States south of New London to Cape Hatteras essentially dictates
that most recurving storms must either make a landfall south of Cape
Hatteras, or pass New England well offshore to the southeast. The majority
of storms tend to follow the path of the oceanic Gulf Stream and pass well
to the southeast of New London. However, occasionally storms take a more
northerly track instead of recurving to the northeast. An example is the
disastrous 1938 hurricane which advanced rapidly up the east coast
offshore, passing Cape Hatteras, moving over central Long Island and New
Haven, CT before proceeding north-northwest into Vermont. Thus, instead of
passing New England offshore, the hurricane remained on a northerly track
and accelerated to an average rate of advance of over 50 kt. It passed by
Cape Hatteras at about 8:30 am local time on 21 September and reached
Connecticut at about 4:00 pm on the same day. Such a speed of advance
would be difficult to handle for storm preparations even with today's more
sophisticated warning methods. Figure
VIII-8 illustrates the very rapid approach of four exceptionally
fast moving hurricanes which caused destruction at New London.
With today's advances in meteorology, it is possible to identify those
circumstances which lead to the rapid acceleration of tropical cyclones
towards the north, although rarely would a 50 kt speed of advance (SOA) be
forecast. Figure VIII-8
shows that a hurricane can be offshore between Jacksonville, FL and Cape
Hatteras, NC before its track begins to indicate it is heading for
southern New England. A storm may be within 24 hours travel time from New
London when its departure from a normal recurvature track to a rapid
north-northeast track is detected.
Figures VIII-9,
VIII-10,
VIII-11 through
VIII-12 address the
probability of tropical cyclones affecting New London. The hurricane
season has been divided into four periods: June, when most storms
affecting New London come out of the Gulf of Mexico; July and August;
September, the month of maximum frequency of occurrence; and October and
November.
The solid lines in each figure represent the "percent threat"
of a tropical cyclone in a given location entering the 180 nmi threat
radius around New London. The dashed, heavy lines represent approximate
time in days for a system moving at a climatological average speed to
reach CPA to New London. For example, in
Figure VIII-10, during
the months of July and August a tropical cyclone located at 25°N 75°W
would have an approximate 35% probability of entering New London's 180 nmi
threat radius, and would reach CPA to New London in approximately three to
four days. The times to CPA presented in Figures
VIII-9, VIII-10,
VIII-11 through
VIII-12 should be used
with caution since it is not the average moving storm but the
exceptionally fast moving ones that are the greatest danger to New London.
For example, Figure VIII-11
indicates that a September storm located near 27°N 74°W should
reach New London in about 3 to 4 days (84 hours), based on climatology.
However, the 21 September 1938 hurricane traveled the same distance in
less than half that time, approximately 32 hours
(Figure VIII-8).
7.2 WIND AND TOPOGRAPHIC EFFECTS
Wind records for the Groton Airfield (Trumbull Airport) are available
for the years 1950 through 1995, but are of limited use due to the station
closing during tropical cyclone threat periods. Because of the closings,
critical wind data during the passage of many tropical cyclones are not
available. Other observation sites near New London/Groton include Block
Island, RI, approximately 25 nmi east-southeast of New London Harbor,
Naval Air Station (NAS) Quonset Point, RI, located on Narragansett Bay
approximately 30 nmi east-northeast of New London Harbor, New Haven, CT,
about 35 nmi west of New London Harbor; and a Coast Guard facility at
Point Judith, RI, located approximately 25 nmi east of New London Harbor
on the southwest side of the entrance to Narragansett Bay. No information
is available as to the exposure of the wind measuring equipment at each
site.
Table VIII-3 has been
prepared to offer a comparative listing of winds observed at three of the
sites during the passages of some of the more significant tropical
cyclones to enter New London/Groton's 180 nmi threat radius during the
period for which observations are available for the sites listed. It must
be understood that the exposure of each of the sites is different, and may
not be representative of winds occurring over the entire area of southern
Connecticut and Rhode Island.
Figures VIII-13,
VIII-14,
VIII-15 through
VIII-16 depict segments
of the tracks of tropical storms or hurricanes when sustained winds
22
kt, 34
kt, 47
kt, and 64
kt were observed at one or more of the three locations listed in
Table VIII-3. The period
of record is 1943-1996. Index numbers shown in
Figures VIII-14,
VIII-15 through
VIII-16 correspond to
those listed in Tables
VIII-1A and VIII-1B.
As can be seen in Figure
VIII-3, most of the sustained winds 22
kt began while the storm centers were within 180 nmi (3 degrees of
latitude) of New London, but one storm center was over 420 nmi away when
the 22
kt winds started. As shown in Figure
VIII-14, all of the instances of sustained wind.
34
kt began when the storm center was inside a 180 nmi radius. Similarly, all
of the 47
k wind events shown in Figure
VIII-15 occurred when the storms were inside a 100 nmi radius of New
London. During one storm, Hurricane Gloria (Index number 75), winds
47
kt lasted until the center of the storm was approximately 270 nmi north of
New London. Five of the 6 storms depicted in the figure passed west of New
London. Figure VIII-16
depicts the tracks of the four hurricanes that caused winds of hurricane
force (64
kt) at one or more of the three recording sites. As can be seen in the
figure, the center of three of the four storms had passed New London
before the hurricane force winds began at the observing stations. All of
the hurricane force events were caused by storms that passed west of New
London.
Because of its location in a sharply defined and narrow river valley,
New London Harbor has considerable topographical shelter from most wind
directions, except those with a strong northerly or southerly component.
The western part of the lower portion of the main harbor, especially from
Winthrop Cove southward, is exposed to winds from southeast and south. As
stated in the prior evaluation of New London as a hurricane haven, the
exposure was confirmed by the considerable damage inflicted by the 1938
hurricane. The New London "official" anemometer (location
unknown) indicated 85 kt winds before
the cups were carried away. Fishers Island reported an east wind of 91 kt.
The "Marsala," a 300-ft, 5-masted barkentine, dragged two
anchors (one 8-ton and one 10-ton mushroom) while anchored off Shaw Cove.
The 1,057-ton lighthouse tender 'Tulip" was carried ashore, by the
combined effects of wind and surge, so that its bow was left atop the
railroad tracks in front of the New London railroad station
(Figure VIII-2).
The Thames River is a natural, north-south oriented channel for the
funneling of hurricane force winds through New London Harbor. The SUBASE
and Coast Guard Academy piers are particularly exposed to the north winds
that occur in the less dangerous, western semicircle of a north-moving
hurricane passing east of the harbor. The main harbor and, to a lesser
degree, the SUBASE would be exposed to southerly winds for a short
duration if a north-moving storm passed west of the harbor. However, the
SUBASE is well protected by topography from the strongest northeast
through southeast winds that exist in the dangerous, right hand semicircle
of a north-moving hurricane passing west of the harbor.
Figure VIII-17
illustrates portions of the tracks of major hurricanes which have affected
New London. With one exception since 1871, the most destructive hurricanes
have made landfill to the west of New London along the Connecticut coast.
The lone exception is the September 1944 hurricane which passed over Rhode
Island, a short distance east of New London. Approaches to the west of New
London generally produce easterly winds at New London, veering to
southerly. The topography around New London should significantly reduce
the easterly components until the wind shifts to the southeast thus
delaying the full impact of the storm.
7.3 WAVES
New London harbor is well protected from most hazardous wave motion.
Long Island and Fishers Island significantly reduce the fetch for seas
which could enter New London harbor. The islands also act as barriers in
protecting the harbor from large deep ocean swell. A west wind can
generate waves over the Long Island Sound's 75 nmi fetch west of New
London, but except for its extreme south end, the harbor channel is well
protected from such waves. Local authorities state that the largest waves
they can recall from any direction at the port are limited to
approximately 6 to 8 ft at the mouth. of the Thames River, and 4 to 6 ft
in the river adjacent to the Coast Guard Station.
7.4 STORM SURGE
As stated in the previous evaluation of New London as a hurricane haven.
New London is one of the few major east coast Navy ports to have
experienced a major storm surge in this century. The storm surge of 21
September 1938 caused considerable destruction. Witnesses reported that
the storm surge had the appearance of a tidal bore (wall of water), likely
attributable to the high rate of forward movement of the storm. Such an
event may account for the extensive damage to vessels and facilities in
the lower harbor area. Storm surge levels varied from approximately 9.7 ft
from the mouth of the Thames River up to the highway and railroad bridges
to 10.5 ft at the SUBASE. Surge levels are in reference to
National Geodetic Vertical Datum (NGVD),
which approximates to mean sea level. Hurricane Carol caused a surge of
approximately 9.1 ft above NGVD in 1954. Lesser storm surges from other
storms occurred in 1960 (Donna) and 1985 (Gloria), but exact water level
data are not available.
Mystic, CT, a small harbor located 5 nmi east of New London, experienced
a storm surge of 11.6 ft above MLW during the 1938 hurricane, 10.1 ft
above MLW during Hurricane Carol in 1954, 7.0 ft above MLW during
Hurricane Donna in 1960, and 6.4 ft above MLW during Hurricane Gloria in
1985 (Treworgy, 1996). If Gloria's surge had
coincided with high tide, the water level would have been 8.5 ft above
MLW.
Figure VIII-18
illustrates a tidal flood profile for New London Harbor and the Thames
River. When applying historical high water marks to present day
facilities, approximately one-half foot should be added to the NGVD values
in the figure to account for the changing sea level since 1929. The 1938
storm surge was slightly greater than that expected once in a hundred
years (the 100-year flood level).
A storm surge calculation computer program called SLOSH (an acronym for
Sea, Lake and Overland Surges from Hurricanes)
indicates that the New London area is susceptible to greater surge levels
than that indicated by the 100-year flood level in
Figure VIII-18.
Table VIII-4 lists
computed surge heights for three locations: (1) The mouth of the Thames
River, (2) Thames River adjacent to the State Pier, and (3) Thames River
adjacent to the SUBASE. The locations are indicated by circled numbers 1
through 3 on Figure VIII-2.
Calculations were made for hurricane categories 1 through 4 on the
Saffir-Simpson scale, a hurricane rating system commonly used to classify
hurricanes according to their strength. See
Table VIII-5.
Table VIII-4 indicates
that the SUBASE can expect storm surge levels that are approximately one
to two feet higher than those farther south in the Thames River. Because
the constriction at the bridges and widening of the river above the
bridges, the area north of the bridges escapes the destructive kinetic
effects of storm surge, but it is not immune to the flooding
associated with a hurricane strike. The prior evaluation of New London as
a hurricane haven stated that during the 1938 hurricane, a 100-ft
two-rnasted schooner was sunk at the Coast Guard Academy docks after being
battered by strong winds. The SUBASE sustained mainly water damage from
the same storm, along with minor damage to piers and the sinking of a
floating crane (YD). The evidence suggests that although the SUBASE area
experienced flooding during the 1938 hurricane, the areas close to the
river's mouth suffered the more destructive impact of the storm surge. It
is further theorized that the crane and schooner were submerged by taut
mooring lines at their berths.
Astronomical tides are of only limited concern in New London other than
their timing relative to the occurrence of a storm surge. The 1938 storm
surge occurred about two hours before the predicted high tide, thus
causing a storm surge to be added to a tide that was near its high level.
The tidal range at State Pier is only about 2.6 ft from mean high water to
mean low water. The most recent tide tables should be consulted for exact
values. River currents, although relatively strong on the ebb, are not of
major concern at the SUBASE since tugs are routinely required for all
submarine docking and undocking evolutions.