1938 Hurricane

Geological Impact of the 1938 Hurricane

Perhaps the greatest long-term impact on Long Island of the Great Hurricane of 1938 was its creation of the Shinnecock Inlet and the widening of the Moriches Inlet to the west. (The Moriches Inlet was most recently formed by a powerful northeaster in 1931.) The picture to the right shows the Shinnecock Inlet one month after being formed by the incredible storm surge of the hurricane. At least 12 inlets were created by the hurricane and all but Shinnecock were filled with wrecked cars, broken trees, structural debris, and millions of tons of sand. Over the centuries, the strip of barrier beach along Long Island's south shore has been shaped by storms, surf, and currents. Inlets form and close with time due to these events and Shinnecock was no exception. Because of the creation of the Shinnecock Inlet and the widening of the Moriches Inlet, the natural transport of sand from east to west was interrupted. To compensate for the loss of sand to the west of Shinnecock a groin field was created along the beaches of Westhampton to try to slow beach erosion. The end result: these two inlets and the groin field have caused increased beach erosion in the Fire Island Barrier Beach region (Spencer & Terchunian). Spencer and Terchunian report that over the past 25 years an estimated sand loss of 8-10 million cubic yards has occurred in the 32 miles of Fire Island. That volume of sand represents a loss of 100 feet of beach and a depth of 12-16 feet along the entire Fire Island beach zone. Shinnecock Inlet - 1938

Longshore Current - Click for Larger Image
(Monroe & Wicander, 1992)
Sand is pushed at an angle up the beach face but is carried seaward in the backwash perpendicular to the long axis of the beach. Therefore, individual sand grains move in a zigzag pattern toward the west. The longshore current, the phenomenon by which sand is transported along the shoreline by this motion, is causing sand to move westward along the coast away from Montauk Point and toward New York City.
Beach Analysis - Click for larger Image Shinnecock Inlet, when opened in 1938 by the hurricane, immediately began trapping sand. The amount of sand trapped annually inside Shinnecock Bay was calculated to be approximately 60,000 cubic yards per year between the years 1955 and 1969 (McCormack, 1973). The U.S. Army Corps of Engineers confirm that Shinnecock Inlet is trapping between 55,000 and 120,000 cubic yards of sand annually and the shoreline to the west of the Inlet has been eroding as a result of the sand trapping (Spencer and Terchunian). The figure to the right (Taney, 1961) shows the historical shoreline change (advance vs. retreat) and the rate of shoreline change from the years 1838-1933 (before the hurricane) and from the years 1940-1956 (after the hurricane). Click the image for a larger view. Taney's study showed that the natural rate of beach erosion (due to the littoral transport) of the area west of the future Shinnecock Inlet location was 1.2 feet per year. After the inlet was created by the hurricane and stabilized by jetties in the 1950's, the rate of erosion to the west of the inlet increased to 8.2 feet per year. To the west of the Moriches Inlet, the Fire Island Sea Shore also shows increased rates of erosion most likely caused by the trapping of sand by the two inlet jetties and the Westhampton groin field.

Recent aerial photographs (source: Library Map Collection, SUNY at Stony Brook) illustrate the sand-trapping ability of the Shinnecock Inlet. One can clearly see the trapping of sand to the east of the jetties (wider beaches closer to jetty) and the trapping (delta) inside Shinnecock Bay due to the flood tide into the bay. Please click the two photographs below for a larger image.

Shinnecock Inlet (1970)
Shinnecock Inlet (1970)
Shinnecock Inlet (1988)
Shinnecock Inlet (1988)
A review of maps of the Shinnecock Inlet (source: Library Map Collection, SUNY at Stony Brook) over time also shows a buildup (trapping) of sand to the east of the inletand inside the bay. Much of this sand would naturally move westward to the Westhampton Beach area if the inlet had not formed and been stabilized by jetties.
Shinnecock Inlet (1942)
1942
Shinnecock Inlet (1955)
1955
Shinnecock Inlet (1991)
1991
Moriches Inlet to the east was created by a powerful storm in 1931. Between 1933 and 1938, Moriches Inlet widened and deepened as tidal currents deposited large sand delta on both the ocean and bay side of the inlet. The 1938 Hurricane further widened the Inlet to over 4,000 feet (Kassner & Black, 1982). When just this inlet was open, the water from both the Moriches and Shinnecock Bays flowed through the Moriches Inlet causing scouring and widening. When the Shinnecock Inlet was opened by the 1938 Hurricane, tidal pressure was halved and the slower tidal currents allowed shoaling within the inlet area (Czerniak, 1976).

The jetties at both inlets were constructed and the channels maintained by local interests until the 1980s or 1990s. Locals were keen to have these inlets in place, and the Federal government was not initially involved. At some point, the local interests had Congress pass laws authorizing the Corps to assume responsibility for maintenance of the inlets. Thereafter, the jetties were rehabilitated and the channels dredged as necessary by the federal government (Kraus, 2004). The inlets were stabilized for navigation and tidal flushing. Once these jetties were in place, the westward movement of sand due to the longshore current was extremely inhibited. The U.S. Army Corps of Engineers estimates that Moriches Inlet has annually trapped 178,000 cubic yards of sand since 1953 (Spencer and Terchunian).

The Westhampton groin fields (a series of stone structures much like jetties that project seawrd from the beach) were constructed in the mid-1960's and the early-1970's to stop the erosion of sand caused by the Shinnecock Inlet trapping to the east. Downdrift from these groins was an average loss of 55,000 cubic yards of sand per year (Long Island Regional Planning Board, 1989). The known total volume trapped by 1991 was 2.4 million cubic yards (Nersesian et al., 1992).

The 1938 Hurricane has had a "domino-effect" on coastal erosion. By opening the Shinnecock Inlet several situations developed - all of which have caused rapid erosion of beaches to the west of Moriches Inlet:
  1. Longshore current disrupted and sand begins to be trapped in the bay.
  2. Weaker tidal pressure causes shoaling in both the Moriches and Shinnecock Inlets so jetties are constructed.
  3. The Shinnecock jetties further inhibit the natural westward propogation of sand so the Westhampton groin fields were constructed.
  4. With all three sand traps, the beaches to the west along Fire Island are losing sand much more quickly than sand can be naturally replaced by the longshore current. The result is less beach to protect the homes in that region.
Fire Island
Newsday, 1992/David L. Pokress
In Saltaire and much of Fire Island, a devastating nor'easter on Dec. 12, 1992, washed away much of the beach, leaving the shoreline severely eroded and houses toppled into the sea.

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Scott A. Mandia, Professor - Physical Sciences
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