Long Island's Coastal Processes

Long Island is surrounded by water, the Sound to the north, the Atlantic Ocean to the south and east, and the East River to the west. Additionally, bays separate the Twin Forks and the barrier islands from the mainland. Water and wind interacts with the shoreline in a variety of ways. 

Waves are formed as a result of wind acting on the water’s surface and pushing the water in one direction or another. Wave size depends on the “fetch”, which is the distance of water surface that the wind moves over, and the strength of the wind. For example, larger waves form if a waterbody is larger and provides greater surface area for wind to travel across, and/or if the wind is stronger. As waves travel towards shore, they begin to interact with the ocean floor, forcing the wave to grow in height. Waves will continue to increase in height until they become top heavy and tumble over, or crash. In Long Island’s embayments, waves are predominately formed by local winds, but especially at the ocean beaches, waves hitting the shore may have been formed by storm winds thousands of miles away. For example, a hurricane in the Caribbean could be sending the long, low period swell waves that surfers like to the south shore. 

Long Island has semidiurnal tidal cycles, meaning two high and two low tides occur within each 24-hour period. The most dominant force acting on the Earth resulting in tides is the gravitational pull of the Moon. Other factors also play a part, including the gravitational pull of the sun, the Earth’s rotation, the latitude and longitude, and the bathymetry, which is the shape and depth of the ocean floor. The water level of tides varies based on the lunar cycle. When the Earth, Sun, and Moon are aligned during full and new moons, the gravitational pulls are the strongest, resulting in higher-than-normal high tides, or spring tides (See more about spring tides in chronic flooding). When the Sun and Moon are at right angles to each other, the solar and lunar gravitational attractions partially cancel out and the high tides are a little lower than normal and low tides are a little higher, this is known as neap tides. The difference between high and low tide is known as the tidal range and can vary depending on your location. For instance, the tidal range at Port Jefferson (Long Island Sound) is about 5 feet and at Patchogue (Great South Bay) around 1 foot. 

Chronic, also sometimes referred to as nuisance, fair-weather, king tide, or “sunny day” flooding is the flooding of low-lying areas during normal high tides, unrelated to storm events impacting an area, and is a result of overall sea level rise. This type of flooding most commonly occurs during spring high tide, which is the highest high tide of every month. Due to the increases in water level associated with sea level rise caused by climate change, sunny day flooding is becoming more common and more areas are being impacted. Impacts from nuisance flooding are expected to get worse as sea levels continue to rise (See  Chapter 5  for more information on sea level rise). 

Various types of currents are superimposed on the tides. As tides flow in and out of inlets, they can create a strong current where the water is moving through Long Island Sound and swiftly through the inlets between the ocean and Long Island’s bays. Waves also interact with and play a major role in the movement of sediment along and across the shoreline. At the ocean beaches, waves can create a longshore current that moves along the beach. If you have ever gotten out of the water and found you were further down the beach from where you entered, then you have experienced this current. Rip currents occur as water moves swiftly away from the shoreline and can be dangerous to beachgoers who may become trapped and taken to deeper water or who try to swim against them and become exhausted and unable to get back to shore. 

Storm surges create flooding during coastal storms, such as nor’easters or tropical storms. Wind-driven currents can push ocean water causing it to pile up against the shore. In addition, storms are low-pressure systems which move over water, creating a “bulge”, or increase in the water level underneath them. As the storm interacts with land, the combination of wind-driven currents and the air-pressure bulge move onshore, resulting in flooding. At the shoreline, each storm surge is added to the usual tide, creating a “storm tide” of elevated water levels. The flooding can be exacerbated substantially if the storm-fall corresponds with a high tide, as was the case when Superstorm Sandy impacted Long Island in 2012. Alternatively, if storm surges move onshore during low tide, their impact can be lessened. 

The wind affects nearshore coastal processes. As aforementioned, waves form as a result of wind pushing on the water’s surface, which in turn can redistribute shoreline sediments. Additionally, the wind can move unconsolidated sediments on the beach. Vegetation is the main component of stopping and trapping wind-blown sand, but sand fencing can also be used to achieve this. 

Where the land meets the water at the shoreline, both the water and coastal material, typically sand, is mobile resulting in dynamic processes, such as flooding and erosion (See  Chapter 4  for more information on the impacts of these processes). 

Sand moves along the shoreline when waves hit the beach at an angle. As waves reach the shore, they pick up sediments and pull them offshore where they are captured by the next wave and brought back to the beach. This process creates a zigzag pattern and allows the sand grains to move from one location to the next, typically in a dominant direction. For example, along the south shore the dominant direction is from east to west; however, occasionally the transport could reverse at times. Sand can also be moved across the shoreline, which is known as erosion and accretion. Ultimately, these processes result in sand being constantly moved along the beach from the dunes to beyond the offshore bar. 

When sand is removed from the shoreline, it has been eroded away. Usually this sand is stored temporarily offshore in sand bars and eventually returns to the dry beach resulting in accretion. Large storm events, such as Superstorm Sandy, can also cause significant erosion in a shorter period of time. In these instances, some sediment may return to the beach, but some sediment may be moved far enough that it is permanently lost from the system. Erosion can also occur by an imbalance in the rate of longshore sediment transport. If more sand is driven by waves along the shore out of one end of a stretch of beach than is being supplied by waves at the other end, the imbalance results in beach erosion. The presence of hardened structures along the shoreline can impact these natural processes and alter the rates at which erosion and accretion occur. 

While many people think the beach refers to the dry sand that they visit in the summer, the term beach refers to the entire system from the dune to the sloping sand surface submerged offshore. A particle of sand can move anywhere along the beach profile from dune to sand bar and back. Sandy beaches along the south shore of Long Island undergo seasonal erosion and accretion cycles. As steeper waves impact the beaches during the winter months, sediment is moved from the dry beach and stored offshore in sand bars. During the summer, gentler waves allow the sand bars to migrate back onshore and reattach to the beach. As a result of this action it is normal for Long Island’s beaches to be narrower in the winter months and wider during the summer. Major storms, like hurricanes in August, can also create a temporary “winter” beach in the summer, or unusually calm periods in the winter can result in a “summer” beach in January. 

Dunes are naturally occurring shore-protective features. A benefit of dunes is that they act as protection for landward areas by holding waves at bay and acting as a levee to the storm surge. They also naturally store sand that can replenish the beach if erosion occurs. However, if storm surge reaches the dune system, waves will remove sediment from the dunes and redistribute it onto the beach. If surge overtops the dunes completely, sediment will be pushed landward. On barrier islands, like Fire Island on Long Island, this sand combines with sand carried in by the tides through inlets to create the process of barrier island ‘roll-over’, which results in the barrier island moving landward and is a natural response to sea level rise. 

Read more about Healthy Dune Systems  here .

Tidal marshes or tidal wetlands line the bays and other water bodies around Long Island. These areas of the coast flood and drain by the tidal movement of the adjacent waterbody. Similar to dune rollover, marshes have the ability to build upon themselves to remain in place in the tidewaters or to ‘migrate’ landward as sea level rises. However, if the marshland keeps up with the rate of water increase, or if it is blocked from migrating landward by adjacent development or a hardened shoreline, there is nowhere for the marsh to move and erosion occurs, ultimately resulting in conversion to open water.  

Long Island has many streams and rivers that enter into the bays, Sound, and ocean. These can be tidally influenced closer to the adjacent waterbody and are subjected to flooding during coastal storms and large precipitation events. Many of the banks of these streams and rivers have been hardened by bulkheads or other types of revetments, which have altered the natural processes of these coastline features (read more about shoreline management in  Chapter 3 ). 

Coastal bluffs are composed of unconsolidated sediments ranging in grain size, from silts and clays, to large rocks and boulders. These features are similar to dunes as they act as a natural replenisher by providing sediment when they erode to the beach below. However, unlike dunes and marshes, once bluffs erode, they are unable to recoup those lost sediments.