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Home , Coastal erosion, Sound (geography)

Coastal Processes on Long An Introduction to www.nyseagrant.org

Kathleen M. Fallon, Ph.D. Coastal Processes and Hazards Specialist email: [email protected]

Most Long Islanders enjoy the many benefits of living on or near our ocean, or bays. However, the fact that some of our land is being lost to the sea each year due to erosive pro- cesses is apparent to most, particularly those who own coastal land. What people must consider is that a particular section of the shoreline is part of a much larger system that relies on the movement of within it. is constantly being rearranged – moving offshore then back on again (sometimes at a different location) and moving along the shoreline from east to west and/or vice versa. These processes are normal; however, when a smaller amount of sediment is permanently removed from the system, the deficit is considered to be an erosion problem.

The process of is defined by the removal of , , and/or bluff by the physical forces of wave action, , currents, high winds, or a combination Top photo: This bluff is eroding; the face is unvegetated and slumping. As a result, the houses on top are in danger of sliding off unless immediate action is taken. of these. Shoreline erosion can be very complex and most Bottom photo: This bluff is stabilized by terracing the slope and installing a landowners are not familiar with the multitude of factors that at the toe. Image credit: Jay Tanski are creating their erosion problems. There are treatments for erosion, perhaps not cures, but the choice of a particular tactic depends on a clear idea of the cause(s) of erosion, as well as what asset needs protection.

The purpose of this fact sheet is to assist landowners in identifying and understanding the primary causes and processes responsible for their erosion issue. With a general understanding of these causes and processes, the owner can move forward, with the appropriate professional assistance, and effectively deal with the specific erosion problem they have encountered.

New York Sea Grant 146 Hall Stony Brook University Stony Brook, NY 11794-5002 New York’s Sea Grant Extension Program provides Equal Program and Equal Employment Opportunities in association with Cornell Cooperative Extension, U.S. Department of Agriculture and 631.632.8730 U.S. Department of Commerce and cooperating County Cooperative Extension Associations. Longshore Sediment Transport On both the North and South of the main cause of erosion and accretion1 is wave action. Waves are responsible for the movement of sediment along the beach; beach sediment is constantly being rearranged, resulting in At Risk changes in the shoreline’s shape and beach Structures profile. Most commonly, waves impacting Long Island are gentle. However, Sand Build-up coastal storms like nor’easters bring large, steep waves to our . Variations in the strength of wave attack produce movement Erosion of sand perpendicular to the shoreline, or cross-shore transport. Sand is transported Groin and temporarily stored offshore in large sand bars, which move back onto the dry beach once smaller waves return and the beach will Longshore Transport Direction grow in width and height. Therefore, eroded sediment is very rarely lost to deeper waters; the beach width shrinks and grows with Figure 1: The groin, a hard rock structure, blocks the sediment transport resulting in accumulation on the respect to a seasonal cycle. It is quite normal updrift side and increased erosion rates downdrift. Image credit: Loriann Cody for the beach to change nearly 15 feet in transport is interrupted by an or other surges will occur. The erosion that results elevation and up to 180 feet in width over hard structure, the sediment will be trapped from a storm can cause more damage in a the course of one year. on the updrift side and the beach will widen. shorter amount of time than would occur When waves approach the shoreline at However, this process increases the erosion during normal conditions over several years. any angle, currents are created that move rates at the downdrift beach as the Sometimes, Long Island is impacted by larger the sand in the same direction (Figure 1). resumes and subsequently carries away new than normal storms, such as Superstorm This phenomenon is called the “longshore sediment because the structure prevented Sandy in 2012. A storm of great magnitude sediment transport” and it carries beach and the sand from reaching this beach. will bring with it waves that are steep enough dune sand into the and moves Storm-Related Damage to remove the sediment from beaches and parallel to the coastline. This sediment is deposit it past the ‘depth of closure’. This Most of the erosion on Long Island is is defined as the depth at which sediment picked up by the wave action, transported, associated with storm impacts. As previously and later deposited further along the beach. cannot return to shallower water by further mentioned, large waves will erode the wave action. Once the sediment is past this With changes in wave condition, beach sand beach and higher water levels from storm in one place might be shifted to other places designated offshore water depth, the gentle along the shore, moved offshore, or even waves cannot return it pushed inland from the beach. If for some to the beach and it is reason more sand moves out of a stretch of permanently removed beach than moved into it due to longshore from the system. This transport, the beach will erode. results in a net loss of sediment from Along the South Shore of Long Island the beach and the when waves come from the southeast sand shoreline recession will move from east to west; this process commonly seen by typically moves sediment along the beach at property owners. a rate of up to 500,000 cubic yards each year. Additionally, the rates Although the average direction of longshore of are a transport is to the west, sand can move to concern and will be the east if and when the wave conditions discussed later in this regularly change. Due to the irregularity of fact sheet. the ’s shoreline, sediment moves in variable directions at a much slower rate of 100,000 cubic yards2 each year. On either shoreline, when sediment Aerial view of the breach created by Superstorm Sandy at Old Inlet, Fire Island National Seashore. Image credit: Kathleen Fallon

22 Dune Erosion Bluff Erosion If a storm’s surge is large enough, the During heavy rainfall, entire expanse of beach could be sand can be washed flooded and waves will begin to attack down the face of the the . Once the dunes have been bluff if it is not vegetated, flattened, infrastructure located adjacent similar to the beach and to the beach will become vulnerable dune erosion processes to large storm waves. In order to grow just discussed. Storm- dunes, vegetation can be planted; beach related increases in grass traps wind-blown sand and roots water height can also be aid in keeping the sand in place. Not detrimental to coastal only do dunes act as barriers, like , bluffs. When waves against but they also begin to attack the play an important role in natural beach base of the bluff, which nourishment. Sediment stored in dunes is normally protected can replace the sediment that was lost from wave conditions When people walk on bluffs, they risk trampling plants and/or loosening sand to erosion. Washovers are a concern by a beach, sediment in the face. Image credit: Cornell Extension Marine Program for barrier and occur when will be removed from waves cut away the beach resulting in the base (i.e., toe) breaches that connect the ocean water resulting in undercutting and leaving to the area. These have occurred on a vertical scarp. This will make the separates larger chunks of sediment, which Long Island’s barrier system in the past, entire bluff unstable due to continual slide down the face. Property owners can most notably when Superstorm Sandy slumping of bluff material from above usually remedy erosion issues caused opened multiple inlet breaches along the eroded toe, which in turn removes by groundwater by installing a drainage Fire Island. the vegetation that provides stability system that will intercept and transport and protective cover, furthering erosion. the water down the bluff face in a pipe. Typically, the addition of a bulkhead or Landowners can take some measures revetment installed at the toe can aid in in order to lessen the amount of surface protecting against erosive wave attack. runoff running over and groundwater To provide further erosion control, moving through their bluff. One practice additional processes that are also acting would be to build a berm or ridge of soil on the bluff must be considered. One along the top of the bluff, preventing such influencer is water entering the water from flowing over the edge. To stop ground, whether by natural causes such the water from penetrating the soil and as rainfall or human-induced causes like flowing through the bluff face, it should be sprinkler systems. Long Island’s coastal directed to a drain pipe that can carry the bluffs are composed of a wide variety of water to the bottom and prevent further sediment ranging in from fine erosion. Another option would be to plant and clays to large . As the vegetation along the edge and bluff face groundwater moves, it loosens and picks to slow runoff. up material; erosion will result when the Bare, unvegetated portions of the bluff water flows out of the bluff removing face are very susceptible to erosion by sediment with it. The coarser the bluff rain and wind that pick up and remove material, the more easily this can occur. unconsolidated materials. To lessen Gullies, which form as a result of surface the effect of these processes, the bluff water flowing down or through the slope could be graded or terraced and bluff face, indicate groundwater-related vegetation planted; roots would aid in problems. In freezing temperatures, keeping the soil in place and the plants the groundwater will become trapped This dune was recently stabilized with beach grass. Once would take in excess water in the bluff. the plants mature, their roots will both trap sand to grow in the bluff and expand as cracks. This the dune and keep sand from blowing away. Image credit: Jay Tanski

3 Montauk NY Gauge 0.60

Linear Mean Sea Level Trend 0.45 Monthly Mean Sea Level (Average Seasonal Cycle Removed)

0.30

0.15 Figure 2: Sea level has risen steadily at Montauk, the eastern most point on Long Island. 0.00 Current Sea Level Rise rate is approximately 3 mm/yr. Image credit: Loriann Cody METERS -0.15

-0.30

-0.45

-0.60 1940 1950 1960 1970 1980 1990 2000 2010 2020

While property owners on bluffs want areas becoming more susceptible to Conclusion to protect their infrastructure and in- flooding. Dunes will shift further inland Coastal erosion processes on Long vestment, they need to consider that as the barrier islands attempt to “roll- Island can be complicated. Therefore, bluff erosion is the primary source over” and move closer to the mainland. before attempting to solve the erosion of material for the beaches below. Coastal property owners on Long Island problem, it is important for landown- Bluffs can be protected with a variety already experience flooding during full ers to identify the main cause of their of measures, however these tech- moons and storms; with higher sea lev- erosion issue. A quick fix might result in niques may also deprive the beach els, storm surges from tropical systems a greater issue down the line. A better of sediment. Therefore, complete and nor’easters will only be greater. understanding of erosion processes will stabilization may result in the reduc- result in selection of the right solution. tion of beach size and eventually in its Landowners who are already struggling disappearance. with erosion of their coastal bluffs will More information about coastal and unfortunately only face more issues. As bluff erosion and on erosion control higher water levels flood their beach, methods is available from Sea Grant’s Sea Level Rise waves will attack further up the bluff Marine District Extension Office at On Long Island, sea level is rising close eroding away more sediment and (631) 632-8730. to the global rate3. Rising water will resulting in a greater amount and faster Foot Notes: have many adverse impacts, however frequency of slumping. Although sea 1Accretion is by definition the opposite of this fact sheet focuses on the impact levels are rising slowly (currently about erosion; the widening of a beach due to the permanent addition of sediment onto the shore. higher sea levels will have on erosion one inch per decade) on Long Island, processes. As sea levels rise, shorelines this is something property owners 2A standard dump truck holds about 20 cubic yards of sand; therefore, the rate of will move further landward resulting should be considering now as they deal sediment movement on the South Shore in permanent inundation and dry would be equivalent to the loads of 25,000 with their current erosion issues. trucks and on the North shore, 5,000 trucks. 3The global sea level is rising at an average rate of 3 millimeters per year.

www.nyseagrant.org

March 2018

New York Sea Grant is part of a nationwide network of 33 university-based programs working with coastal communities through the National Oceanic Atmo- New York Sea Grantspheric is part Administration of a nationwide (NOAA). network Sea of 33 Grant university-based research and programsoutreach programs working with promote coastal better communities understanding, through conservation, the and use of America’s coastal resources. National Oceanic AtmosphericSea Grant is Administrationfunded in New York(NOAA). through Sea GrantSUNY research and Cornell and Universityoutreach programs and federally promote through better NOAA. understanding, conservation, and4 use of America’s coastal resources. Sea Grant is funded in New York through SUNY and Cornell University and federally through NOAA.