A short history of the fall and rise of beaches and dunes on the developed coasts
We don’t think about it much when we enjoy the beach, but the dune-beach-nearshore system’s most important natural function is to dissipate wave energy, to protect the land. Mangroves, coral reefs and sea cliffs serve the same function in other parts of the coast.
The wave energy comes from storms and the ocean waves carry this energy up to thousands of miles from the origin almost without energy loss. Only when they get to shallow waters of the nearshore do they start to feel friction of the bottom. Moving the sediment of the nearshore and beach (usually sand or cobbles) and breaking allow the waves energy to finally dissipate.
The dunes form the reserves of sand to help handle the large storm where the energy available overwhelms the beach and tears into the dunes. The sand pulled off the dunes in a major storm gets dragged to the nearshore, extending the area of the shallow nearshore where the energy dissipation takes place. Offshore sand bars often from from the sand washed out the dune in a storm
When the storm passes, the normal wave energy slowly pushes the from the sandbars back onto the dry beach. There the occasional onshore winds strong enough to pick up and move sand will blow it inland, much of which is trapped by the wrack and dune vegetation to rebuild the reserves for the next major storm.
Complicating things is the fact that during the wave dissipation, the sand moves along the shore in what is known as littoral drift. The direction of the drift varies with seasons and even day to day but there tends to be a dominant direction of sand flow. If sand is being moved off a section of the beach in one direction, the beach and dunes will only be able to maintain themselves if the same amount or more gets moved in from the other or updrift direction.
Over the millennium before the arrival of human engineers, the beach and dunes reached an equilibrium position, with occasional temporary upsets. New sediment arrived by rivers from the breakdown of continental mountains and the slow erosion of coastal cliffs or coral reefs to balance that swept into and left in estuaries or deep sea canyons. With all in balance, the nearshore, beach and dunes worked together to survive hurricanes and Nor’easters year after year without losing beach long term.
But sailors up and down the coasts came to want deeper channels for navigation. Engineers built jetties and dug deeper channels which opened navigation but served to block the natural supply of sand. As the beaches and dunes began to erode because they were starved for sand, engineers built groins and breakwaters to slow the loss. The engineers did not create any new sand, so their effort only sent erosion downdrift.
Structures that were once protected by the beach and dune soon found themselves at risk of being swept away by the next storm waves. Seawalls, retaining walls and stone revetments were built to protect coastal property, helping to reduce the risk of destruction. However, they reduced the value of the beach and dune as habitat and increased the sand losses by reflecting wave energy and sand back out to sea, not dissipating it and keeping the sand on the beach. In the US, this process peaked in the mid 1970 when the “Beach” in Miami Beach was gone
Soft Engineering Approaches
Coastal scientists and engineers started to recognize that the “hard engineering” approaches often caused much more harm than good and we started to look to soft approaches. Sand being “softer” than concrete or rock, it was the first thing explored and indeed the Miami Beach problem was solved by pumping about 25 million cubic feet of sand on the shore to nourish the beach and dunes back to life. Later, when Super Storm Sandy wiped out the malnourished dune system and caused billions of dollars of damage to New Jersey and New York, the solution was to build a much larger dune system with sand and bulldozers
What are natural and nature-based solutions?
While soft engineering solutions work better in most cases than the “Hard” , it soon was recognized that the natural coastal work of the waves, winds and currents continued, unabated regardless of the solution type. So it seemed appropriate to try to work with, instead of against, the natural processes. It became recognized that beach nourishments worked best when the sediment provided the beach most closely matches the natural sediment. In many cases it encourages retreat from developed locations that the natural forces will just overwhelm.
On the beaches and the dunes it recognized that when sand is available, that nature builds dunes when the winds move sand off the beach and it gets trapped and then anchored in a dune by the native vegetation. This vegetation is most famously sea oats and American beach grass on the east coast of North America but several species play an important role. On the Mediterranean climate of west coasts of California and Mexico, topography can play a very large role in dune creation and stabilization.
Natural dune formation process
Aeolian (wind) processes form the dunes in nature. Onshore winds strong enough to move dry sand from the beach carries along the ground until it is slowed by hitting wrack (wave deposited most organic trash) materials or vegetation. The slowed wind lacks the energy to move the sediment further and the sand drops, adding to the dune. Smaller sand particles get moved easier and further than larger ones and at lower wind velocities, leaving dunes sand much finer than the source beach berm
The dune building vegetation includes very hardy plants in their ability to handle the salty air and wind, dry conditions and occasional salt water inundation. They can also survive constant burial in sand. The wrack materials left by waves at higher tide levels also forms a natural wind block and dunes can start to form there. The wrack also provides organic nutrients to help feed the dunes vegetation.
Nature based process of dune formation
Assuming availability of sand, wind and room, dunes will establish or reestablish themselves on the beach, relying on vegetation that floats in or grows in on its own. The waves will assure that the dune building does not take place too close to the ocean itself
We can try to imitate natural processes and speed them along in several ways. These are listed in the order of preference, at least in the eyes of the Dune Science Group:
We can plant the dune pioneer plants above the wrack line. This will encourage the building of a strong dune without depending on chance for the “right” variety of vegetation arriving.
Again while the sea oats and American beach grass get the most attention and legal protections in several states, other plants like panic grass, morning glory varieties, beach elder play their roles in a natural -like dune.
The natural vegetation used in Dune Science Group restorations are selected and positioned for the specific beach and dune condition to create the strongest most natural dune with the highest habitat value as quickly as possible. Once planted, the vegetation is selected to build the dune at the location to as tall as can be developed without additional human input
Sand fences simulate the vegetation slowing the sand carrying wind to form a dune. Frankly they can initially do it quicker than recently planted vegetation. Fence also can be an effective barrier to keeping people out of dune restoration, especially where high foot traffic is involved. However, simple post and rope demarcations with signage often proves very adequate to protect the dune restoration from unintentional damage from people.
The disadvantages of sand fences are that they do not root and they make it difficult for later arriving vegetation to root all the way down to the water table. They also do not grow with the growing dune and need to be pulled out and raised when they are covered to depth of ¾ of their height. They form a barrier to wildlife and can be a particular problem for nesting sea turtles. And because dunes are meant to be eroded to some degree in storms smaller than “THE BIG ONE” they often form trash in a post storm cleanup. The Dune Science Group only employs sand fencing under special conditions when there is no other way to keep sand from being blown off the beach and only temporarily until replaced by vegetation.
Since sand is more natural of a beach flooding barrier, we can classify a manmade dune as a last resort structure. Such dunes should be vegetated and contoured to duplicate as much as possible real dunes Growing dune vegetation on even a contoured dune can be difficult because the water table will be lower in the dune and the plants will have a difficult time growing roots down to it. Irrigation will likely be required to permit the normal hardy dune vegetation survive
Sand fencing use may be more acceptable on an artificial dune but growing vegetation on these dunes is critical to prevent the high and dry sand from blowing away and inland ( or back into the sea.) It is also important to improve the beauty of the dunes and the habitat function to make concern over blocking views of the beach more palatable.
The manmade dune needs a lot of work and attention but they are preferable to seawalls and revetments after a major storm strips away all protection against storm waves. Natural processes can take a decade or more to restore natural protection to pre-storm levels. After Super Storm in New Jersey, it was determined to select the last option available, a one-size fits all manmade dune running the length of a considerable part of the state’s shoreline.
The rise of dunes and beaches on developed coasts
By imitating nature and restoring sand supply via beach nourishment and sand bypassing work, removing hard engineering structures and artificially building dunes, we have been able to restore some positive natural and nature-based beach and dune processes.
A classic example is on Amelia Island in far Northeast Florida. This barrier island, the most southerly of the so called Sea Islands of the Georgia Bight. The 1770 British map of the island shows a “High hill of sand making a natural dyke or fence against the sea” running the entire 13 mile length of the Atlantic beach. But a hundred years later the channel on the St Marys River on the north end was too shallow for the new steamships. So engineers started building the jetty to deepen the channel, which led to dredging the channel to make it deeper yet and then to maintain the depth.
These actions blocked the natural continuous sand supply needed to sustain the beach and dunes and over the next hundred years, the beach and dunes retreated an estimated 450’. When Hurricane Dora made landfall 50 miles to the south at St Augustine in 1956, the storm surge wiped out the remaining protective dunes at several spots on the island.
The US Army of Corps of Engineers would then build what would become their standard rock revetment to protect what got rebuilt near the beach.
Around 1978, the Navy realized that sand which kept filling its channel on the St. Marys was the same sand that nature had been pushing south to Amelia Island and points south throughout most of the Holocene. So they started dredging and moving it to the nearby north end of the beach not unlike nature intended. A detailed beach nourishment plan would take about 30 years to come to fruition, resulting in the 2008 beach nourishment, the first completed by the USACE-City-County sponsored Nassau County Shore Protection Project. Since the beginning of sand nourishment on the island about 18 million cubic yards of sediment have been place on the beaches of the island
The result is that with wind and sand again available, the natural processes that never stopped have been rebuilding the protective dunes. The rock revetment has been buried under many feet of sand dune. Some of this happened without any human participation but more came with the assistance of landowners and neighbors planting dune vegetation, building walkovers across the dune, and installing sand fences
As much progress as has been enjoyed, in 2017, several hundred beachfront property owners found they did not have a large enough dune in front of them to keep themselves out of the FEMA Special Hazard VE 100 year flood zone. And other beachfront property owners came to realize that during the half of the year that is the Atlantic tropical cyclone season that you can’t have too much dunes to protect you. Especially in the face of climate change and sea level rise .
So the Dune Science Group was formed to use science to help Nature build and protect our dunes. We believe that not only does Nature build the strongest flood protection but it also creates a very unique habitat for very special flora and fauna. We were founded in the beautiful recovered dunes of Amelia Island where most of our work and our research continues, but we are anxious to share the lessons and approach with other communities.
Note: See News section for latest Dune vegetation catalog