How Do You Rebuild Sand Dunes?
How beaches work
We don’t think about it 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. How do you rebuild sand dunes to serve this purpose after they’ve been destroyed by natural or manmade events? To answer that question, you need to understand how beaches work.
Wave energy comes from storms. Ocean waves carry this energy as much as thousands of miles from their origin. That power is transferred with almost no energy loss. Only when waves reach shallower nearshore waters do they begin experiencing friction from the bottom.
Contacting and moving the sediment of the nearshore and beach – usually sand or cobbles – and “breaking” allow wave’s energy to finally dissipate.
Dunes form the reserves of sand to abate large storms where available energy overwhelms the beach. Dunes are a shield. A second, more substantial line of defense for the land from the most powerful of storms. They are designed by nature to be destroyed by storms and then rebuild during their interim.
Sand pulled off the dunes in a major storm is dragged to the nearshore, extending the area of the shallow nearshore where the energy dissipation takes place. Offshore sand bars often form from sand washed out of dunes in storms.
When storms pass, normal wave energy slowly pushes the sand from the recently created sandbars back onto the dry beach. There, occasional onshore winds are strong enough to blow sand inland, much of which is trapped by dune vegetation to rebuild the ecosystem’s sand reserves for the next major storm.
How nature builds sand dunes
Aeolian (wind) processes form sand dunes in nature. Onshore winds strong enough to move dry sand from the beach carry that material along the ground until it is slowed by hitting wrack – wave-deposited, mostly organic detritus such as washed up seaweed – or vegetation. The slowed wind lacks the energy to move the sediment further and the sand drops, little at a time, adding to the dune’s mass. Smaller sand particles move more easily and further than larger ones and at lower wind velocities, leaving dune 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.
How do you rebuild sand dunes? By naturally recreating these processes.
Dunes at the Ritz Carlton Amelia Island
Littoral Drift
Complicating this process is that during wave dissipation, sand moves along the shore in what is known as littoral drift. The direction of the drift varies with seasons and even from 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 millennia, before the arrival of human engineers, beaches and dunes reached an equilibrium 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 sand swept into and left in estuaries or deep sea canyons. With nature in balance, the nearshore beach and dunes worked together to survive annual hurricanes and Nor’easters without losing beach dimensions long term.
How man destroyed beaches and dunes
Upsetting this harmony were sailors and industry up and down the coasts wanting deeper channels for safer navigation and the ability to bring larger and larger vessels into port. Engineers built jetties and dug deep channels which opened navigation but upset the natural supply of sand. As the beaches and dunes began eroding as a result of being starved for their natural replenishment of sand, engineers built groins and breakwaters to slow the loss. Coastal engineers did not create any new sand, so their efforts only sent erosion down-drift.
Structures that were once protected by the beach and dune soon found themselves at risk of being swept away by powerful storm waves no longer dissipated by nature. Seawalls, retaining walls, stone revetments and other intrusive and expensive “gray infrastructure” was built to protect coastal property, helping to reduce the risk of catastrophic destruction. However, these unsightly manufactured impediments reduced the value of the beach property and destroyed dunes as a natural habitat. Furthermore, they increased the sand losses by reflecting wave energy and sand back out to sea as opposed to dissipating it temporarily and keeping sand on the beach.
In the United States, this process peaked in the mid1970s when the “Beach” in Miami Beach was gone. This left entire city’s and, in fact, the nation wondering: how do you rebuild sand dunes?
How are sand dunes rebuilt naturally?
Coastal scientists and engineers started recognizing that their “hard engineering” approaches often caused more harm than good and started exploring more natural, “soft approaches.”
Sand being “softer” than concrete or rock, it was the first option explored. 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. Decades later, when Super Storm Sandy wiped out the malnourished dune system in New Jersey and New York resulting in billions of dollars of damage – $30 billion in New Jersey alone – the solution was to build a much larger dune system.
While “soft” engineering solutions work better to restore dunes and beaches in most cases than the “hard” solutions, it soon was recognized that the natural coastal ebb and flow of waves, winds and currents continued, unabated, regardless of the solution type.
It then seemed appropriate to try working with, instead of against, these natural processes. That’s the key to understanding how do you rebuild sand dunes. Beach nourishments became recognized as working best when the sediment provided – the extra sand added to the beach – most closely matched the natural sediment.
On beaches and dunes where sand is available, nature rebuilds dunes through wind moving sand off the beach which then becomes trapped and anchored by sand dune native plants. This vegetation is most famously sea oats and American beach grass on the east coast of North America, but dozens of species play an important role.
Natural dune restoration
While the sea oats and American beach grass garner the most attention and legal protections in several states, other plants like panic grass, morning glory varieties and beach elder play important roles in natural dune restoration.
Natural vegetation restorations are selected and positioned for specific beach and dune conditions to create the strongest, most natural dune with the highest habitat value as quickly as possible. The native plant designs build the dune by anchoring it with their deep roots and capturing blowing sand at that location to grown as tall as possible without additional human input.
Sand, of course, is key when wondering how do you rebuild sand dunes, but native plants do the work.
Sand fences
Sand fences simulate vegetation by slowing sand-carrying wind to form a dune. Initially, they work more quickly than planted vegetation. Fence can also be an effective barrier to keeping people out of sensitive dune restoration areas, especially where high foot traffic is involved.
Sand fencing’s advantages are limited, however, in that they do not root into dunes, anchoring them like native plants. The quickly piled up sand made possible by dunes can also make it difficult for later arriving vegetation to root all the way down to the water table. Planting native vegetation in coordination with installation of sand fencing is preferred.
Sand fencing also does not grow with the growing dune and needs to be pulled out and raised once covered to depth of ¾ of its height.
Sand fencing additionally forms 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 smaller storms, they often end up as trash in a post-storm cleanup.
Sand fencing should only be employed 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.
Assuming availability of sand, wind and room, dunes will establish or reestablish themselves on the beach, relying on vegetation floating on shore or growing in on its own. Waves assure that the dune building does not take place too close to the ocean itself.
Coastal engineers imitate natural processes and speed them along in several ways.
Dune pioneer plants are planted above the wrack line. This encourages the building of a strong dune without depending on chance for the right variety of vegetation arriving.
Sea oats and native plants help anchor sand dunes
Native plants installed as part of an Amelia Island dune restoration
Manmade sand dunes
Manmade dunes are a last resort structure. Such dunes should be vegetated and contoured to duplicate real dunes as much as possible.
Growing dune vegetation on even a contoured dune, however, can be difficult because the water table will be lower in the dune and plants will have a difficult time growing roots down to access 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 again, 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 aesthetic beauty of the dunes and their ecosystem services to make concern over blocking property owners’ views of the beach more palatable.
For all their problems, manmade dunes are vastly preferable to seawalls and revetments for rebuilding sand dunes after major storms strip away natural protections. Natural processes can take a decade or longer to restore protections to pre-storm levels. After Super Storm Sandy, 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. This best of a series of poor options was the result of the state and property owners ignoring their deteriorating coastal sand dune for decades. As is often the case, an ounce of prevention is worth a pound of cure.
Amelia Island dune restoration
By imitating nature and restoring sand supply via beach nourishment and sand bypassing work, removing hard engineering structures and artificially building dunes, Dune Science Group been able to restore natural and nature-based beach and dune processes.
A classic example is on Amelia Island in far northeast Florida. This barrier island is the most southerly of the so-called Sea Islands of the Georgia Bight.
A 1770 British map of the island shows a “High hill of sand making a natural dike or fence against the sea” running the entire 13-mile length of the Atlantic beach. One hundred years later, the channel of the St. Mary’s River on the north end became too shallow to allow for larger, deeper draft steamships. Coastal engineers built a jetty to deepen the channel, which led to dredging the channel to make it deeper yet, and then continued dredging to maintain the depth.
These actions blocked the natural and continuous sand supply needed to sustain the area’s beach and dunes. Over the next hundred years, the beach and dunes retreated an estimated 450 feet. When Hurricane Dora made landfall 50 miles to the south at St. Augustine in 1956, the resulting storm surge wiped out the remaining protective dunes at several spots on the island.
Amelia Island and the City of Fernandina Beach had a crisis of how to rebuild sand dunes on their hands.
In response, the U.S. Army of Corps of Engineers built what would become their standard rock revetment to protect properties rebuilt near the beach.
In the late 1970s, the Navy realized that the sand which kept filling its channel on the St. Mary’s River was the same sand that nature had been pushing to Amelia Island and points south for eons. New dredging efforts began moving this sediment to the nearby north end of Fernandina 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, roughly 18 million cubic yards of sediment have been placed on the beaches of Amelia Island
The result is that with wind and sand again available, the natural processes that never stopped have been rebuilding the protective dunes. The ugly, old 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 out of the FEMA Special Hazard VE 100 year flood zone. Other beachfront property owners came to realize that during the half of the year that is the Atlantic tropical cyclone season, you can’t have too much dunes protecting your family and property – especially in the face of climate change and sea level rise .
Not only does nature build the strongest storm and flood protection, but the unique habitat created for the area’s animals and plants is of great value as well.
Amelia Island beach before natural dune restoration
Amelia Island after natural dune restoration