Shorelines are always changing and require coastal erosion solutions to prevent land loss or structural damage. The severity of the erosion can vary significantly depending on the site conditions and season.
It should be noted that some beach erosion occurs naturally during storm seasons, however, at a ‘healthy beach’ the sand will 'self-re-nourish’ over time. Human interference at the site (or nearby sites), can disrupt the natural, dynamic cycle and therefore a coastal erosion solution (or multiple solutions) are required.
Coastal engineers have developed processes and procedures to evaluate the causes or erosion and determine the most suitable outcomes for the site, taking into consideration things like:
The natural site conditions
Coastal erosion solution type (hard armour, softer solutions including green and blue)
Cost
Aesthetic
Disruption to surrounding areas
Longevity and Durability
The following article reviews the causes behind erosion, the erosion process, different coastal erosion solutions and the method by which the best suited design should be determined.
What is Coastal Erosion?
In most cases, coastal erosion relates to beach loss. This can lead to property loss and severe damage.
Defined as the mechanical grinding and wearing away of natural surfaces, erosion can happen in a variety of ways and over a range of time. Beach erosion does occur naturally and, in some cases, can be temporary.
It is possible to measure and explain the retreat of the shoreline by using the tides, the seasons, and other short-term cyclic events in a site investigation study. This will be used to determine the best solution for the site.
Coastal Erosion Causes
Coastal erosion is the process by which sand, soil and rocks are removed from the coastline by natural forces such as water, waves, currents, tides, wind-driven water, waterborne ice, and other types of storm impacts.
Depending on where you are in the world it could be any one or a combination of forces that is causing the erosion. Each process can have different impacts. There are however, a lot of beach erosion prevention solutions.
Effects of Coastal Erosion
Different types of coastline are affected differently by erosion.
In areas where rock layers meet the sea, coastal erosion produces rock formations. Softer sections erode much faster than tougher ones, resulting in landforms like as natural bridges, pillars, and columns.
Typically, the coastline levels off with time. The softer areas get filled with silt and sand eroded from the harder areas, and the rock formations are eroded away. Leaving headlands.
These kinds of large-scale erosion happen over very long periods of time and in most cases coastal communities or private properties are concerned with the more short-term effects that are already causing land loss or damage to property.
Without proper design coastal erosion solution, the land loss and damage can intensify quickly, especially heading towards storm season.
How is Erosion Caused by Rivers?
In many coastal areas where rivers meet the sea, there are waterways directly affected by river erosion.
When water erodes the banks of a river or waterway, it is termed river bank erosion. Although river bank erosion is a natural process, it may be accelerated by human activity. Common elements leading to river and waterway bank erosion consist of:
Removal of natural vegetation along the river bank
Excess flooding and rain
Interference with the natural tidal flow of the water way
The consequences of erosion on river banks are not confined to the area in which the erosion is taking place; rather, they are likely to be felt farther downstream as well. As a result of erosion, more debris is carried downstream, which may change the course of the river and obstruct navigation routes.
There are several ways coastal erosion solutions specifically for river banks/canal ways and marine waterways that will be discussed further below.
What are the Effects of Wind Erosion?
The natural process known as "wind erosion" involves the movement of soil from one location to another by the force exerted by the wind. It is possible that it may do significant damage to landscape and infrastructure.
Wind erosion may be caused by even a gentle breeze that moves soil particles over the surface, but a strong wind can produce dust or sand storms by lifting a significant number of soil particles into the air.
Even though wind erosion is more common in arid regions, coastal sand dunes, and beaches, certain geographical features may also cause wind erosion. Therefore, wind is a primary agent of erosion; nevertheless, the topography and condition of the land are mostly to blame for the wind erosion that does the greatest damage.
Reducing the impacts of wind erosion are one of the considerations covered in the design of coastal erosion solutions and are discussed in more detail in the sections below.
Sea Level Affecting Erosion?
According to The Intergovernmental Panel on Climate Change coastal erosion will accelerate globally as a result of sea level rise induced by climate change, resulting in severe changes to coastlines and low-lying coastal regions.
There is no doubt that as sea level rises and storm severity increases there will be significantly more strain on coastline and coastal properties/infrastructure.
In coastal engineering design (especially in the case of coastal erosion solutions) sea level rise is taken into consideration.
There are various computer model predictions used around the globe to look at expected sea level rise amounts over the coming 100 years. There are also accepted sea level rise values that have been adopted in different locations which should be used in the design process by coastal engineers.
Examples of Coastal Erosion Processes
There are three main types of coastal erosion processes:
Hydraulic action - this is the force of the waves when they slam on the rock.
Abrasion occurs when pebbles scrape on a rock platform, similar to sandpaper.
Attrition occurs when boulders carried by the water collide with one another.
Each process result in slightly different outcomes. And in many cases, site erosion can include a combination of all three. The design process for coastal erosion solutions can take into consideration the different erosion process to bes determine which solution will give the best outcome.
How to Prevent Coastal Erosion
In different parts of the world, people have tried a wide range of different strategies to cut down on the amount of beach erosion. Although a few of these tactics have shown to be very successful, each one comes with its own set of benefits and drawbacks.
They typical process to prevent coastal erosion is to first study the site, find out why the erosion is occurring, then review different strategies and solution methodologies to determine which (or what combination) will work best for the site.
Coastal erosion solutions should be designed by professional coastal engineers as it has been proven time and time again that incorrect installation of erosion control methods can actually do more damage and cause greater levels of erosion both locally at the site and in nearby sites.
Coastal Erosion Solutions
The only real way to stop erosion from happening is to stop the natural process (ie. stop the wind and waves). In some cases, this is possible, where high visual impact is not a concern, however, in many cases to completely stop the natural conditions will drastically impact the site (both visually and environmentally), and therefore a level of reduction would be acceptable.
There are a few different approaches to coastal erosion solutions, including the following main types:
Hard coastal erosion solutions
Hard Coastal erosion solutions are considered to be a form of structural defence that are designed by coastal engineers to stop or reduce wave or tidal impacts on shoreline or existing structures.
There are several different types of hard structures:
Seawalls
Seawalls are not only used as a beach erosion prevention method, but they can be used in a variety of site scenarios including reclamation and river banks/waterways. Seawalls can be made from a varity of material and designed in different ways to perform as required on site. Some of the different types of seawalls are highlighted below:
Curved Face Seawall
A seawall with a curved face is intended to resist intense wave action. Curved faced seawalls are typically made from concrete and direct wave energy up the curve to dissipate the energy rather than reflect the energy (which happens in vertical walls).
Stepped Face Seawall
A seawall with a stepped face is used to moderate wave action. This type of seawall is made out of reinforced concrete sheet piles that are put together with tongue-and-groove joints (it can also be made using gabions or geotextile sand containers). Between the piles, the spaces are either filled with grout to make a sand-proof cut-off wall or geotextile fiber is installed at the back of the sheet pile to make a sand-tight barrier. Putting down geotextile is a good idea because it lets water seep through and stops water pressure from building up.
Rubble Mounded Seawall
Design and development properly this seawall layout may be simpler and less expensive that the others mentioned above. It can withstand very powerful wave energy as it’s permeable finish allows for good wave dissipation. They are also slightly ‘flexible’ in that even though the beach becomes eroded at the base of the structre, the seawall's quarry stone may be readjusted and settled without structural collapse.
Bulkhead or Quay Walls / Vertical Walls
Bulkheads may be made from concrete, steel, or wood. There are two primary types: gravity structures and sheet pile walls. Vertical walls general suite locations that are not subjected to very powerful wave movements and their primary function may be to retain soil, but the designer must consider scour at the structure's base. Cellular sheet pile bulkheads are used when rock is near to the surface and sufficient penetration for an anchored bulkhead cannot be attained. Sheet pile design should be done by coastal engineering specialists that take into consideration the various strain moments on the wall.
Groynes
Gryones (or Groins) are shoreline protection structures that reduce coastal erosion by altering offshore current and wave patterns. Groyne may be constructed from concrete, stone, steel, or wood, and their classification depends on their length, height, and permeability. Groynes impact the natural longshore flow of sand and cause a jigsaw like finish to a shoreline and must be designed correctly otherwise they can create down drift erosion problems.
Breakwaters
Breakwaters can be either connected to shore or completely dethatched from shore (offshore breakwaters). They are popular form of coastal erosion solution for areas that want uninterrupted access to the beach front (unlike groynes that disrupt the beach flow).
Breakwaters can be designed and built to have varying levels of wave energy reduction. From full wave reduction (when the breakwater crest emerges fully out of the water high enough to block storm waves). They can also be low-lying (semi submerged) which can significantly reduce the wave energy without being visible all of the time (and also using less material to build resulting in cheaper installation costs).
Artificial Reefs
Built offshore and out of site, these can be some of the best solutions for sites that want low visual impact. Artificial reefs are designed to be multifunctional and can both reduce wave energy on shore while enhancing the local marine habitat and environment.
Unlike offshore breakwaters that can be highly emergent out of the water and reduce all the wave energy, artificial reefs are always submerged and allow some wave energy to pass over. Depending on the size of the artificial reef (height below the water level and crest width) the reef can be designed specifically to take out certain amount wave energy to allow for energy reduction while keeping water flow/circulation at the site.
Designing Multi Purpose Artificial Reefs can have significant improvement on coastal resilience.
Floating Barriers
Compared to the more traditional fixed breakwaters, floating breakwaters provide an alternate approach to the problem of protecting a site from waves. It is more likely to be successful in coastal regions when the wave environment is relatively calm. As a result, they are more often used with the purpose of reducing erosion at a waterway entrance and preserving small boat harbors and marinas. A few of the factors that work in favour of floating breakwaters are as follows:
Deep water – for sites with deep water can be a cheaper alternative to use floating breakwater to save on large volume of material required to breach the surface
Less disruption to flow of fish species – the floating barrier allows for marine life to pass with minimal disruption, compared to large, solid structures
Different Types of Material Used for Hard Coastal Erosion Solutions
There are several different coastal erosion solutions that can be used at one site. And often a combination will render the best results.
There are several different materials that can be used to build the different design options. And there is no ‘best’ option as each site will be different and have different requirements both for the outcome of the erosion protection, costs, visual and environmental impacts.
Rock
In many different coastal environments around the world, rock is used for coastal erosion solution structures. The process of rock design and construction is very well document and analysed with precise formulas derived to determine suitable rock wall slope, height, width and layer thickness.
Depending on where in the world the site is located, there may be access to different grades of rock. Rock typically used for large scale construction comes from quarries and is broken specifically into a variety of sizes for use in the rock wall design.
Each rock type has different density and therefore will have different design qualities. Rock is so commonly used because of a few reasons:
It is permeable, which means some of the wave energy can actually pass through the rock itself, which acts as a way to dampen the wave energy without completely reflecting the wave energy as per a flat surface.
It is slightly flexible, which means that if small shifts in the seabed occur the rock has the ability to settle into gaps without loosing structural integrity. Unlike a solid concrete surface for instance that can crack and loose shape if it shifts slightly due to sand loss underneath the structure.
It is relatively easy to build. Staking rocks is typically a simpler installation process than some of the more technical product alternatives
Sand Filled Geotextile Containers
More and more, shore protection structures, especially along sandy coasts, are being asked to have less of an effect on the environment and the way things look than traditional structures like groynes and revetments. Also, these measures of reinforcement and protection must be cost-effective. This means using local materials and no heavy equipment, especially when the necessary infrastructure is not there.
As a fill material, geo-containers have the advantage of being able to use the sand that is native to the area even in places where there is no access to rock material. As a direct result of this, it is possible that transportation costs and the environmental impacts they cause will be reduced. Because geocontainers may be transported up to a certain size without the need for heavy equipment, this might lead to a reduction in the costs associated with constructing. In contrast to conventional revetments and other types of hard structures, geocontainer constructions can easily be removed if needed (for temporary emergency erosion protection).
Large scale geotextile containers can also be used for coastal protection structures. The size of the tubes can vary depending on the requirements of the design, however, can be large (a few meters high) and weigh thousands of tones once filled.
These large tubes can be used on shore or underwater and can either be a standalone structure or tied into a rock design whereby the sand filled geocontaienr is used as a filler or core for a rock design to reduce the rock volume required.
There are also different shaped geotextile container frames that are more rigid and stand up to be filled in long barriers. These can be stacked or integrated into other coastal designs.
Concrete
Concrete has been used in coastal erosion solutions for a very long time. It can fundamentally take on any shape or form and the results of the protection structure can vary significantly.
The benefits of using concrete in the marine environment, is that it is often easily to source and builders are experienced with using it. It can also be poured on location into many different kinds of moulds or shapes as needed.
The downside of using construe is that while it is strong and durable, it is also brittle and does not handle flexibility which is critical in the marine environment. This will depend greatly on the actual design and where the concrete structures are located.
Structures made of reinforced concrete that are placed in maritime settings often experience deterioration in the early stages of their service lives. This happens most quickly in the splash zone, where there is a lot of oxygen, which speeds up corrosion, and where wet and dry conditions make chloride penetration worse. Moisture in the concrete also makes it better at conducting electricity, which leads to rust pitting, a type of aggressive localized corrosion. This causes steel parts to break off quickly and the concrete to crack and chip.
In tidal and underwater areas where the concrete is saturated with water, oxygen levels are low because the concrete pores are always being filled with water. But corrosion can still happen in places where there isn't much concrete, which makes it hard to fix.
Wood
Wood has been traditionally used in coastal erosion solutions for centuries and is still popular in certain locations around the globe. It is used because of cost and aesthetic.
Wood has been used to make a variety of coastal erosion solutions for a very long time. This is because wood has a lot of good qualities that are important for building in water. Some of them are
For how much it weighs, it has a lot of strength
It is often easy to source
It's easy to use and keeps going for a long time
It doesn't get damaged easily.
Wood can be cut to any size, which makes it easy to use for any project and makes it very useful.
Overall, hardwoods are better than softwoods because they can last longer and stand up to wear and tear better.
There are many great ways to use tropical hardwoods. Ekki and Greenheart are two of the most popular ones. But Balau and Jarrah, which are also tropical hardwoods, have also been used for water projects.
Gabions
Gabions are wire baskets with a mesh manufactured from galvanized steel wire or heavy duty marine plastic. The "boxes" are filled with hard rock pieces and piled to create a gravity wall or other coastal erosion solutions.
Gabion walls rely largely on the compactness and weight of the rock pieces to ensure interior stability and withstand hydraulic and earth forces.
Gabions are permeable, like rock in coastal design which allows for better absorption of wave energy. They also require relatively low-level construction process and minimal machinery which can work great for remote sites.
Soft Coastal Erosion Solutions
Using natural processes to safeguard the coastline, soft engineering may be a more sustainable, long-term, and can be more cost-effective method to coastal defence depending on the site.
‘Soft’ basically means that no ‘hard’ or structural coastal erosion solutions are built to directly reduce wave energy.
Examples of soft erosion solutions include:
Beach replenishment entails importing beach-quality sediments to "top up" beaches.
Sand dune management may include the construction of walkways, ladders, and boardwalks to avoid human deterioration of the beach.
Locally lowering the water table under the beach face so that sand accumulates over the drainage system.
Nearshore nourishment is a great way to achieve mass volume for lower cost
Green/Blue Coastal Erosion Solutions
Nature has developed some amazing, natural beach erosion prevention methods. These can be considered green (for terrestrial/land based vegetation) and blue (marine/underwater vegetation/flora). Here are are a few examples of different types of natural coastal erosion solutions.
Artificial Reefs
These also fit into a blue/green solution and can be eco-engineered into promotion of local marine growth and tailored specifically to attract local fish species.
Artificial reefs can vary significantly in design, to be large masses which create offshore volume to significantly reduce wave energy offshore. Or they can be small purpose built products that individually have low impact, but when added in mass can create significant change.
Coral Planting
Natural coral reefs protect shorelines from waves, storms, and floods, hence preventing loss of marine life, damage to property, and erosion. When reefs are damaged or destroyed, the lack of natural barrier may enhance the damage caused by regular wave action and major storms to coastal settlements.
Natural coral reefs are amazing barriers for wave energy reduction and have been shown to reduce wave energy by up to 97%.
In areas where natural corals have significantly died, coral fragments can be planted. These can come from either coral farms (onshore or offshore) or directly re-positions in an act called coral transplanting.
While planting corals on a reef may have a long-term improvement on wave energy reduction, it will rely heavily on the outcome of the corals ability to survive.
In most cases where the coral has degraded to the point that it is no longer acting as a wave reduction barrier. It is likely got to that point by an array of external influences, whether natural causes or human influenced. Unless these external factors are also addressed it is likely the newly planted corals may also face the same level of eventual decline. Therefore, coral as a solution on its own may not be enough for a coastal erosion solution.
Mangrove Planting
Despite the fact that mangrove forests are often situated on coasts with low wave energy, they can experience greater waves during storms, hurricanes, and times of severe winds. Flooding and damage to coastal infrastructure may be caused by high winds and surge waves. Mangroves may possibly lessen related damage by lowering wave energy and height.
Wave height may be reduced by between 13 and 66 percent when mangroves are present at distances of more than 100 meters. Near the border of the mangrove forest is where waves experience the largest rate of wave height reduction per unit mile as they begin their trip through the mangroves.
In addition to mangrove trees, oyster reefs within the mangrove system area a great nature based solution.
Dune Vegetation Planting
With beach nourishment (soft solution) artificial sand dunes can be created or built back up to a desired level. Combined with a hard solution (dune barrier which is buried under the new dunes as an emergency last resort), the dunes can also be planted with regionally available dune vegetation. These plants 'hold' the dune sand in place and help to reduce loss from wind erosion.
How to know which coast erosion solution is right for your site?
Determining the right coastal erosion solution for your site can seem overwhelming at first. There are a whole myriad of options of varying costs and style with varying results.
For the majority of cases, you are going to want to use a professional coastal engineer to review the site and determine which option are the most suitable.
In many cases, the most effective beach erosion prevention approach will be to implement a combination of coastal erosion solutions.
The basic process in which best to determine is as follows this step by step process:
Step 1. Initial discussion with coastal engineer
This will help to determine what is happening at the site, the desired outcome of the design so that the engineer can get local knowledge of the site
Step 2. Price proposal made
This is when the coastal engineers offer a price proposal to investigate further/do the required design works
Step 3. Option Assessment & Concept Designs
Concept designs are developed for the site, looking at a few select options with their potential outcomes and costs
Step 4. A review of the concepts (client and engineer)
This offers the client time to review the concept designs to see which the preferable method of approach based on costs, predicted outcomes, aesthetic, etc.
Step 5. A detailed design for construction
Detailed design is done by the coastal engineer which includes design drawings that can be taken by a marine contractor and used to build the design on site
Considerations in the design process also include:
Sit specific information
Depending on the location of the site, a sit survey may be required to collect both land and underwater survey levels, tide, wind and wave data
Local design sand environmental standards
Different regions of the world have different design standards that may need to be adhered to
Approvals
Generally, in marine design and construction there is an approval process required before the works can take place. The approval takes into consideration environmental impacts on the site and surrounding area