To protect beach amenity value, the prevention of coastal erosion is critical. With factors like climate change and sea level rise there's never been more need for well designed measures against coastal erosion. Seawalls are a great example of one solution. But is a seawall suitable for your site/project? This article reviews the advantages and disadvantages of seawalls and how to determine suitability for you site.
What is a seawall?
A seawall is a manmade barrier constructed where the land meets the sea. Their main purposes are:
To prevent land loss through the coastal erosion process
To hold the land in position, creating a permanent/fixed border in the case of marine infrastructure, like marinas or harbours.
There are many different types of seawall (both in design and material makeup), that perform differently under various exposed conditions. And there is not necessarily one ‘best’ kind of seawall, as each site has its own unique requirements and interacts with the dynamic coastal environment in their own way.
In most cases, seawalls are exposed to the natural conditions and can therefore be seen. This can disrupt the aesthetic of a location. However, without the seawall, the natural conditions may be subject to serious erosional damage and land loss. This may be due to natural causes, such as high wave conditions, or man-made changes, such as disruption to the coastline/the natural sand and water flow which can create erosion ‘hotspots’ that need coastal erosion solutions.
Types of Seawalls
While the advantages and disadvantages of seawalls can vary depending on the design, several factors influence which type is best suited for your site. These factors include:
Cost
Visual aesthetic
Sit location in the coastal zone
Functionality and usability
Impact on the site/environment
These factors should be taken into consideration when choosing the right type of seawall for the site to deter from unwanted disadvantages of seawalls.
The most common types of seawall include:
Sloped Sewall
Stepped Seawall
Vertical Sewall
Curved Sewall
Stacked or Module Seawall
There are also combinations of the above mentioned. And, each of these different types of seawalls can be made of different types of material (discussed in the following sections).
Emergency or Temporary Seawalls
In many coastal locations around the globe the beaches are considered government or state land. Typically, in these instances, government approvals are required before there can be any construction of seawall structures (especially if they are going to directly touch/impact the beach).
In some cases, where there is a direct threat to property or infrastructure, an emergency or temporary seawall may be installed/constructed without going through the typical approval process.
Note that regulations can very significantly between states and local governments, so it is always best to check on which approvals are required (as some locations implement what is called 'managed retreat' whereby you may not be able to build any kind of seawall, temporary or not).
Emergency or temporary seawalls can be built quickly and are often made using material that is easy to remove if/when it is necessary. These include materials like sand filled geotextile containers and other emergency flood protection filling modules.
Advantages and Disadvantages of Seawalls for Emergency Works
Like all coastal erosion protection structures there are advantages and disadvantages, with the emergency or temporary works there are a few things to consider.
Advantages:
Typically, works can be performed quickly bypassing the lengthy approval process required for some other types of seawalls
Generally, as the emergency or temporary works are not designed for longevity, they can be built cheaper (depending on the materials used/site conditions) that traditionally, long-term, exposed seawalls
In many cases, the emergency works could be removed if necessary.
Disadvantages:
Quick installation of emergency works may not be the best long-term solution and therefore, the temp works may need to be removed at some point for better long term solutions
Quick, cheap solutions may not have the best aesthetic to match the site
Exposed Seawalls
This is the most common type of seawall. These seawalls are designed to be permanently exposed to the ocean and wave environment.
Seawalls interact with the coastal processes of a sandy beach in terms of onshore/offshore sand transport and local longshore sand transport when present in the active zone.
Numerous papers evaluate the influence of seawalls based on their location in the active prism and the long-term beach stability – accreting, stable, nourished, or eroding [2]. Nevertheless, a popular and too simplistic idea prevails that seawalls cause coastal erosion and destroy beaches. Consequently, seawalls are often disregarded during option evaluations.
An exposed seawall in the surf zone under wave attack would result in reflection and scour in front of the wall and/or accelerated erosion along the seawall, despite eroded volumes being 60 percent of what they would be without the seawall [1].
Refer to ICM’s Coastal Conference Paper on Terminal Seawalls for more info.
Advantages and Disadvantages of Seawalls that are Exposed
Advantages
The biggest and most obvious advantage of exposed seawalls is that they mitigate wave energy from hitting the landmass
Exposed seawalls can be used to reshape natural or man-made coastal areas by creating a solid edge/definitive line
In most cases, seawalls are used to prevent land loss behind the wall, thereby preserving property or infrastructure
Disadvantages
Seawalls by design stop/reduce wave energy from passing through or over the wall. This wave energy therefore is either reflected or redistributed somewhere else. Often, part of the wave energy is reflected back to the sea which can create an erosion hotspot at the base of the seawall itself (referred to as scour). Through proper coastal engineering design, scour can be accounted for and therefore built into the seawall design to reduce the scour effect
If seawalls are built out of the natural beach alignment the can act as a kind of groyne and disrupt the longshore sand transport to beaches/properties on the leeward side of the wall
Exposed seawalls look man-made (as they are) and can therefore take away from the natural beauty of a site (atheistic interruption)
It should be noted that proper designed seawalls by experienced coastal engineers can improve site protection and mitigate negative impacts.
Get in touch today to speak with real coastal engineering experts in the field of rock wall design .
Terminal Seawalls
Seawalls that are situated as far inland as possible from "normal" beach changes are termed terminal seawalls. These structures are only active during severe erosion events and remain buried under normal circumstances.
A terminal wall, which is often buried within the dune buffer zone, limits erosion during severe events and serves as a clear planning boundary between the active beach and permitted development.
Due to the fact that these occurrences may only occur for brief periods a few times per hundred years, the potential for negative impacts on the beach is equally brief.
Advantages and Disadvantages of Terminal Seawalls
Advantages
One of the greatest advantages of terminal seawalls over exposed seawalls is there low visual impact
As the seawall only becomes exposed during an extreme event, the storm demand is sourced from almost the entire upper beach profile and not just scour at the base of the seawall. The waves are generally depth limited and of smaller magnitude, resulting in a smaller structure with lower design requirements, less toe scour, and less overtopping
Disadvantages
Terminal seawalls only come into effect during extreme events when the rest of the beach profile has become eroded and are therefore a ‘last line of defences’ approach
Think a terminal seawall may be suitable for your site?
Materials used for seawalls
As mentioned previously, there are a wide range of materials used for seawalls. Each will have their own advantages and disadvantages depending on the site.
Hard engineering design often refers to materials in seawalls such as:
Rock
Concrete
Steel
Gabions (rock baskets)
Wood
Composite Materials
There are also ‘softer’ approaches using technologies such as:
Sand filled geotextile containers
Self-standing sand filled modules
Green solutions are considered dune vegetation or landscaping. While this would not be considered a stand-alone seawall solution, it is often incorporated into the design process to reduce the visual impact and add a more ‘natural’ look to the site on completion.
How Effective are Seawalls?
Seawall effectiveness comes down to design and installation execution and can vary significantly (depending on how well the design is done).
Seawalls can be very effective at protecting landmass from wave impact/erosion. In most cases it is not a question of the seawalls effectiveness of protecting the immediate site, however, how the seawall interacts with the surrounding coastal environment and adjacent sites is often the area of concern.
When designing and installing seawalls, careful consideration should be taken into the long-term effects of the wall on the surrounding area.
In conjunction with other coastal protection measures like beach nourishment, seawalls can be very effective in maintain both stable beaches and secure property lines.
Do Seawalls Stop Tsunamis?
Seawalls can be designed for a wide range of impact possibilities and wave conditions. From small waves (boat wake and wash), to large wave events such as Tsunamis and significant storm surge events.
Experienced coastal engineers can determine which wave conditions need to be considered when designing seawalls at a site and can accommodate this into the design process.
Designing and building seawalls to stop Tsunamis would require for instance, larger/heavier rock or modules with thicker overall width and higher design crests.
Do Seawalls Erode Beaches?
As discussed, there is a misconception that seawalls only erode beaches. One of the disadvantages of seawalls (if not designed properly) is that they can have negative effect on the immediate beach, through wave reflection and scour. They can also have negative effects on the surrounding beaches and adjacent properties.
If designed properly inconjunction with a hollistic approach to coastal protection, they can be very effective.
Therefore, it is critical to engage with experienced coastal engineers when considering seawalls as a solution for your site.
Are Seawalls Sustainable?
Sustainability in design can consider a few different elements:
The materials used
The impact on the site (and surrounding sites)
Sustainability relative to the materials themselves will vary greatly depending on the material source relative the proposed site.
For example, quarried rock is often used is seawall design and construction. In areas with accessibility to quarried rock (assuming the quarries themselves are operated in a sustainable way, which may relate to things like volume of rock available vs. time impact on the environment, etc.) the rock may be a viable choice with relatively low transport costs associated.
In areas where no quarried rock is available, there may be options to ship in rock or use locally available material such as coral rock, or sand (into containers).
In summary, there is no clear-cut answer to sustainability in seawalls as each site and design will vary significantly. It should be noted that sustainability in design is something that needs to be considered for a holistic approach.
Are seawalls expensive?
The cost of a seawall at different sites can vary significantly. Factor effecting cost include:
The design itself
Some sites will require larger seawalls to protect against high power wave/storm conditions
Some sites will require smaller walls
The material
Depending on which material type is used it will determine the constructability and associated costs, transport to site costs, etc.
In all coastal erosion protection design there are few different costs to consider:
Capital cost
The upfront cost to design and build the structure
The maintenance cost
Any ongoing maintenance that may be required
In general, higher capital cost require lower ongoing maintenance. Whereas lower capital cost may require more ongoing/higher maintenance costs.
Are seawalls affordable to maintain?
As mentioned above, the maintenance cost of seawalls will depend heavily on the type of design and material used.
For example, at a remote site where no quarried rock is available, coral rock may be used which would be a lower capital cost than importing quarried rock. However, over time, the coral rock will not likely hold up in storm conditions as long as quarry rock. Therefore, the coral rock seawall may need to be maintained with additional coral rock or completely replaced.
These kinds of capital vs. long term costs will need to be considered when deciding on seawall material and design.
What are the Advantages and Disadvantages of Seawalls?
In summary, there are plenty of advantages and disadvantaged of seawalls. For the most part, seawalls are a very effective way of maintaining a structural line for land and property protection.
The main disadvantages of seawalls are that they can create localised erosion. This can be at the base of the seawall itself or at adjacent properties.
If designed properly in conjunction with a hollisitc approrach to site resilience building, seawalls can be very effective.
Looking at implementing a seawall to your property? Be sure to consult with an industry professional (experienced coastal engineer).
Or if you're looking for seawall alternatives, consider a variety of coastal resilient measures like Multi Purpose Artificial Reefs in conjunction with nearshore nourishment.
Contact us today for consultation to determine which seawall is right for you.
Read more about artificial reefs or sand bypassing systems as a coastal erosion solution.
References:
[1] Barnett, M.R., "Laboratory Study of the Effects of a Vertical Seawall on Beach Profile Response," UFL/COEL-87/005, University of Florida, Coastal & Oceanographic Engineering Department, Gainesville, FL, May, 1987.
[2] Dean, R. G. and Dalrymple, R. A. (2004). Coastal Processes and Engineering Applications. Cambridge University Press. pp. 404-406