Seawalls

What are seawalls?

A seawall is a type of shoreline hardening that is designed to protect property from shoreline erosion. Seawalls are typically made of hard materials including concrete, rock, and wood. 

Seawalls are often used to protect against storm surge. Seawalls are man-made and used in emergency shoreline erosion situations. Seawalls are only one method for shoreline protection out of many, but are often used in emergencies because of their efficiency. 

Seawalls have their pros and cons. For example, seawalls can also lead to worsening erosion known as “accelerated erosion.” This type of erosion happens faster and more severely than natural erosion. Attempting to install your own shoreline erosion control methods may cause more damage than it prevents — and, depending on your region, it could be illegal. Make sure to check your state’s guidelines and permit requirements.

In Understanding, Living With, and Controlling Shoreline Erosion: A Guidebook for Shoreline Property Owners Third Edition, experts state: “many erosion control projects which have been constructed are either inappropriate or ineffective…” (The Tip of the Mitt Watershed Council, 2019).

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Types of seawalls:

  1. Vertical Seawalls — Vertical seawalls are basically walls that are built to block waves from hitting the shore and causing erosion. These walls are effective at blocking large, consistent waves, but require frequent repairs and maintenance.
  2. Curved Seawalls — Curved seawalls follow the curve of the shore. These walls are more custom, and therefore, more costly.
  3. Mound Seawalls — Mound seawalls are made from sandbags, or natural materials like rock and dirt. TrapBags® can be used to create a mound seawall. By filling our TrapBag® Fabric Forming System with concrete you create a permanent seawall to protect your property. Once the concrete settles you can remove the TrapBag® Fabric Forming System. The benefit of using a TrapBag® Fabric Forming is it is simpler, less expensive and less time consuming compared to installing large boulders/rip-rap or sheet piling.

How effective are seawalls?

While seawalls can be effective at protecting the shore from wave erosion in the short-term, in the long-term they create more issues than they solve.

What are the disadvantages of a seawall?

Seawalls block the absorption of wave energy. This means that when a wave hits a seawall — instead of natural vegetation or sand/soil — the energy is deflected rather than absorbed. The energy does not just disappear, it is redirected elsewhere. 

Wave energy deflected by a seawall is redirected in two possible ways. The first is downward. This creates an event known as “scouring.” When wave energy is deflected downward towards the lake or sea bottom, it scoops out the soil. Eventually, scouring causes habitat loss. It also renders the seawall completely ineffective because, when the water becomes deeper, waves are able to undercut the seawall. Long-term effects include more algae and cloudy water.

The second way wave energy is redirected after hitting a seawall is called “wave flanking.” Wave flanking is when the energy is directed sideways. The energy moves towards neighboring properties and homes where it causes accelerated erosion. This in turn creates a domino effect. When one homeowner decides to install a seawall, they are essentially forcing their neighbors to do the same until everyone in the area is forced to install an expensive and mostly ineffective unnatural barrier. The lake or sea suffers and the seawall eventually fails.

In order to ensure you are being responsible with your seawall installation, consult the U.S. Army Corps of Engineers as well as a professional team before starting your project. The U.S Army Corps of Engineers has plenty of resources and documentation that can help you decide which type of shoreline protection will be most beneficial to you and least harmful to neighbors and the environment.

How long does a seawall last?

Depending on the type of seawall, the area where it’s installed, and the materials used, seawalls can last between 20 and 50 years. If the seawall is not maintained, however,  it will fail much sooner.

The TrapBag Fabric Forming System is what is used to create the seawall. The material the TrapBags are made out of has a life of at least five years but when you install a TrapBag seawall you remove the TrapBag material once the concrete cures. A finished TrapBag seawall will look like the attached photos (concrete blocks tied together with cables). This type of seawall can last just as long as a typical seawall and maybe longer.

How do you build a seawall on a lake?

As there are many types of seawall, there are also several ways to build them. Before conducting any type of shoreline stabilization or hardening, it is important to get the right permits. Shoreline stabilization of your own property affects not only you, but other property owners and the natural environment. 

To install TrapBags® to protect your property from shoreline erosion:

  1. Drill a 1 ½ inch hole through the center of each TrapBag bundle. This hole is for the cable. Pull the cable through the hole.
  2. Pull the TrapBag bundle out to its full length of 50ft. If you are installing multiple TrapBag bundles this is the time to make the connections.
  3. To connect the TrapBag bundles you will need 2×4’s and deck screws. Create H-Frames within the last cell of the 1st bundle and the first cell of the 2nd bundle (see attached photo).
  4. Connect the H-Frames by inserting deck screws from both directions. Once you’ve made all of your connections you are ready to begin filling.
  5. Make sure the TrapBags are pulled tight with the cable running through the center and begin filling the chain of cells 1 cell at a time in 1ft lifts. Fill the whole chain of cells with a 1ft lift before beginning the 2nd lift, finish the 2nd before beginning the 3rd and so on. (4ft tall = 4 lifts)
  6. If you are stacking more TrapBags on top, during your 4th and final lift insert the 1”x12” dowels 6” into the center of each cell.
  7. Allow the concrete to cure.
  8. If you are stacking more TrapBags on top, repeat steps 1 through 4.
  9. When placing the 2nd row of TrapBags on top of the 1st row, make sure the chain of cells is pulled tight and pierce the bottom of the 2nd row of TrapBags with the dowels that are inserted into the 1st row.
  10. Repeat steps 5 through 7. If you are filling 2ft tall TrapBags there will only be 2 total 1ft lifts.
  11. Once you are done with the installation and all of the concrete has cured, you may begin removing the TrapBag material. You can remove the material by cutting it with a razor knife. Any places where it is tough to cut you can burn with a torch.

How do you build a seawall on a lake?

As there are many types of seawall, there are also several ways to build them. Before conducting any type of shoreline stabilization or hardening, it is important to get the right permits. Shoreline stabilization of your own property affects not only you, but other property owners and the natural environment. 

To install TrapBags® to protect your property from shoreline erosion:

  1. Drill a 1 ½ inch hole through the center of each TrapBag bundle. This hole is for the cable. Pull the cable through the hole.
  2. Pull the TrapBag bundle out to its full length of 50ft. If you are installing multiple TrapBag bundles this is the time to make the connections.
  3. To connect the TrapBag bundles you will need 2×4’s and deck screws. Create H-Frames within the last cell of the 1st bundle and the first cell of the 2nd bundle (see attached photo).
  4. Connect the H-Frames by inserting deck screws from both directions. Once you’ve made all of your connections you are ready to begin filling.
  5. Make sure the TrapBags are pulled tight with the cable running through the center and begin filling the chain of cells 1 cell at a time in 1ft lifts. Fill the whole chain of cells with a 1ft lift before beginning the 2nd lift, finish the 2nd before beginning the 3rd and so on. (4ft tall = 4 lifts)
  6. If you are stacking more TrapBags on top, during your 4th and final lift insert the 1”x12” dowels 6” into the center of each cell.
  7. Allow the concrete to cure.
  8. If you are stacking more TrapBags on top, repeat steps 1 through 4.
  9. When placing the 2nd row of TrapBags on top of the 1st row, make sure the chain of cells is pulled tight and pierce the bottom of the 2nd row of TrapBags with the dowels that are inserted into the 1st row.
  10. Repeat steps 5 through 7. If you are filling 2ft tall TrapBags there will only be 2 total 1ft lifts.
  11. Once you are done with the installation and all of the concrete has cured, you may begin removing the TrapBag material. You can remove the material by cutting it with a razor knife. Any places where it is tough to cut you can burn with a torch.

Check out the video below to see how TrapBag® was installed along the Gulf Coast in Southwest Florida for erosion control. 

How much does a seawall cost?

Seawalls typically cost about $150 per linear foot. This of course depends on the materials used.

Cost will vary depending on the type of seawall system that is needed. We provide the TrapBag® Fabric Forming System and all of the geogrid, geotextile, cables, and dowels needed for the job. All other cost will be given by the contractor to perform the installation. (labor cost, concrete cost, etc.)

Can I use bags of concrete for a seawall or retaining wall?

TrapBag® Seawalls are always filled with concrete.

TrapBags® as a material for seawalls?

TrapBag® is a series of pentagon-shaped bags that are sloped on one side, vertical on the opposite side and open at the top for filling. Each of the cells are connected side by side like an accordion, each cell has a common wall with the next cell, and are collapsed during storage and deployment. The cells are made of high-strength textile. Each of the cells are self-contained yet rely on the next cell for added strength. If one of the cells is compromised, it will not affect the rest of the barrier, which will remain standing strong.

TrapBag® uses 40% less fill material than a stacked sandbag wall, but more importantly a single 100 foot section of 4ft high TrapBag® replaces approximately 8,000 sandbags making TrapBag® Barriers an excellent alternative to small and large sandbags. 

You can't always predict,
but you can always be prepared.

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