Hard Facts on Flexible Artificial Reefs

The ability of artificial reefs to collect and concentrate pelagic fish is well known to the sport and commercial fishing communities. Sinking ships, concrete debris and other derelict material to create sites of interest for SCUBA divers is also a well established tradition. Here, the main purpose is to provide something of interest to dive on, though the ability of the wreck to attract fish is recognized as an equally important function. Some artificial reefs use materials specifically designed for their purpose, such as the concrete domes made by Reef Ball Development Group (right). However, when scrap cars and other debris are simply dumped in the ocean, these sites take on the look of underwater junkyards.

 
Reef Ball(TM) Designed Artificial Reef

Whatever their principal function, artificial reefs are almost always created in relatively deep water at sites only accessible by boat. But no one has constructed a diving reef accessible from the beach, in shallow water, specifically for use by snorkelers and SCUBA divers. The Florida Department of Parks and Recreation is planning just such an underwater marine sanctuary as part of the new State Park at Navarre Beach, FL. This is an exciting, ground-breaking idea that will open up the beauty and wonder of the Gulf of Mexico to many that would otherwise never experience it. The intent is to simulate the beauty and environmental variety of a coral reef. However, the surf zone places some severe constraints on the "reef" that are not factors in deeper water.

  Structural - Breaking waves in the surf zone cause enormous loads on rigid structures, compared to the same waves in deep water. Ensuring adequate strength raises costs.
  Foundation - Gravity foundations are ineffective in holding a structure in place. Pile foundations work well, but must be substantial, and require heavy equipment to install.
  Sand movement - Hard structures create scour and subsidence. Even large structures quickly sink into the bottom. Later, storms can uncover and move them on or offshore.
  Littoral interference - Hard structures can interrupt alongshore or cross shore sand movement, resulting in shoreline erosion.
  Inadvertent impact - Even when waves are low, divers in shallow water are subject to oscillating wave motions that can make contact with hard structures unavoidable and painful.

Compliant or articulated structures are an effective solution to these constraints. When subject to high lateral wave forces that could break a rigid structure, the compliant structure simply bends over.

The effective surface area is greatly reduced, and the remaining force acts along the strong, longitudinal axis. Because they cause minimal flow disturbance, scour and shoreline effects are avoided.

The BETA Tower (seen at right) is one example of an articulated platform installed in 30 ft of water off of Florida. It survived a direct hit from Hurricane Elena with no damage. Below are two conceptual designs for a compliant "snorkeling reef".

 

 

The first compliant reef design is not modeled on a coral reef, but a forest - a kelp forest. A kelp bed is an incredibly rich habitat that looks like an underwater jungle. They are generic to colder coastal waters, and are one of the most productive and beautiful of marine environments.


Natural Kelp Forest

 

The artificial kelp bed is made from a grid of buoyant, synthetic rope, such as polypropylene, "planted" in the sea bed. While it could be used in a wide range of water depths, this design is particularly suited to shallow water, say from 3 to 12 feet deep.


Artist's rendering of Artificial Kelp Bed
Link to larger view

 

 The second design is modeled after a special, compliant type of coral - the soft corals, like sea fans, though it looks more like an underwater bamboo grove. This design is best suited for somewhat deeper waters, say 10 to 20 feet deep.


Link to larger view



There are many variations on the compliant reef concept that utilize the wide variety of inexpensive, rugged synthetic fibers and materials. For example, in deeper water pipe sections could be joined using standard pipe connections in a trunk and branch shape. Attaching rope to the ends of the branches creates a very tree-like structure. Such open frames become colonized with coral much faster than flat surfaces because of improved flow. The result is closer to a swim through an underwater garden, and less like negotiating an underwater debris field.

 

Subject matter reference links:
http://www.indiana.edu/~scuba/artificial.html
http://life.bio.sunysb.edu/marinebio/kelpforest.html