Keeping Sharks and Rocks Away: A few of the countless applications of nets

 

 

nets-frei-otto
Some of Frei Otto’s sketches from the Institute for Lightweight Structure’s “Netze in Natur und Technik” (1975)

 

Nets have been a perennial source of fascination in fields as diverse as engineering, architecture, art, and mathematics. As such, thinkers in these fields have come up with a dazzling array of applications and uses for nets, which force us to expand upon our preconceptions of what nets are and what they can be used for.

Pause for a moment – how many applications of nets can you think of? The late Frei Otto had a well-known interest in nets and their applications to structural engineering. A flip through a 1975 publication from the University of Stuttgart’s Institute of Lightweight Structures (of which Frei Otto was a director) reveals pages of sketches (see above and below) on net elements, forms, typologies, and applications. The applications range from the prosaic (tennis racquet, hammock) to the extraordinary (stadium roofs, bridges), to the bizarre (airplane barrier, anti-U-boat net).

Personally, my research concerns underwater cable nets, and I’m currently assisting with the design of a net with a very unique application: preventing shark attacks.

La Reunion, a French island in the southern Indian Ocean, is renowned for its surfing and beautiful beaches. However, this paradise has been suffering from a surge in shark attacks in recent years. Since 2011, there have been nineteen attacks, of which seven were fatal. The attacks peaked in 2013, which forced authorities to temporarily ban aquatic activities. As a result, the island’s economy has been strained, with beach-front businesses bearing the heaviest losses.

img_8195
“Net Out Of Order,” reads a sign on the empty Boucan Canot beach.

A solution to the shark problem soon arrived in the form protective nets, which were installed on the beaches of Roches Noires and Boucan Canot. At first, the results seemed promising, and other municipalities soon wanted to install such nets on their own beaches. However, it turned out that the conditions so sought by surfers were causing a significant level of wear and tear on the nets designed to protect them, meaning that the nets have been plagued by constant and costly maintenance issues. Then, on August 27 of this year, several holes were discovered in the net at Boucan Canot, prompting lifeguards to close the beach. Despite this, some people stayed in the water, until a young surfer lost a foot and arm to a bull shark. Since this accident, the beaches have remained closed, and the nets out of order.

img_0663
The net at Roche Noire, currently protecting nobody due to the swimming ban.

 

As part of my master’s project, I am developing numerical models to study the failure mechanisms of these nets and had the opportunity to work with Prof. Marcelo Pauletti (Sao Paulo University, Brazil), an expert in numerical modeling of non-linear systems and Prof. Adriaenssens (Form Finding Lab, Princeton University, USA). In early November, I had the opportunity to travel to La Reunion to see the nets myself, as well as to meet with Prof. Khalid Addi, an expert in contact mechanics (Reunion University, France) and Seanergy, the company overseeing the project.

The nets suffer the most damage in shallower waters, where breaking waves cause more movement. See if you can spot the problem areas in the video below:

In deeper waters, the nets experience less movement:

This visual evidence, along with my meeting with the company confirmed previous suspicions that a significant issue is fatigue and friction. Therefore, it is important to study the motion of the joints. I am currently working on a hydrodynamic model of the net using data collected from current profilers installed alongside the net at Boucan Canot Beach. Such a model describes how the net displaces due to the wave action, and should provide the necessary input information for a future detailed joint model.

After leaving the meeting with Seanergy, I drove down La Reunion’s historic Route du Littoral. (Learn about the current construction here.) This scenic coastal highway meanders along the base of steep cliffs and is notorious for constant closures due to rock falls. Besides the amazing views, one can’t help but notice the vast amount of rock retaining netting. After a 100 ton rock fall blocked the highway in 2002, authorities covered the cliffs with, of all things, anti-submarine netting salvaged after World War Two. Frei Otto’s more bizarre sketches no longer seemed so outlandish…

10581529-17403628
A minor rock slide on the Route du Littoral, November 2016.

 

Author: Olek Niewiarowski

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s