Saturday, October 22, 2016

Exploring dolosse: walking on and off the perilous Crescent City jetty

A pair of dolosse reinforcing the harbor jetty of Crescent City, Del Norte County, California
The Crescent City jetty is a long and reinforced breakwater with excellent views of the harbor and the Battery Point Lighthouse islet The latter can be reached on foot at low tide. Low tide and a calm sea is what you are looking for, if you cannot resist the temptation to stroll or jog on the jetty. A danger sign warns about deadly waves at any time. In general, walking out onto the jetty is discouraged. It literally is a walk on the edge—with powerful waves unexpectedly crashing over. Unfortunate visitors have died or were badly injured here. This makes me wonder how safe I was when walking on the nearby isthmus between Battery Point and the lighthouse islet to visit the picturesque lighthouse.

During summer month, waves typically are not so hazardous. Fishermen, recreationists and tourist then take the risk. Go out when the tide is going out and the seawater level drops to its low.

The jetty was built to protect the harbor of Crescent City. The dolosse you will see in great numbers toward the end of the jetty provide further protection by dissipating the energy of strong ocean waves and weakening their erosive force. A paper posted next to a path on the lighthouse islet informs about the dolosse (using a different spelling: doloes): they were put in place on the jetty in 1986 to break up the force of the water during tremendous winter storms (when you definitely do not want to be on the jetty). Each dolos (spelling on post: dolo) consists of concrete with steel rods, weighing 42 tons (84,000 lbs.). 750 dolosse are in place, 20 red ones with transmitters to monitor possible movement. Note that even a dolos with 400 times your weight (or more) may fail to stay in place when hit by a really forceful wave. Therefore, dolosse are individually numbered to track their displacement.

A single dolos: H-shaped concrete block engineered with one side turned through 90 degrees

Invented in South Africa, a dolos roughly has the geometry of a letter H with one side turned through 90 degrees to create the distinctive shape that ensures interlocking with each other when arranged by a crane. Although entangled, a “dolos wall” leaves holes such that wave energy gets dissipated. Depending on position and wave action, it happens that one dolos hammers a neighbor dolos into pieces. 

End of the walkable jetty: turning around at the dolos jungle
Near the end of the jetty you can marvel at the geometric shape of the concrete blocks and their odd-looking assemblies.

References, resources and more to explore
[1] The full story behind the dolos and its SA creator. IOL, August 2016 [].
[2] Gary L. Howell: Measurement of Forces on Dolos Armor Units at Prototype Scale [].
[3] Diana Tolerico: Crescent City. Life on the Open Road, September 2010 [].

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