Thanks for the dialogue on docks

Posted: Thursday, July 26, 2001

Last year when a large portion of the new pier at Irakleio, Crete - the portion being used by large passenger ferries - collapsed on a Sunday morning, the construction firm that built the pier Domiki Kritis said that the pier's foundations were laid in a shallow depth of eight meters and were therefore unsuitable for large vessels.

The leading cause of catastrophic bridge collapse with no prior warning is from bridge scour. Sediment around the pilings is softened and carried downstream. Juneau divers have taken advantage of scour for years by going under the cruise ship docks to look for crockery after departing ships have desilted the bottom.

The bottom under the cruise ship pilings is not flat. It drops off, rather sharply in places. Anyone who has built a retaining wall can tell you hills creep downward. As bottoms is removed, either by dredging or by propellers of large vessels, the uphill sediment must inevitably slough down to replace it. What is more, bow and stern thrusters of the large new vessels are large enough for a grownup to walk into and able to shoot out enough water to knock over a bus.

The effect of such force, combined with the torque put on a dock by large vessels tying up in high fall winds after a summer of scouring presents Juneau with unique circumstances. It is certainly right of our City Assembly to question the wisdom of bringing ever larger vessels to the docks. Thanks to them for the dialogue.

Dick Callahan


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