Sea Technology

DEC 2018

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www.sea-technology.com December 2018 | ST 19 Part of the challenge with manag- ing and sustaining this part of U.S. infrastructure is the fact that the av- erage age of dams in this nation is over 56 years old. Therefore, it is no surprise that the ASCE Report Card identifies more than 15,000 dams as "high hazard," while 11,000 more are listed with "significant hazard" potential, meaning a fail- ure would not necessarily cause a loss of life but could result in significant economic losses. Many of us are only vaguely aware of what dams do for their local communities. Dams come in a variety of sizes and serve a number of purposes. Our nation's dams provide essential benefits such as drinking water, irrigation, hydropower, flood control and recreation. The public most commonly thinks of engineering marvels like the Hoover Dam in Nevada rather than a smaller structure that, for example, creates a lake at the center of a planned community, provides power for a small city, or ensures flood control for a valley. No matter how large or small, dams have a powerful presence that frequently is overlooked until failure has occurred. By 2025, seven out of 10 dams in the United States will be over 50 years old. Fifty years ago, dams were built with the best engineering and construction standards of the time, but today's standards are different. Many dams are not expected to safely withstand current or future predicted large floods and earthquakes. One of the major challenges of certifying that Ameri- ca's dams can be upgraded to ensure their continued vi- ability is surveying underwater. Doing this kind of survey with divers is slow, expensive and essentially hazardous. The danger of using humans to survey the underwater portion of dams is not well understood. Many dams have violent, high-speed and high-volume currents that even the strongest divers cannot cope with. Until recently, the technology needed to do a proper survey of the underwater portion of dams without putting humans at undue risk simply did not exist. Now, there are new and emerging technologies such as sturdy and reliable unmanned vehicles equipped with sophisticated sonars. A Case Study: The Keokuk Dam One recent example of how emerging technology has been employed to survey dams involved marrying an unmanned surface vehicle with a high-resolution multi- beam echosounder to conduct an underwater survey of the Keokuk Dam on the Mississippi River near Keokuk, Iowa in October 2017. The Keokuk Dam is considered a nine out of 10 on the scale of danger and difficulty due to the high currents and eddies caused by water flowing through the dam. This made an appropriately equipped USV the ideal solution to examine the dam and provide precise under- water structural imaging. By way of background, the Keokuk Dam was com- pleted over a century ago in 1913. The movable portion of the dam is more than 4,500 ft. long, with 119 sep- arate 30-ft. rectangular, steel-skin-plated, sliding gates. The gates are either installed or removed, and river flow is controlled by the number of gates installed. The gates are removed by a gantry crane that travels on the service bridge above the dam. At the time it was completed, the Keokuk Dam was second in length only to the Aswan Low Dam on the Nile River. Due to the age of the Keokuk Dam, as well as the high risk to divers due to the strong currents and eddies, the facility's owner and operator, Ameren Missouri, contract- ed with a Florida USV manufacturer, Maritime Tactical Systems (MARTAC) Inc., to conduct a comprehensive survey of the underwater portions of this dam. MARTAC produces a family of MANTAS unmanned vessels built on a catamaran hull. Ameren Missouri selected a 12-ft. MANTAS for this underwater bathymetric imaging. The MANTAS was equipped with a Teledyne RESON T20 multibeam echosounder. (Top) Representative scans of dam structures from the Keokuk Dam survey. (Bottom) Expanded view of a Keokuk Dam structure.

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