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LABORATORY STUDY OF SEICHING INDUCED ON AN OFFSHORE SHELF.
NAVAL CIVIL ENGINEERING LAB PORT HUENEME CALIF
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An exploratory investigation was made in the NCEL wave tank to determine whether with existing equipment an oscillation seiche of the water over a submerged shelf just offshore could be induced by an incident train of shorter waves and if so, whether the run-up from the wave system on the beach would thereby be affected significantly. This study was a phase of current work at NCEL dealing with the run-up and other shore effects of surface gravity waves from underwater explosions and was supported by the Defense Atomic Support Agency under its subtask 14.083, Effects of Underwater Nuclear Explosions on Waterfront Structures. When the wave-maker was set to generate short waves, a low-amplitude dispersive train preceded the regular periodic train. Weak low-frequency oscillations in the run-up record occurring prior to the arrival of the periodic train were detected in seven of eighteen runs analyzed, the frequencies agreeing with estimates of first-mode shelf-seiche frequencies. A special run-up factor was evaluated for four of these seven runs it indicated maximum run-up in the long-period wave spectrum alone of from 35 to 85 of the maximum run-up in the spectrum of the incident dispersive waves. The highest value was associated with the highest incident train. From these results it was concluded that a seiche existed in some of the runs. It is believed that the principal reason why no oscillation was detected in other runs is that the heights of the incident dispersive waves which could not be increased were too small. When the wave-maker was set to generate long waves, the initial waves at the shelf were those of the periodic train. Author
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