FREQUENCY RESPONSE OF TIDE GAGES.
CALIFORNIA UNIV BERKELEY HYDRAULIC ENGINEERING LAB
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The most common source of data on tsunamis is from the records from permanently installed tide gages. A tide gage commonly consists of a float in a stilling well. The float is connected to a mechanical recorder, and the stilling well serves to eliminate the effects of wind waves and swell from the gage response. This damping of the higher-frequency wind waves is achieved by an orifice connecting the float chamber of the stilling well to the sea. This orifice acts as a filter, damping out the higher wind-wave frequencies, while allowing the much lower tidal frequencies to pass without loss in amplitude. Tsunamis, however, fall between the wind-waves and the tides in the frequency spectrum, and thus it is important to know the response of a tide gage at typical tsunami frequencies. These results show that for most ordinary purposes, the tide gage stilling wells described -- i.e., a 12-in. diameter well, with either a 34-in. or 1-in. diameter orifice, give a good representation of a tsunami, if the periods are larger than 5 min. for tsunamis 2 ft. high, or 15 min. for tsunamis 20 ft. high. The graphs provide a means for estimating both the reduction in height and the time lag of the peak. Author
- Physical and Dynamic Oceanography
- Test Facilities, Equipment and Methods