Tide Gage Frequency Response

Theoretical calculations of the frequency response of a tide gage stilling well are made, and compared with recorded tsunami heights and periods. While the frequency response only affects very short-period, large-amplitude tsunamis, the nonlinearity of the response (i.e., dependence on amplitude) places limitations on the interpretation of spectra obtained from tide gage records. The nonlinear interaction between the orifice used as a filter on the stilling well and Stokes' second-order waves is calculated numerically for a steady-state case, and is shown to result in small errors in the tide gage reading. The possibility of the transient case resulting in spurious seiche or surf beat records from groups of higher waves is suggested.