Low Frequency Modes on Two Fringing Reefs: Observations & Dynamics

Abstract

Surface gravity waves are an important physical feature of the ocean that affect nearshore processes, such as coastal inundation. Wave driven inundation results from increased coastal water levels due to high and low frequency waves, breaking wave set-up and the tide. To understand and predict wave driven inundation over a fringing reef, it is necessary to relate incident wave conditions to the response at the coast. In this study, we focus on observations and dynamics of the high and low frequency waves. Bottom pressure measurements across two fringing coral reefs in the Republic of the Marshall Islands are analyzed in an effort to relate incident wave energy measured on the fore reef to the energy at the shoreline. Time series of significant wave height in the high and low frequency bands are examined, and the records with large wave energy are identified and analyzed spectrally. The largest observed shoreline response is due to low frequency motions on the reef flat. Spectral peaks during the largest events occurred at a period T0 = 4L/√gh which is consistent with the excitation of a quarter wave length normal mode on the reef flat. The amplitude of this normal mode increases shoreward leading to enhanced low frequency energy at the shoreline.