Transformation of wave energy across the fringing reef of Ipan, Guam

Measurements from a cross-shore array of pressure sensors and current-meters, deployed on the fringing reef of Ipan, Guam as part of the PILOT (Pacific Island Land-Ocean Typhoon) experiment, were analyzed to understand the processes driving the large water level oscillations observed at the shore during energetic wave events. Offshore, sea and swell (SS: 3 to 20 seconds period) energy is dominant, while on the reef flat, energetic oscillations are observed with periods in the infragravity (IG: 20 to 200 seconds) and far infragravity (fIG 200 to 1000 seconds) bands. The nonlinear processes that contribute to this low-pass transformation of wave energy across the reef are analyzed in terms of the energetics of the wave field in each frequency band. Wave transformation across the fringing reef is characterized by the strong breaking and dissipation of the incident SS energy which provides a driving force for the waves at lower frequencies. The SS energy on the reef flat is shown to be strongly dependent on the total reef flat water level that includes wave induced setup. The non linear transfer of energy between the SS and low frequency (LF: IG and fIG) bands is responsible for both the forcing and the loss of low frequency energy at the reef crest. In IG band, the low frequency oscillations resulting from the breaking SS envelope work against the incident bound waves and energy is transferred to the SS band similar to the nonlinear transfer observed on sandy beaches. The SS envelope oscillations at fIG frequencies force free fIG waves across the surfzone. Across the reef flat, low frequency oscillations at normal mode frequencies are preferentially excited. The development of standing waves on the reef flat is controlled by the strong depth dependent frictional dissipation of the LF waves reflected at the shoreline. During tropical storm Man-Yi, resonant standing modes in the fIG band were excited when a large increase in water level over the reef occurred due to wave setup that both reduced the effects of friction and the period of the fundamental mode.