Observations, Forecast, and Modeling of 0.5-200 Min Infragravity Oscillations in Hale'iwa Harbor Region, Hawai'i

Abstract

The scientific objective of this study is to identify the dominant phenomena, their generation mechanisms, and the corresponding energy pathways that result in 0.5-200 min infragravity (IG) wave energy in Hale’iwa Harbor, North Shore of O’ahu, Hawai’i. We meet these objectives with spectral analysis that is applied to sea level and currents data obtained from historical and recent observations, and high-resolution numerical modeling, inside the harbor and at the coast. Validation and calibration of the model results with harbor and coastal observations have confirmed many of our observational results. Furthermore, analysis of model output from cross-shore and alongshore arrays at several coastal sites has improved our understanding of the generation mechanisms and dynamics of IG waves at the coast; and, detailed maps of energy, coherence amplitude, and coherence phase at the coast and inside the harbor, revealed unique standing wave patterns at several IG periods bands. Interestingly, the greatest observed variance was found to be in different period bands for sea level and currents; ~5-15 min for sea level, and ~3-8 min for currents. When SS forcing is non existent, we observe a suite of coastal and harbor modes that could potentially be forced in different ways (e.g., wind, internal waves). We find that the coastal modes range between 1~23 min (in agreement with other model studies in that region), and the harbor modes between ~40 sec and ~6 min. The model results further support observations indicating that the harbor’s gravest mode potentially oscillates at ~5-6 min. As the SS forcing increases to high levels, our anticipation was that the energy of the modes would grow linearly, as was observed in similar studies by others. Surprisingly, we find that the energetic spectra is predominantly non-modal with uniform levels across a wide band of IG periods, suggesting that other processes (non-modal) are at least as important. The observed high spectral levels extend from periods of minutes to several hours, corresponding to scales much longer than the gravest coastal mode of 23 min. The overwhelming amount of energy hitting the North Shore coast and the irregularities of the bottom and the coastline, are likely strong factors in this observed response. At periods shorter than ~30 min under strong SS forcing, we obtained abundant evidence of bound IG waves offshore of the SS break point, and dominance of free IG waves within the surf zone. The free IG wave field appears to be mostly composed of leaky waves, but at one site we also found evidence of short-period (∼45-60 sec) low-mode edge waves. Inside the harbor, the IG wave field is free, and could be forced by the leaky or edge waves from the coast. Observational evidence several kilometers from the harbor suggests that a SS- driven setup mechanism drives energetic coastal oscillations at periods from minutes to hours. Modeling results much closer to harbor suggest that such a mechanism could potentially force (T<20 min) oscillations inside the harbor. Overall, the scientific portion of this study suggests that inside the harbor, coastal oscillations may be as important as harbor oscillations. This leads us to the conclusion that, at least for harbors with a similar environment as the one in Hale’iwa Harbor and the North Shore, understanding the dynamics of energetic harbor oscillations requires understanding of the dynamics at the coast. For the practical objective of this study, we developed a forecasting system of energetic 0.5-40 min IG oscillations that result in large sea level amplitudes and strong currents in Hale’iwa Harbor. Observations of sea level offshore of the harbor and sea level and currents inside the harbor are used to determine a statistically-optimal relationship between offshore SS forcing and harbor IG response. Using this we establish transfer functions relating the offshore sea level with the harbor currents. Given an input of sea level forecast at an offshore site, the transfer functions are used to generate a forecast of surge currents inside the harbor. This output is expressed in terms of an index that, compared against threshold levels, provides a sense of the danger levels inside the harbor given a particular forecast of offshore forcing conditions. The threshold levels were determined using two exceptionally strong SS forcing events during which we documented the aftermath inside the harbor. These SS forcing events serve as calibration of our forecast and provide us a good sense of the response inside the harbor given particular offshore SS forcing conditions.