COASTAL OBSERVATIONS ALONG OAHU’S NORTH SHORE DURING A STRONG EL NINO YEAR

Observational data of coastal processes is critical to improving our understanding of shoreline response to ocean forcing. In Hawaii, El Nino Southern Oscillation (ENSO) mimics some aspects of anticipated climate change-related impacts, such as alterations to the prevailing northeasterly trade winds. Thus, ENSO-related impacts to Hawaii beaches may be better understood using coastal observation data collected during an ENSO period.

Existing observational data for the North Shore of Oahu, Hawaii is limited due to the logistical challenges presented by working in this highly-energetic wave climate. Anecdotal evidence relates some understanding of seasonal changes: strong North Pacific swells dominate the winter, and shorter-period trade wind-generated waves drive shoreline change in summer. Less documented, and less understood, are the short-term seasonal and inter-seasonal changes that occur.

A coastal observation program was developed and implemented for a 2.5km stretch of coastline surrounding Sunset Beach on the North Shore of Oahu during 2015-2017, including measurements of ocean forcing and shoreline change. Monthly beach profiles were collected between January 2015 and March 2016 to quantify shoreline elevations and document geomorphic shoreline response when subjected to various seasonal wave conditions. Shoreline observations continued on an approximately bi-monthly basis between April and November 2016 using Unmanned Aerial Vehicles to collect photographs, in combination with limited beach profiles and ground control points. These observations continue.

Ocean forcing was measured through the use of bottom-mounted Acoustic Doppler Current Profilers (ADCPs) in the study area. Two ADCPs were deployed to characterize wave and current parameters in summer and winter, respectively. The winter ADCP was recovered on the windward side of Oahu in March 2016. The instrument was disconnected from the external battery during an extreme big wave swell in January 2016, recording wave heights of 8.1m prior to equipment failure. The unique data set described herein will support future coastal research aimed at improving our understanding of shoreline change in the region, as well as predictions of how ENSO and climate change may impact Hawaii beaches in the future.