Understanding coastal flood thresholds for Hawaii

The UH Sea Level Center is developing a new methodology for determining elevations of where impacts from high sea levels are observed, with respect to the coastal water levels measured by the nearest tide gauge. Our objectives are to develop an efficient surveying methodology using established GPS technology, quantify the vertical uncertainty of the elevation measurements that we acquire, and determine if this is a feasible activity to include in future projects. We anticipate this effort leading to the development of a new opportunity for referencing the elevations of vulnerable coastal sites with respect to known tidal datums (e.g., Mean Sea Level or Mean Higher High Water) and water level observations from existing tide gauges. We are developing the methodology so that it could be applicable in many Pacific Island locations.

Preliminary testing by UHSLC shows promising results. We are consistently measuring to within ±0.03 m the elevations of coastal sites that are known to be vulnerable to sea level inundation and/or wave run-up, and these measurements were acquired efficiently (i.e., using existing equipment and 1–2 technician-hours per site). Testing has primarily been performed in three locations: 1) around the NOAA-NOS tide gauge in Honolulu Harbor where we validated the GPS solutions compared to the established tidal benchmark surveys; 2) along the Ala Wai Canal wall where we measured low points of 24 cm above MHHW, which is comparable to the observed Honolulu tide gauge water levels when water splashes on top of the wall; and, 3) at low points on the sidewalk fronting Ala Moana Beach Park (84 cm above MHHW), which is inundated compounding events of high astronomical tides, sea level anomalies, and waves. Other than the first testing site, which was chosen for validation purposes, the other places are sites known to be impacted by so-called “King Tide” impacts. After further testing, these preliminary results could be used to communicate about “Impact Datums”, or elevations, by including the information on existing water level displays.