Musings: Highest Wash of the Waves

I waited all weekend for the rain, which I could feel was out there, and finally it arrived last night, freshening up the world and creating that beautiful music on the leaves and roof that has been ominously infrequent this winter.

Not so with the big winter swells, and another hit the North Shore this past weekend. As I noted in last Tuesday’s post, such swells are common enough that they should indeed be counted as the seasonally highest wash of the waves that marks the boundary of the public beach.

Now here’s a little data to bolster that claim collected from buoy #51001, which is the closest one to Kauai with the longest record — 25 years — although it is currently not reporting.

It shows that on average over the last 25 years, significant wave heights at that buoy exceeded 6 meters (20 feet) 41 times each year, and more than 1,000 times since 1981. The maximum significant wave height on record is at the buoy is 12.3 m (40 feet), recorded in 1988, and the mean annual maximum significant wave height is 7.9 meters (26 feet).

There’s also a rather interesting scientific paper on the subject that you might want to read, which was written by Sean Vitousk for “Pacific Science.” In it he notes:

In October 2006, the Hawaii Supreme Court issued a ruling (Diamond v. State of Hawai‘i) that the shoreline should be established ‘‘at the highest reach of the highest wash of the waves.’’

He then discusses at length the scientific calculations used in coming up with such a determination before concluding:

Thus our recommendation for the annually recurring significant wave height in Hawai‘i is 7.7G0.28 m (25 ftG0.9 ft), and the top 10% and 1% wave heights during this annual swell are 9.8G0.35 m (32.1 ftG1.15 ft) and 12.9G0.47 m (42.3 ftG1.5 ft), respectively.

These values should be considered the maximum annually recurring wave height for open north- and northwest-facing shores such as Kaua‘i and O‘ahu, where swell is directly incident to the shoreline and blocking from neighboring islands is minimized.

Seems it's not so far-fetched to claim that the public shoreline does, indeed, extend farther mauka than some of the certified shorelines might have us believe.