ERROR: there is an error at 54:04. The significant wave height Hs is the mean of the wave heights between the highest wave height and the H1/3. The H1/3 is the wave height value for which 33.3% of the waves have equal or higher heights.
In another words the Hs is s the average of the highest one-third (33%) of wave heights.

pdf: http://www.ipt.ntnu.no/~stanko/files/Courses/TPG4230/2022/Notes/23.pdf
Reading material:
-Chapter 6 (from section 6.1.4) from compendium https://drive.google.com/open?id=1dkRpQOBfkr9tqkSS6f9MDjO4XW-Sy3DS
Part of playlist:
https://www.youtube.com/playlist?list=PLXfmJjG2tXbrcHizWgYbdEDnytfSXlLIQ