Back to the Beach: Sand Transport at Waikiki
I have discussed beach erosion and topography in several previous posts (e.g. Australia’s east coast and The North Sea); and there is no reason paradise should be exempt from the ravages of wind and waves. Those two powerful tools of nature are on the minds of everyone who lives in Honolulu, as revealed through desperate efforts to keep these beaches from eroding.
Where is Waikiki Beach?
What does the beach look like?
The wind blows from a southeasterly direction for about six months of every year, generating moderate waves that strike Waikiki obliquely from the SE. The result is along shore drift of sand. One quickie solution to this problem is to construct resistant stone or concrete barriers perpendicular to the beach. They are called groins.
So, what happens on the upflow side of a groin, like that seen in Fig. 4? We can see the erosion on the down flow side (remember the waves are coming towards the camera), but what happens when moderate waves hit a solid wall?
The damage moderate waves can do over years, even decades, has been demonstrated, and Figs. 4 and 5 support those results. The wind blows steadily from the south-southeast between April and October on Waikiki beach. What is the impact?
The westward transport of sand by the SSE wind is obvious at any obstacle to its unimpeded flow, such as sidewalks protected by low walls (Fig. 3), where sand accumulates on the windward side and spills over.
The state of Hawaii is aware of the problems identified in this post, and their solution is beach replenishment. That would explain why the Waikiki beach I visited in 1991 is no longer white, but now as dirty as the beaches of Mississippi Sound, where beach replenishment from offshore sources has been the standard for decades.
Beaches are dynamic zones, where wind, waves, land, and biology interact in a never-ending dance, which is the primary reason (in my opinion) that people are drawn to the seashore. I expected this when I lived on the Gulf coast, but I am surprised to see reality showing its ugly face in paradise…