Back to the Beach: Sand Transport at Waikiki

Figure 1. View looking along Waikiki Beach toward Diamond Head. Everything looks wonderful from this perspective, a lovely tropical beach with gentle waves lapping at poorly sorted sand with a steep beach face. Things are not as cozy in paradise as they appear.

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?

Figure 2. Waikiki Beach is on the island of Oahu, labeled as “Honolulu” in this image. It faces south and is subjected to southeasterly winds between April and September. Note that it is 2500 miles from Los Angeles, at a latitude of 21 degrees north. Tropical storms are rare and there are no cold fronts or other meteorological anomalies, but the wind does blow from the ESE between April and September.

What does the beach look like?

Figure 3. View looking south on Waikiki beach, showing stone retaining wall (aka groin) Note the sand collecting against the eastern face (left side of photo), flowing onto the sidewalk. This sand has been transported by wind, which blows for months at a time from the east .

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.

Figure 4. View looking east toward Diamond Head, showing a groin jutting out into the nearshore. Note the deep erosion on the camera side of the obstruction. This is a typical response of a nearshore wave regime to a groin. Also note the steep beach face seen in the nearer part of the image. This is not an equilibrium condition between waves and sediment type, but rather an artificial situation caused by the sidewalk (hardened back shore) not allowing the beach to reach a more appropriate angle (see Fig. 1).

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?

Figure 5. View looking seaward at the groin shown in the distance of Fig. 4. Note the rock that forms the backbone of the structure, seen to the upper right of the image. A wall similar to that seen in Fig. 3 is exposed, as is the upwind edge of a threatened walkway.

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?

Figure 6. This rather boring image shows sand accumulating on the back shore of Waikiki, blown by the stead ESE wind and pushed back by brooms and gas-powered blowers. This process is obvious at the western end of Waikiki beach.

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…

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