Beach Erosion at Kailua

Figure 1. Map of SE peninsula, showing Kailua with the push pin.

This is a quick post to summarize some effects on the beaches of the windward side of Oahu, where basalt rocks of the Ko’olau volcano don’t protect the coast. Kailua isn’t far from Honolulu (Fig. 1) and the climate is similar. The wetter coastline, such as at Nu’uanu park, doesn’t extend this far. Kailua is a broad flat area, unlike the deep valleys of the north shore. The caldera is set back much further from the coast and the basalt is buried.

Figure 2. View looking inland at Kailua beach. The stream that passes through the break in the sand dune is typical for Oahu, in that it doesn’t reach the sea during the dry season. Note the erosional scarp at the top of the sand dune, which may be a seasonal feature that is healed during the rainy and stormy season. If I had to guess, however, I’d say it is a long-term feature.
Figure 3. Photo of dune face, showing consistent scarp. Note also the erosion around the life guard station. This doesn’t look like a case of summer/winter beach profiles. For one thing, this photo was taken in early October, the end of the summer season. Persistent erosion such as this indicates a lack of either sediment (most likely cause) or waves and wind to return it to the beach face. This is what is supposed to (theoretically) happen in the summer.
Figure 4. Close-up of typical sediment at Kailua beach (5x magnification). This is a poorly mixed assortment of calcite from the offshore coral reef and related organisms. If the beach is sediment starved, it could be a lack of growth or erosion from the reef. I can’t tell but (again, if I had to guess), I bet the reef is stressed and not as productive as it once was.

There is nothing surprising about what we saw at Kailua beach. Beach erosion is ubiquitous around the world; for example, it takes years for scarps like that seen in Fig. 3 to recover from a tropical storm in the Atlantic Basin. Recent studies suggest that in general, sandy beaches are being eroded; the proximate cause is a lack of sediment or increased wave energy, but the root cause most-often blamed is climate change and sea level rise.

We need to stop blaming the climate and reconsider all of the dams and diversions we’ve constructed on rivers that feed the world’s beaches, and ill-considered engineering projects completed in coastal zones.

The beach isn’t a play pool…

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