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The Growing Volcanic Beach, Pohoiki, Hawaii

1:33 AM · Jan 11, 2020

FINDINGS Sand and sediment at Pohoiki has continued to build up following the end of the 2018 eruption, with the thickest accumulations now extending over 450 feet into the ocean from the old coastline and filling Pohoiki Bay to where it was previously 10 feet deep. The shallow areas around Pohoiki Bay were rapidly inundated by fresh and eroded lava rock fragments in August 2018, starting nearest the boat ramp at its northern margin. Sediment continued to deposit and accumulate southwest into Pohoiki’s ‘Second Bay’ through the end of 2018. Once Second Bay was filled, sand spilled around Lae o Kahuna, the point between Second and Third Bay, driven by the predominant longshore current to the southwest. Over 2019, the beach roughly doubled in length, from 1/4 mile to 1/2 mile long, and now stretches past Mahinaakaaka Heiau. The beach covered 8.2 acres of the bay over the final 5 months of 2018. In the next 12 months, it covered an additional 7.7 acres, for a total coverage of 15.9 acres by the start of 2020. By this measure, the rate of coverage slowed from 1.64 acres/month to 0.64 acres/month, though to compare accumulation it would be necessary to include the upwards growth of the beach by stacking. It’s worth noting that specifically at the northern part of Pohoiki beach in the area of the boat ramp, it appears that sediment deposition is approaching an equilibrium, with greatly slowed, but still visible seaward growth in the last 6 months. Currently we have no reason to suggest that the sand has been receding in any area between the lava flows northeast of Pohoiki Bay and southwest of Third Bay, in contrast to some anecdotal accounts. There are some smaller scale trends that relate to recent surf swells and their direction and intensity, yet the overall trend of steady growth remains. Finally, it would not be unusual if the beach widens further during the summer months, following the typical pattern seen elsewhere in Hawai‘i and across the world. Most likely, we will know Pohoiki has reached its sediment capacity once the beach is seen to lose sediment during the winter months, unlike in 2019. METHODS Using aerial imagery taken of Pohoiki Beach from August 9th, 2018 to January 1st, 2020, we apply pixel analysis to determine the rough progression of sediment buildup following the 2018 LERZ eruption of Kīlauea. The current sand/rock buildup forming the present beach was verified by measurements taken on the beach in late December, 2019. LIMITATIONS The aerial footage used in this study was not captured with our analysis in mind, anmd as such the angles and equipment vary. Also, the tides and surf swells of the ocean vary between each flight. These limitations reduce precision of the map subsequently created. The final limitation comes from a lack of available aerial imagery and the irregular intervals between flights, which creates some additional variance in mapping the sand movement in chronological order. All in all, the margin of error for each pixel is estimated to be no greater than 30 ft. BACKGROUND The 2018 eruption of Kīlauea created a significant deposit of new lava extending the coastline between Pohoiki and Kapoho, a distance of roughly four miles. Lava flows claimed the ‘Shacks’ and ‘Bowls’ surf spots on the northern area of Pohoiki, but stopped short of claiming the Pohoiki Boat Ramp. Following the eruption, sand and rock began to be swept into Pohoiki Bay from the predominant currents. The sand quickly blocked the boat ramp as it built a new black sand beach in Pohoiki’s First Bay. The State of Hawai‘i has explored options for relocating and restoring access to the Pohoiki Boat Ramp, which remains the only ramp on the east side of Hawai‘i Island. VIDEO: Video Sources Used: - Jeremiah Osuna, Strange Hawaii, - Mick Kalber, Tropical Visions Video, - USGS Hawaiian Volcanic Observatory. - Hawaii County Civil Defense.


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