The pounding waves, the relentless tides, the winds, the shape and geology of the shoreline give us the unrivaled beauty and diversity of the Hawaiian beaches . Their ever-changing nature is breathtaking: black, white, green, rocky or sandy, quiet or roaring, they are magical. Their dynamic personality is rivaled perhaps only by Pele and her home, Kilauea, the place where fluid magma becomes solid earth.
A visit to the Hapuna Beach on the Big Island of Hawai’i during low tide on a beautiful day in May turned into an invitation to explore the space at the intersection between the native Hawaiian knowledge of the place and the approaches of Western science.
Hapuna Beach at low tide, looking towards South.
Hapuna, a crescent shape, white sand beach located on the NW shore of the Big Island, is one of the best known beaches of the Hawaiian chain. It was named ‘#1 Beach in the US’ in 1993 by Dr. Beach, the coastal research scientist from Florida International University. Hapuna makes a good location for research because it is close to Kawaihae, a place steeped in Hawaiian history and tradition wonderfully captured by the Pacific Worlds at their comprehensive website. Kawaihae is also one of the two NOAA data collection stations on the Big Island.
NOAA observation stations at Kawaihae, north of Hapuna Beach. Image captured with MotionX GPS iPhone App.
NOAA collects tide and current data at an offshore station (HAI1128), and meteorologic measurements at an onshore station (161 7433). The only other NOAA station on the Big Island is located in Hilo area, on the east side.
The Winds
Usually in the morning, wind comes from the East. Then at a certain time, around noon, it turns around and comes from the West. So these two winds, they just circle in like this. (William ‘Papa’ Akau, 1959 – courtesy of Pacific Worlds/Kawaihae)
The kama’aina (local Hawaiians) careful observations document two dominant wind directions: Apa’apa’a is a very strong wind that sweeps over the top of the mountain and down the slope. The Naulu is a convection wind, caused by heated air moving up the slope.
Rose diagram showing wind direction during the month of April, 2017. Data from NOAA Tides&Currents, plotted with Stereonet9 developed by Rick Allmendinger)
The NOAA wind data from April 2017, displayed using a rose diagram, shows the Apa’apa’a WNW wind and the Naulu easterly, convection wind. The speeds are typically up to 13 m/s, with gusts up to 20 m/s.
The currents, tides and waves
Water current direction varies with depth. Measurements by NOAA from March 2011 show near surface currents (6m water depth) with directions similar to the tides, winds and waves, while the deeper currents (38m water depth) are parallel to the the coastline.
Curent directions measured during March 2011 by NOAA, in Kawaihae area. Data from NOAA tides and currents
Hapuna Beach is a micromareal environment, with tide ranges typically below 2m.
Tides at Kawaihae NOAA station during the month of April 2017. Data source NOAA Tides and currents.
But see, all the three waves out here and the shore line, you hear people talk. Because when they hear what kind of surf’s happening, they quick know which place. Ka‘ewa is a heavy, rough, pounding. Pua ka ‘Ilima is a nice soft. And Kukui is a nice long, starts from way out. That’s where Kamehameha surfed. Kukui. They surfed at night time. So this is the story. (William ‘Papa’ Akau via Pacific Worlds/Kawaihae).
Wave direction is predominantly from the north-west, with wave heights up to 15 m.
Map showing the mean (white arrows) and variability (red ellipses) of wave directions and heights, in meters, during the 2010–2011 NOAA winter experiment for the Kawaihae area, Big Island of Hawaii. All of the sites showed a predominant wave direction from the northwest, with the largest wave heights measured in the more-exposed central portion of the bay. Image from USGS Report
The rocks
The host rocks at Hapuna Beach are Pleistocene basalts of the Hamakua Volcanics series. They are 250-65 thousand years old and were sourced by lava flows from the Mauna Kea volcano. These basalts are obviously not the source of the sand on the beach, given that the Hapuna sand is white and most likely transported by currents from the coral reefs to the north (and south?).
Geologic map (left) and close-up picture of the basalt rocks cropping out around the Hapuna beach area. Olivine crystals are easily visible in the basalt, and they form small pockets of green sand.
Some of the questions to address in future posts are: what is the sand composition and what controls the sediment transport? Are there seasonal variations in beach configuration? What is the effect of storms on the beach geometry in comparison to the fair weather processes?