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"More Than 50% of the sediments making their way into the ocean are coming from the pillbox trail."
Dr. Kim Falinski
Conservancy marine science advisor
The windward O‘ahu community of Lanikai, traditionally known as Kaʻōhao, was once a quiet suburb of Kailua. Nestled between a crescent-shaped hillside and a white-sand beach, with a picturesque pair of offshore islands guarding its turquoise waters and coral reefs, Lanikai was an idyllic place to live.
But about a decade ago a not-so funny thing happened. Lanikai was discovered. The offshore Mokulua Islands became everybody’s favorite postcard and the beach was ranked among the world’s best.
The notoriety brought scores of visitors, and along with them a rash of problems—everything from parking and traffic congestion to trespassing and noise. To that list, you can now add an incipient water quality problem.
In 2016, after longtime residents noticed the ocean water didn’t look as clear as it used to, the Lanikai Beach and Park Foundation asked The Nature Conservancy to investigate. Dr. Kim Falinski, one of the Conservancy’s marine science advisors and a water quality expert, worked with the community over three months last summer taking samples and recording observations. Their findings confirmed what many residents suspected: water clarity was poor in the nearshore area along the beach.
Finding the Source
Their surveys found that the primary cause of turbidity (i.e., murky water) in the nearshore area was sediment generated by erosion from a popular hiking trail leading up to the pillboxes atop Ka‘iwa ridge, the hillside that overlooks Lanikai. A hundred or more visitors a day hike to top to take in a panoramic view of windward O‘ahu.
“More than 50% of the sediments that are making their way into the ocean are coming from the pillbox trail,” says Falinski, who used a computer model to estimate sediment inputs from various sources. “Another 25% is due to sea level rise and shoreline erosion caused by waves cutting into the beach.”
There are two main drainage systems running down to the sea in Lanikai. One is on Lanipō Drive at the community’s far end; the other is off Ka‘elepulu Drive, which is near the entrance and is the starting point for the pillbox trail. Researchers found that storm water drains right down the pillbox trail and onto the street, where it is washed into storm drains and empties out onto the beach.
Although more sediment washes into the ocean at Ka‘elepulu, the turbidity is slower to clear at the Lanipō end, likely due to seawalls which interfere with the natural water flow.
On summer weekends, an average of 2,300 people a day use Lanikai Beach, and with no public restrooms there was concern that beachgoers were relieving themselves in the ocean. But nutrient levels in the water were relatively low and within state standards.
Going forward, the Lanikai Beach and Park Foundation plans to work with the Conservancy and the broader community to develop a community action plan, which will prioritize and develop solutions for restoring coastal waters.
“For the pillbox trail, it makes the most sense to invest upstream by revegetating the hillside to stop sediment at the source,” says Kitty Courtney of the Lanikai Beach and Park Foundation.
Sediment and nutrients can damage coral reefs and fisheries. They also impact the quality of snorkeling and other beach experiences. Solving the problem means cleaner water and a healthier reef for everyone to enjoy.