By Ruby Pap
Few people think of Waimea when describing the quintessential Kaua‘i beach experience. I’ve even heard the beach described as “dirty,” in reference to its dark sand and murky waters, but that’s unfair. Those waters are actually transporting sand to the beach, and the beach is pretty special when it comes to its geologic and recent human history.
While most of Kaua‘i’s beaches contain white(ish) carbonate sand derived from coral reefs, Waimea’s greenish-gray sand comes from the land. This terrigenous sand is derived from weathered lava rock from Waimea Canyon and carried to the shore by the Waimea River. According to geologist Charles Blay, Waimea contains the state’s largest volcanic beach sand reservoir extending 2.5 miles from the river west to Oomano Point near Kekaha.
Located about half way down the beach, Kikiaola Boat Harbor was constructed in 1959. Prior to its construction, a relatively uniform, narrow beach extended over the entire stretch, according to Blay. Over the last 65 years, it became lopsided, leaving a wide beach east of the harbor and a narrow one west of the harbor.
What harbor engineers failed to appreciate was the westward along shore currents that distribute the sand from the river mouth to the rest of the beach. The harbor physically blocks this sand from moving further west, starving downdrift areas of sediment. This combined with waves and tides has caused high rates of coastal erosion to the west, threatening homes, properties and even a cemetery.
In 2014, after many years of hand wringing, the state implemented a “sand bypass” project, trucking 60,000 cubic yards of beach sand from the east around the harbor to the west. Sand bypassing is a “beach nourishment” technique that protects shoreline development, replenishes the beach with sand, and allows for natural beach processes to continue. Another option to restore beach processes would perhaps be to remove the obstruction, but that does not appear to be in the cards.
An important part of any beach nourishment project is seasonal monitoring to understand how the sand behaves. From April 2014 to October 2016 (before and after the bypass) Blay conducted beach profiles at seven survey sites on the beach, measuring beach width and elevation. The study utilized funding and volunteers from Kaua‘i Community College, Sea Grant, and the Hawai‘i Department of Land and Natural Resources.
The study has some interesting findings so far, and monitoring is expected to continue through 2017. While 60,000 cubic yards of sand were removed from the eastern beach, it has remained relatively stable. Waves have reshaped the beach, but there hasn’t been a significant build-up of sand like before. This is a good reminder that it took 65 years for that sand reservoir to build up against the harbor, so it will take some time to accumulate.
The monitoring found that the western beach sand moved and eroded a bit at first, yet maintained a moderate degree of stability. Recently there has been renewed coastal erosion indicating that something will need to be done, perhaps more replenishment, to protect shoreline development.
Beach locations further west of the sand deposit locations did not appear to get naturally replenished with sand by the currents. This might mean future bypassing efforts would need to place the sand directly on those beaches.
Next time you’re in Waimea, why not spend some time at the beach? Perhaps you will have a different perspective on its shifting sands. I certainly wish they had all this insight back when the harbor was built, but hindsight is 20/20, it seems.
- Ruby Pap is a Coastal Land Use Extension Agent at University of Hawai‘i Sea Grant College Program. She can be reached at firstname.lastname@example.org.