Options mulled to reduce sediment flowing to reefs
Working to stem ocean runoff: West Maui residents favor natural alternatives over some man-made solutions
LAHAINA — Retrofitting sediment basins, restoring historical loi terraces and repurposing flood plains are among several solutions the Army Corps of Engineers is considering to reduce sediment flowing into West Maui reefs.
Honolulu District Corps officials presented their initial list of solutions to a crowd of about 40 people during a community meeting Thursday night at Veterans Memorial Center in Lahaina.
While residents were receptive to some of the more natural alternatives proposed by the Corps, those involving man-made structures or discharging sediment far offshore were shot down immediately.
“I can tell you the one going out to the ocean is a nonstarter, and I think that’s true for everybody,” said Dana Reed, West Maui team leader for community water quality group Hui O Ka Wai Ola. “I don’t think there’s one solution. I think there’s multiple ones, and I suspect they think that too.”
The first alternatives are part of the West Maui Watershed Management Study, which aims to help in the restoration, enhancement and resiliency of West Maui coral reefs and nearshore waters by identifying methods of reducing sediment carried in streams from the summit of Puu Kukui to the outer reef.
The study covers five watersheds: Wahikuli, Honokowai, Kahana, Honokahua and Honolua.
The Corps is working with the state Department of Land and Natural Resources Division of Aquatic Resources to produce the study expected to be completed within two years. The study will include feedback from stakeholders and members of the public as well as a cost-benefit analysis for each solution.
The management study arrives as brown-water advisories have become commonplace in West Maui. Although storms and heavy rains typically wash down sediment, researchers have found that even small rainfalls have created visible plumes over a few hours in Honolua and Honokowai bays.
Army Corps hydraulic engineer Mitchell Moore referenced a 2015 U.S. Geological Survey study that found streambanks are eroding into historical terraces of sands, silts and clays — causing increased runoff into the ocean. He said that during heavy rain events West Maui stream valleys filled with decades of sediment are flooded and produce plumes in the ocean.
The worst outcomes are fine-grain plumes because the sediment is the first to be picked up and last to settle in water.
“The overwhelming majority (of pollution) is coming from agricultural fill terraces inside the valley,” Moore said. “Those valleys are pretty small compared to the agricultural fields, especially when you look uphill and you see all the fields. But the hot spots are those streams, and as you walk up them you see the sediment just waiting to be picked up. There’s a lot of it. A lot of clay and a lot of fine grain material not being captured right now.”
The Army Corps’ goal is to deal with the in-stream erosion, and Moore provided residents and local groups three practical options, along with three possible solutions and two long shots.
The two impractical solutions were constructing an underwater pipe to shoot the sediment past nearshore coral reefs and to route all high-flow water into a single watershed basin during heavy rains. Both were shot down immediately by Thursday’s crowd.
The three possible solutions included constructing upstream basins retrofitted with flocculants, which clump fine sediments together, and installing pumps to divert high-stream flows into temporary geotextile bag structures that catch fine sediment. Both projects would require minimal construction, expand upon existing maintenance programs and all existing basins would be potential candidates.
It is unclear, however, what would be the environmental consequences, maintenance demand and long-term use of the projects.
The other possible solution involved manually removing sediment from streams with an all-terrain vehicle and vacuum. The accessibility of heavy equipment, unknown volume of sediment and possible instability of the bank post-removal pushed this alternative down.
The first of the three most practical solutions calls for improving existing basins, which are not functioning optimally, Moore said. He said that when the basins are overtopped, fine sediments make their way to the ocean.
Improving basins would be less invasive and expensive, while utilizing existing maintenance programs through the county Public Works Department. Issues would be permitting and increased maintenance costs for the county.
Moore identified Honokowai and Kahana Nui as the best locations for the plan, and possibly Kaopala, Mahinahina and parts of Napili.
Another practical solution involves constructing and restoring loi terraces, which have historically proven to manage sediments. The alternative would embrace cultural values, agricultural production and have minimal environmental impacts.
Loi have not proven, though, to manage large storm events and may raise water rights issues, access challenges and require an enormous amount of manpower for operation and maintenance. Possible locations would be Honokowai and Honolua.
The final practical plan would be to utilize available flood plain space to hold stormwater and sediment. The “soft alternative” limits and prevents development in flood plains and can be turned into a park, community garden or other recreational use. Wahikuli and Honokowai are possible locations.
To comment on the West Maui Watershed Management Study, contact Army Corps Project Manager Jessie Pa’ahana at (808) 835-4042 or email@example.com, or hydraulic engineer Mitchell Moore at (808) 835-4148 or firstname.lastname@example.org.
Reed said she found some of the solutions “intriguing,” but she gravitated to the most practical ones presented by the Army Corps. She said the final plan would likely involve multiple solutions, and she believes some are “worth exploring further,” while others should be “taken off the table.”
“I like the idea of having the water not flowing into the ocean and staying on the land so it sinks back down to the aquifer,” she said. “That’s a win-win situation, and I like solutions that go that route.”
Mark Deakos, executive director of the Hawaii Association for Marine Education and Research, agreed that each watershed might need its own solution, but he said he hopes Army Corps officials look to get West Maui “back to a more natural state of affairs.”
“I think it’s a little more complex than a one-type solution for everything,” he said. “Certainly I would prioritize the natural approach at all possible because I think from a sustainability standpoint, if you can restore the functionality that was there, it doesn’t require continual maintenance and cost and effort. You can back away from it and let the natural system take over. But these hard engineering solutions will require continuous maintenance and money.”
Thursday night’s crowd complained of private landowners leaving West Maui fields fallow over the years and not cleaning their water systems or the streams.
Deakos said the greatest threat to marine life has been humans and the abandonment of lands. He said the restoration of loi and ownership of the lands may help mitigate issues upstream.
“If lands are managed effectively, you at least buffer whatever issues are happening upstream,” he said. “Even if large landowners are unwilling to manage it, I’m sure there’s plenty of people willing to farm lands or reforest to at least try to restore some vegetation and moisture to the soil so they’re not just eroding away.”
While residents and agencies prepare to study long-term solutions for West Maui reefs, Deakos is concerned about immediate impacts from recent massive brush fires.
Last month, fire scorched more than 2,000 acres and well over a dozen homes. Firefighters fought to extinguish hundreds of more acres burning Saturday in Olowalu.
Deakos said wildfires are “having a tremendous effect on soil erosion” in West Maui and have also “devastated” Maalaea Harbor in recent years. He said he plans to have a meeting with Fire Department officials soon about decreasing fires and exploring power lines as the culprit.
“Power lines and high winds are major contributors to these fires,” he said. “You can imagine the effort of replanting and reforesting hundreds of thousands of acres and two years later a big fire comes and you’re at square one again.
“There’s no development in front of Olowalu reef, and it’s being hammered by sediment and I believe it’s from the pali and all those fires.”
Thursday’s meeting was presented by Ridge 2 Reef Initiative, a partnership of the Corps, DLNR and other federal, state and local agencies and organizations. R2R initiatives support the U.S. Coral Reef Task Force’s designation of the West Maui watershed as a watershed partnership initiative priority study area.
For more information about the meeting and the West Maui R2R Initiative, contact Tova Callender, West Maui watershed coordinator, at email@example.com.
* Chris Sugidono can be reached at firstname.lastname@example.org.