The U.S. Army Corps of Engineers, Honolulu District, is recommending the removal of a revetment, installation of a preformed scour hole — a form of sediment and erosion control — and implementation of a flood warning system, according to the draft Iao Stream Flood Control Project Engineering Documentation Report Amendment.

The purpose “is to address ongoing flood hazards caused by the long-term damage during repeated floods to the existing project structures since its construction completion and to provide the authorized level of reduced flood risk to Wailuku,” according to the Army Corps.

The current design is anticipated to fail within the next 50 years if left unresolved, officials reported.

The flood control project is located in the lower reach of Wailuku River, which extends about 2.5 miles upstream of the river mouth to Kahului Bay. About 30 percent of it is lined with concrete channels with the remaining sections consisting of sediment and soil.

The area of concern is primarily within a 1-mile-long portion upstream of Waiehu Beach Road that includes a slow deterioration of the boulder-concrete invert at the upper concrete channel, according to the draft report.

The failure of the invert at this site is mostly caused by scour of the foundation material. Analysis by the Corps shows that this increases exposure and erosion of the material, and sudden changes in elevation of the channel bed creates turbulent waters. As the foundation material of the lined channel is eroded by these waves, pieces of the boulder-concrete invert are falling in.

“If left unaddressed, the entire channelized reach would eventually be compromised, and the concrete retaining walls could fail,” according to the report.

Costs for the entire recommended plan is $5.5 million, and will involve “substantial improvements” to community safety and long-term reductions in operation, maintenance, repair, rehabilitation and replacement costs. To date, the project has prevented over $50 million in damages.

The project was completed in October 1981 by the Corps. The local sponsor is the Maui County Department of Public Works.

Information regarding the timeline of the project and its amendments were not readily available.

However, the design deficiency has caused water to flow quickly through the channel and created debris, which produces significant impacts like erosion, property damage and risk to community safety during a heavy rain event.

During observation, debris completely filled and overflowed the project’s debris basin.

“Damage to the levees from erosion and scour occurred as a result of high velocity flows and debris movement,” according to the report.

Other improvements will include redesigning of a few failing levees, which are designed to prevent a storm surge from flooding nearby towns and infrastructure.

The report cites the 2020 Maui County Multi-Hazard Mitigation Plan, which indicates that there’s a 90 percent chance or higher that a flood event will occur annually within the Wailuku-Kahului Community planning area, which includes Iao Valley and the Iao Stream Flood Control Project area.

While the name “Iao Stream” is recognized by the federal government, the state Board on Geographic Names unanimously voted to return the name of the stream to its original “Wailuku River” in 2015.

Nonprofit Hui o Na Wai ‘Eha has been actively involved with any projects and initiatives pertaining to waterflow over the years, and has testified at ongoing public meetings with the Army Corps, the state Department of Land and Natural Resources and the water department.

The organization advocates for mauka-to-makai streamflow restoration in Waikapu Stream, Wailuku River, Waiehu Stream and Waihe’e River.

Hui President Hokuao Pelligrino said Sunday that “I will have to say, on the plus side, there have been some positive changes,” pointing to the proposed plans of removing the revetment.

“We support any and all efforts to remove man-made channelization in Wailuku River to allow for native aquatic species to thrive and for more greater access for traditional and customary rights, and just for the ability for water to flow in its natural state to allow for percolation into groundwater aquifer,” Pelligrino said.

“What we didn’t see and requested and still have grave concerns about is the action plan about how this work will be implemented,” he added.

Because a timeline and other specifications have not been provided yet, and public testimony is coming to a close, it “won’t allow for any community members to comment, so that is a little bit concerning for us.”

In order to receive more support for the project, Hui o Na Wai ‘Eha is seeking more information regarding best management practices of Wailuku River and for better communication with the community about when work will be done throughout the project’s implementation “so that we know what’s going on.”

More studies also need to be done on aquatic resources to ensure that there’s no spawning occurring during construction, he said.

“We understand, the safety of our community is of utmost importance, but now that we have water restored, we have a greater abundance of Native Hawaiian species that we didn’t have for over a hundred years and people are gathering once again to all areas of the stream,” he said.

To view the full draft report, visit https://www.poh.usace.army.mil/Missions/Civil-Works/Civil-Works-Projects/Iao-Stream/.

* Dakota Grossman can be reached at dgrossman@mauinews.com.