Repeated floods have eroded Iao Stream’s channel bed and levees over the past three decades to the point where the U.S. Army Corps of Engineers is proposing new structures and stream diversions to protect surrounding homes and businesses.

Iao Stream stretches about 2.3 miles from an upstream sediment basin to Kahului Bay. About 30 percent of the flood control project is lined with concrete channels with the remaining sections consisting of sediment and soil. It’s the latter areas that have the issues.

The proposed Iao Stream Flood Control Modification Project would slow the velocity of stream flows during storms by diverting water to a naturally existing flood plain and other renovations, according to a draft environmental assessment appearing in Tuesday’s edition of the Office of Environmental Quality Control’s “The Environmental Notice.”

The average channel velocity of Iao Stream exceeds 20 feet per second during floods, which has caused severe damage to the flood control system. The proposed actions would minimize damage to the channel bed and levees – especially to the right bank levees. (Left and right banks are determined by downstream views.)

Since it was built in 1981, the Iao Stream Flood Control Project has prevented an estimated $49.6 million in flood damage as of fiscal year 2013, according to the draft environmental assessment.

“The flood control system has instilled a sense of security in the growing community of Wailuku,” said the draft report. “However, Iao Stream is in danger of reverting to a flood hazard zone due to deterioration/scour of the right bank and undermining of the levee toe resulting from changes in the stream bed dynamic and upstream watershed use/development that have occurred during the past 30 years.

“A failure in the deteriorating levees would cause floodwaters to inundate the Iao Stream drainage basin, and loss of life and extensive property damage would be inevitable.”

The construction area for the project would be within a 0.4-mile stretch of the stream between Waena and Imi Kala streets. Construction is expected to take about 21 months.

The flood plain, or overflow channel, is in the area of Piihana Farms on the left bank of the stream. The area has been unused for more than two decades and is thought to have been used historically in agriculture production.

The reconnection of the stream to the flood plain would involve lowering a portion of the left bank by about 5 to 9 feet and grading the overflow area to disperse flow into the flood plain, the draft report said. The construction of an 18-foot-high by 15-food-wide diversion wall will force flood flows to enter the flood plain; a portion of the left bank would be raised downstream to contain the overflow within the flood plain.

Farther downstream, the left bank would be lowered to allow the return of the overflow into the main channel. The proposal also includes replacing a 290-foot-long revetment along the left bank with a natural earth embankment and replacing a severely eroded barrier along the right bank with a 470-foot-long concrete retaining wall.

The release of 10 million gallons of water per day into the stream by Wailuku Water Co. also was acknowledged in the draft report. The release was part of a settlement won by Hui O Na Wai Eha and returned diverted water to the four great waters of Na Wai Eha – Wailuku (Iao), Waihee, Waihee and Waikapu streams.

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” last month. The restored name is currently up for approval by the U.S. Board on Geographic Names for federal recognition and a decision could be made in the next couple months.

The Iao Stream Flood Control Modification Project is up for public review and comments must be sent by July 23 to the county Department of Public Works Engineering Division, 200 S. High St., Wailuku 96793.

For more information, contact Kristi Ono at 270-7745 or kristi.ono@co.maui.hi.us.

* Chris Sugidono can be reached at csugidono@mauinews.com.