An environmental assessment of a flood-control project of the Waiakea and Palai streams in Hilo was released Friday with a finding of no significant impact.
The integrated feasibility and final EA study was done by the U.S. Army Corps of Engineers in partnership with the county Department of Public Works. Its purpose is to address risks to life, structures, property and public infrastructure due to periodic flooding by those streams.
“A high risk of flooding exists within the watershed due to the magnitude and intensity of rain events, the limited capacity within stream channels, and the tendency of flood flows to disperse broadly … within developed areas” once streams overflow, the EA states. “The risk of flooding is exacerbated by the flashy nature of the streams in the watershed, with heavy rains flowing downstream extremely quickly due to steep topography and debris accumulation.”
Several alternatives were explored in the plan, with the one recommended by the Army Corps including a levee/floodwall with water detention basin at Kupulau Ditch and a water detention basin at Hilo Municipal Golf Course, both in Waiakea Uka. The EA notes a new residential development under construction near the southeast corner of the Hilo golf course.
“While the development is outside of the proposed footprint of the detention basin feature, the height and alignment of the detention basin may require refinement to ensure the recommended plan will not negatively impact the housing development,” the document states.
According to the EA, The implementation of the plan would result in “no loss of wetlands or other special aquatic sites, no significant adverse effects to protected species, and no significant impacts to commercially important species or protected marine mammals … anticipated to occur.”
Projected cost of the plan is $10.8 million, with the feds to pick up 65% of the tab and the county 35%.
The EA detailed a history of flooding in the area as justification for the project.
During a 24-hour period on Nov. 1 and 2, 2000, an upper-level low-pressure system combined with the remnants of Tropical Storm Paul and caused 26.22 inches of rain to fall at Hilo International Airport. Damages totaled approximately $70 million on the island of Hawaii, including approximately $6.3 million in damages in the Waiakea/Palai floodplain. In addition, an estimated $12.4 million in municipal property damages, clean up costs and emergency costs were incurred by the county within the Waiakea/Palai watershed.
The November 2000 flood triggered disaster declarations from then-Gov. Ben Cayetano and then-President Bill Clinton.
In August 2018, the center of Hurricane Lane passed well south of Hawaii Island, sparing it from major wind damage, but outer rain bands caused torrential rainfall described as of “almost biblical proportions” by U.S. Rep. Kai Kahele, then a state senator representing Hilo. Mountain View, upslope of Hilo, received 51.53 inches of rain in a four-day span.
Flooding from Lane caused dozens to be rescued and damages were a reported $20 million to county infrastructure alone, not taking into account state infrastructure and more than 150 homes and 30 businesses reported damaged.
That includes about $5 in damage to the Waiakea Stream flood channel.
In February 2008, about 16 inches of rain were recorded in a 24-hour period. Approximately 150 homes were damaged by floodwaters rising up to 4 feet deep in Hilo.
Earlier flooding episodes include August 1994, when about 4 inches of rain were recorded with damages estimated at $1 million, and March 1980, when about 25 inches of rain fell in a 72-hour period with damages estimated at $3.8 million.
A phone call Friday to Ikaika Rodenhurst, the county’s Public Works Director, wasn’t returned in time for this story.
Email John Burnett at jburnett@hawaiitribune-herald.com.
