Waiakea-Palai Streams Flood Reduction Project

This integrated feasibility report and environmental assessment presents the results of a U.S. Army Corps of Engineers (Corps) study undertaken to identify and evaluate flood risk management (FRM) problems and opportunities on the Waiakea and Palai Streams near Hilo, Hawaii. The study is authorized under Section 205 of the Flood Control Act of 1948, as amended. The non-Federal sponsor for the study is the County of Hawaii Department of Public Works. This report provides documentation of the plan formulation process to select a recommended flood risk management plan, along with environmental, engineering, and cost details of the recommended plan, which will allow additional design and construction to proceed following approval of this report. This Integrated Feasibility Report and Environmental Assessment was developed under the NEPA rules of 1978. All analysis, coordination, consultations, as well as outreach was complete prior to the implementation of the new NEPA rules effective September 14, 2020. This Integrated Feasibility Report and Environmental Assessment is intended to be consistent with the State of Hawaii Chapter 343, Hawai‘i Revised Statutes. The study area encompasses the Palai Stream watershed and the Waiakea Stream watershed near the town of Hilo, Hawaii, located on the northeastern coast of the island of Hawaii. The purpose of the study is to address the risks to life, structures, property, and public infrastructure from periodic flooding in certain locations within the vicinity of Waiakea and Palai 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 as sheet flow within developed areas once streambanks overtop. 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. The plan formulation process identified several structural and non-structural measures to address flood risk in the study area. An initial array of eight alternatives underwent early rounds of qualitative screening. Additional evaluation, comparison, and optimization of alternatives assisted the study team in identifying and evaluating the final array of four action alternatives. The Tentatively Selected Plan presented in the Draft FR/EA has been modified following agency, technical, and public review of the report as well as completion of feasibility-level design refinements including site visits. While the Draft FR/EA presented the Corps’ tentative proposal for a selected plan, this Final FR/EA presents the Corps’ recommended plan. Based on subsequent coordination efforts between the Corps, the non-Federal sponsor, and local landowners in the study area, the alternative including Kupulau Ditch Levee/Floodwall with Detention and Hilo Golf Course Detention is presented as the recommended plan. This plan does not include the Ainalako Diversion feature. The National Economic Development (NED) Plan is the alternative that reasonably maximizes net benefits while remaining consistent with the Federal objective of protecting the environment. The alternative with the highest net benefits is the combination plan that includes a detention basin and levees at Kupulau Ditch, and a detention basin at the Hilo Golf Course. This plan maximizes annualized net benefits at $1.6 million. At the FY 2020 discount rate of 2.75 percent, the total project first cost of the NED Plan and Recommended Plan is $10.8 million with a benefit cost ratio of 3.5.

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