INTRODUCTION 

The Ala Wai Watershed is at high risk for widespread flooding across the basin. The risk of flooding endangers the lives and livelihoods of the 200,000 residents living along the Makiki, Mānoa, and Pālolo streams, the Ala Wai Canal, and the wider community within the City and County of Honolulu. Also at risk are a wide variety of commercial interests representing a significant portion of Hawaii’s economy. A catastrophic flood would damage commercial and residential structures, disrupt transportation and critical infrastructure, and cause widespread displacement and loss of income for businesses and residents. The risk of such flooding is significant and likely to increase with climate change.

This study provides an opportunity to reduce that risk by leveraging federal support and the tools developed and data collected during previous studies to take a fresh look at the problem and come up with the solution that works best for Honolulu. The study aims to produce recommendations to build a safer, more resilient and economically viable community, both today and into the future.

On June 30, 2021, the U.S. Army Corps of Engineers (USACE) and the City and County of Honolulu (City) signed a Feasibility Cost Sharing Agreement (FCSA) for the Ala Wai Canal Flood Risk Management General Reevaluation Study. The FCSA was signed by USACE Honolulu District Commander Lt. Col. Eric Marshall on behalf of the Department of the Army and Honolulu Mayor Rick Blangiardi on behalf of the City.

The FCSA allows USACE and the City to evaluate flood risk reduction strategies and identify solutions that balance costs and environmental impacts. The study is federally funded up to $3M and is expected to take less than three years to complete.

The ongoing partnership between the City and USACE continues to build resiliency within O'ahu communities.  The FCSA can be viewed here: https://www.honolulu.gov/rep/site/may/Ala_Wai_General_Reevaluation_FCSA_executed_210630.pdf

The Ala Wai Flood Risk Management Draft General Re-evaluation Report and Integrated Supplemental Environmental Impact Statement were published on Friday, November 24, 2023. You can view the published report by clicking here: https://files.hawaii.gov/dbedt/erp/Doc_Library/2023-11-23-OA-DSEIS-Ala-Wai-Canal-Flood-Risk-Management-Project.pdf

LOCATION

The watershed encompasses 19 square miles (12,064 acres) and extends from the ridge of the Ko‘olau Mountains to the near-shore waters of Māmala Bay. It includes the Makiki, Mānoa, and Pālolo streams, which flow to the Ala Wai Canal, a 2-mile-long, man-made waterway constructed during the 1920’s to drain extensive coastal wetlands. This construction and subsequent draining allowed the development of the Waikīkī District.

The watershed’s highest elevation is about 3,000 feet, before dropping to 300 feet at valley floors and rolling to sea level — all over the span of about four miles. Given the combination of sheer slope, considerable rainfall — up to 150 inches a year in the Ko‘olau Mountain Ridge — as well as the dense Waikīkī population and growing climate-change and sea-level rise concerns, the waterway is designated as high risk for flash flooding.

GENERAL REEVALUATION STUDY

A general reevaluation (GR) study is a congressionally authorized opportunity to reevaluate a previous study. The Ala Wai Flood Risk Management (FRM) project began in 1999 and was authorized and funded for design and construction in 2018. A 2020 Engineering Documentation Report (EDR) provided the technical basis for modifications necessary to the authorized project. Subsequent analysis found that the modifications in the 2020 EDR were not cost effective and lacked the economic justification necessary for continued federal involvement. The current GR study allows the project team to reevaluate whether a federal project is feasible, potentially at a different level of protection.

In addition, the USACE and City teams identified opportunities for community input early in the GR study process. This relaunch allows for a fresh look at the engineering challenges, while considering construction costs and community input throughout the process. The proposed plan to execute this reevaluation is based on the following principles:

Strong partnership

The study team comprises both USACE and City representatives working on every aspect of technical analysis and study execution in order to identify optimal solutions. The mayor and the USACE District Commander coordinate with one another, conducting regular engagements with their staff to ensure project success from start to finish.

 Community relationships

 It will take a whole-of-community approach to deliver a meaningful flood risk management plan to the people of O’ahu. That is why USACE and the City are pursuing a comprehensive community outreach initiative. Planned engagements will include workshops to identify problems and brainstorm solutions as well as formal comment periods for recommended actions. Moreover, both USACE and the City will maintain open channels for receiving public input throughout much of the study’s duration, including via mail, email, and virtual and face-to-face meetings (pending Covid-19 restrictions).

The team will listen to public concerns and considerations before launching the development phase of the flood risk management plan. The community’s input may include local knowledge that will enhance the validity of technical analyses as well as preferences and values that will be important for any viable proposal to take into account.

Comprehensive, Science-Driven Solutions

The general reevaluation study of the Ala Wai watershed will incorporate existing, verified technical analysis along with a full reassessment of potential measures to mitigate flood risk. The goal of the study is to identify solutions that maximize the return on investment, reduces risk to the health and safety of residents and minimizes any potential environmental impacts. Every alternative considered will be evaluated against criteria that include costs, benefits, environmental benefits or impacts, environmental justice, climate resiliency, and life safety. An implementable recommendation will seek to maximize overall benefits across these categories.

STUDY TIMELINE

USACE and the City estimate that the draft GR study report will be released for formal public review and comment by the end of 2022. There will be ample opportunity for the community to engage and provide feedback throughout the process prior to and following the release of this report.