One Water Honolulu: Addressing Climate Resiliency in Low Lying Areas Subject to Flooding in Honolulu, HI

Introduction to One Water Honolulu
In December 2020, the Mayor of Honolulu signed a One Water Ordinance to address climate change concerns and codify the responsibilities of the previously-established City of Honolulu Office of Resilience (City), including developing and implementing a climate adaptation plan to proactively prepare for the physical and economic impacts of climate change and organize a One Water Panel of city water, wastewater, transportation and other agencies to build consensus on One Water policies and projects that the City should prioritize. The Resilience Office is also tasked with implementing a coastal and water program, a climate resilience and equity program, and a food security and sustainability program. The coastal and water program will coordinate actions related to mitigation and adaptation, protecting coastal areas, promoting resiliency and natural infrastructure development, and integrating water and urban forest management. This will include developing a coastal monitoring program, participating in the National Flood Insurance Program's Community Rating System, tracking water use by agencies, and providing technical assistance to departments and agencies.
Flooding in Mapunapuna
One of the most critical climate resiliency projects identified by the One Water Panel is the Mapunapuna area of Honolulu, HI, a low-lying industrial neighborhood (see Figure 1) built on fill, that is subject to subsidence and periodic flooding caused by a combination of heavy rainfall and storm drain system backups during high tides, especially king tide events. Commercial and industrial properties in the area have dealt with nuisance flooding over many decades, and the Honolulu Board of Water Supply has experienced more than 50 water main breaks in this area over the past 30 years, primarily due to corrosion and settlement. The nuisance flooding, which also impacts other buried utilities and infrastructure, roads, emergency vehicle access, and property usage, will continue to worsen with sea level rise (see Figure 2).
Assessment Methodology
The City has been addressing the flooding problem by installing backflow prevention devices ('duckbill valves') on the stormwater outfalls to block backflow from the ocean and stream during high tide and large storm events. With the looming threat of climate change and sea level rise, where climate models show a possible increase in sea level of over three feet by mid-century, more adaptations are needed if Mapunapuna wants to remain a viable industrial hub. Across the U.S., coastal cities like Miami and Boston have wrangled with similar flooding issues due to sea level rise. Options to address these issues include: armoring, or building grey and green flood barriers to protect infrastructure; raising or adapting the low-lying infrastructure to work in a flooded environment by elevating infrastructure or installing pump systems to remove water or allowing flooding on first floors of buildings; or relocating or retreating at risk infrastructure and population areas, where people or facilities may be relocated to higher elevations or some low-lying areas may be abandoned altogether. Many communities are considering a combination of these to cost-effectively mitigate existing flooding problems and adaptation for future climate conditions. As a catalyst project for One Water Honolulu, an initiative that promotes valuing all water forms and finding efficiencies and co-benefits of collaboration between agencies, a multi-agency Working Group considered important issues associated with climate adaptation and resiliency in Mapunapuna, including impacts on internal and external stakeholders and evaluating the cost, benefits, and potential timelines for adaptation development. The solutions investigated include: 1) building a flood resilience barrier for physical separation from the flooding potential of Moanalua Stream through the construction of additional backflow prevention devices and raised roads or a floodwall along the stream and southern end of the neighborhood, 2) a pumping system that can address the worse areas of flooding, and 3) the potential of raising streets and utilities to keep important infrastructure out of the reach of flood waters. These solutions were evaluated in the context of flood stages of the Moanalua Stream and potential sea level rise using FEMA watershed models, the potential benefits and the costs of the solutions are presented, and the timeline for implementation is explored.
Findings
Based on the economic benefits and cost analysis, the implementation of a floodwall and stormwater pumping system was not found to be feasible. But there are additional considerations and variables that should be weighed in any decision to move forward with flood mitigation or retreat. An adaptive management approach, shown in Figure 3, was taken to target solutions and decisions that will mitigate flooding for critical areas in the near-term to extend the useful life of the area as well planned and proposed studies that would be used to support these decisions. Long-term, the impact of climate change will be re-evaluated and the approach will be refined with a phased approach for armoring, raising and relocating combinations for large mitigation projects in Mapunapuna. The alternatives and mitigation/adaptation plan will be refined through the public participation process. Any successful solution for the Mapunapuna area will require the active participation of local stakeholders, including landowners and tenants along with public and private funding to implement the mitigation and adaptation measures. The approach regarding when and how to engage stakeholders in the findings of numerous studies was explored by the Working Group as well.