CONCLUSION

This study examines flood mitigation design criteria for a new rail transit hub community that is undergoing rapid urban planning and development—the Waipahu Transit Oriented Development (TOD) area on Oahu.

The report includes maps showing the existing and new planned development that are anticipated to experience temporary flood shocks from storm surge, king tides, intense rainfall, and storm water runoff. Additional challenges for the area also include the long-term flood stressor of SLR and groundwater inundation in low-lying areas. The September 2017 Waipahu Town Action Plan identified three priority actions, which included addressing area-wide flooding.

This study provides a replicable methodology to analyze flooding from various sources: comparing flood maps to existing and planned development; identifying relevant flood mitigation strategies and built examples; and surveying flood mitigation design criteria. It offers potential planning scale flood mitigation measures for future development at the town planning scale, city block scale, and building scale. The report presents concepts, shares built examples, and shows potential future designs specific to the Waipahu TOD site.

Town planning scale strategies include retreat from flooded areas, development on higher ground, widened floodways, intentionally floodable open spaces, and large below grade water detention areas. City block planning scale strategies include bioswales and pervious pavement. At the building scale, various guides on DFEs are compared. At the Waipahu TOD site, flooding from rainfall, SLR, and storm surge should be considered simultaneously when determining the appropriate building occupied floor elevation. Also at the building scale, techniques for elevation, wet floodproofing, and dry floodproofing are presented.

Relevant flood related portions of existing standards and benchmarks are summarized for ease of reference by future design teams including FEMA, LEED, RELi, Fortified, Enterprise Housing, and NIBS.

This document should be a useful reference for future planners, policy makers, and design teams addressing the Waipahu TOD area.