EXECUTIVE SUMMARY

The purpose of this study is to examine 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. As part of a larger study, this report will do the following: identify the extent of anticipated flooding; propose proof-of-concept level design criteria to mitigate flood damage to roadways and buildings; present built examples of proposed strategies; and identify relevant flood design standards and emerging resilience rating systems. The faculty and students at the University of Hawaii School of Architecture’s Community Design Center (UH CDC) conducted this study for the State of Hawaii Office of Planning. The Waipahu TOD area is a pilot region to establish a design framework that better informs the funding of capital improvement projects in the future.

This study provides a replicable methodology to: analyze flooding from various sources; compare flood maps to existing and planned development; identify relevant flood mitigation strategies and built examples; and survey flood mitigation design criteria. While the information in this report is specific to the Waipahu TOD site, the process may be replicated at other sites. This study begins with a site analysis of the Waipahu TOD area, a half-mile radius surrounding the new rail station. This section includes descriptions and maps of the topography, current buildings, future development plans, roadways, new rail line, rail station, demographics, waterways, major green spaces, and State of Hawaii-owned land parcels.

Anticipated flooded areas are visualized through a series of maps depicting various levels of sea level rise (SLR), riverine flooding, and Flood Insurance Rate Maps (FIRMs). At the site, flooding of roadways and buildings occurs when elevated water levels overtop streams and canal banks. Occurring in isolation or simultaneously, temporary flood shocks include: storm surge; king tides; intense rainfall; stormwater runoff as well as the long-term flood stressor of SLR; and groundwater inundation in low-lying areas. Flooding threatens the existing and proposed roadways, buildings, and some parts of the rail transit area. Impervious buildings, roads, and slow-draining clay soils exacerbate flooding in the area. The September 2017 Waipahu Town Action Plan identified three priority actions, including “Address Areawide Flooding”.

The section on Flood Resilience Design Criteria begins with a comparison of the above flood maps and proposed future development maps. It offers potential planning-scale flood-mitigation measures for future development at the town planning-scale, city block scale, and building scale. At the town planning-scale, the first consideration is retreating existing built development in areas affected by three feet of SLR. Second, encouraging future development in areas not affected by six feet of SLR. Third, designing floodable open spaces, such as Waipahu District Park, for temporary detention of water from rainfall or riverine flooding.

At the city block planning scale, measure include designing water management strategies to temporarily detain flooding. Relevant flood mitigation strategies in Waipahu may include green infrastructure methods such as widening the floodway adjacent to streams; stormwater management in bioswales and rain gardens; and multi-use floodable landscapes and public spaces. For example, many of the properties affected soonest by SLR are along a stream, which creates an opportunity for building retreat and creating a wider floodway adjacent to the waterway that may serve multiple purposes such as flood control and recreation (e.g., walking or biking). At the building site scale, flood mitigation strategies include green infrastructure, gray infrastructure, and building construction measures. Green infrastructure examples include bioswales to direct and infiltrate water on a building site. These strategies may be combined with gray infrastructure flood mitigation strategies, such as water detention in underground cisterns and building flood protection.