30 Minute Presentation
Urban Stormwater Retrofits in Hawai‘i
The unique characteristics of urban O‘ahu create several challenges for the implementation of stormwater retrofit best management practices (BMPs). Storms in Hawai‘i are typically of high intensity and short duration, which generates high peak flows in a relatively short period. BMPs need to be designed to handle both the treatment flow and have a reliable overflow for peak storm flows. Because we are on a relatively small island, available space to install stormwater BMPs is severely limited. In addition, congested underground utilities in public rights-of-way can result in costly utility relocation and/or limiting BMP selection. Microclimates on O‘ahu, like the other Hawaiian Islands, vary dramatically, which then influences plant selection for BMPs. Lastly, unique archaeological, cultural and endangered species concerns pose significant challenges to address urban stormwater treatment. The City and County of Honolulu MS4 permit requires that 10% of all the City’s MS4 structures be retrofitted with permanent BMPs, with an emphasis on the implementation of low impact development (LID) practices. With about 65,000 structures in the City’s MS4, achieving the permit target of 10% is significantly challenged. To address these challenges, an action plan for the urban stormwater retrofit program was first developed. The action plan inventoried all completed, ongoing, and planned retrofit BMP projects across all the City’s various stormwater and non-stormwater programs from 2006 to 2016. The plan identified the type, amount, and costs of BMPs installed or planned throughout the drainage system and geocoded projects in geographic information system (GIS) shapefiles. An implementation schedule for future BMP retrofits was included in the plan. The effectiveness of each completed, ongoing, and planned BMP project was evaluated using a prioritization and tracking system that included multiple factors, such as pollutants to be addressed, maintenance, access, cost, environmental impacts, and community acceptability. In addition, a desktop analysis of the existing drainage system was then conducted, identifying potential retrofit BMPs in industrially zoned areas. The desktop analysis utilized the same prioritization and tracking system to evaluate the potential effectiveness of the proposed BMPs. To further encourage MS4 retrofits, a technical design manual with standard details and specifications for eleven retrofit BMPs was developed. These eleven BMPs were grouped as low impact development, detention and debris basins, and manufactured treatment devices, incorporating both green and gray infrastructure. This manual will serve as a guide for City personnel to plan, select, design, construct, and maintain retrofit BMPs. In addition, data transfer forms were developed so that BMP information is collected at the planning, preliminary design, and completed construction phases of a project and installed retrofit BMPs can be counted towards the 10% permit requirement. Finally, standard details for a future catch basin retrofit demonstration project in the Kaka‘ako area were developed. The demonstration project will address catch basin repair and access and incorporate permanent BMPs.