Stormwater Risk Assessment in Portland, Oregon

The City of Portland, Oregon partnered with DHI Water and Environment, Inc. (DHI) to develop an innovative approach to assessing risk and managing the assets of their stormwater system, both in the built and natural environment. Instead of initially focusing on physical assets as is often done in traditional asset management, the City first collaborated with stakeholders to understand the stormwater system and the services it provides. This insight allowed the City to look at the risks associated with failure to provide the identified services. The stormwater system in Portland is a complex network of engineered and natural assets that provide conveyance, protect water quality, and provide and protect habitat and biological communities. Assets in the stormwater infrastructure inventory include hundreds of miles of pipes and ditches, thousands of sumps, flow control and pollution reduction facilities (PRFs). Additionally, Portland relies on thousands of private facilities within the stormwater network that are not owned or controlled by the City as formal assets. Further, Portland's stormwater system depends on the management and expansion of the city's tree canopy and natural areas that intercept rainfall, keeping it out of pipes and filtering it naturally. Finally, acres of wetlands and thousands of miles of natural streams and drainageways are a critical part of the stormwater conveyance network. Applying asset management to the stormwater system, therefore, requires a comprehensive risk framework that considers ownership boundaries, natural resources and functions, multiple services and a myriad of risks born from the failure of those services. Asset management in the stormwater system requires a broader, holistic view of risk, and to do that Portland needed to expand the understanding of the services they provide, the assets they use to provide them, and a common language to evaluate impacts when those services or assets fail. Risks related to stormwater services include failure to safely convey water leading to nuisance flooding, erosion, landslide exacerbation, as well as habitat degradation, poor water quality, and community development impeded by system deficiencies. As these aspects of the stormwater system were explored and understood, DHI worked with the City to develop the Risk Assessment Framework Tool (RAFT). This dynamic software tool delivers automated GIS analysis of the stormwater system using dozens of input datasets, including LiDAR, ground cover, soils, pipe asset data, stormwater management permits, habitat data, and water quality samples, to produce comprehensive risk maps citywide. On the backend is an automated series of scripts, programs, and geodatabases which run weekly as input datasets are updated. This tool produces live risk maps that present information based on the latest data available. RAFT has proven to deliver valuable insight through several key outputs including a fully connected network of stormwater flow across the city via surface drainage, through piped infrastructure and, ultimately, to rivers or treatment plants. This output allows tracing of stormwater flow across the city as well as precise delineation of watersheds at all scales, thus the source and destination of stormwater at any location can be determined. RAFT also enables PRFs and other stormwater control structures to be included in the analysis. The tool allows the City to assess risk across service categories. A significant effort was made to make risk comparable across these categories. For example, levels of risk to water quality were made comparable to landslide risks or risks of nuisance flooding. This allowed the RAFT tool to combine risk from all service categories and produce a single map showing 'total risk.' With this map, hot spots can be identified, and users can select locations warranting further detailed analysis. The analysis is then used to propose and implement risk mitigation projects with the highest impact. This approach has helped to illuminate the multi-faceted nature of stormwater risk and the value of adapting asset management principles to instill transparency and efficiency into Portland's risk reduction strategies. Assessing stormwater risk requires looking broadly at service provision and the assets the City relies on to provide those services. This means valuing natural resources as both assets that provide services and suffer consequences of service failure. It can also mean that the lack of an identifiable storm system can lead to a multitude of problems and solving them has the potential to reduce multiple risks and provide multiple benefits. By taking a holistic, comprehensive view of the stormwater network and risk potential, the City of Portland is better prepared to plan and implement successful mitigation strategies.