Quantifying the Socioeconomic Impacts and Benefits of Nature Based Floodplain Projects in a Flood Prone Urban Watershed in Pittsburgh, PA

Saw Mill Run Watershed near Pittsburgh, Pennsylvania is a highly urbanized watershed with steep slopes, narrow ravines, minimal upstream stormwater management controls and natural floodplain buffers. For more than a century, the main stem of the stream has been historically prone to hazardous flash flooding conditions. More recently, public records from 2010 to 2020 indicate that there were 49 days of individual flash flooding occurrences resulting in a combination of road closures, damages, cleanup, and emergency management services. Due to the frequency of flooding, large portions of the watershed, primarily along the main stream corridor, have become subject to blight and/or abandonment of homes and businesses. The stream has been largely neglected, highly channelized, and encroached upon by development and transportation projects. To understand the prolonged negative effects of continued flash flooding of Saw Mill Run, a social and economic analysis of the main stream corridor was performed for the purposes of understanding the economic disinvestment and related social injustices to adjacent neighborhoods. The study was broken into two phases over a year and half: Phase I (completed in 2023): This phase included the development of a detailed two dimensional rain-on-grid watershed flood prediction model, a flood damage and socioeconomic assessment of the existing properties, and an economic green benefits screening assessment of the parcels corridor. Phase II (ongoing and due to be complete in early 2024): This phase consists of a more rigorous and in depth future economic evaluation of two site locations for potential nature based flood plain strategies. Strategies include detailed hydraulic modeling to determine flood reduction and water quality benefits along with an economic impact analysis to the local watershed and the surrounding area. As part of Phase I, flood inundation extents of the study area were determined using a full watershed two-dimensional rain-on-grid flood model developed using HEC-RAS 6.2 and publicly available USGS QL2 LiDAR data. The results of the model were compared against historical flood reports to confirm predicted flood extents and validate the model results. Using the validated model, the model was then simulated for a range of return frequency rainfall events under both present day and future climate scenarios to determine the limits of flood inundation within the study area. The modeled flood inundation levels were then used in conjunction with existing building structure GIS data to develop flood damage frequency curves and annualized flooding costs. In addition to the flooding economic impacts, existing socioeconomic data was compiled to determine social justice impacts of flooding in the study area. This included demographic, employment, and county tax assessor's parcel data to understand property values and associated economic output in the corridor. Results indicate social justice areas, high vacancy, and low property values are correlated with the flooding results. Property values in the flood prone areas are shown to be reduced in comparison to the Allegheny County average. On average commercial and residential properties in the study area were 50% lower in value as compared to the County wide average indicating reduced desirability for new and redevelopment efforts due to reoccurring flooding. Using the baseline flooding inundation mapping results and the socioeconomic results, parcel locations were prioritized for potential nature-based solutions using a GIS based economic greening benefits screening analysis using FEMA approved approaches. The economic greening benefit screening analysis helped identify initial locations within the study area where the potential economic greening benefits exceeded the property acquisition value of the parcel. Results indicated flood prone vacant property clusters are the most advantageous location for potential future floodplain greening strategies. Using the green benefits screening analysis, two focus areas with high vacancy and high greening benefit to cost acquisition ratios were chosen for the development of nature based floodplain strategies as part of the Phase II portion of the project. As part of Phase II (to be complete early 2024) conceptual design workshops will be held for each of the focus areas to identify and prioritize floodplain nature based strategies and economic redevelopment opportunities. Strategies will be specifically tailored for each site and the needs of the surrounding community based on feedback from the design workshops. The flood mitigation strategies will be inserted into rain-on-grid watershed flood model to estimate flood damage reduction economic benefits using both present day and future climate change projections. Within each focus area, softer economic benefits (social and environmental) will be estimated using IMPLAN economic modeling software. These economic benefits included potential increased property values, increased tax revenues, value of economic redevelopment/increased jobs/economic output, and potential ecosystem benefits such as value of increased green space, carbon/air pollution reductions, improved water quality, recreational opportunities, and heat island effects. This presentation will provide conference attendees with approaches for how to build an economic case for community driven nature based floodplain strategies using socioeconomic data and state of the art flood modeling software tools for a real world case study in a densely built urban watershed.