Present and Future Damage: Quantifying Urban Flood Impacts in Philadelphia

Learning Objective: This presentation will include an overview of 1D/2D modeling conducted in key sewersheds across Philadelphia. Decision making required during both sewershed selection and the modeling process will be presented with a focus on optimizing model results and level of effort. Model results and accompanying analysis determining locations of sewer backups, basement flooding, and first floor elevation flooding will be presented in addition to considerations for application of similar programs in other locations. BACKGROUND Water service in Philadelphia began in 1801. The Philadelphia Water Department (PWD) is now tasked with mitigating the risk of its aging infrastructure through evaluation and recommendation of both structural and non-structural solutions. Aging or undersized infrastructure is an increasing concern across many municipalities - and will only accelerate in the future. Targeted flood modeling across their service area is a key pillar of PWD's asset evaluation, risk assessment, and long-term capital planning. 1D/2D modeling has been utilized on an individual sewershed scale to determine urban pluvial flood risk within the city. To date, 6,100 acres across approximately 40,400 acres making up the city's service area have been modeled, with the ultimate goal to model all sewersheds. Sewersheds are prioritized and selected by PWD based on known flooding issues, known upcoming development, or customer complaint logs. OBJECTIVES The ultimate objective of this ongoing project is to determine areas across the city prone to flooding, sewer backups, or basement/first floor elevation flooding. As in all modeling, data quality directly impacts quality of model results. What implications should be considered if input data is outdated or modeling is being conducted in an area of rapid redevelopment and infrastructure changes? What is the best way to leverage existing models (such as existing 1D planning level models) to accelerate creation of flood models? METHODOLOGY Jacobs began supporting PWD with their linear asset management planning program in 2022, though it has evolved over the last several years. Progress includes: - a comparison of a 1D/2D PCSWMM 1D/2D and a 2D only HEC-RAS approach - addition of a basement flood analysis methodology - refinement of model expansion techniques from planning level models to more representative 1D networks - incorporation of climate perturbed rainfall data for 2100 Once a sewershed is selected, an integrated 1D/2D PCSWMM model is built with an approach designed to optimize a variety of considerations including overall level of effort, accuracy, and model run time. PWD's existing 1D network models are utilized with incorporation of 2D terrain, inlet connections, varied rainfall, and expansion of pipe network, and building footprints. Validation of the model is conducted using a historical event time series, documented backflow preventor installations, and problem areas which received flood calls from the public were utilized to confirm areas of flooding. RESULTS AND CONCLUSIONS Final outcomes of this work are flood depths across the modeled area, identified locations of sewer backups and locations of basement and first floor elevations. The following results will be summarized and presented: - Model setup and validation (Figure 1) - Flood model results - Analysis of sewer backups, basement flooding, and first floor elevation flooding (Figures 2 and 3) Once flood modeling is completed, the use and application of model results is just as important as the model build itself. For PWD, these flood model results are utilized to help enforce private development stormwater standards called the Public Health and Safety Release Rate, which dictates that a maximum release rate based on sewer capacity must be used in design in certain watersheds where known flooding occurs.