See it before we build it - How Fairfax County utilized high resolution 2-D Model to support a stormwater drainage improvement design to address 100-year flood!

INTRODUCTION Fairfax County is the most populous county in the state of Virginia with approximately 1.15 million residents. The County, like many other communities around the world, is experiencing increased flooding due to climate change and extreme wet weather related events. The County is implementing various stormwater improvement projects at the neighborhood level for communities that are exposed to severe flooding risk. Tucker Avenue Neighborhood Stormwater Improvement Project is one such project the County initiated to reduce localized flooding and erosion, improve stormwater drainage conditions and infrastructure, and improve water quality and protect the local stream. Tucker Avenue is a residential neighborhood in Fairfax County, Virginia. The project is in the Dranesville District in the Pimmit Run Watershed. The project area (approximately 67 acres) is divided into three different areas: Upper, Middle, and Lower Tucker Avenue with each area presenting its own challenges. All these areas suffer from inadequate drainage systems that cause street and yard flooding and reported house flooding. In addition, portions of Lower Tucker Avenue do not have any curb or storm inlets and they are located in the Federal Emergency Management Agency (FEMA) floodplain along Pimmit Run-creating a drainage problem of its own. The July 8, 2019 storm (~500-year return period) resulted in basement and backyard flooding and had more than 6 inches of ponding on Tucker Avenue. METHODOLOGY To address the flooding issues in the Tucker Avenue Neighborhood, the County proposed a storm sewer system design that would alleviate flooding in the neighborhood for a 100-yr return period storm. However, the County desired to demonstrate the alleviation of flooding and extents with the proposed design which is adding new storm inlets and storm sewers in the project area. A 1-D stormwater model developed to support the proposed design met the design criteria, however, it could not demonstrate the surface flooding experienced on Middle and Lower Tucker Ave. Therefore, an integrated 1-D/2-D hydraulic model was proposed to validate the surface flooding observed during existing conditions and demonstrate the improvements with the proposed design. The integrated model represented both surface and subsurface drainage for the Tucker Ave neighborhood. This type of integrated model has the ability to simulate the dynamics between three systems: the surface overland drainage, the underground stormwater sewer, and the outfall at the Pimmit Run. Figure 1 illustrates the relationship between three drainage systems within an integrated model. RESULTS An integrated 1-D/2-D hydraulic model was developed in Computational Hydraulics International's (CHI) PCSWMM software for the existing conditions and the proposed design conditions. The 1-D/2-D model validated the flooding observed at various locations along Tucker Ave for the existing conditions as shown in Figure 2. In addition, the 2-D model supported the design by identifying portions of Tucker Avenue where storm sewers should be upsized to reduce the hydraulic grade line and demonstrated the reduction in street and backyard flooding for the 100-yr return period storm. The final design of the project is completed, and the construction is expected to begin in 2024. BENEFITS AND LESSONS LEARNED This paper will demonstrate the benefits of developing integrated 1-D/2-D hydraulic models to support the design of stormwater drainage improvements and how models can be used to demonstrate the alleviation of flooding extents with proposed improvements, and identify additional improvements required to meet the desired level of service. In addition, this paper will summarize the types of data needed to develop an integrated 1-D/2-D model and the importance of validating the model predictions. It will include how combined 1-D and 2-D hydraulic models provide a clear understanding of flooding, the associated risks, causes of flooding, and above and below ground wet weather flow behavior. CONCLUSION The integrated 1-D/2-D hydraulic models support decision making during planning and design to evaluate multiple solution options and alternative paths to meet the desired levels of service and demonstrate the alleviation of flooding extents with proposed improvements. It is concluded from the 1-D/2-D modeling effort that the proposed sewer improvements design provided significant improvement and reduction of flooding extents and depths under 100-yr return period storm conditions. The final design of the project is completed, and the construction is expected to begin in 2024.