2D or Not 2D: That is the Question

The Fairbank Silverthorn Storm Trunk Sewer System (FSSTSS) project is one of the Basement Flooding Protection Program which began in 2005 implemented by the City of Toronto. The program separates the City into study areas investigated through individual Environmental Assessments to develop 1D dual drainage models to evaluate potential flood reduction measures. Solutions identified are required to meet specified level of service criteria during the 100-year design storm event comprising a minimum freeboard of 1.8 m in storm and combined sewers and a maximum surface water depth of less than 150 mm within the road right of way (ROW) with reverse sloped driveways (RSDs) or 300 mm otherwise. The FSSTSS project focuses on reducing overland and primarily combined sewer system flooding in a dense suburban neighbourhood. The primary scope is to separate or reduce stormwater into the combined sewers and to construct a 4.5 m diameter storm storage tunnel which discharges into Black Creek. Other flood reduction measures are proposed where it was not cost effective for a direct connection to the storm tunnel, including one local storm storage facility adjacent to Woodborough Park, a small City-owned green space located within a commercial/industrial neighbourhood. Based on field observations and photographs taken during recent extreme storm events in 2018 and 2020, significant overland flooding was observed within the park causing visible damage to the local playground, and ponding depths of approximately 0.5 meters at low points within the park illustrated in Figure 1. The design developed using the 1D dual-drainage model where the overland system is represented using 1D links between nodes recommended a large underground storage facility consisting of a 190 m long 3.6 wide x 3.0 m high concrete box culvert to store about 1210 m3 of storm runoff collected via 10 high-capacity inlet (HCI) catch basins. This design alternative presented a significant constructability challenge as locating the large storage facility within the park would limit the park and baseball field use by local residents, and fitting this storage facility within the local road ROW would create substantial conflicts with other utilities and disruption along a busy road. To validate the observed flooding within the park, gain a better understanding of existing flood pathways and ponding areas, and optimize the size and location of the storage facility, it was decided that a small scale 2D surface model (39 ha) would be integrated within the existing 1D model, illustrated in Figure 2. The 2D model was completed using high resolution digital terrain model data to develop a 2D flexible mesh for the catchment area draining to Woodborough Park. The domain includes a full representation of surface roughness and a higher mesh resolution within the road ROW where most surface flow is being captured. Through the implementation of the small 2D domain it was possible to validate flooding conditions within the park using observed rainfall data and confirm primary flow paths for the storm runoff to the park. Additionally, it was confirmed that the original 1D model implicitly introduces conservatism due to the significantly faster routing time for overland flows to low points in the topography, as they do not provide sufficient resolution for capturing local depressions, flow paths, and obstacles for overland flow. This is best shown through a comparison of the hydrographs for the overland flow captured by the HCIs at the entrance to the park between the 1D and integrated 2D models shown in Figure 3. The attenuation and better routing of flows by the 2D domain enabled the design to be optimized by reducing the overall size of the underground storage and capturing storm runoff in the road ROW more effectively at locations where runoff was observed to pond and enter the park on the refined 2D domain. The additional understanding and resolution on flow path gained through the 2D model allowed the park to be utilized better for overland storage through minor topographic refinements in the park to redirect flow to the natural low point during extreme storm events thus avoid damaging to the existing playground as historically observed. Figure 4 and 5 shows the optimised solution using the integrated 2D model and the corresponding 100-year design storm performance which meets the City's freeboard and ROW overland requirements. A summary of the final design relative to the original design is provided in Table 1. This project demonstrates how innovation and digital technology advancement in modelling can be used to mitigate flood risk using real time evidence of increased storm severity and frequency in an ever-changing climate. It also demonstrates how the utilization of the latest 2D modelling technology through an additional CA$30,00 spend can support nature-based and green infrastructure solutions while providing economic savings of about CA$2.5 million. In this paper, we will discuss the approach and techniques used to integrate the 2D model with the 1D model and corresponding learnings from sensitivity analysis of various 2D modelling elements. The comparison of solutions derived using the full 1D model versus the integrated 2D and the corresponding benefits and limitations of using the two different models for solution development will allow us to conclude on that question. 2D or Not 2D?