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Evaluating Benefits
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Evaluating the Benefits of Future Improvements to the City’s
Storm Drainage System

The potential benefits of improvements to the Bridge Street and Wellington Avenue project areas will be evaluated using calibrated models for the City’s storm drainage system. These models provide insight on the causes of historic flooding events. They also provide a platform to quantify how future system improvements may reduce the frequency and magnitude of future flooding events. In order to identify the alternatives that best meet the community’s objectives it is important that potential improvements be evaluated for a wide variety of realistic conditions. Correspondingly, this study will use what is known as a “typical year” for these evaluations. The benefit of simulating the system’s performance for an entire year is that alternatives are tested against a wide range of scenarios, such as a small storm at high tide and a large storm at low tide. Initially, the models are used to establish a bench mark for how the existing system performs for the “typical year”. Later, the models are used to demonstrate the benefits of potential improvements to the storm drainage system. To identify a “typical year” for this study and to establish a baseline for the system’s performance, the model was used to simulate the performance for the Bridge Street and Wellington Avenue study areas for the last 10-years of recorded of precipitation and tide data. Rainfall data collected at the Newport State Airport and tide data collected from NOAA Station: 8452660 were used. Results from the model simulation for the Bridge Street system are shown in Table 1 and Figure 1 and for the Wellington Avenue system are shown in Table 2 and Figure 2. Key observations from the 10-year simulation include:

Comparison of the frequency and causes of flooding over the last 10-years demonstrates the unique characteristics for each study area. The more frequent tidal flooding shown for the Wellington area can be attributed to two locations with lower elevations than are found in the Bridge Street study area. The more frequent rain caused flooding in the Wellington area can be attributed to conveyance capacities within its drainage system.

Next Steps

Although simulation of the system’s performance for a 10-year period provides a foundation for understanding historic trends, to accommodate the evaluation of a wide variety of improvement scenarios for each study area, a “typical year” will be used. Although the data presented in Tables 1 and 2 show wide variations in annual precipitation volumes and tidal conditions, 2013 was identified to most closely meet the project’s planning needs. It includes 74 precipitation events ranging from trace amounts up to 3.7-inches, and a peak intensity of 2.4 inches per hour. It also represents recent trends in tide and sea level rise. Going forward, the models will be used to establish a baseline for each study area and then to quantify the benefits associated with candidate improvement projects for the storm drainage system. Use of a “typical year” for these evaluations will ensure that alternatives candidate projects are tested against a wide range of storms and tidal conditions.