Abstract
The UNH Stormwater Center has conducted three years of monitoring of the stormwater catch basin at the intersection in Tisbury, MA known as “Five Corners.” The intersection is known to all residents and many visitors to the island for its frequent flooding. The goal of this project was to gather empirical data, observe, and characterize the rainfall events to quantify the conditions which result in the flooding at the intersection. The monitoring was performed with level loggers in the catch basin to measure the water level within the catch basin as well as when it exceeded the street level (flooding).
Summary of Results
- 64 runoff events observed over the 2.2-year monitoring period.
- 7 events resulted in flooding of the intersection; 57 events did not flood.
- The Mann-Whitney U test was used to compare the rainfall characteristics of the flooding vs non-flooding events. The rainfall duration was not statistically significantly correlated with the result of flooding. However, the rainfall depth and rainfall peak intensity were correlated to the flooding.
- The rainfall characteristics of the two correlated groups were used in a bootstrap analysis to find the 95% confidence intervals of the depth and peak intensity expected to result in flooding.
Conclusion
The 64 stormwater runoff events monitored at the Five Corners intersection indicated that 7 (11%) of the events resulted in flooding at the intersection. Rainfall depth and peak intensity were statistically significantly correlated to the flooding at the 95% confidence interval. The bootstrap analysis resulted in upper bound values for the non-flooding group of 1.3 inches of rainfall and 0.9 inches/hour peak intensity. In other words, total rainfall depths larger than 1.3 inches and peak intensities larger than 0.9 are expected to result in flooding of the intersection. We recommend using these values as “action numbers” for the Tisbury public works staff to preform preventative maintenance at the intersection and particularly at the clogged or buried outfalls at the end of Beach Street Extension to reduce the incidence and severity of flooding.
Date Created
2/27/2024
Publication Date
2-27-2024
Grant/Award Number and Agency
EPA Agreement Number SE-00A01003
Document Type
Report
Recommended Citation
Macadam, Daniel and Houle, James J., "2024 Monitoring Program Update for Five Corners Intersection, Tisbury, MA" (2024). UNH Stormwater Center. 134.
https://scholars.unh.edu/stormwater/134