Estimating Future Changes in 100-year Floods on the Connecticut and Merrimack Rivers - Report from MassDOT and UMass
Hydrologist and meteorologists do not often use the term 100 year flood because it is somewhat misleading in terms of how often a significant precipitation event might occur. 100-year floods are a probability that a significant precipitation event will reach a peak stage, however, this is based on historical data. Instead, It is essential to understand the recurrence interval and how it is instrumental and determining the magnitude and probability of a precipitation event to occur on that same scale at the same location in the future.
The USGS defines recurrence interval as: “The average number of years between floods of a certain size is the recurrence interval or return period.” But is quick to note that the actual number of years between floods of any given size can vary based on changing climate or man-made changes such as upgrades or removal of flood mitigation infrastructure.
This report published by MassDOT and UMass considered how widely accepted models used to predict climate change may be applied to the communities along the Connecticut and Merrimack Rivers during the 21st century. Specifically, the research focused on 100-year, 24-hour extreme precipitation events and what the impact may be on existing infrastructure. In other words, what will happen if events such as Tropical Storm Irene occur more frequently?
In summary, the report notes that, for Massachusetts, these events are predicted to increase 25% over the near term and later, towards the end of the century as much as 50%. The research indicates the northeastern and southwestern portions of the state may be most adversely impacted.
Most importantly, this shift in patterns “suggests that infrastructure designed on historical hydrological events/records may not be adequate to sustain the 100-year, 24-hour flows that may occur during the latter half of the century”. The infrastructure the report refers to includes culverts, roads, bridges and tunnels.
The reports provides research that indicates these changes to climate are already well underway, for instance, metrics noting that the number of extreme precipitation events along the Connecticut River have increased almost 20% between 1950 and 2011. This may be reflected in different ways depending on time of year.
Built sometime around 1890, the 45-foot long wooden arch Bowers Bridge over the Mill Brook in West Windsor, VT was rebuilt after it was washed 200 yards downstream during flooding from Tropical Storm Irene on October 18, 2011. It was later damaged by an overweight truck and repaired again in 2014.
Photo from FEMA at the National Archives
The report concluded that the Connecticut and Merrimack Rivers are two of the largest in Massachusetts and the infrastructure and communities along them could be significantly damaged by more frequent 100-year, 24-hour extreme precipitation events. Further, it noted that there are challenges in predicting just how well the 3000 dams in the Connecticut River basin and 850 in the Merrimack will protect these communities but perhaps most importantly: “The insights gained from this study suggest that applying the 100-year, 24-hour precipitation event of the past in designing future infrastructure will result in underestimates of the most likely 100-year storms in the future by a significant percentage.” (p.28)