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.
One way in which geologists categorize flood types is by historic and recorded, the former having been documented by personal accounts and relative magnitudes to previous events and the latter as measured by gauging stations. Stories of historic floods have been captured by historians and kept in records at town halls and historical societies and documented by reporters in newspapers for hundreds of years and are typically characterized by height of water and often marked in a common location on a particular date. But discharge rate is also important, commonly measured in seconds per feet and was often not able to be qualitatively captured in the same capacity as the height was.
Records of historic floods in New England date back to prior to 1620. Measurements of major rivers and tributaries began around 1904 and became the standard over the next decade. While never a substitute for a personal account, gauge stations, particularly those monitored by academic institutions and government agencies provide a consistent, reliable, quantitative metric that allows for the development of predictive models and planning to some degree.
Flooding remains the most commonly occurring and most costly disaster to impact the cities and towns in New England. There have been dozens of events over generations The floods of 1927, 1936 and 1938 are all remarkable in their own right but it is important to briefly distinguish how they each differ from one another as well.
But flood mitigation systems were built in response to the damage sustained during these events. The US Army Corps of Engineers, in some cases, literally moved rivers and constructed miles of flood walls and dikes and on the coast, erected hurricane protection barriers and raised seawalls.
But floods continue to create damage on a larger scale than any other hazard in New England and likely will do so over this century.
Flood mitigation efforts had been taken on prior to 1936 at the federal level through various iterations of the Flood Control Act going back as far as 1917 but arguably most significantly after the 1936 Flood. But the program got a boost following the flooding in 1936 and was newly infused with support and funding through a variety of New Deal programs. Unfortunately, in spite of the enthusiasm and action taken on the heels of 1936, most of the flood mitigation infrastructure was on a scale that could not be fully completed before the region was hit again, this time more isolated by the Hurricane of 1938 - the Long Island Express. Until 1936, the flooding event in 1927 was the standard by which other storms were graded but in fact, these were different meteorological events.
But given the efforts that went into preventing flooding in these communities why did subsequent events happen on a major scale in 1955, 2006 and even 2011? And given this modern approach, what might we do now differently that may prevent or at least reduce the damage, disruption and loss of livelihood and life going forward the next time New England Floods? These are the issues we’ll be considering.
Thomson, M., Cannon, W., Thomas, M., & Hayes, S. Historical floods in New England. Retrieved from https://pubs.er.usgs.gov/publication/wsp1779M
Arnold, J. L. (1988). The Evolution of the 1936 Flood Control Act. United States: Office of History, U.S. Army Corps of Engineers.