Nantucket tide-gauge data show accelerating high‑tide flooding, presenter tells advisory committee

Chuck Larson, the Town of Nantucket manager of strategic projects, told the Coastal Resilience Advisory Committee on March 25, 2025 that a 60‑year tide‑gauge record for the island shows an accelerating increase in days that exceed local nuisance‑flood thresholds.

Larson presented statistical analysis of daily higher‑high‑water values from the Nantucket tide gauge, showing roughly 0.82 feet (about 10 inches) of local mean sea‑level rise over 60 years and a rapid, nonlinear increase in the frequency of high‑tide exceedances at lower thresholds used to mark nuisance flooding. "The theme is really with 60 years of data, what can the past tell us about the future?" Larson said.

The study separates the record into 19‑year ‘‘epics’’ and compares frequency histograms and cumulative frequency curves for each epic to show how the distribution of daily high tides has shifted rightward. Larson said the shift of the distribution tail across thresholds explains why days of nuisance flooding have risen much faster than the roughly linear change in mean sea level. He showed that a local threshold of about 1.1 feet mean higher high water corresponds to observed nuisance flooding on Easy Street and Oak Street and that exceedances at that level have risen from a handful per year to several dozen per year in recent decades.

Larson used NOAA tide‑station datums and noted upcoming national tidal datum and geodetic datum updates; he emphasized that the tide‑station datum is a time‑independent reference useful for comparing long records. He cautioned that his count measured days above a threshold (frequency), not the duration in hours. "I did not do that — I'm just looking at the number of days that we went over that level," he said.

Committee members asked for operational context. Doug Rose, a committee member, summarized Larson's point about frequency: "A hundred days a year where there will be some kind of nuisance flooding. Yep. At minimum. Yeah." Larson and others said the 1.1‑foot threshold corresponds to wet sidewalks and shallow ponding (inches), whereas higher thresholds such as 1.8 feet correspond to deeper flooding that can impede vehicle traffic at known locations.

Larson told the committee he is preparing a technical paper that will include additional variance and seasonal analyses and said he will share drafts for peer review. He recommended more work on flood‑duration metrics and seasonal clustering as next steps. He also noted that groundwater and soil saturation effects can amplify the practical impact of higher high tides in low‑lying sandy areas like Brant Point.

Why it matters: committee members said the tide‑gauge results provide local evidence for urgency in downtown flood planning and for Easy Street mitigation work now under design. Larson's analysis was presented as a data foundation for near‑term design choices and longer‑term adaptation timing.

The committee did not take a formal vote on Larson's presentation; members asked to receive the draft paper and to revisit details as the Easy Street mitigation and downtown barrier planning continue.