Relative Sea Level, Tides, and Extreme Water Levels in Boston Harbor From 1825 to 2018

Using newly-discovered archival measurements, we construct an instrumental record of water levels and storm tides in Boston (MA) since 1825. After ascertaining the 19th century datum and correcting for a 0-0.03 m bias in the modern tide-gauge record, we show that local, decadally-averaged relative sea level (RSL) rose by 0.280.05 m since 1826, with an acceleration of 0.0230.009 mm/yr(2). Tide range decreased by 5.5% between 1830 and 1910, due in large part to dredging and filling of Boston Harbor, and trended slightly upward thereafter. An evaluation of storm events since 1825 suggests that trends in flood risk are driven by RSL rise, with a small contribution by tidal trends. Sea-level rise also interacts with the 18.6 year nodal cycle in tide amplitudes to produce decadal fluctuations in hazard. Conditional sampling of the 1825-2018 record shows that storm tides with a 0.01-0.5 annual probability (100 and 2 year events) are 0.1-0.2 m larger during periods with above-average tidal amplitudes. Similarly, the once-in-25 year event during elevated tidal forcing becomes a once-in-100 year event during periods of reduced tides. A plurality of historic flood eventsincluding floods in 1851, 1978, and 2018occurred near the peak of the tidal nodal cycle. Projections to the year 2100 suggest that decadal fluctuations in tide characteristics will interact with relative sea-level rise to produce a fluctuating hazard over time, with periods of relative stationarity (e.g., the 2020s) bracketed by relatively abrupt increases in flood hazard (the early 2030s). Plain Language Summary We show that sea-level in Boston (MA) rose by nearly a foot (0.28m) over the past 200 years, with most occurring since 1920. The underlying tide measurements we analyzed were made, in part, by local civil engineers in the 1800s and early 1900s who measured daily high and low tides to help solve design problems and protect infrastructure from flooding. One of those structures, dry-dock number 1 from 1833 at the Charlestown Navy Yard, currently houses the USS Constitution and-if you know where to look-contains a benchmark from 1867 that allows historic measurements to be compared to modern data. The fact that this dry-dock was flooded twice in early 2018 is visceral evidence that sea-level is higher now than it used to be. The old data also shows the interesting-and unexpected-result that some historical extremes were larger in Boston than they would be today because tides were larger. Looking forward, we show that the 18.6 year astronomic cycle in tides also impacts flood risk. During the 2020s, tides will be lower than average, partially mitigating against sea-level rise effects; In the early 2030s, tides will be larger (as in 2018), exacerbating projected sea-level rise effects.