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There is widespread concern that rapidly rising sea levels may drown salt marshes by exceeding the rate at which these important ecosystems can build elevation. A significant fraction of marshes reside within backbarrier estuaries, yet little attention has been paid to how changes in inlet geometry influences estuarine tides and marshes. In 1898, a coastal storm eroded a new inlet through the barrier beach that fronts the North‐South Rivers Estuary in Massachusetts, USA. The new inlet shortened the North River by 5.6 km and lengthened the South River channel by the same amount. Modern measurements of tidal attenuation suggest that channel shortening abruptly increased mean high tide along the North River by at least 30 cm. Foraminifera communities within North River marsh sediments indicated an environmental change from infrequent to frequent inundation at the time of the 1898 switch in inlet location, which supports this hypothesis. Increased mineral sediment deposition after the inlet switch played a dominant role in allowing marshes along the North River channel to adjust to greater inundation. Following the inlet switch, sediment accreted in North River marshes at 2–5 times the rate of sea level rise (SLR). The North River channel widened by an average of 18% relative to pre‐1898 conditions to accommodate the increased tidal prism. The role of mineral sediment accretion in making this marsh resilient to an abrupt increase in inundation depth highlights the importance of maintaining adequate sediment supplies in coastal regions as SLR accelerates. Plain Language Summary: Salt marshes form along sheltered coastlines, incorporating mud and dead roots atop a grassy platform at about the elevation of high tide. It is unclear whether marshes will build their platforms fast enough to keep up with expected increases in sea level. In 1898, a coastal storm eroded a new opening in a beach, behind which a salt marsh extends inland along two tidal channels. The new inlet shortened the distance from the open ocean to parts of the salt marsh, causing high tides that flood the salt marsh to increase more than 30 cm. We considered this "natural experiment" in increased tidal flooding to understand how other marshes might survive rapid sea level rise (SLR). Sediment core samples from the marsh showed that higher tides after the inlet switch resulted in lots of mineral sediment deposited on the marsh platform. This shows that mineral sediment is very important to salt marsh survival when sea level is rising fast. Increased tidal flow after the inlet switch eroded and widened channels. Some of that eroded channel bank sediment deposited atop the marsh, but most of the sediment that allowed the marsh to survive rapid SLR came from the ocean. Key Points: Inlet and barrier dynamics can have first order controls on tidal propagation and impacts to marshesDeposition rates three to five times the rate of sea level rise (SLR) allowed for continued marsh resilienceMinerogenic sediment supply allowed for marsh resilience under rapid rates of effective SLR [ABSTRACT FROM AUTHOR]

Tracking paleoenvironmental change and past event deposits is very important to evaluate the natural hazard spatially. This paper presents how the environment changes and implies the event deposit depending on the diatom assemblage change. To investigate paleoenvironmental change and identify the difference between tsunami and typhoon deposits, we analyze diatoms from the sediments in two coastal lakes in southern Japan where flood deposits have been linked to historical typhoon and tsunami events (Lakes Kawahara and Ryuo). The sediment cores extend from B.C.E. 500 to approximately C.E. 1000 and the downcore variation in diatom assemblages indicates a series of transitions from saline to fresher conditions in both Lake Kawahara and Lake Ryuo between approximately C.E. 500 and 1700. We observe an obvious deviation in diatom assemblages in event deposits previously identified to be either of tsunami or typhoon in origin. For the most prominent event deposit preserved in Lake Ryuo by the Hoei tsunami of C.E. 1707, the deposition of marine diatoms serves as evidence of marine flooding, while the subsequent deposition of soil and freshwater diatoms indicates the mobilization of terrigenous sediment during returning seaward flows. In contrast, the most prominent event deposit in Lake Kawahara is associated with freshwater flooding by the Kamikaze typhoon of C.E. 1281 and contains very low diatom abundances and a peak of freshwater taxa, followed by a peak in diatom counts potentially due to greater biological activity induced by a resultant influx of nutrients and re‐oxygenation during the event. [ABSTRACT FROM AUTHOR]

We demonstrate that long‐term tidally induced changes in extreme sea levels affect estimates of major flood hazard in a predictable way. Long‐term variations in tides due to the 4.4 and 18.6‐year cycles influence extreme sea levels at 380 global tide gauges out of a total of 581 analyzed. Results show coherent regions where the amplitudes of the modulations are particularly relevant in the 100‐year return sea level, reaching more than 20 cm in some regions (western Europe, north Australia, and Singapore). We identify locations that are currently in a positive phase of the modulation and therefore at a higher risk of flooding, as well as when (year) the next peak of the long‐term tidal modulations is expected to occur. The timing of the peak of the modulation is spatially coherent and influenced by the relative importance of each cycle (4.4 or 18.6‐year) over the total amplitude. An evaluation of four locations suggests that the potentially flooded area in a 100‐year event can vary up to ∼45% (in Boston) as a result of the long‐term tidal cycles; however, the flooded area varies due to local topography and tidal characteristics (6%–13%). We conclude that tidally modulated changes in extreme sea levels can alter the potentially inundated area in a 100‐year event and that the traditional, fixed 100‐year floodplain is inadequate for describing coastal flood risk, even without considering sea‐level rise. Plain Language Summary: Interannual and decadal (long‐term) variations in tides modulate extreme sea levels, increasing the risk of flooding at specific and predictable times. Here, we use a data set of 581 tide gauges to estimate the magnitude of the long‐term tidally induced changes in extreme sea levels as well as to identify the timing of the peak of the modulations. We find significant amplitudes of the long‐term tidal modulations in extreme water levels. We conclude that the long‐term modulations in tides influence the total inundation area and, therefore, they should be taken into account when assessing coastal vulnerability. Key Points: Long‐term tidal cycles (4.4 and 18.6‐year) modulate extreme sea levels and affect estimates of flood riskThe timing of the peak of both modulations is influenced by the relative importance of each cycle over the total amplitudeTidally induced changes in extreme sea levels affect estimates of total inundation area [ABSTRACT FROM AUTHOR]