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Earth’s coastlines are shaped by geophysical and human dynamism. Waves, tides, currents, and sea level change reconfigure coastal environments on hourly to centennial timescales, and the coast is experiencing the fastest economic and population growth rates in the world. This coexistence of a dynamic environment and human development makes coastal communities uniquely vulnerable to natural hazards. Climate change is expected to exacerbate flooding and erosion hazards in the future; thus, it is critical that we understand the underlying physical drivers of coastal change. The overarching goal of this dissertation is to improve the mechanistic understanding and quantification of dynamic processes that shape the coastal environment. Specific topics range broadly from earthquake modeling to coastal flood statistics and salt marsh sediment dynamics. Chapter 1 aims to constrain a worst-case earthquake and tsunami along southwestern Japan’s Nankai Trough using novel earthquake rupture modeling techniques. The work is motivated by a mismatch between lacustrine sedimentological records of tsunami inundation in the region and modeled earthquake and tsunami scenarios in published literature. Recent advances in space geodesy enable high-precision measurements of crustal motion that can be used to estimate the degree of frictional locking along fault interfaces, known as interseismic coupling. We demonstrate methods for scaling modern interseismic coupling to coseismic slip to construct Nankai Trough earthquake scenarios. Results show that coupling-based models produce distributions of ground surface deformation and tsunami inundation that are similar to historical and geologic records of the largest known Nankai earthquake in C.E. 1707 and to an independent, quasi-dynamic rupture model. This implies that contemporary coupling mirrors the slip distribution of a full-margin, 1707-type rupture, and GPS measurements of surface motion are connected with the trough’s physical characteristics. Chapter 2 presents a new statistical methodology for calculating non-stationary flood height-frequency relationships that account for modulation of flood hazard by predictable tidal cycles. We developed the methodology after the winter of 2018, when two Nor’easters with relatively moderate storm surges led to the first and third highest water levels recorded at the Boston tide gauge in 100 years due to their overlap with anomalously high tides. Applying the new method in the Gulf of Maine, we find significant tidal forcing of winter storm season flood hazard by the 18.6-year nodal cycle; for example, the nodal cycle forces decadal oscillations in the 1% annual chance storm tide at an average rate of ±13.5 mm/y in Eastport, ME; ±4.0 mm/y in Portland, ME; and ±5.9 mm/y in Boston, MA. Appendix C is a contribution to the City of Boston’s climate projections report that combines the Chapter 2 methodology with probabilistic sea level rise to generate probabilistic flood projections that account for both sea level and tidal non-stationarity. We outline mechanisms of both extreme and tide-only coastal flooding; describe the impacts of climatic and natural tidal variability on flood hazard; give context for the record-breaking 2018 floods; and provide projections of flooding through 2100. Projections show that increases in the frequency and magnitude of flood hazard driven by sea level rise will plateau during decades when the 18.6-year nodal cycle is in a negative phase (2019-2027 and 2037-2046) but accelerate in the subsequent decade as the nodal cycle enters a positive phase and increased tide range combined with sea level rise amplifies flood hazard (2028-2036 and 2047-2055). Beyond mid-century, the importance of tide range on probabilistic flood projections becomes less important, as background sea level rise becomes the dominant influence on flooding. Chapter 3 presents an observational study of sediment delivery to a New England tidal salt marsh. An external clastic sediment supply is a key factor in determining salt marsh resilience to future sea level rise, yet information on sources, mechanisms, and timescales of sediment delivery are lacking for most marsh systems. We show that marine sediment mobilized and delivered during coastal storms is the primary source to the North and South River, a mesotidal bar-built estuary typical to New England. On the marsh platform, deposition rates, clastic content, and dilution of fluvially-sourced contaminated sediment by marine material all increase down-estuary toward the estuary inlet, consistent with a dominantly marine-derived sediment source. Marsh clastic deposition rates are also highest in the storm season. We observe that periods of elevated turbidity in channels and over the marsh are concurrent with storm surge and high wave activity offshore, rather than with high river discharge. Flood tide turbidity also exceeds ebb tide turbidity during these high turbidity intervals. Timescales of storm-driven marine sediment delivery range from 2.5 days (5 tide cycles) to 2 weeks, depending on location within the estuary; therefore the phasing of storm surge and wave events with the spring-neap cycle determines how effectively post-event suspended sediment is delivered to the marsh platform. This study reveals that sediment supply and the associated resilience of New England mesotidal salt marshes involves the interplay of coastal and estuarine processes, underscoring the importance of looking both up- and downstream to identify key drivers of environmental change.

A tsunami generated by the C.E. 1707 Hōei earthquake is largely thought to be the flood event of record for southwestern Japan, yet historical documentation of the event is scarce. This is particularly true in northwestern Shikoku within the Bungo Channel, where significant inconsistencies exist between historical records and model-derived tsunami heights. To independently assess flooding from the C.E. 1707 Hōei tsunami in the context of the region’s long-term flooding history, we present complementary reconstructions of extreme coastal inundation from three back-barrier lakes in the northern Bungo Channel: Lake Ryuuoo, Lake Amida, and Lake Kamega. At all sites, the most prominent marine overwash deposit of the past ~1,000 years, as defined by grain size, density, and geochemical indicators, is consistent with the timing of the 1707 tsunami, providing strong evidence that the event caused the most significant flooding of the last millennium in this region. At Lake Ryuuoo, modern barrier beach elevations and grain sizes in the tsunami’s resultant deposit provide ~4 m as the first physically based height constraint for the 1707 tsunami in the northern Bungo Channel. Around 1,000 years ago, a concurrent and abrupt transition in lithology observed at all three sites is also consistent with rapid, regional geomorphic change. At Lake Ryuuoo, a marine overwash deposit comparable to the 1707 deposit directly overlies this transition. A 1,000-year-old lithological transition or deposit has not been observed at sites closer to the mouth of the Bungo Channel, suggesting that the deposit in Lake Ryuuoo is more consistent with a tsunami generated by local seismicity along the Japan Median Tectonic Line than with a Nankai Trough-derived tsunami. Our findings are significant in that they provide three new millennial-scale tsunami inundation reconstructions for a relatively understudied region of Japan, along with the first physically based height constraint for the Hōei tsunami in the northern Bungo Channel.