How changing tidal properties impacts coastal flood hazards - Stefan Talke (cal poly)
Abstract: Over the past 100-200 years, tides in estuaries and tidal rivers have changed at rates not explained by astronomical forcing. Rather, the observed evolution of tides and other long waves, such as storm surges, is influenced primarily by anthropogenically-caused changes to the roughness, depth, width, and length of embayments, estuaries, and tidal rivers. In this presentation, I review the current state of knowledge of evolving tides, using examples from around the world, and explore the implications for coastal flood risk and flood duration. While tidal amplitude changes in estuaries are sometimes slight (<1%) or even negative, changes are often measured in meters. Several types of systems that are particularly prone to tidal amplification: (a) shallow, strongly damped systems, in which a small increase in depth produces a large decrease in effective friction, (b) systems marked by significant shifts in either total or partial wave reflections, caused for example by tide barriers and (c) systems that are near a resonance condition. Other important factors include convergence, wetland coverage and inlet configuration. The changes in tidal properties also influence storm surge dynamics, in many cases leading to larger water levels during coastal storm events. Larger tides lead to longer-duration events. But, because long-term geometric shifts have often reduced river flow effects, hydrological hazard can decrease at the same time that marine-sourced hazards increase. The changing barotropic dynamics within estuaries therefore pose a challenge to traditional hazard assessments that assume stationarity, particularly when sea-level rise and its effects on dynamics are also considered.
