Trends in North American tides and storm surge, 1844-present

Using hundreds of station-years of archival tide data found in the US National Archives and digitized by students, we investigate the hypothesis that depth and other topographic changes to estuaries and tidal rivers have increased tidal constituent amplitudes and other long-waves such as storm surge and river floods. In the Columbia River Estuary, both numerical models and data suggest that channel deepening has increased the M2 tide by ~10% since 1854, and resulted in faster and less dispersive flood waves. In Wilmington (NC), measurements and an idealized model show that the M2 tide has doubled since the 1880s, leading to large increases in the modeled storm surge. Analysis of other archival records from Boston (1847-present), Providence (1872-present), Philadelphia (1901-present), Norfolk (1844-present), Charleston (1850-present) , San Diego (1854-present), and San Francisco (1858-present) suggest that perturbations in harbor tidal properties are the rule, rather than the exception; in particular, comparison with nearby gauges suggests that coastal tides are much less changed over time, as measured for example by trends in overtides. A primary cause is increased channel depth, which has approximately doubled in many locations since the mid-19^th century. Since depth is inversely proportional to friction in the linearized shallow water equations, increasing depth (by sea-level rise or channel deepening) has the same effect as decreasing friction. Such changes may help explain observations in New York harbor, where the elevation of the once-in 10 year storm surge has increased by nearly 0.3m since the mid-19^th century. Other factors include altered river flow and resonance; for example, both analytical models and measurements suggest that the M2 tide has increased by ~3% in Long Island Sound since 1892. To sum up, changes in tides can be used as an indicator of system sensitivity and can help determine whether the risk from extreme events such as storm surge will outpace sea-level rise.