Tide-Storm Surge Interactions in Highly Altered Estuaries: How Channel Deepening Increases Surge Vulnerability

We develop idealized analytical and numerical models to study how storm surge amplitudes vary within frictional, weakly convergent, nonreflective estuaries. Friction is treated using Chebyshev polynomials. Storm surge is represented as the sum of two sinusoidal components, and a third constituent represents the semidiurnal tide (D2). An empirical fit of storm surge shows that two sinusoidal components adequately represent storm surge above a baseline value (R2 = 0.97). We find that the spatial transformation of surge amplitudes depends on the depth of the estuary, and characteristics of the surge wave including time scale, amplitude, asymmetry, and surge‐tide relative phase. Analytical model results indicate that surge amplitude decays more slowly (larger e‐folding) in a deeper channel for all surge time scales (12–72 hr). Deepening of an estuary results in larger surge amplitudes. Sensitivity studies show that surges with larger primary amplitudes (or shorter time scales) damp faster than those with smaller amplitudes (or larger time scales). Moreover, results imply that there is a location with maximum sensitivity to altered depth, offshore surge amplitude, and time scale and that the location of observed maximum change in surge amplitude along an estuary of simple form moves upstream when depth is increased. Further, the relative phase of surge to tide and surge asymmetry can change the spatial location of maximum change in surge. The largest change due to increased depth occurs for a large surge with a short time scale. The results suggest that both sea level rise and channel deepening may also alter surge amplitudes

Nineteenth‐Century Tides in the Gulf of Maine and Implications for Secular Trends

Since the early twentieth century, the amplitudes of tidal constituents in the Gulf of Maine and Bay of Fundy display clear secular trends that are among the largest anywhere observed for a regional body of water. The M2 amplitude at Eastport, Maine, increased at a rate of 14.1 ± 1.2 cm per century until it temporarily dropped during 1980–1990, apparently in response to changes in the wider North Atlantic. Annual tidal analyses indicate M2 reached an all‐time high amplitude last year (2018). Here we report new estimates of tides derived from nineteenth century water‐level measurements found in the U.S. National Archives. Results from Eastport, Portland, and Pulpit Harbor (tied to Bar Harbor) do not follow the twentieth century trends and indicate that the Gulf of Maine tide changes commenced sometime in the late nineteenth or early twentieth centuries, coincident with a transition to modern rates of sea‐level rise as observed at Boston and Portland. General agreement is that sea level rise alone is insufficient to cause the twentieth‐century tide changes. A role for ocean stratification is suggested by the long‐term warming of Gulf of Maine waters; archival water temperatures at Boston, Portland, and Eastport show increases of ∼2 °C since the 1880s. In addition, a changing seasonal dependence in M2 amplitudes is reflected in a changing seasonal dependence in water temperatures. The observations suggest that models seeking to reproduce Gulf of Maine tides must consider both sea level rise and long‐term changes in stratification