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 (D-2). An empirical fit of storm surge shows that two sinusoidal components adequately represent storm surge above a baseline value (R-2 = 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. Plain Language Summary Many estuaries around the world are heavily altered from their natural state. Wetlands have been reclaimed, and shipping channels widened and deepened to accommodate large container ships. The effects on storm surge and flood risk are just beginning to be explored. In this paper we employ a theoretical approach to understand how the characteristics of a storm surge-such as how fast it is moving, how big it is, and whether it happens on flood or ebb tide-change how it behaves in an estuary. Our results show that storm surge generally gets larger when channels are dredged and deepened; the largest amplification is observed for fast-moving storms with a short time scale, within estuaries that are highly frictional. Other characteristics-such as the timing relative to the tide and the shape of the estuary-also impact the amplitude and the amount of sensitivity to changing conditions. We find that channel deepening effects are negligible at the coast and far upstream. In between, a region of maximum sensitivity to dredging occurs. Thus, changes in flood risk due to channel deepening and sea level rise can be spatially variable, even within a single estuary.