The Tide is High and I'm Moving On

Flag Harbor, aftermath of Hurricane Isabel 

Phys.org, Chesapeake Bay's storm surge tides can be 47% higher than the open ocean

When hurricanes or strong storms sweep up the United States' East Coast and meet the shores of the country's largest estuary, Chesapeake Bay, the familiar pattern of storm activity gets a little more complicated. A new study, published in the Journal of Geophysical Research: Oceans, shows that water levels inside the bay can spike far more dramatically than along the open ocean, raising flood risks for coastal and inland communities.

Ph.D. researcher Jenero Knowles, of North Carolina State University, and colleagues focused on storm tides, these being the combined height of regular tides and the additional water pushed in by intense winds and low air pressure causing storm surges. In extreme cases, the storm tide magnitude can be many meters high, such as peaks of nearly 9 meters along the Mississippi coast during Hurricane Katrina in 2005.

Using a high-resolution simulation of the Lower Chesapeake Bay region, the researchers compared how extreme water levels might differ at open coast zones (like Virginia Beach) versus more sheltered locations inside the bay (such as Hampton and Norfolk).

They modified storm parameters such as wind strength, storm size, speed and track to generate a wide range of plausible storm scenarios, with the range of possible peak storm tides in the bay being up to 47% greater than those at the open coast. This translated to an almost 2 meter higher tide than normal.

My slip and dock box after Isabel

the water was down considerably at this point

 

While it may seem obvious to assume that a large estuary like the Chesapeake, partially protected from the full force of the ocean, would experience lower extremes, the study shows the opposite—enclosed or semi-enclosed water bodies like bays can actually intensify flood risk under certain conditions. The key lies in a mix of physical factors: the shape and geometry of the bay, how tides and storm surges interact, and how sensitive the surge is to small changes in storm behavior.

In Chesapeake Bay, the researchers used a model validated against observed water-level records from 2011's Hurricane Irene to ensure the simulation was realistic. By modifying storm characteristics (for example, shifting a storm's track slightly closer or further from the bay, or changing the wind intensity), they found that water levels inside the bay responded far more variably than along the coast. In many simulated cases, this resulted in storm tides considerably higher than typical open ocean surge predictions.

This heightened variability and amplification arises because bays and estuaries are complex hydrodynamic systems. Their shape, depth, and the narrowing or widening of channels can cause water to pile up or flow differently than on a straight, open coastline. When the entering surge interacts with the regular tidal cycle, and when wind and storm tracks align just so, the result can be larger than expected water levels.

Water levels were much higher further up the Bay for Hurricane Isabel. Basically, the Bay acted as a funnel, and as the wave of the tide moved up the bay, it concentrated. It was much worse for Annapolis and Balmer.