STORM SURGE

Coastal Flooding: Storm Surge vs. Swell

Hey Kyle,

Great question!

With tropical cyclones persistent strong winds push the surface water toward a common direction, along with creating a vertical circulation in the water column. In deep water the vertical circulation is unobstructed, but as the storm moves over more shallow coastal waters the interaction of the vertical circulation and the seafloor causes the water to pile up above the "normal" sea level. As well, the surface low pressure at the center of tropical cyclones can cause a surge of its own, though the amount is generally negligible. When you combine this with local bathymetry and coastal geography (areas with gently sloping continental shelves, i.e. much of the East Coast, are more susceptible to storm surge), the sea level will rise above normal and the ocean will surge inland inundating low-lying areas.


Fig. 1 Storm surge/storm tide schematic. Courtesy of NOAA

As well, the most significant storm surge takes place on the windward side of the storm (side where the wind direction matches the direction of the storm's movement), and nearest the center of the storm where the strongest winds are typically located along with the secondary pressure surge. This is also the part of the storm where winds are "onshore", allowing the water to pile up as it interacts with the coastline and local bathymetry. When a tropical system's landfall and surge coincides with a high tide, a large swell, and increased rainfall ahead of the system, the total water level increases.


Fig. 2 Image showing main factors that lead to storm surge (not total water level). Courtesy of NOAA/The COMET Program

The total water level is the main thing that forecasters look at when determining coastal impacts. In reality, its the surge, tide, and wave processes that combine forces to lead to 3 main coastal impacts: erosion, overwash, and inundation. When you compare total water level predictions to beach elevation data, you can effectively make a forecast for these coastal impacts.


Fig. 3 Diagram showing the main elements that make of total water level and lead to coastal impacts: surge, tide, and wave processes (runup and setup). More on this HERE from the USGS.

While "surge" is often the component of total water level that gets the most press, it is not uncommon to see the effects of waves (through processes known as wave runup and wave setup) be a more significant contributor to total water level than surge and tide combined! Then you have to deal with waves on top of the total water level that also can cause significant damage and just exacerbate the problem...

The East Coast frequently has to deal with the complexities of all three of the contributors to total water level (surge, tide, and wave processes) during tropical cyclone landfalls and Nor'easters. In fact, this very recently led to significant damage from the Outer Banks through New York during Superstorm Sandy.

While Hawaii can and does have to deal with this problem, the frequency is much lower than on the East Coast. It is obviously not uncommon for Hawaii to be impacted by large surf especially during the winter, but the frequency of coastal flooding is much lower. Much of the wave energy during high surf events in Hawaii is dispersed out on the reefs, dissipating upon finally reaching the sand. These wave events also do not often coincide with extreme tide or surge events. However, it does happen and large wave events very much can be primary factors in coastal flood events in the Aloha State. Here is an article that looks into that.

Hope that helps...


Fig. 4 Total water level in action. Courtesy of USGS


- The Surfline Forecast Team

Categories: Forecasting and Meteorology, Hurricanes