Factors Determining the Size of the Storm Surge
You may wonder why a storm like Katrina, which made landfall as a category 3 storm, had such a huge storm surge compared to say, Harvey, which came ashore as a category 4 storm. When a hurricane is approaching a shoreline, the communities in its path need good information about what to expect. But surge predictions are tricky.
Refer back to the Saffir-Simpson Scale. These are estimates that are not 100% reliable because there are many factors determining the height of a surge. They include:
- the size, intensity, speed, and angle of approach of the storm itself;
- the width and slope of the offshore shelf and the slope of the shoreline itself;
- the shape of the shoreline and coastal features present.
The intensity (wind speed) of a storm is directly related to the storm surge – with the relationship shown in the Saffir-Simpson Scale. The speed of a storm has an impact on the size of the surge, but it is not a simple relationship. A fast-moving storm can generate a higher surge, but when a slow-moving storm pushes a surge into a coastal bay or other enclosed water body, it can cause the surge height to magnify. A large storm (in terms of diameter) can produce a larger surge than a smaller storm because the winds affect a larger area of the ocean. The communities along the Gulf Coast are keenly aware of another feature of hurricane storm surges: The greatest surge occurs in the northeast quadrant of the storm, to the east of the eye of a northward moving hurricane. Hence, Bay St. Louis, Mississippi received a greater storm surge than New Orleans in Katrina.
If the offshore continental shelf is wide and gently sloping, the storm surge will be greater than if the shelf is narrow and steeper. If the land also has a very gentle gradient, then there will be little to stop the storm surge from flooding far inland. This is the case on the Louisiana coast as well as other locations along the Gulf coast. This topic will be revisited in Module 8. Narrow inlets along a coastline will tend to magnify a storm surge as the water is funneled into the opening and “piles up”. This was evident on the Mississippi coast when Katrina’s storm surge came ashore in 2005. See the SLOSH model animation below. Here, the surge pushed into Bay St. Louis with an almost 30 ft storm surge. Conversely, features such as offshore barrier islands can act to reduce the surge height before it comes ashore, although these features take the brunt of the energy of the surge, resulting in massive amounts of erosion.
For more information
The Sea, Lake, and Overland Surge from Hurricanes (SLOSH) Model of Katrina’s storm surge, found at Historical SLOSH Simulations, shows a time series reconstructing the storm surge height as the storm came ashore, pushing the storm surge into the bays and estuaries. It helps to play it at a slower speed and repeat the loop several times to appreciate how the water behaved.