Explaining the big difference in Collier and Broward County’s evacuation zones

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Examining a hurricane evacuation map of Florida shows some major differences from one part of the state to another.

One of the most significant differences between two neighboring regions is the evacuation zones in Collier County on the Gulf coast with Broward County on the east coast.

In Collier County, every part of the county is in some kind of evacuation zone. Next door in Broward, the only permanent evacuation zones are near the coast to the east of US1.

As you know, evacuation zones are drawn based on the risk of salt water flooding from storm surge, and that is important to stress. Collier County is far more vulnerable to surge flooding than Broward County is.

How deep and how far inland storm surge can go varies from storm to storm. How fast a system moves, how wide a storm is, at what approach it makes to a coastline, what the shape of the coast looks like and where the high tide cycle is at can all impact surge depth and inland extent. But a major factor determining who is at risk for salt water surge flooding is what the shape of the Earth looks like underneath the surface of the water. This is difficult to convey to many people because it’s something impossible to see just by standing on the beach.

To visualize what the Earth looks like underneath the water you need to dabble in something called bathymetry. That’s the fancy term given to analyzing how deep bodies of water like the Gulf and Atlantic are. A depiction of Florida’s bathymetry is shown below. In this map, deep water is shown in dark purple. Shallow water is noted in red.

Looking at this map it’s clear as day there’s a giant difference in how deep the water is off the Gulf coast of Florida where we are as compared to areas like Palm Beach, Broward and north Miami-Dade (Biscayne Bay however is a different story).

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That is a major reason why storm surge flooding is more likely here than it is on the Atlantic side of South Florida. The deep and steep continental shelf on the other side of the state offers a form of natural storm surge protection by holding back the water. That’s something the slow and gradual drop down into the deepest part of the Gulf doesn’t do.

A way to visualize this is by looking at a model cross section of the Collier and Broward County coasts. Today on NBC2 News at 3, Meteorologist Rob Duns built such a model to show how the continental shelf depth varies between the two coasts.

In the model seen below, take note of the gradual slope off the Collier coast and the steep slope off Broward. Since a steep slope reduces storm surge potential, Collier County’s gradual slope isn’t able to hold back much of the water. This is why salt water can get easily pushed onshore in Collier County and go farther inland than in Broward, requiring Collier to have much larger evacuation zones.

Are you ready for hurricane season? Now’s the time to make sure you know what to do in case a storm comes our way. Get caught up on your tropical weather knowledge by visiting our NBC2 First Alert Hurricane Guide here.

Tags: Weather blog

 

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