Flood plain

 

Florida is known for many things, but huge hills is not one of them. What there are in the way of hills are often manmade, being either prehistoric shell middens (vast collections of discarded oyster shells and other refuse dating from ancient times) or modern day landfills. One example of natural hills in this coastal state are sand dunes. Stabilized sand dunes (those overgrown by mature woody vegetation) can even be found far inland from the ocean due to the rise and fall of sea levels over thousands of years. The tallest ancient sand dunes at Fort Matanzas (now existing within a mature hardwood hammock forest) are about 25 feet in height. In fact, it’s theorized that the colonial Spanish used these tall dunes as bases upon which to build wooden watchtowers from which they could effectively scan the ocean for enemy vessels. Once the Matanzas Watchtower (fort) was built out of sturdy coquina stone in the early 1740s, the temporary wooden structures became redundant and were abandoned.

Most of the terrain encompassed within the national monument does not reach the lofty heights of a towering sand dune, however. Most of the park is closer to sea level in elevation and is subject to being flooded. In fact, the Federal Emergency Management Agency places the majority of the park into a zone called High Risk Special Flood Hazard Area (AE) “Areas subject to inundation by the 1% annual chance flood event determined by detailed methods”. The rest of the park falls under a High Risk Coastal High Hazard or a Moderate to Minimal Risk Area. That is a somewhat bureaucratic way of saying the park is located within a floodplain. A simple definition of a floodplain is an area of low-lying ground adjacent to a river that is subject to flooding during periods of high discharge. “Periods of high discharge” certainly occurred when hurricanes Matthew and Irma passed close to the park in 2016 and 2017 respectively. But a nor’easter coinciding with a high tide coinciding with a new or full moon can also cause flooding to occur. And with sea levels rising due to global climate change, even relatively minor storms have a leg up, so to speak, and can induce more flooding as they compound the problem of already elevated water levels.

But are there any advantages to floodplains? They are, after all, a natural feature of the landscape. Perhaps ironically, they actually reduce flooding to areas downstream by slowing and soaking up floodwaters that would otherwise race along the river’s course. And when left in their natural state, floodplains are often comprised of biologically diverse wetlands that provide excellent habitat for wildlife of all sorts. From an agricultural standpoint, they contain some of the most productive farmland available. The problem comes when humans build hardened structures within floodplains that are very expensive to rebuild if they are destroyed or damaged in a flood. Not every floodplain is going to flood every year by any means, and risks need to be managed and mitigated, but by definition a floodplain has flooded in the past and will almost certainly do so again some time in the future. There are many benefits to living by large waterbodies, as evidenced by the near constant presence of Native Americans, European colonists, and Americans in the area of Fort Matanzas going back thousands of years. But waterbodies flood, and we can’t say we haven’t been warned.