So, the Florida Department of Environmental Protection’s web site has an Ask a Geologist section, of which I took full advantage while researching a factoid about beach erosion for a talk I’m giving on the history of Key West this week.
The great thing? Real, live geologists answer your question.
The horrible thing? Real, live geologists answer your question.
To wit:
My question:
A million years or go (give or take), a college professor taught me about erosion and how the state’s barrier islands were building up on one side of the state as they eroded on the other. This is, of course, without human alterations such as beach renourishment and seawalls.
I cannot remember on which side of the state the sands erode as new barrier islands tend to want to develop on the other side. Does this make sense?
How Science answered me:
The only constant in nature is change and barrier islands and beaches are no acceptation. The barrier islands and beaches of Florida are constantly evolving in reaction both to the actions of man and to natural events. The changes they undergo generally occur rather gradually over the time span of a person’s life. These natural responses, both accretion and erosion, are typically only noticed by the public when they occur as localized dramatic change due to exceptional events, within a person’s life time, such as hurricane landfalls and extreme northeasters. Those events are not however exceptional when viewed over the span of geologic time.
Sand is in continual movement on the beaches of Florida. In the main, with some notable exceptions, sand on the beaches and in the waters immediately adjacent to the beaches of the east coast of Florida move southward. Barrier islands on the east coast of Florida classically tend to accrete on the north side of inlets. The flip side of the coin is that they tend to erode on the south side of inlets. Barrier islands also tend to move landward in response to sea level rise through, among other mechanisms, storm event over wash. The following link is to the most recent beach critical erosion report.
http://www.dep.state.fl.us/beaches/publications/pdf/critical-erosion-report-2012.pdf
Man’s efforts have caused both erosion and accretion to occur where such would not naturally happen. Take a look in Google Earth at the inlets of the east coast of Florida. Those that have been extensively modified by man tend to exhibit distinct landward offset on their south side. This is due to the groins placed on the north side of inlets blocking the passage of sediment into ebb tidal deltas. Ebb tidal deltas are the lobe of sand bars on the seaward side of inlets created by tidal flushing. The inlet offsets are further exacerbated by the dredging of and disposal of sand from ebb tidal deltas to maintain navigation channels. Nature would have otherwise moved that sand out of the delta onto the beaches of islands further south of inlets. We maintain navigational channels across ebb tidal deltas and nature continues to fill in the holes we create thus interrupting the nature southward progressing of sand.
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I think it is totally awesome I have access to this man’s brain… now, if I only could make sense of how he answered. Still, well done, FDEP. Mostly.
