Inlets

Welcome to the Geology Spot!
Welcome to the first post from the Geology Spot blog.  The goal of this blog is to discuss some scientific topics that I think are neat and break them down to a level that everyone can understand.  My name is Chris Seminack and I'm a Ph.D. student at George Mason University.  I specialize in coastal geology, in particular inlet geomorphology.  So I figured I'd kick off the blog with a posting about inlets.

Google Earth image of the barrier islands along the Maryland and Virginia coast

What is an inlet?
An inlet is basically a water-way that separates two barrier islands and connects the ocean to the bay.  A barrier island is essentially a really big sand bar that hugs the coast.  The Eastern United States has the longest chain of barrier islands in the world.  The only reason that inlets exist is because of tides.  Every 6 hours the tides change (high and low tide).  Think of the tides as the ocean swashing back and forth.  As the high tide rushes in, water flows through the inlets and into the bay.  Six hours later, the water that has collected in the bay from the high tide needs to get out.  The water in the bay then flows back through the inlet and into the ocean.  It is the tidal cycle that keeps the inlets open.  Inlets are essentially just passage ways so that the water in the ocean can flow into the bay.

Google Earth image of Drum Inlet, located in the Outer Banks of North Carolina

How do inlets form?
Inlets form as a result of severe storms like hurricanes or nor'easters (East Coast US terminology).  Storms are low pressure centers.  Because of the severe low pressure and winds from storms, the water beneath the storm tends to bulge up slightly (sometimes up to several meters for bad storms).  This is called a storm surge.  A storm surge forces a lot more water into the bay than what it is normally used to.  As a result, when it needs to get out, this water has no where to go except to spill over the barrier island, and thus, an inlet is born!

This picture was taken by the USGS immediately after Hurricane Irene along North Carolina.  The photo shows a large amount of erosion as a result of the hurricane.  A small temporary inlet was formed from the hurricane.

Why are inlets important?
Inlets are very important for many reason.  Certain marine animals depend on inlets to stay alive.  Water in the bay tends to be not as salty as ocean water.  This is known as brackish water.  Think of it as half way between fresh water and salt water.  Inlets control the salt levels in the bay.  If an inlet closes, the water may not be salty enough for the organisms living in the back bay.  And vice verse if a new inlet opens up.  An example of this occurred in the late 1800s along Assateague Island, MD.  The bay behind Assateague Island is known as Chincoteague Bay.  This bay was famous for its oyster farming.  Oysters tend to live in brackish water.  As a result of an inlet closing up along the island, the water in the bay became to fresh for the oysters and caused a die-off.  Inlets also bring in nutrients from the ocean into the bay.  Another example of why inlets are important is for boat owners.  If you've ever been on an ocean-going boat, chances are that you have traveled though an inlet.  Most marinas are located behind barrier islands because the waters are calmer and are protected from large storm waves.  Inlets act as a highway between the ocean and the bay.  Fish also use inlets in a similar way.  Many species of fish lay their eggs in the calmer, protected waters of the back bay.  They use inlets as a means to travel into the bay.

The Green Run Inlet along Assateague Island Maryland/Virginia before and after its closure.  This inlet caused the oyster die-off due to the bay water became too fresh.

Why do people study inlets?
People, like myself study inlets for many different reasons.  First, prehistoric inlets in the rock record have been known to hold oil.  Knowing how to look for barrier islands and inlets in the rock record is very valuable to petroleum geologists.  Another reason is to reduce property damage along the coast.  There are many million dollar homes along the beaches of the US and around the world.  Geologists can help reduce property damage by knowing how inlets form and where they are likely to form next.  Finally, studying inlets can help us learn more about the formation of barrier islands.  This is currently a very hot topic.  The ocean levels are rising at about 2 mm per year.  This may not sound like a lot, but it is.  At this rate, some of our barrier islands could literally be washed away in as little at 2000 years.  Studying inlets can teach us how the barrier island formed and how it reacted to sea level rise in the past.