15 Pensacola Beach Nourishment Project

Over Easter break, my family and I went to Pensacola Beach, FL. We like to stay in an old hotel right on the beach. However, after being there a little while, it occurred to us that it certainly seemed farther to the ocean than last year. No doubt we’re getting older, but age alone seemed an unlikely explanation. In fact, our previous experience indicated that each year the ocean got closer to the hotel, not farther away!

Early the next morning, I awakened before the kids for a walk down the beach. I like to make some coffee, go to the nearby pier, and stroll out to watch the waves and see if the fish are biting. As I strolled out on the pier and looked east, I saw a large boat close to shore, unlike any I’d seen near the beach before. Nearby on the beach were large metal pipes. Bulldozers rumbled over where tourists usually sunbathed. As I asked around, I discovered that Pensacola Beach was undergoing a major beach renourishment program. Hurricanes Erin and Opal in 1995, Georges in 1998, and Tropical Storms Hanna, Isidore, and Lilli in 2002 all eroded the beach, so much so that the State of Florida designated Pensacola Beach as a critically eroded shoreline. Approximately $20 million were now being spent to widen an 8.5 mile stretch of the beach.

Besides erosion due to storms, beach loss occurs due to natural subsidence, sea level rise, and longshore currents. The beaches of the northern Gulf of Mexico lie upon thick packets of sediment deposited throughout millions of years of streamflow. These sediments continue to consolidate and dewater, reducing their volume and leading to subsidence. Not surprisingly, the sediments that subside the most are the newest geologically, such as nearshore barrier islands, including Santa Rosa Island where Pensacola Beach is located (Click for map.). Many of these sediments have been deposited and reworked since the last ice age, about 18,000 years ago, a short time ago geologically. It is natural for coastal sediments to move. In the northern Gulf of Mexico, the predominant current is east to west, causing sediments to migrate in a westward direction. Offshore islands tend to erode on their eastern end and build on their westward end. In addition to these currents, sea level rise has modified coastlines. Islands become more frequently overtopped by water during storms, leading to more erosion. Old shorelines become submerged. A submerged shoreline is the source of sand for the restoration project at Pensacola Beach. Sand from about 65 feet water depth is used to replace sand lost to erosion. The sand was chosen because of its location and to approximate the color and physical characteristics of sand already present on the beach. A barge is used to churn up the sand which is mixed with water so as to create a slurry. That slurry is pumped through large pipes onto the beach. Once there, bulldozers place and smooth the sand, creating new beach several tens of feet into the ocean.

Protecting coastal zones is crucial to us here in Louisiana. Barrier islands, similar to Santa Rosa Island, and coastal wetlands protect us from hurricanes. The Louisiana coast is also incredibly rich in resources and culture. We have the US’s busiest port (by volume), more offshore oil wells than Texas, the second greatest concentration in the world of petrochemical plants, a huge seafood industry, and cities that tourists flock to. In our efforts to protect coastal Louisiana, we must seek to understand the successes and failures of efforts throughout the U.S.

Notes on Coastal Processes:

Shorelines are dynamic—they are where the water meets the land. Relative to a specific location, it’s impossible to see whether a change in shoreline location is due to water level changing of land level changing. The land elevation can drop due to erosion or subsidence. It can rise due to tectonic sources such as plate collision or isostatic rebound. Sea level can change due to currents, slow changes in ocean-basin size during tectonic movements, and changes in the size of polar ice caps. The change in global average sea level change is called absolute sea-level change.

On a human timescale, the melting of polar ice caps is the most significant factor in sea level rise. During ice ages, sea level drops by hundreds of feet. Ice ages have three major causes:

plate tectonics: Ice tends to accumulate only when there are land masses near the poles, such as current-day Greenland and Antarctica. When the continents have been clustered nearer the equators, such as during the time of the dinosaurs, ice doesn’t accumulate.

Amospheric gases: The greenhouse effect refers to how certain atmospheric gases, such as carbon dioxide, methane, and water vapor, at like the glass on a greenhouse, allowing visi-ble light to reach the Earth’s surface but preventing some infrared radiation (heat) from exiting. A buildup of those gases is like improving the insulation on the greenhouse, trapping more heat and warming the Earth. Humans have been adding carbon dioxide and other greenhouse gases to the atmosphere in large volumes by burning fossil fuels, such as coal and petroleum.

Milankovitch cycles: Over the scale of tens of thousands of years, the Earth slowly changes its orbit around the Sun. The shape of its pathway and the tilt and orientation of its axis all change at different predictable ways. For instance, the pathway around the Sun alternates slowly over about 100,000 years between more circular and more elliptical and back again. When these three cycles line up together, winters become more extreme, and ice accumulates. We find a record of these changes preserved in ice dating back 800,000 years or more recovered from the ice caps. Besides continental ice sheets, glaciers in mountains, Alpine glaciers, also grow and retreat, marking changes in climate. See [here.]

Most waves are due to wind, except for tsunamis that are mostly caused by earthquakes. In the open ocean, as a wave passes the water moves in a circular motion. The father down into the ocean one goes, the smaller the circular motion becomes, petering out at the wave base. The depth to the wave base depends on the wave length, which is the distance from wave crest to wave crest.

As a wave approaches shore and the water becomes more shallow, the wave base reaches the bottom of the ocean and begins to drag and slow. Gradually the top of the wave gets ahead of the bottom, over-steepens, and breaks onto the beach, and the water runs back down the beach.

If you go walking along a beach and watch the waves, they seem to come in nearly straight-on. That’s because they are bent as the portion nearest the beach starts to drag first and slow, bending the wave— wave refraction. However, the waves don’t hit the beach perfectly straight-on, and the water runs back due to gravity down the beach slope. A grain of sand is washed up the beach by the wave, then carried back down by the retreating water, making a zig-zag path that creates movement along the shore. This longshore drift is a major movement of sand and drunken tourists along the beach from in front of one hotel to another.

A enclosed body of water near the ocean, connected to it, but protected from its full force is an estuary. Estuaries are very rich biologically, being a safe place for reproduction, full of potential food, and flushed of waste by tides. Biological organisms can eat, poop, and reproduce. Lake Potchartrain in New Orleans is an example of an estuary.

Tides are due to the gravitational attraction of the moon and sun. The daily times of high tides vary because the moon Orbits the Earth in about 27–29 days, depending on how you count it, giving the appearance of rising later each day, the amount depending on latitude. The tide size depends on the shape of the local coastline, the most famous spot being the Bay of Fundy in Canada, where the tidal range can be over 50 feet!

Organisms that live in the tidal zone are adapted for the changes in water level. For example, an organism that lives in the zone between average high tide and average low tide has an average day sometimes submerged and sometimes out of the water. It has to be able to handle both. But between average high tide and annual high tide is a zone that on an average day is not submerged. But once in a while it is, and any organism living there has to be able to handle this occasional occurence,