UGA Skidaway Institute scientists map Wassaw Sound

A research team from the University of Georgia Skidaway Institute of Oceanography has completed the first high-resolution, bathymetric (bottom-depth) survey of Wassaw Sound in Chatham County.

Led by Skidaway Institute scientist Clark Alexander, the team produced a detailed picture of the bottom of Wassaw Sound, the Wilmington River and other connected waterways. The yearlong project was developed in conjunction with the Georgia Department of Natural Resources.

This shows a wide view of the Wassaw Sound survey map. Shallow areas are shown in orange and yellow, deeper areas in green and blue.

The survey provides detailed information about the depth and character of the sound’s bottom. This information will be useful to boaters, but boating safety was not the primary aim of the project. The primary objective was to map bottom habitats for fisheries managers. DNR conducts fish surveys in Georgia sounds, but, according to Alexander, they have limited knowledge of what the bottom is like. “One of the products we developed is an extrapolated bottom character map,” Alexander said. “This describes what the bottom grain size is like throughout the sound. Is it coarse, or shelly or muddy? This is very important in terms of what kind of habitat there is for marine life.”

A second goal was to provide detailed bathymetric data to incorporate into computer models that predict storm surge flooding caused by hurricanes and other major storms. Agencies like the United States Army Corps of Engineers, the Federal Emergency Management Agency and the National Oceanographic and Atmospheric Administration use mathematical models to predict anticipated storm inundation and flooding for specific coastal areas. A key factor in an accurate modeling exercise is the bathymetry of the coastal waters.

“You need to know how the water will pile up, how it will be diverted and how it will be affected by the bottom morphology,” Alexander said. “Since we have a gently dipping coastal plain, storm inundation can reach far inland. It is important to get it as right as we can so the models will provide us with a better estimate of where storm inundation and flooding will occur.”

Funded by an $80,000 Coastal Incentive Grant from DNR, Alexander and his research team, consisting of Mike Robinson and Claudia Venherm, used a cutting-edge interferometric side-scan sonar system to collect bathymetry data. The sonar transmitter/receiver was attached to a pole and lowered into the water from Skidaway Institute’s 28-foot Research Vessel Jack Blanton. Unlike a conventional fishfinder, which uses a single pinger to measure depth under a boat, the Edgetech 4600 sonar array uses fan-shaped sonar beams to both determine water depth and bottom reflectivity, which identifies sediment type, rocky outcroppings and bedforms, in a swath across the boat’s direction of travel.

Skidaway Institute of Oceanography research coordinator Claudia Venherm logs survey activity on board the R/V Jack Blanton

The actual process of surveying the sound involved long hours of slowly driving the boat back and forth on long parallel tracks. On each leg, the sonar produced a long, narrow strip indicating the depth and character of the sound bottom. Using high-resolution Global Positioning System data that pinpointed the boat’s exact location, the system assembled the digital strips of data into a complete picture of the survey area.

All the other sounds on the Georgia coast were mapped in 1933, but for some reason data from that time period for Wassaw Sound was unavailable. When the team began this project, they believed they were conducting the first survey of the sound. However, just as the researchers were finishing the project, NOAA released data from a 1994 single-beam survey that had been conducted in advance of the 1996 Olympic yachting races that were held in and near Wassaw Sound.

“This worked out very well for our project, because we are able to compare the differences between the two surveys conducted 20 years apart,” Alexander said. “We see areas that have accumulated sediment by more than 2 meters, and we also see areas that have eroded more than 2 meters since 1994. Channels have shifted and bars have grown or been destroyed.”

Because of advances in technology, the current survey is significantly richer in detail than the one conducted in 1994. “We can zoom down to a square 25 centimeters (less than a foot) on a side and know the bottom depth,” Alexander said.

The survey produced a number of findings that were surprising. The intersection of Turner Creek and the Wilmington River is a deep, busy waterway. Although most of the area is deep, the survey revealed several pinnacles sticking up 20 feet off the bottom. “They are round and somewhat flat, almost like underwater mesas,” Alexander said.

The researchers determined that the deepest place mapped in the study area was a very steep-sided hole, 23 meters deep, in the Half Moon River where it is joined by a smaller tidal creek. They also found several sunken barges and other vessels.

The survey data set is available to the public on the Georgia Coastal Hazards Portal at http://gchp.skio.usg.edu/. Alexander warns that while boaters should find the survey interesting, the information is intended for habitat research and storm surge modeling, not for navigation. “Because the bottom of Wassaw Sound is always shifting and changing, as our survey showed, don’t rely on the data for safe navigation,” he cautioned.

Alexander has already received a grant for an additional survey, this time of Ossabaw Sound, the next sound south of Wassaw Sound. He expects work to begin on that survey in early 2015.

The Skidaway Institute of Oceanography is a research unit of the University of Georgia located on Skidaway Island near Savannah. The mission of the institute is to provide the state of Georgia with a nationally and internationally recognized center of excellence in marine science through research and education.

New R/V Savannah video

We just completed a new video on our Research Vessel Savannah. It is posted on YouTube.

Sea Turtle Release Video

Here is a video of the release Delta, the loggerhead sea turtle from the Tybee Island Marine Science Center. Delta was carried to the Gulf Stream on board the R/V Savannah as a “piggy back” on an already scheduled science cruise.

Teacher At Sea

Carmen Andrews

One of the participants in NOAA’s Teacher at Sea program spent some time on board the R/V Savannah this summer.  Her blog makes for some interesting reading and nice description of life on a science research vessel. You can read her blog here.

Teachers at sea

It has been a busy summer on board the R/V Savannah. One cruise involved some teachers, who blogged about their experience. This is fairly interesting.

Some nice news coverage

Clark Alexander’s project on erosion on the Intracoastal Waterway generated some good news coverage.

Mary Landers from the Savannah Morning News wrote this story that was published last week.

Alice Massimi, from WSAV-TV (local NBC affiliate), shot this story before Christmas, but it aired yesterday.

Enjoy!

 

High school students spend a day on the water

A group of students from Johnson High School in Savannah spent last Friday on a mini-research cruise on board the Research Vessel Savannah. It was part of a joint project among the local school system, Savannah State University and Skidaway Institute.

A photographer from the local CBS affiliate, WTOC-TV, went along. Here is a video of the trip.

Skidaway Institute scientists study Intracoastal Waterway erosion

The banks of the Atlantic Intracoastal Waterway (AIWW), an artificial channel running through Georgia’s marshes behind the barrier islands, are steadily eroding, and there are several possible causes, including wakes from recreational boats. That is the conclusion of a year-long study by scientists at the Skidaway Institute of Oceanography.

 “Our goal was to quantify the impact that waves are having on the Georgia segment of the AIWW,” said Skidaway Institute professor Clark Alexander. “We also wanted to see if the salt marshes that line much of the waterway were expanding or retreating.”

Georgia contains more than one third of the salt marsh on the eastern coast of the United States and more than 90 percent of its AIWW shoreline is salt marsh. These marshes are essential habitat for fish and crustaceans because they play an important role in the life cycle of most local commercial and recreational species. The AIWW was designed to support both recreational and commercial vessel traffic.

“The major environmental impact of boats on the estuarine environment is the erosion of the channel margins from wakes,” said Alexander. “In Georgia, this diminishes the extent of the salt marsh habitat and causes the channels to widen – in some cases, at rates of up to half meter a year, which is pretty significant.”

Wakes undercut the marsh, causing to them to fail and collapse, particularly at low and mid-tides. Frequently, intertidal oyster bars are buried by eroded sediment, and oyster larvae are hindered from settling because shell material is not available upon which to settle.

Erosion is a natural process in salt marshes. However, in a natural setting, when one side of a tidal creek erodes, the other side usually accretes. Along the AIWW this was typically not the case. Alexander found extensive stretches where the shoreline was eroding on both sides of the channel.

Alexander and his team used historic and recent charts and aerial photography to track the erosion and accretion along the entire 91 mile length of the waterway between South Carolina to Florida. They also used a combination of high-definition video camera connected to a Global Positioning System (GPS) receiver to document both sides of the waterway along its length.

The research team examined shoreline change over two time periods, the first from 1933 to 1976 and the second from 1976 to 2004. The team limited their study to the eight relatively narrow main sections and six alternative sections of the waterway, avoiding the sounds where wind and storm waves might have a significant impact on shoreline change.

“Erosion has become increasingly significant and widespread in the 1976  -2002 time period,” Alexander said. “That isn’t to say that every section is eroding, but most of them are.”

Comparing the earlier time period to the later, the study found a strong trend towards more erosion in the more recent time period. Boat traffic and their wakes provide a mechanism for bank erosion.

“We don’t see commercial boating as being significant because the number of ships and tonnage in the AIWW has gone down by about 80 per cent in the past 18 years,” said Alexander. “But recreational boat registrations in coastal counties (currently about 29,000) have continued to increase.”

Alexander also has another explanation that cannot be ruled out with current information. Except for two short segments, the Corps of Engineers is no longer dredging the AIWW to maintain its target depth and sea level is rising at about 1 foot per century.  The channel could be widening because it is becoming shallower but must still transport and contain the same amount of water.  “Boating is most likely the immediate primary erosion force, but rearrangement of the channel cross section may contribute as well,” he said. “We just don’t know absolutely at this time.”

Busy day in Grove Creek

The Skidaway Institute campus was busy earlier this week, as researchers began a month-long field study in the Grove Creek salt marsh adjacent to our campus.

The project has two purposes:

1.) To test a model for predicting how salt marshes absorb and discharge contaminants.

2.) To test a new low-power, high-frequency radar system that is designed to monitor water flow through a salt marsh.

Dr. Dana Savidge next to one of her radar antennae.

The start of the field work was visually impressive.
It began with the release of a concentrated red dye into Grove Creek. As the tide came in, the dye was pushed into the marsh. The research team used a helicopter, boats and sensors to observe the way the die was distributed.

From 3,000 feet, it was easy to watch the way the die flowed through the tidal channels and into the intertidal zone.

Grove Creek salt marsh from 3,000 ft.

While this may have been the most visual part of the study, the most valuable scientific data will come over the next month. The marsh will absorb some of the die and release it on the outgoing tide a little at a time – like rinsing soap out of sponge. The scientists will observe this process and then compare it to their model to see how well their predictive model works.

Alexander lab takes delivery of new research vessel.

The Alexander lab is gearing up for mapping the intertidal morphology of Groves Creek.  The non-vegetated, soupy areas cannot be surveyed by foot, so a new research vessel was purchased that, along with a survey grade echosounder tied into the lab’s new RTK-GPS, will be used to map the bottom elevations at higher tides.

A similar boat

The new, very stable vessel, a 4 feet wide by 10 feet long pontoon boat with an electric motor in each hull, will allow them to collect data in areas as shallow as one foot deep.  To see a video of the vessel in action go to this link.