Salt Marsh Watch – Dean Hardy

How will rising sea levels affect coastlines? As this is written, rate of sea level rise is about 3 milimeters per year worldwide.

The main video above is a quick “snapshot” of the tidal flow in a Georgia salt marsh replete with smooth cordgrass (Spartina alterniflora) and marsh fiddler crabs (Uca spp.) scurrying about over the mud. The perspective of the camera, between two and three feet above the marsh sediment, can be thought of as from that of a Great Blue Heron, a frequent visitor to the marsh. The location is a few miles north of Darien, just off of Buzzard Roost Creek. I used a friend’s dock to access this site. The short video captures about one quarter of the full twelve and a half hour tidal cycle using time-lapse photography over about a three and a half hour period of the incoming tide.

As part of my PhD research on vulnerability to sea-level rise in coastal Georgia, I decided to immerse myself in the region by cycling along the coast to acquire a better sense of the landscape’s people, places, and ecology. To kick it off, I cycled about 25 miles around Tybee Island in July 2013 following the three foot contour line as closely as possible (much of this line cuts across private property, but I stuck to the roads). In August, I headed to McIntosh County and cycled 70+ miles in the 100-degree heat! The contour I followed on these rides is referenced to current high tide, a.k.a. mean higher high water (MHHW), meaning that the contour I was sticking close to is measured from the average upper reaches of high tide. I chose this contour because this is approximately where forecasts of sea-level rise predict the future extent of high tide to reach around the year 2100. This estimate is based on current climate change modeling, which factors in greenhouse gas emissions, polar ice sheet melting, and thermal expansion of the oceans due to their warming from heat diffusion from the atmosphere, among other contributors.

United State’s Georgia coast is unique for the area, having rather large, mixed semi-diurnal tides (up to 10 ft or more in some places!). This means that there are two high tides and two low tides every lunar day occurring approximately six hours and fifteen minutes apart. Semi-diurnal is what signifies that there are two high tides per lunar day, while mixed means that one high tide is a little larger than the other high tide. The National Oceanic and Atmospheric Administration (NOAA) does a good job of briefly explaining the three types of tides, and the U.S. Coast Guard Auxiliary unit’s Savannah Flotilla explains Georgia’s tides in detail if you’re interested in learning more about what makes them mixed and semi-diurnal.

If you’re interested in the details of how I did the time lapse photography, you can read those details on my Vimeo channel, Salt Marsh Watch.

The following video is a pilot, of sorts, with snippets taken from the cycling route on Tybee Island. The map in the upper left shows my location relative to current and possible future extents of high tide. A full map of the route is available on my blog. The green shading in the map shows the current extent of tidal water. The blue’ish purple shading on the map represents areas that are predicted to become tidal over the rest of this century from sea-level rise (visit NOAA’s sea-level rise viewer), assuming no human modifications to the shoreline, such as armoring it with seawalls or other structures. This prediction also does not take into account existing seawalls and armoring that may already prevent the sea from encroaching on some these areas. They are, however, measured as being below three feet in elevation.

Cycling Tybee Island’s 3′ Contour from Dean Hardy on Vimeo.

Every time the tide rolls in, it brings loads of alluvial sediment with it. These sediments are transported by the rivers (e.g. the Ogeechee, Altamaha, and Satilla) that empty their waters into Georgia’s estuaries. The incoming high tides, or flows, transport these sediments into the marsh. As the sediment-laden waters move across the salt marsh, the sediments settle out due to friction from the blades of the smooth cordgrass slowing the water’s movement and the momentum of the sediment particles. The settling sediments vertically stack up, or accrete, on the marsh’s substrate, effectively growing the height of the marsh’s base. But there is counter force that leads to erosion. As the tide moves out, or ebbs, the water transports sediment out of the marsh, slowly eroding the substrate. If accretion rates are higher than erosion rates, the marsh grows vertically. If the inverse is true, the marsh’s base slowly becomes lower relative to the tide.

This is an important process driving salt marsh processes and movements, especially regarding the effects of sea-level rise on the salt marsh. Without going into too much detail, the rates of sea-level rise are forecast to increase over the coming century, having been an average of 2.5 mm/yr along Georgia’s coast since the early 20th century based on rates measured near Savannah, GA and Fernandina Beach, FL. If the rates of sea-level rise sufficiently exceed the marsh accretion rates, the Spartina may die off due to extended inundation times at high tide. The science is still ongoing regarding the effects of sea-level rise on Georgia’s salt marsh. Many scientists affiliated with the National Science Foundation’s Georgia Coastal Ecosystems – Long-term Ecological Research program are currently investigating many aspects of sea-level rise (visit the UGA Skidaway Institute of Oceanography’s coastal hazards portal) and the potential changes rising seas may trigger along Georgia’s coast and our nation’s (visit USGS’s Coastal Change Hazards Portal).

The next two really short videos are from early experimentation with filming the tide using time-lapse photography. The first is of the salt marsh south of U.S. Highway 80 headed out to Tybee Island. The second video is of the salt marsh around Horsepen Creek photographed from the dock at the Burton 4-H Center on Tybee Island.

U.S. 80 Tidal Flow from Dean Hardy on Vimeo.

Horsepen Creek Tidal Flow from Dean Hardy on Vimeo.

To learn more about my research, and see new videos of the salt marsh as I post them, check out my personal website and blog and follow me on twitter.