Change Analysis
Overview of Shoreline Change Analyses
Sullivan’s Island has been the beneficary of sediment transport alterations since about the 1950’s. An overall positive balance continues to this day, but it is not spread evenly over the island. Parts of the island are beginning to show signs of erosion that has not been seen in the past 20 years. The area between about Station 16 and 18 has seen statistically significant erosion, i.e., it is changing much differently than in the past 20 years. Green areas on the map below are accreting at statistically significant levels, which is not a bad trend; but it does highlight changes in patterns.
May 2020 high water line color-coded by change from the existing 20 year trends. Green areas are accreting at rates beyond the existing normal variance, red areas are eroding at rates outside of the normal variance. Light blue areas are changing within the normal variance of the shoreline.
Shoreline Change Techniques
If you are interested in the more subtle ways to look at shoreline change - keep going.
So at this point you may have seen all the ‘lines’ on the GIS representing past shorelines. Interesting but also somewhat noisy and confusing, which is why shoreline change analysis techniques were developed.
Traditional Shoreline Transects -1 Dimension
This is the most common technique and often done with an automated technique, (e.g., AMBUR or DSAS). I do not use these fully automated software packages/techniques but instead use a GIS technique called the ‘nearest node’. Basically it is the same idea as the automated ones; but instead of having a ‘set’ transect that does not vary (always 90 degrees to baseline) the nearest node technique can change angle to find the shortest distance. Kinda like getting dropped off a boat and swimming the shortest distance to shore.
Interpolation of shoreline positions through time
Once all the numbers are in (i.e., the measurements between shorelines) there are several ways to go about looking at making sense of them. In the present case I have used linear interpolation to define the slope of change on a yearly basis (see figure above). This technique finds a ‘best fit’ line that represents all the change included. It is not going to honor all the inter-yearly changes but does it’s best to represent all of them as a whole. For example in the profile above the overall trend would suggest that the shoreline is on the baseline in 2020 (offshore baseline in this case; see map below), but as you can see the last point (2019) is like 75 m inland of the baseline. This is a good example of the limitations of using long-term trends, i.e., things change and the conditions that created the big differences between 1950 and 1990 may no longer exist.
In the map below I included all the shorelines from 1939 to present and also from 2000 to present. The baseline itself is the location of the maximum extents of shorelines (the best of all years to date) so if you see the 2019 shoreline close to it, that is good; if it far from the baseline that indicates, in the broadest sense, erosion is likely higher there than in previous years in the past.
Beach Volumes - 2 Dimensions
Volume change analyses are more common now and provide a measure of the real differences occurring on the beach. Shoreline location differences through the years, although a partial outcome of volume changes, only catches a fraction of the total change. This measure of the beach evolution gives you a more complete accounting of the beach’s ‘health’ and ability to protect adjacent habitats and structures. The information can only be provided through precise and timely measurement of the topography, which can be achieved using a host of on-the-ground surveys (traditional and GPS), drone surveys, or Lidar (terrestrial, mobile, or aerial).
To highlight longer-term changes linear change trends over the past 20 years of lidar data have been calculated and highlighted in this map. This provides a good look at what has been, and also sets the stage to look at how things are evolving - as they are.
Shoreline Location Likelihoods
This is a unique analysis and is based on the changing beach volumes. It will highlight areas that are changing outside of the ‘normal’ variance over the past 50 or so years. Having just started looking at the data, the southwest end of the island is an area that bears watching, something is dramatically different here.
Shorelines and SLR
In Progress
TBD
Thinking out loud, but maybe a wet-dry likeliness.