We’re going to look at some beach morphology data from Benson Beach, just north of the North Jetty on the Columbia River. The beach is accessible from Cape Disappointment State Park near Ilwaco, WA.
In 2010 and 2011, the USGS, OSU, WA State Department of Ecology, and the Army Corps of Engineers did a joint study of the impacts of beach nourishment. The Army Corps traditionally dumps dredging materials offshore, outside of the reach of littoral sediment transport. In the summer of 2010, they deposited these materials on high-energy, often erosive Benson Beach.
Today, we’ll do a difference analysis of beach elevations before and after the beach nourishment. Data was taken with foot- and Polaris side-by-side-borne GPS, and with jetskis equipped with GPS and sonar.
Start by copying the file S:\Geo599\Data\SEXTON_Difference_Tutorial into your own folder.
We’re going to start with the point data. It’s been cleaned, but otherwise has not been processed. I had some trouble with opening it, so please let me know if you have any issues – I’ve got a back-up plan.
The S01 data is the “before nourishment” baseline survey, performed on July 11, 2010. The S06 data is the “after nourishment” survey, performed September 22, 2010.
In the catalog, navigate to where you saved the folder. Find the S01 folder and right click on the file: S01_20100711_topo.
Create a feature class from XY table.
Choose Field 1 for X, Field 2 for Y
Leave Z as “none” (we have Z data, but it got real mad when I told it here).
Click the Coordinate System of Input Coordinates button:
Select “Projected Coordinate Systems,” then “State Plane,” then “NAD 1983 (Meters).”
Navigate to and select the coordinate system called:
Choose where to save your output feature class. I called mine: XYS01_20100711_topo.shp
Hit OK and cross your fingers – this is where your program might crash!
Once it’s done thinking (and maybe not responding for a bit – don’t panic if that happens, it kept working for me), navigate to the new shape file in your folder and drag it into your working area.
This should be pretty exciting, except that all the symbols are one color. To better visualize, right click on the layer, and find properties. Click the “Symbology” tab and then “Quantities” on the left. Change the “Value” to “Field3” and select a color ramp. Field three is your Z values, and it will color by elevation!
PS: when you tell it to use Field3, it gives you an angry message. I just hit OK, and it moved on and looked fine. If anyone has any suggestions, I’m all ears.
While this was super cool, I thought we’d short cut pulling in the rest of the XY data, and I’ve provided you with shape files of the rest.
In the S01 folder, there is a help folder. In there, you’ll find my version of the S01 topo shape file (which you likely just completed) and aS01 bathy shape file, called XYS01_20100711_bathy. Drag that into the work space. You can change the colors to see the elevation, but it’s not necessary to our next steps.
You’ll also need to drag in the S06 shape files. I still gave you the XYZ text files in the S06 folder, but again provided a “help” folder with shape files of each point set. Drag these files into your work space:
You should now have four layers: two bathy, two topo; one from each survey.
Now we’re going to make some surfaces.
In the toolbox, open 3DAnalyst, then Data Management, then TIN, and find the “Create TIN” tool. (or search for it…)
Name your first TIN S01
You’ll need to choose the projection again: NAD_1983_StatePlane_Washington_South_FIPS_4602
Then choose the two S01 feature classes, bathy and topo.
When they appear in the list of input features, you’ll need to change the Height Field to Field3!
Hit OK and it will make a TIN.
Hopefully, it looks like this:
Repeat the process for the S06 data set. Don’t forget to specify the Height is in Field3.
Lastly, we’re going to perform a difference analysis. This will compare the baseline beach survey with the “after nourishment” beach survey and find where sand was deposited, and where erosion happened after the deposition (since the survey wasn’t directly after).
In the 3DAnalyst toolbox, expand the “Triangulated Surface” options, then open the “Surface Difference” tool.
Set the Input Surface to “S06”
Set Reference Surface to “S01”
Name your output feature, I called mine “Beach_Change”
Expand the Raster Options and name an output raster, I also called this one “beach_change”
Leave cell size as default (10)
Click OK! This might take a minute.
The resulting raster can be recolored to more readily show differences.
The feature class generated currently shows only where there sand was deposited (blue), eroded (green), or unchanged. While this might be helpful, I wanted elevation changes in a feature class.
If you’d rather a shape file:
Open the attribute table for surface_dif1 feature class
Add a field, call it “ele_dif” and make it a “float”
Open field calculator and calculate the elevation change by multiplying volume and the code (negative means loss of sediment, positive means gain), and dividing by “SArea.” Then click okay.
Close the attribute table and open Properties. Choose the “Symbology” tab, then “Quantities.”
Choose the new “ele_dif” as the value, and increase the number of classes (I chose 9). When you hit okay, the feature class will show the elevation changes!