Regional area of interest (AOI), which is comprised of the southeast coast of Lake Mendota [Google Earth].
Orthophotos after Georeferencing process. The red line denotes the Regional AOI and the yellow star denotes the Local AOI (Alpha Iota).
Alpha Iota lakefront with the bluff crest, bluff toe, and shoreline highlighted. Bluff crest is the top of the bluff slope, while bluff toe is the base of the bluff slope. Shoreline is where the waterbody meets the land surface.
Example of digitized bluff crest lines, baseline, and transect lines. The yellow star marks Alpha Iota.
It is important to understand how the coastal area is evolving temporally and spatially on a regional scale. If regional patterns of erosion are discovered the implications of these results may be applied to the Alpha Iota site. The regional analysis’ area of interest (AOI) is the southeast shoreline of Lake Mendota, which is bounded by the UW-Madison Campus and Tenney Park, as shown in Figure 1. This AOI was assumed large enough to capture regional scale trends of erosion, while small enough to limit erosion processes and coastal areas that are drastically dissimilar to the Alpha Iota lakefront.
A local erosion analysis is important for understanding exactly how the Alpha Iota site has evolved, both in comparison to and independent of the regional erosion analysis results. The regional analysis does not capture the coastal evolution with a high enough resolution to make detailed inferences about the site on a local scale. Regardless of the different erosion analyses' resolution – which will be discussed in more detail – both were performed using similar means and methods.
Means and Methods
The first step in the erosion analysis process was to compile historical aerial imagery of the SE coast of Lake Mendota. Aerial images from the following years were used for this analysis: 1937, 1955, 1968, 1987, 1995, 2000, 2005, 2010, and 2015. All images from 1937 to 2000 were provided by the County of Dane, while images from 2005 to 2015 were collected from the WisconsinView Data Portal. Any aerial images collected for the erosion analysis were required to be orthophotos, which means they have the same lack of distortion as a map. This is crucial for acquiring accurate erosion information. Once the aforementioned orthophotos were compiled, all years were Georeferenced to ensure they were referenced to the same coordinate system NAD 1983 HARN Wisconsin CRS Dane (meters). A graphic showing the result of this process is shown in Figure 2.
After all orthophotos were Georeferenced, the next step in the erosion analysis was the digitization process. The digitization process was performed using the ESRI tool ArcGIS. In short, the digitization process involved translating raster data (orthophotos) into vector data (polylines). Specifically, the coastal elements of the bluff crest, bluff toe, and shoreline underwent the digitization process. An image of the Alpha Iota site showing the previously mentioned bluff elements is shown in Figure 3.
​
In order to translate the bluff crest, bluff toe, and shoreline into vector line data, each of these coastal elements were digitally traced in ArcGIS. This process of digitizing the three coastal elements was then completed for each of the orthophotos. While the coastal elements were being traced they were linked to their respective orthophotos, and therefore, the respective years they would represent. For example, if the coastal elements on the 1955 orthophoto were to be digitized, the three polylines for the bluff crest, bluff toe, and shoreline would represent the year 1955.
Once the bluff elements for each of the orthophotos were digitized the erosion rates could be calculated by using a software extension to ArcGIS called Digital Shoreline Analysis System (DSAS). To use the software, a baseline is created to act as a reference point. Once the baseline has been created, the DSAS extension was used to cast transects lines perpendicular to the baseline, as shown in Figure 4.
​
The DSAS extension functions by calculating the distance from the baseline to an intersection between a transect and digitized line. This distance from the intersection point to the baseline is calculated along the respective transect. From these distances, the total change in bluff crest, bluff toe, or shoreline between any of the analyzed years can be determined.
​
For the regional erosion analysis, a transect spacing of 10 meters was chosen as sufficient for representing the coastal changes on a regional scale. It was determined that a transect spacing of 10 meters would not be sufficient for the local erosion analysis. With a 10 meter spacing the coastal changes at the Alpha Iota lakefront would only be represented by five transects, which could potentially misidentify and distort the overall results. Therefore, a transect spacing of 1 meter was used for the local erosion analysis, which will capture the coastal changes with higher detail through this higher resolution.
Erosion Analysis
There were two erosion analyses performed for this project. The first analysis was a regional erosion analysis of Lake Mendota’s southeast (SE) coastal area. The second analysis was a local erosion analysis of the Alpha Iota lakefront. These analyses are critical for assessing the past and present condition of the site under inspection.