Summary table of the regional erosion analysis results. Displays the mean, standard deviant, and the range (minimum/maximum) for each of the three coastal elements under inspection.
Bluff crest results from the local erosion analysis. The average rate of change was -0.030 m/year (-3.0 cm/year).
Bluff toe results from the local erosion analysis. The average rate of change was -0.058 m/year (-5.8 cm/year).
Shoreline results from the local erosion analysis. The average rate of change was -0.096 m/year (-9.6 cm/year).
The regional analysis’ area of interest (AOI) stretched a length of 2.87 km along the southeast (SE) coast of Lake Mendota, bounded by the University of Wisconsin - Madison and Tenney Park. Zoning in the regional AOI consisted primarily of residentially owned properties and one public park (James Madison Park).
A summary table of the regional erosion analysis results is shown below in Table 1. The respective average rate of change for the bluff crest, bluff toe, and shoreline were -0.9, -1.3, and -1.4 cm/year. More detailed visuals of how the coast was evolving are available by following the link below ('Regional Erosion Analysis Visuals'). Overall, these average rates of erosion are relatively insignificant compared to other known rates of erosion. For example, on the Great Lakes in high risk erosion areas the shorelines are changing at rates greater than -30 cm/year [4]. On a regional scale the SE coast of Lake Mendota has remained relatively consistent for the past 78 years. Therefore, the regional erosion analysis results suggest that overall, the regional AOI is not under threat of severe erosion and coastal recession.
Although the regional erosion analysis did not suggest there was any threat due to erosion, the local erosion analysis results proved to be contradictory to this analysis. From the local analysis, the average rate of change of the bluff crest, bluff toe, and shoreline were -3.0, -5.8, and -9.6 cm/year, respectively. A visualization of how these bluff elements evolved locally from 1937 to 2015 can be viewed below in Figures 1, 2, and 3. Several conclusions can be made from these results; the most obvious of which is that the Alpha Iota site has not behaved consistent to the regional AOI.
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At first glance the crest results from the local erosion analysis appear significantly larger than the regional analysis. The average rate of erosion at Alpha Iota (-3.0 cm/year) was found to be more than three times the average results found in the regional AOI (-0.9 cm/year), but this result must be taken with caution. Through further inspection, it was discovered the majority of this erosion took place between 1937 and 1968. Although never confirmed, it was hypothesized that between the years 1955 and 1968 the bluff crest was stabilized. This may have been the time period when the current stone retaining wall was constructed. From 1968 and on there was little movement of the bluff crest line.
Likewise to the bluff toe results, it is important to note that the shoreline at Alpha Iota has shown a greater rate of erosion than the regional AOI. Also similar to the toe results, there are more significant results that may be drawn. Specifically, the rate of change of the shoreline (-9.6 cm/year) was more than one and a half times that of the toe (-5.8 cm/year) at Alpha Iota. The area between the shoreline and the bluff toe represents the beach; beach width is an important factor that affects bluff erosion processes. Beaches act as a natural armoring for the bluff toe, helping to protect the bluff toe against erosion processes due to wave action. As the beach shrinks the bluff toe becomes more susceptible to wave action and accelerated rates of erosion. This, in turn, could translate to increased slope instability at Alpha Iota.
It is important to note that the bluff toe at Alpha Iota has shown a greater rate of erosion than the regional AOI, but more importantly, the rate of change of the toe (-5.8 cm/year) was almost twice that of the crest (-3.0 cm/year) at Alpha Iota.The area between the bluff toe and bluff crest denotes the bluff face or slope. As the horizontal distance between the bluff crest and bluff toe decreases the steepness of the bluff slope increases. This discovery that the toe has eroded on average twice as fast as the crest is significant; from this result it may be concluded that the steepness of the slope has progressively increased over the past 78 years. As the slope at the Alpha Iota site becomes steeper the slope becomes more instable.
In addition to the erosion analysis performed with ArcGIS, the project team also visited the Alpha Iota site to further investigate any potential erosion issues.
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During the site investigation the revetment at Alpha Iota was visually inspected, and appeared to have experienced some deterioration since its initial construction. An image of the revetment at Alpha Iota is shown below. The stones used to construct this revetment appear to be scattered and random, a clear sign of deterioration. In addition. many of the stones appear to have migrated into the lake, further reducing its effectiveness.
A video was also taken at the Alpha Iota, which shows waves overtopping the deteriorated revetment at the site. The waves on the day this video was taken were not abnormal in size. This overtopping is a key indicator that the revetment is no longer serving its intended purpose and is in need of remediation.