The scientific side of steep slopes


Given the Peek’s Creek landslide, notable erosion problems, and the county’s now tabled steep slope ordinance, residential development on steep slopes has been a hot topic of public discussion in Macon County during the last several years. As in past columns, our purpose here is to discuss available data so that county residents can be informed and develop their own positions. As such, this edition of our column discusses some of the publicly available data about steep slope development in Macon County.

County tax data, the data available in online GIS maps, and the North Carolina Geological Survey’s Downslope Hazard Map help us see which areas of the county are more and/or less vulnerable to unstable slopes. 

The hazard map specifically focuses on the likelihood that heavy rains like the ones we’ve had recently (5 or more inches within 24 hours) will lead to a landslide or other debris flow on a given property.

One key finding from these data is good news: roughly 93 percent of the residential structures in Macon County sit in what the maps term low-risk zones. Essentially, this means that in the case of a five-inch rain event over 24 hours, most structures in Macon County are unlikely to be impacted by a landslide or debris flow. 

Because landslides can result from both human and non-human factors (for example, from either road construction or heavy rains), it is positive that most residences sit in lower risk areas. Nevertheless, the remaining 7 percent might be cause for concern: 4 percent are in high-risk zones and 3 percent are in moderate risk zones. In order to understand why people are building in vulnerable areas and how their vulnerability might be decreased, we began to look for trends related to degree of risk, property value, home ownership, and other factors. One area that stood out was local versus out-of-state residence.

The county tax data lists property ownership by primary address, and we sorted this data according to those owners whose primary addresses were out of state (“out-of-state owners”) and those whose primary address does not have a Macon County ZIP code (“non-local owners”).

One finding is that the residential structures in high and moderate risk zones have higher percentages of out-of-state and non-local ownership as compared to the lower risk zones. In other words, residential structures in high to moderate risk zones are more likely to be owned by out-of-state and nonlocal owners, while residential structures in low risk zones are more likely to be owned by in-state and local owners.

A potential issue with this situation is that most out-of-state and non-local owners are from areas less mountainous than Macon County, and as a result might not know the best practices for residential construction in the mountains. 

Another potential issue is that out-of-state and non-local homeowners may not actually know the risk zones in which their properties sit. Given the projected increase of out-of-state and non-local homeownership in Macon County, these issues have the potential to become more serious in the future.

One final finding is that there is not a strong relationship between a residential structure’s elevation and its risk zone. In other words, just because a house is built high on a mountain slope does not mean it is in a high-risk area—or a moderate or low-risk area, for that matter. This means that, regardless of their property’s elevation, buyers and developers should inform themselves about the downslope hazard risk of their and their neighbors’ property and structures.

To get more information, click on the ‘Map’ link at our website http://listening.coweeta.uga.edu. Our mapping tool shows all of the county’s residential parcels and their downslope hazard risk zones. [Note not in the original printed version: On the mapping tool, use the check boxes on the left side of the screen to show or hide different aspects of the map.]

While the implications of steep slope development for county ordinances are best left up to the community and their elected officials, property owners both local and non-local would do well to know more about their property and steep slope hazards. We would like to hear your thoughts and experiences with steep slope development in Macon County. Please send ideas and observations to the email or postal address below.

 

This column is produced by members of the Coweeta Listening Project (CLP), a branch of the Coweeta Long Term Ecological Research Program. Views expressed here are not representative of the USDA Forest Service or the Coweeta Hydrologic Lab. Please share questions, comments, or suggestions for future topics at cwtlistn@uga.edu or Coweeta Listening Project, UGA, 210 Field St., Room 204, Athens, Georgia 30602.

 

Original Citation: The Coweeta Listening Project. Franklin Press. Column on "Science, Public Policy, Community." Page B4. Feb 15, 2013.