The science of steep slope movements

The topic of a proposed Steep Slope Ordinance for Macon County has been in this paper often recently. While these ordinances are common in other parts of the US, the articles reveal a number of important questions that the people of Macon County are asking such as: “Why do we need this ordinance?” “How does development cause landslides?” “Who does this ordinance really serve?” In this column, we hope that we can offer some basic science about landslides to help inform residents of Macon County about the ecological science related to these questions.

Understanding the causes of landslides is complicated. Since a Steep Slope Ordinance would address development on mountain slopes, residents might wonder what building houses has to do with landslides when landslides happened in this region long before the first house was ever built. We seek to shed light on the question: How can building on mountain slopes potentially destabilize the soil and contribute to dangerous landslides that can affect not only the individual builder, but their down (or up or side) hill neighbors?

Several triggers can cause soil that normally sits calmly and contentedly on mountain slopes to come crashing down (though this can also happen slowly overtime, leaving clues such as curved tree trunks and cracked foundations). These triggers include removal of support, removal of vegetation, addition of moisture, addition of weight, over-steepening and earth-shaking events. Natural or human factors can lie at the root of several of these triggers.

Removal of support happens when material at the bottom of a slope is removed, causing instability in the material above it. This could result from a stream cutting into the base of a slope, or when any part of a hillside is removed to make room for a house or road. It’s like that block-stacking game Jenga — remove a bottom block and risk that those above will come crashing down. Water build-up in soil is one of the most common landslide triggers. It was the back-to-back heavy rains of Hurricanes Fran and Ivan that ultimately caused the Peeks Creek slide in 2004. However, water can be added by forces other than storms. Think road runoff, broken water pipes, changes in water flow due to grading, or sewage and runoff disposal.

Addition of water is tied to another trigger commonly associated with slope development: removal of vegetation. Not only do deep tree root systems hold and stabilize soil perched on the side of a mountain, they also suck up excess water that may transform dry soil into a more fluid, mud-like consistency.

Adding weight to a slope through natural addition of water as rain or snow, new houses or adding fill for a roadbed, is kind of like increasing the force of gravity — it pushes stuff down. Over-steepening can happen when soil is piled at an angle that exceeds what is called the angle of repose. This just means that if soil is piled too steeply, it will start to slide down the mountain. This can happen as a result of excavation, erosion (perhaps when plants are removed to build a house and their roots can no longer grip the soil in place) or grading. Then imagine this soil slowly sliding downwards, piling up more weight in certain places and further destabilizing the soil.

None of the science behind landslides suggests folks cannot build on steep slopes, just that residents need to do further investigation into the soils, steepness, runoff, etc. and employ engineering techniques to protect their family, their investment and their neighbors. Landslides and debris flows have been happening as long as these hills have been around, and it is just now that the density of dwellings on steep slopes is getting to the point where it significantly raises the probability that people are going to be on or within the path of a deadly mass-wasting event.

When fewer folks were in these mountains they automatically chose the flatter and safer building sites, but now those are all taken up, and more and more people build on steep slopes. It stands to reason that as more people locate roads and dwellings on slopes, then there is a greater chance that people can be in the path of a deadly earth movement.

Although the question of whether or not it is best for Macon County to enact a Steep Slope Ordinance lies outside the realm of science, hopefully this information can help Macon County citizens think about how landslides can result from both natural and human triggers. 


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 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. Mar 2, 2012.