Mountain Roads and Erosion

Predicting Erosion and Storm Runoff on High-Elevation Roads

Chattahoochee-Oconee NF
The study took place in the Chattahoochee-Oconee National Forest, northern Georgia. USFS photo, courtesy of Wikimedia Commons

Forests on mountaintops may seem remote. “However, millions of people rely on these forested headwater watersheds for their drinking water,” says Johnny Grace. “For many people in the Southeastern U.S., high-elevation forests are where clean drinking water originates.”

Grace is a general engineer at the U.S. Forest Service, and he recently studied forest roads and their susceptibility to erosion. Native surfaced ‘dirt’ roads crisscross many high-elevation forests. When stormwater runoff encounters these roads, they can erode. Erosion degrades the road surface and can carry soil towards streams.

No one knows how much sediment actually reaches mountain streams. “This is an area of limited understanding and great concern,” says Grace. When sediments are washed off the road surface, they can travel further and further towards streams after each storm. If these sediments are washed into streams, they can remain suspended and impair water quality, or sink to the stream bed and cloak aquatic animals in a layer of mud.

Building mountain roads that won’t erode after storms is challenging. Steep slopes cause rainwater to rush onto the road with greater force than in flatter terrain. Additionally, many forest roads have features like switch-backs. As storm water runs downhill, switchbacks can cause the road and the path of runoff to intersect many times.

“Managers and scientists need more information about erosion, sedimentation, and reliable models to use as decision support tools,” says Grace. “It’s especially important to study these processes in high-elevation forests.”

Listen to a brief audio clip by author Johnny Grace describing this publication. • Text Transcript

Grace, along with SRS engineering technician Preston Steele, measured sedimentation and water quality in nine high-elevation streams on the Chattahoochee-Oconee National Forests, in Georgia. The findings were published in the Transactions of the American Society of Agricultural and Biological Engineers.

The researchers collected measurements for 5 years. During that time, there were 156 storms that generated runoff.

Grace used the data to test the Water Erosion Prediction Project model. “WEPP can be used to simulate a wide range of conditions and scenarios in forested watersheds,” says Grace. “Until this study, the model had not been thoroughly tested in high-elevation forests east of the Mississippi River.”

The model did not do well at predicting storm runoff, probably because there was no way to tell where storm water runoff came from after large storms. However, the model adequately predicted how much sediment would wash off each road section.

monitoring
A roadside ditch monitoring and sampling location. Photo by Johnny Grace, USFS.

Grace also modeled sediment yields for a 25-year period. The results were very different for each stretch of road, but on average every square foot of road surface could produce 75 pounds of sediment per year.

Forest roads make up just 1 percent of the total watershed area, but they produced about 65 percent of the projected sediment yield. Forest roads produced almost twice the amount of sediment as the mature forests that surrounded the roads. At the watershed scale, sediment yield is much lower, as forests prevent erosion in a number of ways.

Roads in high-elevation forests are often located in national forests. They tend to be headwater watersheds that have been least impacted by humans, and are critical to surface drinking water supplies in the Southeast.

“The potential risks or consequences of water resource impacts associated with road management in these critical watersheds may be magnified,” says Grace. “This study shows how important it is to prevent erosion by using best management practices and the remarkable capacity of forests to filter sediment from storm runoff.”

Read the full text of the article.

For more information, email Johnny Grace at jmgrace@fs.fed.us.

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