Applying a combination of wood mulch and compost on highway construction sites can significantly reduce the amount of rain runoff and accompanying erosion, according to research from the LSU AgCenter.

In a recently completed three-year study funded by the Louisiana Department of Environmental Quality, the research showed that using the mulch/compost mixture significantly reduced sediments in surface water, said LSU AgCenter soil scientist David Weindorf. 

“Combating soil erosion on highway rights-of-way requires novel, multidisciplinary approaches that evaluate the issue from vegetative, soil and engineering standpoints,” Weindorf said.

In Louisiana, road construction often involves removing trees from the right-of way. After the harvestable wood is gathered, the rest usually is piled and burned. This project suggests a guideline to grind and make mulch from the wood and save it for application to embankments.

In the study, Weindorf blended wood mulch with compost and applied it to embankments along highway construction sites.

“Compost is more degraded than mulch. Finished compost is stable and provides a rooting medium and nutrient source for vegetation. The mulch, which is larger, locks together and provides an ‘armor’ to hold the mulch and underlying soil in place. The combination also holds soil moisture.” 

The research program addressed several questions, including how thick a layer of material would be needed for best results and how effective the approach would be on various soils and slopes, said Greg Waldron, project manager at DEQ. “We want to be able to provide a cover for soil so it won’t end up in stormwater runoff.” 

Weindorf worked with Waldron to establish the research sites and monitor the results at several test sites along Interstate 49 in Rapides Parish and Louisiana Highway 61 in West Feliciana Parish.

Plots were either covered with a 2-inch or 4-inch mulch/compost mixture or left bare, and water runoff from each type was monitored for sediments. Each of the three combinations also included other variables, including slope of the land, soil texture and soil preparation.

The researchers put out plots at eight sites and collected the water that ran off after rainfall. Because rainfall totals weren’t consistent from one plot to another, the researchers used simulated rainfall in the third year to have results that could be compared among the various sites.

“We found that 4 inches of mulch/compost was more effective than 2 inches,” Waldron said. “We believe 3 inches is optimum.”

DEQ is working with the state Department of Transportation and Development to develop a standard for use in highway construction, Waldron said. “We want to protect surface water with a byproduct of highway construction.”

“We were looking for improvements between the bare plots and treated areas,” Weindorf said. “We had fantastic success. The mulch/compost material stayed in place perfectly.”

The results were particularly gratifying because the researchers intentionally kept the plots bare of any vegetation.

“Erosion control was the ultimate goal – to reduce sediments in surface waters,” Weindorf said. “Even without plants, the process is really highly effective.”