A construction contractor that was demolishing some buildings on Fort Benning, GA, improperly dumped the demolition debris into a gully located at Kelley Hill rather than hauling it to an authorized landfill. Most of the debris had been covered with soil; however, some of it had been exposed by erosion along the bottom of the ravine. Unfortunately, the debris contained huge chunks of painted concrete with protruding rebar, metals, and asphalt, which presented both a public safety hazard and an environmental hazard in the form of toxic paints leaching into the surface water. Additionally, the deposited debris had moved the drainage channel to one side of the gully, causing it to further erode the gully banks and discharge the sediments into state waters downstream. Therefore, the debris had to be removed and properly disposed of in an authorized landfill, and the site needed to be restored.

The Mitigation Plan
The staff from Fort Benning’s Directorate of Public Works (DPW), with contracting assistance from the US Army Corps of Engineers (USACE), prepared a mitigation plan and administered the project. The first step in the mitigation plan called for removing the debris from the gully. The debris was to be sorted. The unpainted concrete would be crushed into sizes appropriate for reuse in the site restoration. The metal would be recovered and recycled, and any nonrecyclable debris was to be hauled to an authorized landfill.

Site restoration was the next step in the mitigation plan. The biggest challenge was the fact that there are two piped discharges that flow into the gully channel. The channel elevation, where the pipes discharge onto the site, are about 55 feet above the site’s outfall into state water, which is located about 700 feet away. This means that the channel has about an 8% gradient.

The site is located in what is called the Georgia Sand Hills, a narrow belt of deep sandy soils with rolling hilly topography consisting primarily of marine sand, loams, and clays deposited over acid crystalline and metamorphic rocks. The soils in this area are extremely susceptible to erosion; therefore, the restoration plan involved constructing terraces across the slope and forming the channel in a serpentine alignment to reduce the channel gradient, and micropools placed on each terrace level to capture sediments. A riffle channel using the previously recycled broken concrete was to be placed from one micropool to the next lower terrace. At the confluence where the site discharges into a state water, a rock filter dam would also be constructed from the recycled broken concrete. This dam would served as a sediment control and energy dissipater during construction, as well as after the restoration was completed. The slopes would be covered with an erosion control blanket until grass was established.

The final step in the restoration would be the planting of black willows, silky willows, dogwoods, elderberry, and ninebark as live stakes along the channel and slopes to provide long-term channel bank and terrace protection. These plants were selected for their ability to thrive in fluctuating wet conditions, to provide wildlife forage and habitat, and because they are native to the region.

From Concept to Construction
USACE contracted with SES Construction and Fuel Services LLC to remove the debris and restore the site. The restoration concept was modified at the request of the contractor because of concern about channel stability during extreme rainfalls. A hydrology analysis indicated a flow rate of 22 cubic feet per second for the two-year frequency rainfall. However, the concern was with the 25-year rainfall with its peak flow rate of 60 cubic feet per second. Therefore, instead of constructing the channel along the back of each terrace with shallow micropools, the revised plan called for the construction of small retention ponds within the entire area of each terrace, and also shortened the travel distance of the flow through the site. The terraces, in effect, formed three large check dams.

Additionally, a field change to the plan was made by constructing a diversion ditch across the top of the slope along the project disturbance limits to prevent the offsite runoff from flowing over the bare soil and causing a slope erosion problem. A channel reinforced with recycled broken concrete brought the flow from the ditch down to the second terrace level.

Site restoration was completed in May 2012.

After the contractor completed the restoration work in May 2012, volunteers from the Fort Benning DPW and the nearby Army unit sprigged the live stakes into the ground late last fall for sprouting the next spring. Presently, 70 to 80% of the live stakes have sprouted.

Conclusion
The project is considered a great success, although it was necessitated by an incident that should never have happened. The project demonstrates that restoration costs can be minimized through proper waste management and recycling of the debris. It also demonstrates that channels can be constructed in ways that are compatible with the environment and that promote natural plant growth and wildlife. This project transformed an unsightly, debris-filled gully into a passive recreational area for use by the soldiers living nearby and into wildlife habitat.

Finally, the project has introduced the concept of using terracing as a landform-type structural BMP to manage stormwater runoff on hilly, highly erodible terrains. It is hoped that future construction projects will consider using this type of BMP as an effective means for both construction-site erosion control and for post-construction runoff volume and pollution control.

About the Author

Randel Lemoine

Randel Lemoine, P.E., is a civil engineer working for the Directorate of Public Works at Fort Benning, GA, where he leads a team in developing Fort Benning’s Post-Construction Stormwater Management Program.