Field Report from the North Carolina Geological Survey Regarding Recent Debris Flows

The North Carolina Geological Survey has completed a field report for the Macon County Emergency Management and a copy of the report has been posted below in the interest of transparency.


Preliminary Field Report
North Carolina Geological Survey
April 24, 2020

At the request of the Macon County Emergency Management Department a team of geologists from the North Carolina Geological Survey conducted field reviews and Unmanned Aerial Systems (UAS) flights on April 13, 14, 15, and 17, 2020 of the areas affected by the debris flows of April 13, 2020. Based on our investigations to date, we offer the preliminary observations and guidance as follows. Additional information may become available as our studies continue.

Considerations for all locations impacted by the April 13, 2020 debris flows:

• Our assessment of the areas did not identify any immediate threats to life safety that would prohibit the occupancy of structures other than those deemed structurally unsafe to occupy by the jurisdictional building inspector. Conditions and circumstances may change, and this assessment in NO way implies that other slope failures in the affected areas would not occur in the future.

• Residual debris could continue to affect the areas particularly after future heavy rainfall events, including runoff and sedimentation from the exposed slopes.

• Because the slopes are over-steepened in the debris flow source areas, the margins of these slopes are unsafe.

• Trees that are leaning, undercut, or have exposed roots as a result of the debris flow activity should be considered hazardous. These trees should be removed where they are near residences or roads.

Valley View Trail and Chavis Road Debris Flows

The debris flows at these locations damaged roads, private properties, and severely damaged at least two residences. We observed groundwater discharge in the slope failure initiation zones at both locations, as well as evidence for surface water runoff from areas upslope during our April 13, 2020 site visit. Increased groundwater and surface water flow in response to the heavy rainfall were factors that likely contributed to the failures. Prior ground disturbance from human activity in the debris flow source areas appeared to be minimal.

Our assessment of the areas did not identify any immediate threats to life safety that would prohibit the occupancy of structures other than those deemed structurally unsafe to occupy by the County building inspector. Conditions and circumstances may change, and this assessment in NO way implies that other slope failures in these areas would not occur in the future. Anecdotal reports by residents indicate that a slope failure occurred in the Valley View Trail area about 15 to 20 years ago.

Additional soil material could fail along the over-steepened slopes in the debris flow source areas. The material would likely move downslope along drainage ways, especially in response to heavy rainfall. Runoff and sediment from the exposed slopes could also affect travel along roads and driveways, and impede or prevent vehicular access in some instances. The debris flow deposits can be thick, wet and contain woody debris and other objects caught up in the flows; therefore, extreme caution is advised for foot traffic in these areas.

Monitoring the steep slopes in the debris flow source areas for any development of new ground cracks that could indicate the enlargement of the unstable areas is advisable. Because the slopes are over-steepened in the debris flow source areas, the margins of these slopes are unsafe, particularly for children. We observed what appeared to be pre-existing, past-active, ground displacement (scarps) in the area immediately upslope for the source area of the Chavis Road debris flow. New ground movement along these scarps may indicate enlargement of the present unstable area.

Dobson Mountain Debris Flow

The debris flow on the northwest-facing slopes of Dobson Mountain is a major event directly impacting an area of about 16.8 acres along a tributary to Cartoogechaye Creek. We understand that the inlet for the town of Franklin’s water supply is in Cartoogechaye Creek. The path of the debris flow is approximately 4,000 feet long as measured along the slope gradient, and is 250-300 feet wide in places. Unvegetated soil subject to continued erosion and sedimentation is exposed along essentially the entire 16.8 acre path. The debris flow initiated on slopes where evidence was not observed of significant road construction or site grading, such as cut and/or fill slopes. Our field investigation of the source area indicates that it is an area of past debris slide activity. Older debris flow deposits of an unknown age were also observed in the debris flow runout zone along the stream banks on the east side of the commercial facility at 595 Maxwell Home Road.

Initial estimates indicate there is a substantial amount of unstable slide debris remaining near the debris flow source area, possibly on the order of 12,000 cubic yards or more. Ponded water present in this area was visible in our April 13, 2020 UAS image, and verified in our April 14, 2020 field investigation. There is a significant potential for future debris flow activity related to this unstable mass in the source area.

There are no known residential buildings in the path of the April 13, 2020 debris flow, or possible future debris flow activity in the same drainage. The commercial facility at 595 Maxwell Home Road was impacted by the April 13th event, and extreme caution should be used in the area where debris flow deposits are piled up behind the main building. This property could be adversely affected in the event of future major debris flows. As our resources allow, the North Carolina Geological Survey is available to provide technical assistance in an Emergency Watershed Protection grant application process, and participate in discussions of possible mitigation measures as they may relate to geologic processes and materials.

Turkey Pen and Enchanted Mountain - Potts Branch Watershed

Four debris flows were identified in the Turkey Pen and Enchanted Mountain areas of the Potts Branch watershed on the southeast-facing slopes of Dobson Mountain. No imminent threat to residential structures was identified in this area.

The largest debris flow damaged the residential buildings at 504 Turkey Pen Road, and caused substantial damage to Turkey Pen Road. Our April 17, 2020 investigation of the debris flow track and source area indicated that it originated on natural, forested slopes where evidence road construction or site grading, such as cut and/or fill slopes was not observed. Oversteepened soil material remains in the scarp of the source area which will likely result in localized instability. Anecdotal reports by residents indicate that a slope failure like a debris flow occurred in the lower Turkey Pen Road area about 15 to 20 years ago. We identified three debris flows in the Enchanted Mountain area that resulted in damage to Enchanted Mountain Road, and to a residential well house and well head adjacent to the road. Future movement of exposed soil from the debris flow path above that location could potentially impact the well head.

Road-related slope failures on Trimont Mtn. Road and Doghobble Drive (Wildflower) Road-related failures at both locations mobilized into rapidly moving debris flows that impacted properties outside of the road corridors. The over-steepened slopes immediately below the road travel ways are unstable, and the potential for further failures, and subsequent loss of the travel way exists until the slopes are adequately repaired. Both locations are generally unsafe for vehicular traffic for the conditions observed on our field visits to Trimont Mountain Road on April 15th and Doghobble Drive on April 17th, 2020.

Evidence of groundwater seepage was observed in the slope failure initiation zones at both locations, and was likely a factor contributing to the failures. Road repair methods and materials should take into account the adequate long-term management of groundwater and surface water at both locations. The culvert was plugged at the Trimont Mountain road location, which likely resulted in storm water traveling over the roadway and onto the road embankment contributing to the failure. It is advisable to replace the existing culvert with a larger diameter one that is better positioned to capture inlet flow, along with adequate erosion protection at the outlet. Ideally, road-related slope repairs at both locations would be done under the guidance of a licensed geotechnical engineer. The debris flows stripped vegetation and surface soil along their paths. Revegetation of the track areas for erosion control is advisable to reduce further erosion and stream sedimentation.

The road failure on Trimont Mountain Road caused sedimentation into the impoundment behind Trimont Mountain dam (LQS Macon-044). A DEQ Land Quality Section inspector visited the dam on Friday, April 17, 2020 and determined that the dam itself was not impacted by the debris flow. The impoundment did receive sediment but there were no impacts to dam-related infrastructure from the debris flow. The Trimont Mountain road failure is upslope of the Trimont Mountain Dam; therefore it is important that the road repair is stable over the long term.

Published at 5:35pm on Tuesday, April 28, 2020