The occurrence or reactivation and acceleration of landslides can occur unannounced and can result in significant impacts to life, property, or the environment. The processes of slope deformation and progressive failure are more active than many realize; rapid slope failures are often preceded by years of erosion, deformation, and smaller failures. In the last 10 years, the use of lidar-derived elevation models has supported the identification of landslides across large regions and is increasing the ability of geoprofessionals to identify precursory signs of failures. During the same time period, advanced computational techniques to numerically compare multiple bare-earth lidar point cloud datasets, known as lidar change detection (LCD), coupled with the development of automated workflows, have resulted in the ability to conduct LCD rapidly across large areas. This paper demonstrates how regional LCD can provide a more complete understanding of landslide hazards and better management of risk. The paper also presents preliminary tests of applying image segmentation techniques to support LCD analysis.

On August 14, 2021, a Mw 7.2 earthquake struck the Tiburon Peninsula of western Haiti triggering thousands of landslides. Three days after the earthquake on August 17, 2021, Tropical Storm Grace crossed shallow waters offshore of southern Haiti triggering more landslides worsening the situation. In the aftermath of these events, several organizations with disaster response capabilities or programs activated to provide information on the location of landslides to first responders on the ground. Utilizing remote sensing to support rapid response, one organization manually mapped initiation point of landslides and three automatically detected landslides. The 2021 Haiti event also provided a unique opportunity to test different automated landslide detection methods that utilized both SAR and optical data in a rapid response scenario where rapid situational awareness was critical. As the methods used are highly replicable, the main goal of this study is to summarize the landslide rapid response products released by the organizations, detection methods, quantify accuracy and provide guidelines on how some of the shortcomings encountered in this effort might be addressed in the future. To support this validation, a manually mapped polygon-based landslide inventory covering the entire affected area was created and is also released through this effort.

ABSTRACT Landslides occur in Polk County, North Carolina, primarily along the Columbus Promontory of Blue Ridge Escarpment (BRE), which has 400 m of topographic relief and slopes typically >20°. Bedrock is characterized as late Proterozoic to early Paleozoic metamorphic rocks within Paleozoic thrust sheets. On May 18, 2018, ∼200 mm of rainfall over a 3- to 4-hour period triggered numerous debris flows and slides along the BRE, causing one fatality and severe damage to homes and roads. The State Emergency Operations Center tasked the North Carolina Geological Survey to assess slope stability ahead of search and rescue operations and assess damage along the North Pacolet River valley. The loss of life and destruction from the 2018 storm and ongoing threats to infrastructure prompted us to map landslides throughout Polk County in 2019–2021 to fully document the 2018 landslides and place them in the context of past and ongoing landsliding. We mapped 920 varied types of landslides and attribute 241 to the 2018 storm, making it one of the largest events in North Carolina since 2004 with respect to landslide numbers and spatial frequency. The highest concentrations of landslide features in Polk County are along the slopes of the BRE, especially the Pacolet River and Green River valleys. These rivers exploit post-orogenic brittle fractures to form linear reentrants where the May 2018 and other landslides are concentrated. This article describes our landslide response and mapping efforts and relates our findings to the geomorphic and geologic framework and to past landslide events in the region.

At 8:07 a.m. EDT on 9 Aug. 2020 a M w 5.1 earthquake located ~3 km south of Sparta, North Carolina, USA, shook much of the eastern United States, producing the first documented surface rupture due to faulting east of the New Madrid seismic zone.The co-seismic surface rupture was identified along a 2-km-long traceable zone of predominantly reverse displacement, with folding and flexure generating a scarp averaging 8-10-cm-high with a maximum observed height of ~25 cm.Widespread deformation south of the main surface rupture includes cm-dm-long and mm-cmwide fissures.Two trenches excavated across the surface rupture reveal that this earthquake propagated to the surface along a preexisting structure in the shallow bedrock, which had not been previously identified as an active fault.Surface ruptures by faulting are rarely reported for M <6 earthquakes, and hence the Sparta earthquake provides an opportunity to improve seismic hazard knowledge associated with these moderate events.Furthermore, this earthquake occurred in a very low strain rate intraplate setting, where earthquake surface deformation, regardless of magnitude, is sparse in time and rare to observe and characterize.

How debris flows erode and deposit material along their paths is difficult to determine in natural settings due to the lack of warning and the rapid pace at which they occur. Post-event field measurements or controlled flume experiments are commonly used to evaluate debris flows between the head and the distalmost deposit. Increasingly available high-resolution lidar data provide another means to evaluate the entrainment and deposition of material during a debris flow. This study utilizes submeter lidar before and after a debris flow event in Polk County, North Carolina, to evaluate for volumetric growth and decay of 54 rainfall-triggered debris flows that occurred during a convective storm on May 18, 2018. Debris flow evolution can be characterized by three discrete phases when viewed according to a volume-distance plot: (1) the initiating debris slide, (2) the entrainment phase, and (3) the depositional phase. The rate of debris flow growth is highest during the first phase, nearly linear during the second, and negative during the third phase. When normalized by distance along the flow, the growth rate decays according to a power law $$E = aX^b$$ ( $$r^2 \ge 0.97$$ ). This new power law relationship may indicate differences in initiating landslide process and should be leveraged in future runout modeling studies.

Modern satellite networks with rapid image acquisition cycles allow for near-real-time imaging of areas impacted by natural hazards such as mass wasting, flooding, and volcanic eruptions. Publicly accessible multi-spectral datasets (e.g., Landsat, Sentinel-2) are particularly helpful in analyzing the spatial extent of disturbances, however, the datasets are large and require intensive processing on high-powered computers by trained analysts. HazMapper is an open-access hazard mapping application developed in Google Earth Engine that allows users to derive map and GIS-based products from Sentinel or Landsat datasets without the time- and cost-intensive resources required for traditional analysis. The first iteration of HazMapper relies on a vegetation-based metric, the relative difference in the normalized difference vegetation index (rdNDVI), to identify areas on the landscape where vegetation was removed following a natural disaster. Because of the vegetation-based metric, the tool is typically not suitable for use in desert or polar regions. HazMapper is not a semi-automated routine but makes rapid and repeatable analysis and visualization feasible for both recent and historical natural disasters. Case studies are included for the identification of landslides and debris flows, wildfires, pyroclastic flows, and lava flow inundation. HazMapper is intended for use by both scientists and non-scientists, such as emergency managers and public safety decision-makers.

High resolution topographic data has become widely available over the preceding decades and increasingly detailed digital elevation models are aiding in nearly every type of natural-resource related research. Digital terrain models (DTMs), which depict the ground surface topography devoid of vegetation or man-made structures, are particularly helpful in stream-related research. Historically, coarse resolution topographic data (e.g., several meters to tens of meters pixel size) did not afford evaluation of meter scale roughness elements exposed above the water surface within stream channels. The purpose of this study is to demonstrate how the integration of water-classified lidar returns in submeter resolution DTM-development may capture stream corridor topography and be useful for further stream-related research. Four reaches of streams draining the southeastern Blue Ridge Escarpment in southern North Carolina (USA) are assessed for reach positioning, length, and gradient. These parameters are chosen because they are foundational to many other forms of stream analysis (e.g., stream power, normalized channel steepness, chi, and others). Water-assigned lidar returns are included in 0.5- pixel size DTMs and compared to both a 0.5-m DTM generated without use of water returns (i.e., bare-earth) and a pre-processed, hydro-flattened 0.9-m bare-earth DTM. In steep bedrock channels, bare-earth only DTMs result in channels 12–23% shorter than water return integrated DTMs. Observations of stream positioning on DTMs that include water returns and comparisons to orthophotographs suggest a more consistent stream center line in relation to boulders and exposed bedrock within stream channels. Small streams do not benefit from the modified analysis methods because water-classified returns are not present in these channels. Nor do low gradient alluvial channels benefit because these streams tend to lack exposed bedrock or large roughness elements that might divert stream flows. Because so many geomorphic parameters are largely dependent on channel length, these findings have far-reaching implications in ongoing stream-related research. The methods presented here do not require new data collection or technology, but offer simple modifications to processing of existing data and should be considered on other high quality lidar datasets.

On August 24th 2019, approximately 100-130 mm of precipitation in a 5-hour period triggered multiple debris flows in the Nantahala Gorge, in Swain County, western North Carolina. First responders and motorists were trapped by debris flows that blocked a 1.2 km section of US 19/74 in at least six places. Multiple state and federal agencies including the NC Geological Survey (NCGS), NC Department of Transportation (NCDOT), Swain County Emergency Management, US Forest Service (USFS), and the National Weather Service collaborated on the initial response. Woody material and debris blocked the Nantahala River in three places, and the USFS removed 6,894 metric tons from the river at a reported cost of ~$255,000. NCDOT reported a cost of $500,000 for highway cleanup and repair, and the Nantahala Outdoor Center lost a reported ~$300,000 in rafting business while the river was closed over Labor Day weekend. Following the event, NCGS produced a series of progressively updated maps on August 27th and 29th, and September 10th to support responders and stakeholders. We used a combination of unmanned aerial system (UAS) imagery, NDVI (Normalized Difference Vegetation Index) imagery, and fieldwork to map the debris flow tracks. Of the 32 mapped debris flows, 23 initiated in areas of moderate or low burn severity from 2016 Ferebee and Tellico fires. To our knowledge, this is the first documented debris flow swarm that directly correlates with 2016 burned areas in the southeastern US. Questions remain in this ongoing study as to the relative effects of bedrock geology on landslide susceptibility. Based on existing geologic maps, there is a geomorphic asymmetry in the gorge likely controlled by Paleozoic bedrock structures. Bedding and foliation dip to the SE, and the anti-dip slope (SE side) is steeper than the dip slope (NW side). Despite this bedrock fabric, the 2019 debris flows occurred on the SE side. Future work is needed to: 1) clarify geomorphic and structural contrasts on either side of gorge; 2) monitor areas in the gorge that burned but did not fail during 2019 debris flow event; 3) test whether fire effects only influence shallow-seated landslides initiating in soil and not deep-seated landslides that start in bedrock; and 4) look for undocumented landslides on the NW side of the gorge where bedding and foliation dip downslope.

Since 1990, the North Carolina Geological Survey (NCGS) has mapped over 3,900 landslides. The dominant type of landslides are debris flows triggered by high-precipitation events such as tropical cyclones or convective storm cells. Since mapping efforts began nearly 30 years ago, our technology and techniques have evolved to include the use of field computers, GPS, and digital mapping tools such as high-resolution digital elevation models (DEMs), public-domain satellite data, unmanned aerial systems (UAS), and a continued focus on field-based verification. The NCGS landslide geodatabase is both an event- and historical-inventory, cataloging landslides of known vintages (e.g., from a particular storm event) and those that have persisted in the landscape with unknown origins. Collected in 2017, quality-level 1 (QL1) lidar point clouds (nominal 8 points/m2) are publicly available for all of western North Carolina. We reduce these data to 0.5-meter resolution bare-earth DEMs, providing a marked increase in terrain analysis efficiency and completeness compared to previously available data. Because landslides occurring since the QL1 data collection are not represented in these DEMs, we use public domain, rapid repeat-cycle, multispectral satellite imagery to observe losses in hillslope vegetation, a potential indicator of mass-wasting activity. In particular, the use of ESA Sentinel-2 data targets decreases in vegetative cover following large landslide events. During routine and emergency landslide responses, we utilize UAS to increase field efficiency and aid interpretation. A key component of the UAS technology is the generation of orthomosaic images to photo-document landslides for use in a GIS. The foundation of this effort, however, is the continued, iterative process of field-verification. More than half of our 2019-2020 landslide inventory entries have been field-verified. Given this level of scrutiny, we have increased the resolution of our mapping to >1:2,000, which would have been unreasonable at the county-wide scale a decade ago. In addition to traditional static maps, NCGS inventories are available as interactive web-map viewers, increasing the accessibility of the data to the public – a primary client in mapping and communicating landslide hazards in western North Carolina.

The Blue Ridge Escarpment is a NNE-trending steep landform that separates the high topography of the western North Carolina Blue Ridge physiographic province from the lower elevation Piedmont to the east. It intersects numerous E-W, NE-SW, and SSE-WNW lineaments along its length, which form reentrants with steep valley walls. During our recent work mapping slope movements in Polk County, we focused on two of these trench lineaments, the E-W-trending Pacolet River Valley and the NE-trending Green River Gorge, where abundant near-vertical fractures in the bedrock, high topographic relief (up to 700 m), and large drainage areas above the escarpment provide favorable conditions for a variety of slope movements to initiate. Many debris flows, debris slides, rockfalls, and rock slides initiate here along high-angle joints that strike parallel to the lineaments. Debris flows often travel through rectilinear drainages that are parallel and perpendicular to these E-W- and NE-SW striking joints, or move parallel to the strike of a pervasive topographic fabric formed by metamorphic foliation that dips gently to the NNE. This foliation, which preferentially crops out on south-facing slopes, may explain the topographic asymmetry of the Pacolet River Valley and why the steeper north side of the valley is eroding by debris flows, while the less steep south side is mantled by composite debris slide, rockfall, and rockslide deposits. Some of the linear features that cross cut the escarpment are driven by fractures but others are formed from metamorphic layers and the associated differential erosional in varying lithology. The orientations of outcrop-scale fractures and foliation match lineament fabrics that are visible in high-resolution lidar and the major topographic reentrants that intersect the escarpment, where it is likely that erosion is enhanced by intersecting fracture sets and minor post-orogenic faults. Similar features found to the north such as Hickory Nut Gorge, the Swannanoa lineament, the Laurel Creek lineament, and the Boone lineament all contain orogen-crossing fractures and faults leading to increased instability and greater abundance of slope movements.

Much of the topography in the southern Appalachians is controlled by post-orogenic WNW-ESE-, E-W-, and N-S-striking fracture and fault zones. To illustrate the influence of these zones on the evolution of the Blue Ridge Escarpment and elsewhere in western North Carolina, we analyzed field data and lidar-derived DEMs from Graham, Polk, Swain, and Watauga counties, which span different rock types of the Blue Ridge and Piedmont geologic provinces and are crossed by topographic lineaments. There are fracture-controlled stream networks with steeply dipping orthogonal fractures that can be traced parallel to streams for hundreds of meters along slot canyons of bedrock being incised as knickpoints migrate headward when debris flows initiate along fractures. In channels where metamorphic foliation or original sedimentary bedding dips upstream and into the hillslope but intersects high-angle fractures there are well-pronounced knickpoints formed by blocky failure of the bedrock. We present numerous examples where knickpoints correspond with initiation locations of modern debris flows that carried rectangular boulders ranging from <1m to 10m in width hundreds of meters downstream. Conversely, knickzones (e.g., low angle, cascade-type waterfalls) are present where foliation or bedding dips downstream and away from the hillslope. There are some knickpoints at the same elevation as bands of fracture-controlled outcrops in the adjacent hillslopes that separate steep topography from flatter landscapes at higher elevations, suggesting that fractures influence stream and hillslope erosional processes. Intersecting vertical joints lead to large blocks of rock toppling into in the surrounding hillslopes. Fractures affect the landscape at multiple levels, ranging from outcrop-scale influences on first- and second-order stream flow directions within rectilinear drainages, to how the regional-scale Blue Ridge Escarpment is migrating more rapidly where it intersects orogen-crossing topographic lineaments controlled by minor faults and fractures.