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Detection of tundra trail damage near Barrow, Alaska using remote imagery
GEOMOR-05760; No of Pages 8 Geomorphology xxx (2016) xxx–xxx
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Detection of tundra trail damage near Barrow, Alaska using remote imagery K.M. Hinkel, W.R. Eisner ⁎, C.J. Kim Department of Geography, University of Cincinnati, Cincinnati, OH 45221-0131, United States
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Article history: Received 29 January 2015 Received in revised form 27 August 2016 Accepted 6 September 2016 Available online xxxx Keywords: Tundra damage Thermokarst Off-road traffic Trails Arctic Alaska Indigenous communities
a b s t r a c t In the past several decades, the use of all-terrain vehicles (ATVs) has proliferated in many Arctic communities in North America. One example is the village of Barrow, Alaska. This coastal community has only local roads, so all access to the interior utilizes off-road machines. These 4-wheel vehicles are the primary means of tundra traverse and transport in summer by hunters and berry-pickers, and by village residents accessing summer camps. Traveling cross-country is difficult due to the large number of thermokarst lakes, wetlands, and streams, and tundra trails tend to follow dryer higher ground while avoiding areas of high microrelief such as high-centered icewedge polygons. Thus, modern ATV trails tend to follow the margins of drained or partially drained thermokarst lake basins where it is flat and relatively dry, and these trails are heavily used. The deeply-ribbed tires of the heavy and powerful ATVs cause damage by destroying the vegetation and disturbing the underlying organic soil. Exposure of the dark soil enhances summer thaw and leads to local thermokarst of the ice-rich upper permafrost. The damage increases over time as vehicles continue to follow the same track, and sections eventually become unusable; this is especially true where the trail crosses ice-wedge troughs. Deep subsidence in the ponded troughs results in ATV users veering to avoid the wettest area, which leads to a widening of the damaged area. Helicopter surveys, site visits, and collection of ground penetrating radar data were combined with time series analysis of high-resolution aerial and satellite imagery for the period 1955–2014. The analysis reveals that there are 507 km of off-road trails on the Barrow Peninsula. About 50% of the total trail length was developed before 1955 in association with resource extraction, and an additional 40% were formed between 1979 and 2005 by ATVs. Segments of the more modern trail are up to 100 m wide. Damage to the tundra is especially pronounced in wet areas, such as ice-wedge troughs. Knowledgeable indigenous people are aware of the problem. Some remediation has been attempted by using heavy-duty PVC matting in areas of greatest damage, but this approach is prohibitively expensive on a large scale. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Arctic tundra vegetation is a fragile mat that protects and insulates the underlying permanently frozen soil. Tundra vegetation is very slow growing and easily damaged, which makes it highly vulnerable to long-term disturbance. Damage to the vegetative cover can result in thawing of the underlying ice-rich permafrost and consequent ground subsidence, a degradational process known as thermokarst. Researchers and members of tundra communities have long been aware of the damage done to tundra vegetation by vehicles (Hambleton and Drescher, 2008). Research has focused mainly on the movement of vehicles involved in industrial and government oil exploration (Jorgenson et al., 2010; Nelson et al., 2003; Brown, 1997; Slaughter et al., 1990; Lawson, 1986). A 2008 study (Eisner et al., 2008) identified the travel routes of both commercial and private overland vehicles in the Barrow region and the resulting surface scaring and thermokarst. ⁎ Corresponding author. E-mail address: [email protected] (W.R. Eisner).
In recognition of the damage caused to permafrost by the activities of seismic exploratory vehicles, the U.S. Fish and Wildlife Service introduced regulations and requires permits for travel on state land by offroad vehicles. Most commercial traffic has been restricted to winter travel and to specified routes since the early 1970s (Jorgenson et al., 2010; Walker et al., 1987). Snow depth and soil temperature along these routes are monitored. Summer travel on State land is also restricted by land use permits and is limited to approved vehicles (Alaska DNR, 2012). The present study focuses on a relatively new source of tundra disturbance: privately owned all-terrain vehicles (ATVs) not regulated by state or federal agencies. Although smaller and lighter than commercial vehicles, they are ubiquitous in arctic and sub-arctic communities. During the snow-free summer (May–September), ATVs replace the snow machine as the primary means of tundra traverse and transport. Hunters and berry-pickers traveling across the open tundra, and village residents accessing summer camps, use these 4-wheel drive vehicles. The deeply-ribbed tires of the ATVs cause damage by disrupting the vegetation, compacting and destroying the underlying organic mat,
http://dx.doi.org/10.1016/j.geomorph.2016.09.013 0169-555X/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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and by disturbing the underlying mineral soil. Long-term or permanent impacts depend on the specific environmental conditions that include vegetation type, the amount of excess ice in the upper permafrost, and soil texture. Impact severity is also influenced by traffic intensity, vehicle weight and tire type, and aggressiveness of the vehicle operator (Hambleton and Drescher, 2008). Our study focuses on the Barrow Peninsula, which is not protected state land. Through the use of time series of high-resolution aerial imagery coupled with on-the-ground observations and field measurements, we have developed a time series of tundra trail damage that spans 50 years of landscape use. We are therefore able to differentiate variations between the unregulated damage caused before 1955 by industrial exploration, and by later ATV damage induced by heavy traffic from residents of the North Slope Borough. Our 2008 study showed that overland travel routes around the village of Barrow generally followed trails similar to those from 50 years ago (Tremont, 1987), but that the use of ATVs has enabled travelers to detour inland and has caused extensive impacts on the landscape (Eisner et al., 2008). We also identified a trend toward increasing use of snow machines on patchy snow or even bare tundra during the spring, which exacerbates deteriorating trail conditions. Coupled with this is an increased lack of geographical knowledge on the part of the ATV operators, and more incidents of disorientation while out hunting. Previous studies identified significant variations in the impact of vehicles on the tundra based on vegetation type, soils, and ice content. Lawson (1986), who studied the effects of vehicle traffic, camp construction, and drilling activities, found that there was a significant difference in the impacts of disturbance in areas with ice-rich soils. Slaughter et al. (1990) showed that vegetation type, topography, season of travel, vehicle weight and type, and the amount of traffic all determined the extent and duration of the damage. Jorgenson et al. (2010) monitored post-1980 winter seismic trails in Northern Alaska over an eighteenyear period and found that, even with a snow and ice cover, such traffic causes damage in these regulated areas. Plant community changes were species-specific, and recovery time depended on the severity of the initial disturbance, ice-content of the upper permafrost, and on the initial plant communities. The objectives of this study are to (a) assess the development of the informal off-road trail system over time using time series of available high-resolution imagery, (2) evaluate the geomorphic setting and geographic patterns of modern ATV trails, and (3) describe some preliminary field studies and remedial efforts to mitigate ATV damage to the tundra. 2. Study area The study area includes the Barrow Peninsula, a triangular shaped region of approximately 2250 km2 that extends from the outlet of the Inaru River at 70.8° N to the northernmost point at Point Barrow at 71.3° N. (see Fig. 1). This flat, low-relief terrain is underlain by permafrost to a depth of about 400 m, and is characterized by low-tundra vegetation, oriented thermokarst lakes, and ice-wedge polygons. Surface thaw begins in May or June, and the active layer is usually 30–60 cm thick by late August. Near-surface sediments are largely ice-rich marine silts and sand above Cretaceous sedimentary rock, although remnant barrier islands of coarser material can be observed as slighter higher beach ridges that arc from northwest to southeast across the upper part of the study area map. The land surface has higher elevations to the west and dips gently eastward. Prominent cliffs are found along the Chukchi Sea that are typically 10–15 m high, and a narrow sand or gravel beach extends discontinuously along the western coast. On the east, the land surface is only a few meters above Elson Lagoon, and there is no beach. The city of Barrow, near the northern point of the study area, is a village with a 2010 population of 4212 people. Only local roads service this remote, coastal community and all access to the interior necessitates the
use of off-road machines. The North Slope Borough, of which Barrow is the administrative seat, does not require licenses or any type of permit for the use of privately owned ATVs or snow machines on the tundra (personal communication, Lt. Phillip Brymer, Village Operations). Villagers commonly practice fishing and subsistence hunting for caribou, geese, and ducks, and there are many hunting camps scattered across the peninsula. Most camps and cabins are found along streams, and fishing with nets is an effective procurement method. In the past, hunting camps were accessed in summer by walking across the tundra along ancient trails (Eisner et al., 2008; Tremont, 1987) or by taking boats from Barrow eastward along the coast into the shallow Admiralty Bay, and then upriver–a mode still practiced. Most villagers, however, travel to their summer camps using ATVs. The routes taken by ATV drivers are not regulated, and certain routes are preferred and heavily used based on convenience and intersection with older traditional trade routes (Eisner et al., 2008). Traveling across the tundra on an ATV is difficult owing to the many water bodies and high microrelief associated with high-centered icewedge polygons. To reach hunting grounds or camps on the western side of the peninsula, the easiest route is to travel southwest along the narrow beach. However, large inlets, lagoons and bays block the route. At Walakpa Bay, some 18 km from Barrow, travelers are forced inland and far to the east to circumvent the dissected terrain and steep banks of the bay and associated inlets and ravines. There are relatively few camps on the eastern side of the peninsula, and most tend to be found on the shores of thermokarst lakes. The majority of camps are found along the Inaru and Meade Rivers, about 50 km south of Barrow. The upper Inaru is too deep to easily cross, but regions south of the river can be accessed by going around, which entails a long and difficult diversion to the west. Most villagers restrict their summer activities to the region north of the Inaru River, so the latitude of the Inaru River outlet defines the southern limit of the study area. 3. Methodology Surveys of tundra trails have been conducted for the past several years by site visits in summer. Trails near Barrow were accessed using ATVs, while those further afield were visited by helicopter. Surveys included measuring the ground thaw depth, width of the trail, and observing the damage done to the vegetation. Winter work entailed conducting ground-penetrating surveys across several trails. This study reports on the analysis of a time series of high-resolution satellite imagery and aerial photographs for the period 1955–2014. As an initial step, trails on the Barrow Peninsula were digitized from high-resolution Google Earth imagery, and 111 trail and trail segments were easily identified as linear features on the flat, treeless tundra. A single passage from an ATV does not leave a trail unless it was recently made in a very wet setting, such as a shallow pond with vegetation. Thus, the trails identified on the imagery are visible because they have been subject to repeated passage of vehicles over the years, and a scar has been left on the tundra that has not yet healed. The 111 trail segments were saved as a kmz file, and imported into ArcGIS for analysis. One objective was to estimate when the trails were formed. To do this, aerial and satellite imagery time series were collected from various sources. These include black-and-white aerial photographs, colorinfrared DOQs, Quickbird, and Worldview images for the period 1955– 2014. The National Snow & Ice Data Center (NSIDC) data sets include aerial photography of the study area which have been geocorrected to Quickbird satellite imagery or Interferometric Synthetic Aperture Radar (IFSAR) imagery. These images are from specific dates in summer (July through August) from 1948 to 1997. Data are in GeoTIFF and ESRI Shapefile formats with FGDC compliant metadata (see http://nsidc.org/ api/metadata?id=arcss306 for more details). The temporal coverage of the NSIDC datasets include 1948, 1949, 1955, 1962, 1964, 1979, 1984, and 1997. Among these years, trails are visible in 1955, 1962, and 1979, as shown in Table 1.
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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Fig. 1. Map of the Barrow Peninsula showing off-road trails and location of hunting and fishing camps and cabins.
High-resolution imagery in National Imagery Transmission Format (NITF) was also obtained from the Polar Geospatial Center (PGC). Quickbird, Worldview, Ikonos, and Geoeye images are georeferenced to the WGS84 datum. Quickbird images (2.4 m resolution) in 2004 and color-infrared (CIR) Digital Orthophoto Quadrangles (DOQs) with a 2.5 m resolution in 2005 are available for the study area. Trails in 2006, 2009, and 2010 were identified using Google Earth (2.4 meter original resolution). Table 1 details the imagery source and horizontal resolution which, with a typical range of 1.4–2.5 m, is adequate to detect a linear feature such as a trail. Each set of imagery was orthorectified, mosaicked, and projected within the GIS as a layer to Universal Transverse Mercator (UTM) zone 5 with the 1983 North American Datum (NAD83).
Table 1 Imagery sources used in this study by year of imagery acquisition, with horizontal resolution indicated. Total number (N) and total trail length (km) of trails first observed in that year, with percentage of total trail length (507 km) in 2014. Year
Source
Resolution (m) N
Trail length (km) % total length
1955 1962 1979 2004 2005 2006 2009 2010 na
b&w aerial photo b&w aerial photo CIR aerial photo HR satellite DOQ aerial HR satellite HR satellite HR satellite na
1.35 1.35 4.6 2.4 2.5 2.4 2.4 2.4 na Total
252.4 2.7 39.1 106.7 95.6 3.7 0.9 0.6 5.0 506.7
47 1 11 14 31 1 2 1 3 111
49.8 0.5 7.7 21.1 18.9 0.7 0.2 0.1 1.0 100.0
By overlaying the current tundra trail network on the imagery, it is possible to determine which trails were in existence at the time of image acquisition. If apparent in a scene for a particular year, it means that the trail was developed prior to that time; if not, the trail was developed subsequent to the image acquisition date. In this way, we could assess when the trails first became visible on the landscape, and determine if the rate of trail network development has recently accelerated. To evaluate the impact of tundra trails on permafrost stability, measurements of maximum summer thaw depth were made at three sites in mid-August 2010. This entailed establishing a transect perpendicular to the trail, which are typically 5–10 m wide. Transects extended 10– 15 m beyond the impacted area on both sides of the trail, so the overall length was 25–40 m. Using a graduated 1-cm diameter steel probe, the thaw depth was measured every 1.0 m along the transect. Further, in April 2011, a ground penetrating radar (GPR) survey was conducted along one transect. 4. Results Analysis of the 111 trails indicates that they total 507 km in length (Table 1), yielding an average trail density over the study area of roughly 0.23 km of trail per square km. However, it is clear from Fig. 1 that trail density is not spatially uniform; there are large areas with no detectable trails, especially in the eastern part of the Barrow Peninsula and to the southwest. Trails to the east are sparse, and were primarily developed before 1955. Newer trails are extensions of the older trail system, and terminate at camps and cabins near large lakes; none go to the coast. Trails on the western side of the peninsula show the same pattern of
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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an early (pre-1955) trail that parallels the Beaufort Sea coastline. This is a gas pipeline service trail that continues on to the village of Wainwright some 150 km further down the coast. The rest of the western region is largely devoid of camps, cabins and trails except for those homesteads along the coast and the upper reaches of the Inaru River. Analysis of the available image time series demonstrates that about half of the total number of visible trails (47) and total trail length (252 km, or 50%) were developed before 1955. These are concentrated around, and have their origin in, the village of Barrow. These trails tend to follow straight lines across the tundra for long distances, before veering to follow a new direction. Several are adjacent to the aboveground gas pipelines that deliver natural gas to the village from the gas fields, and many other trails are easily visible today since sections are long, narrow ponds. These were developed during the natural gas exploration and development period that took place during and after World War II. Trails were made by tracked and wheeled vehicles that caused extensive local thermokarst, as shown in Fig. 2. Trail development between this period and 1979 appears very limited, with only an additional 42 km of trail (7.7%) added over 25 years and most trail development occurring just south of Barrow. By 2004, however, high resolution satellite imagery shows that an additional 107 km of trails was added to the network, or about 21% of the total trail length. These trails are concentrated in the center of the peninsula and extend from Barrow south to the Inaru River. Running north and south, they tend to follow the margins of large lakes. These are “superhighways” that allow villagers to get to southern destinations as quickly and
Fig. 2. Trails made before 1955 using tracked vehicles or dozers; (a) trail with significant linear thermokarst ponds, (b) dozed trail with berms on the right, and modern superhighway on the left of photo.
efficiently as possible. The following year, 2005, records further trail expansion with an additional 96 km (19%) added. Most of these trails appear to link or extend components of previous trails, but some enter into areas previously free of trail such as those in the eastern portion of the peninsula. One segment was established to circumvent a stream crossing, but this issue was subsequently addressed by deploying a portable bridge (location in Fig. 1). Since 2005, further trail development has been limited, with only 5 km (1%) of trails added. These tend to be concentrated near Barrow, and are shortcuts that avoid areas that have been damaged by excessive trail use. 5. Discussion Trails made before 1955 were primarily for the purpose of gas field development and maintenance. These trails are easily visible today due to the vegetation disturbance. In many places, there has been significant local thermokarst that resulted from the destruction of the organic mat with consequent changes to the surface albedo and surface insulation provided by the organic layer. The near-surface sediments over much of the study area are especially sensitive to thaw subsidence since these marine silts are oversaturated with respect to ice; in addition to ice wedges, the upper 10 m is estimated to exceed 80% ice by volume (Sellmann et al., 1975; Brown, 1968). Thaw of the upper permafrost and subsequent ground subsidence beneath trails often results in linear surface ponds, as shown in Fig. 2a. In some cases, it appears that the trail was plowed with a bulldozer, as evidenced by the remnants of soil that form berms on either side of remnant trail (Fig. 2b). To a large degree, these trails were developed without consideration of landscape elements, except that they tend to go around lakes. They cross marshes, ridges, streams and drained lake basins without deviating from their course since these features do not impede the progress of tracked vehicles. Those trail sections demonstrating significant thermokarst are not used for travel today, but other sections of the old trail system are still in use. These tend to be areas of higher and dryer terrain. Near Barrow, the old trail system provides the three most common routes of egress from the village to the interior, primarily because they are an extension of the existing road system in Barrow that allow travelers to take a groomed surface as far as possible onto the tundra. The first is near Fresh Water Lake (Imaiqsaun Lake, “A” on Fig. 1), which is the route hunters take if they intend to access the western part of the peninsula. A second route (“B”) extends south from an existing well head and pumping station several km south of Barrow (Fig. 3b), and is used by those accessing the central part of the peninsula or to reach distant sites on the Inaru River. The western region is accessed by traveling on a groomed road (Cakeater Road, “C”) eastward for 5 km, and using the road as a jumping-off point. There appears to be very little new trail development over the next 25 years (1955–1979), and that which did occur was near Barrow. During this period, snow machines became the primary means of travel across the tundra in winter as machines became more reliable, and oil revenues provided dividend funds to residents that made large purchases possible. ATVs subsequently became more common, and there was significant trail expansion after 1979 reflecting increasing access to summer hunting grounds and cabins. The main goal was to reach the destination quickly and safely. Travel by ATV in summer is constrained by landscape elements including lakes, marshes or drained lake basins containing shallow water, streams, and high-centered polygon terrain. Lakes occupy about 22% of the Barrow peninsula (Hinkel et al., 2003), and must be circumvented. Drained lake basins cover around 50% of the landscape, and most are filled with shallow water that must be avoided by ATVs. Numerous streams cross the tundra. Although narrow in some places, streams often occupy ice-wedge troughs that have progressively deepened from thermal ablation of the underlying wedge ice. Thus, streams
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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Fig. 3. (a) Narrow beach along the Chukchi Sea near Barrow with ATV tracks, and (b) one of the three egress trails from Barrow, labeled “B” on Fig. 1.
are narrow but deep, and safe fording places are not common (Fig. 4a). Finally, regions of high-centered polygons are avoided because they must be traversed slowly and carefully. The relief between the troughs and polygons is typically 1–2 m, with polygons 2–10 m in diameter (Fig. 4b). Traveling in the troughs is not feasible owing to the wet and soft conditions, and the steep-sided troughs makes it difficult and dangerous to ascend the polygon rims for fear of tipping sideways or backward. For these reasons, most new ATV trails follow dry, flat, smooth surfaces; the dominant geomorphic setting is the margins of recently drained lake basins. Here, complete or partial draining has exposed the former lake shelve to stabilization by vegetation and soil freezing. Since the basins drained relatively recently, near-surface ice enrichment and ice-wedge polygon development has been limited, especially in sand-rich sediments. Ice-wedge networks and trough development is usually apparent but at an early stage, so ice-wedge troughs are but minor depressions on the landscape and there are no polygon rims. These lake margins are often found between the modern lake or marshy basin, and the older high-centered polygon terrain further away from the lake (Fig. 4c). They represent the path of least resistance for ATV travelers who wish to quickly and safely traverse the area. They track nearly north–south, preferentially following the shores of large partially-drained oriented lakes, and are concentrated near the center of the peninsula. 5.1. Modern ‘superhighways” Modern tundra trails traveled by ATVs are heavily utilized, especially in the northern reaches near Barrow. Trails generally following the
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smooth lake margins for long stretches, with east-west connectors at the northern and southern end of large lakes or basins that often traverse rough terrain. The primary obstacles are streams, water tracks, and ice-wedge troughs. These require that the traveler slow down to a near stop. Crossing an ice-wedge trough can be hazardous since the heavy machine sinks into the thawed sediment in the trough, which often contains standing water. To get out of the trough requires that the operator accelerate, and the deeply-ribbed tires can quickly destroy the protective organic mat. Over a summer, the trail becomes soft within the trough and near the trough margin, requiring that travelers shift their route to either side of the path. This effectively widens the path at the trough, as shown in Fig. 4d, to create a “braided” trail. Abandonment of a trough crossing does promote recovery of the tundra. Once the organic mat is disturbed or destroyed, the surface albedo is reduced and heat transfer to depth is enhanced. As permafrost thaws and ice is converted to water, the surface subsides. The depression becomes filled with water from precipitation, surface flow, and from lateral flow of soil water above the impermeable permafrost. The thermal conductivity of the saturated soil increases, which promotes further heat transfer to depth and greater thaw subsidence in a positive feedback. Permafrost degradation will continue for years since revegetation is a slow process (Williams et al., 2013; Jorgenson et al., 2010). Over time, as the primary path shifts to accommodate ground destabilization concentrated at ice-wedge troughs and water tracks, the severely impacted zone widens to create a barrier. A similar trail widening process also occurs on the open tundra between ice-wedge troughs. Heavy traffic creates shallow linear depressions on the surface which hold water and, in time, the trampled ground is wet and soft. ATV travelers then shift their path, effectively widening the trail until it becomes a broadly traveled corridor, or “superhighway.” These major ATV transportation trails are easily visible on satellite images since they are 30–100 m wide. Because travel is dispersed over a broad region, visible thermokarst is largely limited to icewedge troughs and water tracks. They account for about one-quarter of the entire length of the modern tundra trail network. Regular trail users, such as indigenous hunters and researchers, claim that major trails have become noticeably wider over the past decade, with discernable impacts on the tundra surface and vegetation (Fig. 2b). 5.2. Field studies of local thermokarst To quantify the impact of superhighways on permafrost stability, measurements of maximum thaw depth were made at three sites near the end of the thaw season in mid-August 2010. The thaw depth in the impacted area was not significantly different from the thaw depth in the undisturbed tundra, and averaged 35 cm. However, as can be seen in Fig. 5a & b, the trail forms a depression owing to ground subsidence following degradation of the upper permafrost, and this displacement is not captured by thaw depth measurements since the ground surface is the datum. This problem is compounded by compression of the organic layer by the heavy ATVs. In and near ice-wedge troughs, it is not uncommon for the fibrous organic mat to be completely destroyed by the ATV tires, leaving an organic slurry residue that is difficult to measure with standard assessment methods. A GPR survey was conducted along one transect the following winter (April 2011) to assess the depth to the permafrost table beneath the trail, and to determine whether the top of the ice-rich upper permafrost has been depressed with respect to the surrounding undisturbed area (J.A. Doolittle, report on Ground-penetrating radar (GPR) study of icewedge networks in Barrow, 9 August 2011). Fig. 5c shows the radar record, with all scales in meters. The depth scale is estimated using a constant propagation velocity of 0.13 m/ns (Hinkel et al., 2001), and has been vertically exaggerated for the purpose of display. The highamplitude, dark-colored, horizontal, planar reflector near the top of the record represents the snow-soil interface. The interpreted depth to this interface ranged from about 0.1 cm in the undisturbed tundra to
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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Fig. 4. (a) Convergence of trails at stream fording place showing evidence of heavy traffic; (b) trail along a drained lake margin at the base of high-centered polygon terrain; (c) as in (b) but with ATVs visible; and (d) heavily traveled braided trail crossing ice-wedge troughs.
81 cm in the trail depression, and averaged 40 cm. In the near subsurface beneath the trail, there appear to be altered patterns or additional radar reflections that are found 10–20 cm deeper than the undisturbed tundra, suggesting a depressed permafrost table.
However, there is too much ambiguity in this interpretation process, and faster and more reliable methods are required if thermokarst effects beneath trails are to be assessed over a large region. High-tech alternatives include acquiring a very high resolution DEM with accuracy and
Fig. 5. Heavily traveled trail near “A” on Fig. 1 showing (a) ground depression and ponding along abandoned older trail, with subsequent development of new trail. The two stakes in (b) mark the end points for the winter GPR survey shown in (c). All units are in m.
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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precision appropriate to the scale of the disturbance (Kääb, 2008), or use of a terrestrial LiDAR scanner at select locations to measure absolute surface displacement over time (e.g. Barnhart and Crosby, 2013; Bodin et al., 2008). Low-tech solutions entail installing frost–defended stakes at both ends of a short transect across the trail. A cable is tightly stretched between the stake tops to create a relative datum from which the distance to the ground surface and frost table can be measured along the transect cable by repeated surveys. This method is currently being applied in the field at sites on the Barrow Peninsula because it is cost-effective, easy to implement, and yields reliable results. Of course, the cable must be removed between field sampling periods. 5.3. Remedial efforts Interviews with indigenous people demonstrate that they are well aware of the damage being done to the tundra, and have taken some preventative and remedial steps to mitigate against further damage (Eisner et al., 2008). A bridge was constructed and deployed across a major stream crossing located stream some 15 km south of Barrow along a primary trail. Built and deployed by Roy Nageak Sr., around 1995, the intent was to limit damage to the stream bank from fording and deepening of the stream bed from heavy ATV traffic (Figs. 1 & 6a). Mr. Nageak subsequently expressed regret for installing the bridge since it focuses traffic on the bridge approaches and concentrates damage on the nearby tundra. A Google Earth image from 2014 shows that the bridge is still at the same location, where it remains a convergence point for all nearby trails. Other remedial efforts involve deploying high-density polyethylene ground stabilizing mesh mats as a protective cover (Schneider, 2011), as shown in Fig. 6b. These molded sections are linked together to create a “hardened trail.” The open cell design permits water and solar radiation to penetrate the mesh to the ground surface, thus allowing grass growth or regeneration while protecting the roots and organic mat. Short test sections have been deployed by the North Slope Borough near Barrow (http://catalog.northslope.org/catalogs/3009-hardened-trail). They have also been used in state and national parks and preserves (Allen et al., 2000), where trails can cause unintended damage to waterways that impact hydrologic processes at the watershed scale (Arp and Simmons, 2012). The use of these and other geotextiles is not a panacea. As noted by the Alaska Department of Natural Resources (2008), trail hardening is expensive; $25,000 to $100,000 per linear mile for a standard ATV trail. Furthermore, when deployed across disturbed ice-wedges, the mats often buckle or separate as the trough continues to subside and deepen over time. Once the sides of the trough become too steep to safely cross for fear of flipping the machine, travelers leave the hardened trail in search of another crossing.
Fig. 6. (a) Stream crossing at bridge deployed around 1995 (location in Fig. 1), with heavy trail damage at approaches, and (b) road hardening with geotextile mats installed along a section of trail at point “A” near Barrow.
6. Conclusions 5.4. Methodological limitations There are several caveats associated with the methodology used in this study. First, there are significant temporal gaps in the imagery coverage, so trail detection indicates only that it was formed prior to the image acquisition year. Although few trails appear to have been added over the 24-year period 1955–1979 (8%), trail expansion during the 25-year period 1979–2004 (21%) was significant. Given that a similar level of expansion also occurred in 2005 (19%), we suspect that the trail network expanded most rapidly after 2000. A second caveat is introduced by cloud cover, which can obscure a portion of the study area in any given year. Third, the horizontal resolution of the imagery varies, and it is sometimes difficult to detect narrow, poorly defined trails, especially if there is some cloud cover. Finally, the GPR transect is time consuming and yields ambiguous interpretations. New methodologies must be developed to assess the impact of concentrated tundra disturbance on local ground subsidence and thermokarst over a large region.
Time series analysis of high-resolution aerial and satellite imagery over the period 1955–2014 reveals that there are 507 km of off-road trails on the Barrow Peninsula. About half of the total trail length was developed before 1955 in association with resource extraction, and an additional 40% were formed between 1979 and 2005 by ATVs with some of the modern trails (superhighways) up to 100 m in width. Damage to the tundra is especially pronounced in wet areas, such as icewedge troughs. Repeated use of tundra trails using heavy ATVs with deep-tread tires destroys the protective vegetative mat. Degradation of the underlying permafrost follows, with consequent ground subsidence and ponding. Temporal gaps in the imagery prevents further refinement in determining when trail network expansion occurred. Furthermore, since registration of ATV s and snow machines is not required, it is not possible to correlate the period of trail expansion to the proliferation of ATVs on the North Slope. However, distribution of oil revenues to residents since the 1970s appears to have contributed to increased purchase and use of
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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ATVs in native communities since that time, and observations by both indigenous people and the authors support this view. In the presence of a slight topographic gradient, low-centered icewedge polygons can evolve into high-center polygon terrain. Preferential pooling of water in troughs will cause deepening as the ice wedges ablate from above, and trough pools eventually coalesce. As the surface hydrology integrates over time and a stream network develops, the pools drain and the high-centered polygons are left as elevated remnants. Human activities can accelerate this process, as observed along the tundra trails. Local trough deepening is hastened by physical destruction of the vegetation and organic mat, and the consequent impact to the surface energy balance and subsurface soil thermal properties. Trail braiding expands the affected area so that noticeable trough deepening occurs in several decades. This is a unidirectional process since the tundra does not heal from this level of disturbance. In time, deep troughs may create an impassable barrier to ATV travel along lake margins that are constricted by the lake on one side and older, highcentered polygon terrain on the other. Lake margins are the optimal surface for rapid ATV travel. These sites have already been developed, and alternative routes are far less favorable for ATV travel. The best approach seems to be preserving existing routes, but taking steps to minimize further damage and promote restoration. One mitigating or preventative approach might be to deploy hardened trails in especially sensitive trail sections. For example, one strategy would be to use protective mats at several undisturbed locations across a single ice-wedge trough to prevent damage before it occurs, and to encourage traffic diffusion across a wider area. Similar trail development has occurred in other villages on the North Slope of Alaska. In Atqasuk, a much smaller community of about 250 people 100 km south of Barrow, a network of newer ATV trails is apparent on recent high-resolution imagery. Local hunters who were interviewed (Eisner et al., 2008) contend that their greater isolation makes them more reliant on subsistence hunting and fishing compared to residents of Barrow, and therefore a higher proportion of villagers use ATVs. The village of Anaktuvuk Pass, on the northern edge of the Brooks Range, has had a more contentious history of limiting ATV use. The village boundaries lie within the Gates of the Arctic National Park. Armed with studies documenting the damage done by ATVs and ATGOs (Racine and Johnson, 1988), the 6-wheeled vehicles with large balloon tires that are the vehicle of choice in this upland terrain, the National Park Service attempted to restrict ATV traffic during the 1980s. The resulting dispute lasted over 10 years and necessitated Congressional redistricting of designated Wilderness areas, Federal lands and tribal lands in order to allow community right-of-way (Norris, 2002). It is clear that best practice is to mitigate tundra damage since it takes an Act of Congress to repudiate the right to use ATVs for subsistence hunting.
Acknowledgements This work was supported by NSF AON grant number AON-1107607. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. We are grateful to the Inupiat Corporation and CPS for administrative and logistic assistance. We thank Jim Doolittle of the USDA for the GPR analysis.
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Brown, J., 1997. Disturbance and recovery of permafrost terrain. In: Crawford, R.M. (Ed.), Disturbance and Recovery in Arctic Lands: An Ecological Perspective. Kluwer Academic, Dordrecht, pp. 167–178. Eisner, W.R., Hinkel, K.M., Jones, B.M., Cuomo, C.J., 2008. Using indigenous knowledge to assess environmental impacts of overland travel routes, Arctic Coastal Plain of Alaska. In: Kane, D.L., Hinkel, K.M. (Eds.), Ninth International Conference on Permafrost, Institute of Northern Engineering, University of Alaska, Fairbanks, AK, pp. 415–420. Hambleton, J., Drescher, A., 2008. Soil damage models for off-road vehicles. GeoCongress 2008: Geosustainability and Geohazard Mitigation, pp. 562–569 http://dx.doi.org/10. 1061/40971(310)70. Hinkel, K.M., Doolittle, J.A., Bockheim, J.G., Nelson, F.E., Paetzold, R., Kimble, J.M., Travis, R., 2001. Detection of subsurface permafrost features with ground-penetrating radar, Barrow, Alaska. Permafr. Periglac. Process. 12 (2), 179–190. 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Detection of tundra trail damage near Barrow, Alaska using remote imagery K.M. Hinkel, W.R. Eisner ⁎, C.J. Kim Department of Geography, University of Cincinnati, Cincinnati, OH 45221-0131, United States
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 27 August 2016 Accepted 6 September 2016 Available online xxxx Keywords: Tundra damage Thermokarst Off-road traffic Trails Arctic Alaska Indigenous communities
a b s t r a c t In the past several decades, the use of all-terrain vehicles (ATVs) has proliferated in many Arctic communities in North America. One example is the village of Barrow, Alaska. This coastal community has only local roads, so all access to the interior utilizes off-road machines. These 4-wheel vehicles are the primary means of tundra traverse and transport in summer by hunters and berry-pickers, and by village residents accessing summer camps. Traveling cross-country is difficult due to the large number of thermokarst lakes, wetlands, and streams, and tundra trails tend to follow dryer higher ground while avoiding areas of high microrelief such as high-centered icewedge polygons. Thus, modern ATV trails tend to follow the margins of drained or partially drained thermokarst lake basins where it is flat and relatively dry, and these trails are heavily used. The deeply-ribbed tires of the heavy and powerful ATVs cause damage by destroying the vegetation and disturbing the underlying organic soil. Exposure of the dark soil enhances summer thaw and leads to local thermokarst of the ice-rich upper permafrost. The damage increases over time as vehicles continue to follow the same track, and sections eventually become unusable; this is especially true where the trail crosses ice-wedge troughs. Deep subsidence in the ponded troughs results in ATV users veering to avoid the wettest area, which leads to a widening of the damaged area. Helicopter surveys, site visits, and collection of ground penetrating radar data were combined with time series analysis of high-resolution aerial and satellite imagery for the period 1955–2014. The analysis reveals that there are 507 km of off-road trails on the Barrow Peninsula. About 50% of the total trail length was developed before 1955 in association with resource extraction, and an additional 40% were formed between 1979 and 2005 by ATVs. Segments of the more modern trail are up to 100 m wide. Damage to the tundra is especially pronounced in wet areas, such as ice-wedge troughs. Knowledgeable indigenous people are aware of the problem. Some remediation has been attempted by using heavy-duty PVC matting in areas of greatest damage, but this approach is prohibitively expensive on a large scale. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Arctic tundra vegetation is a fragile mat that protects and insulates the underlying permanently frozen soil. Tundra vegetation is very slow growing and easily damaged, which makes it highly vulnerable to long-term disturbance. Damage to the vegetative cover can result in thawing of the underlying ice-rich permafrost and consequent ground subsidence, a degradational process known as thermokarst. Researchers and members of tundra communities have long been aware of the damage done to tundra vegetation by vehicles (Hambleton and Drescher, 2008). Research has focused mainly on the movement of vehicles involved in industrial and government oil exploration (Jorgenson et al., 2010; Nelson et al., 2003; Brown, 1997; Slaughter et al., 1990; Lawson, 1986). A 2008 study (Eisner et al., 2008) identified the travel routes of both commercial and private overland vehicles in the Barrow region and the resulting surface scaring and thermokarst. ⁎ Corresponding author. E-mail address: [email protected] (W.R. Eisner).
In recognition of the damage caused to permafrost by the activities of seismic exploratory vehicles, the U.S. Fish and Wildlife Service introduced regulations and requires permits for travel on state land by offroad vehicles. Most commercial traffic has been restricted to winter travel and to specified routes since the early 1970s (Jorgenson et al., 2010; Walker et al., 1987). Snow depth and soil temperature along these routes are monitored. Summer travel on State land is also restricted by land use permits and is limited to approved vehicles (Alaska DNR, 2012). The present study focuses on a relatively new source of tundra disturbance: privately owned all-terrain vehicles (ATVs) not regulated by state or federal agencies. Although smaller and lighter than commercial vehicles, they are ubiquitous in arctic and sub-arctic communities. During the snow-free summer (May–September), ATVs replace the snow machine as the primary means of tundra traverse and transport. Hunters and berry-pickers traveling across the open tundra, and village residents accessing summer camps, use these 4-wheel drive vehicles. The deeply-ribbed tires of the ATVs cause damage by disrupting the vegetation, compacting and destroying the underlying organic mat,
http://dx.doi.org/10.1016/j.geomorph.2016.09.013 0169-555X/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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and by disturbing the underlying mineral soil. Long-term or permanent impacts depend on the specific environmental conditions that include vegetation type, the amount of excess ice in the upper permafrost, and soil texture. Impact severity is also influenced by traffic intensity, vehicle weight and tire type, and aggressiveness of the vehicle operator (Hambleton and Drescher, 2008). Our study focuses on the Barrow Peninsula, which is not protected state land. Through the use of time series of high-resolution aerial imagery coupled with on-the-ground observations and field measurements, we have developed a time series of tundra trail damage that spans 50 years of landscape use. We are therefore able to differentiate variations between the unregulated damage caused before 1955 by industrial exploration, and by later ATV damage induced by heavy traffic from residents of the North Slope Borough. Our 2008 study showed that overland travel routes around the village of Barrow generally followed trails similar to those from 50 years ago (Tremont, 1987), but that the use of ATVs has enabled travelers to detour inland and has caused extensive impacts on the landscape (Eisner et al., 2008). We also identified a trend toward increasing use of snow machines on patchy snow or even bare tundra during the spring, which exacerbates deteriorating trail conditions. Coupled with this is an increased lack of geographical knowledge on the part of the ATV operators, and more incidents of disorientation while out hunting. Previous studies identified significant variations in the impact of vehicles on the tundra based on vegetation type, soils, and ice content. Lawson (1986), who studied the effects of vehicle traffic, camp construction, and drilling activities, found that there was a significant difference in the impacts of disturbance in areas with ice-rich soils. Slaughter et al. (1990) showed that vegetation type, topography, season of travel, vehicle weight and type, and the amount of traffic all determined the extent and duration of the damage. Jorgenson et al. (2010) monitored post-1980 winter seismic trails in Northern Alaska over an eighteenyear period and found that, even with a snow and ice cover, such traffic causes damage in these regulated areas. Plant community changes were species-specific, and recovery time depended on the severity of the initial disturbance, ice-content of the upper permafrost, and on the initial plant communities. The objectives of this study are to (a) assess the development of the informal off-road trail system over time using time series of available high-resolution imagery, (2) evaluate the geomorphic setting and geographic patterns of modern ATV trails, and (3) describe some preliminary field studies and remedial efforts to mitigate ATV damage to the tundra. 2. Study area The study area includes the Barrow Peninsula, a triangular shaped region of approximately 2250 km2 that extends from the outlet of the Inaru River at 70.8° N to the northernmost point at Point Barrow at 71.3° N. (see Fig. 1). This flat, low-relief terrain is underlain by permafrost to a depth of about 400 m, and is characterized by low-tundra vegetation, oriented thermokarst lakes, and ice-wedge polygons. Surface thaw begins in May or June, and the active layer is usually 30–60 cm thick by late August. Near-surface sediments are largely ice-rich marine silts and sand above Cretaceous sedimentary rock, although remnant barrier islands of coarser material can be observed as slighter higher beach ridges that arc from northwest to southeast across the upper part of the study area map. The land surface has higher elevations to the west and dips gently eastward. Prominent cliffs are found along the Chukchi Sea that are typically 10–15 m high, and a narrow sand or gravel beach extends discontinuously along the western coast. On the east, the land surface is only a few meters above Elson Lagoon, and there is no beach. The city of Barrow, near the northern point of the study area, is a village with a 2010 population of 4212 people. Only local roads service this remote, coastal community and all access to the interior necessitates the
use of off-road machines. The North Slope Borough, of which Barrow is the administrative seat, does not require licenses or any type of permit for the use of privately owned ATVs or snow machines on the tundra (personal communication, Lt. Phillip Brymer, Village Operations). Villagers commonly practice fishing and subsistence hunting for caribou, geese, and ducks, and there are many hunting camps scattered across the peninsula. Most camps and cabins are found along streams, and fishing with nets is an effective procurement method. In the past, hunting camps were accessed in summer by walking across the tundra along ancient trails (Eisner et al., 2008; Tremont, 1987) or by taking boats from Barrow eastward along the coast into the shallow Admiralty Bay, and then upriver–a mode still practiced. Most villagers, however, travel to their summer camps using ATVs. The routes taken by ATV drivers are not regulated, and certain routes are preferred and heavily used based on convenience and intersection with older traditional trade routes (Eisner et al., 2008). Traveling across the tundra on an ATV is difficult owing to the many water bodies and high microrelief associated with high-centered icewedge polygons. To reach hunting grounds or camps on the western side of the peninsula, the easiest route is to travel southwest along the narrow beach. However, large inlets, lagoons and bays block the route. At Walakpa Bay, some 18 km from Barrow, travelers are forced inland and far to the east to circumvent the dissected terrain and steep banks of the bay and associated inlets and ravines. There are relatively few camps on the eastern side of the peninsula, and most tend to be found on the shores of thermokarst lakes. The majority of camps are found along the Inaru and Meade Rivers, about 50 km south of Barrow. The upper Inaru is too deep to easily cross, but regions south of the river can be accessed by going around, which entails a long and difficult diversion to the west. Most villagers restrict their summer activities to the region north of the Inaru River, so the latitude of the Inaru River outlet defines the southern limit of the study area. 3. Methodology Surveys of tundra trails have been conducted for the past several years by site visits in summer. Trails near Barrow were accessed using ATVs, while those further afield were visited by helicopter. Surveys included measuring the ground thaw depth, width of the trail, and observing the damage done to the vegetation. Winter work entailed conducting ground-penetrating surveys across several trails. This study reports on the analysis of a time series of high-resolution satellite imagery and aerial photographs for the period 1955–2014. As an initial step, trails on the Barrow Peninsula were digitized from high-resolution Google Earth imagery, and 111 trail and trail segments were easily identified as linear features on the flat, treeless tundra. A single passage from an ATV does not leave a trail unless it was recently made in a very wet setting, such as a shallow pond with vegetation. Thus, the trails identified on the imagery are visible because they have been subject to repeated passage of vehicles over the years, and a scar has been left on the tundra that has not yet healed. The 111 trail segments were saved as a kmz file, and imported into ArcGIS for analysis. One objective was to estimate when the trails were formed. To do this, aerial and satellite imagery time series were collected from various sources. These include black-and-white aerial photographs, colorinfrared DOQs, Quickbird, and Worldview images for the period 1955– 2014. The National Snow & Ice Data Center (NSIDC) data sets include aerial photography of the study area which have been geocorrected to Quickbird satellite imagery or Interferometric Synthetic Aperture Radar (IFSAR) imagery. These images are from specific dates in summer (July through August) from 1948 to 1997. Data are in GeoTIFF and ESRI Shapefile formats with FGDC compliant metadata (see http://nsidc.org/ api/metadata?id=arcss306 for more details). The temporal coverage of the NSIDC datasets include 1948, 1949, 1955, 1962, 1964, 1979, 1984, and 1997. Among these years, trails are visible in 1955, 1962, and 1979, as shown in Table 1.
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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Fig. 1. Map of the Barrow Peninsula showing off-road trails and location of hunting and fishing camps and cabins.
High-resolution imagery in National Imagery Transmission Format (NITF) was also obtained from the Polar Geospatial Center (PGC). Quickbird, Worldview, Ikonos, and Geoeye images are georeferenced to the WGS84 datum. Quickbird images (2.4 m resolution) in 2004 and color-infrared (CIR) Digital Orthophoto Quadrangles (DOQs) with a 2.5 m resolution in 2005 are available for the study area. Trails in 2006, 2009, and 2010 were identified using Google Earth (2.4 meter original resolution). Table 1 details the imagery source and horizontal resolution which, with a typical range of 1.4–2.5 m, is adequate to detect a linear feature such as a trail. Each set of imagery was orthorectified, mosaicked, and projected within the GIS as a layer to Universal Transverse Mercator (UTM) zone 5 with the 1983 North American Datum (NAD83).
Table 1 Imagery sources used in this study by year of imagery acquisition, with horizontal resolution indicated. Total number (N) and total trail length (km) of trails first observed in that year, with percentage of total trail length (507 km) in 2014. Year
Source
Resolution (m) N
Trail length (km) % total length
1955 1962 1979 2004 2005 2006 2009 2010 na
b&w aerial photo b&w aerial photo CIR aerial photo HR satellite DOQ aerial HR satellite HR satellite HR satellite na
1.35 1.35 4.6 2.4 2.5 2.4 2.4 2.4 na Total
252.4 2.7 39.1 106.7 95.6 3.7 0.9 0.6 5.0 506.7
47 1 11 14 31 1 2 1 3 111
49.8 0.5 7.7 21.1 18.9 0.7 0.2 0.1 1.0 100.0
By overlaying the current tundra trail network on the imagery, it is possible to determine which trails were in existence at the time of image acquisition. If apparent in a scene for a particular year, it means that the trail was developed prior to that time; if not, the trail was developed subsequent to the image acquisition date. In this way, we could assess when the trails first became visible on the landscape, and determine if the rate of trail network development has recently accelerated. To evaluate the impact of tundra trails on permafrost stability, measurements of maximum summer thaw depth were made at three sites in mid-August 2010. This entailed establishing a transect perpendicular to the trail, which are typically 5–10 m wide. Transects extended 10– 15 m beyond the impacted area on both sides of the trail, so the overall length was 25–40 m. Using a graduated 1-cm diameter steel probe, the thaw depth was measured every 1.0 m along the transect. Further, in April 2011, a ground penetrating radar (GPR) survey was conducted along one transect. 4. Results Analysis of the 111 trails indicates that they total 507 km in length (Table 1), yielding an average trail density over the study area of roughly 0.23 km of trail per square km. However, it is clear from Fig. 1 that trail density is not spatially uniform; there are large areas with no detectable trails, especially in the eastern part of the Barrow Peninsula and to the southwest. Trails to the east are sparse, and were primarily developed before 1955. Newer trails are extensions of the older trail system, and terminate at camps and cabins near large lakes; none go to the coast. Trails on the western side of the peninsula show the same pattern of
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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an early (pre-1955) trail that parallels the Beaufort Sea coastline. This is a gas pipeline service trail that continues on to the village of Wainwright some 150 km further down the coast. The rest of the western region is largely devoid of camps, cabins and trails except for those homesteads along the coast and the upper reaches of the Inaru River. Analysis of the available image time series demonstrates that about half of the total number of visible trails (47) and total trail length (252 km, or 50%) were developed before 1955. These are concentrated around, and have their origin in, the village of Barrow. These trails tend to follow straight lines across the tundra for long distances, before veering to follow a new direction. Several are adjacent to the aboveground gas pipelines that deliver natural gas to the village from the gas fields, and many other trails are easily visible today since sections are long, narrow ponds. These were developed during the natural gas exploration and development period that took place during and after World War II. Trails were made by tracked and wheeled vehicles that caused extensive local thermokarst, as shown in Fig. 2. Trail development between this period and 1979 appears very limited, with only an additional 42 km of trail (7.7%) added over 25 years and most trail development occurring just south of Barrow. By 2004, however, high resolution satellite imagery shows that an additional 107 km of trails was added to the network, or about 21% of the total trail length. These trails are concentrated in the center of the peninsula and extend from Barrow south to the Inaru River. Running north and south, they tend to follow the margins of large lakes. These are “superhighways” that allow villagers to get to southern destinations as quickly and
Fig. 2. Trails made before 1955 using tracked vehicles or dozers; (a) trail with significant linear thermokarst ponds, (b) dozed trail with berms on the right, and modern superhighway on the left of photo.
efficiently as possible. The following year, 2005, records further trail expansion with an additional 96 km (19%) added. Most of these trails appear to link or extend components of previous trails, but some enter into areas previously free of trail such as those in the eastern portion of the peninsula. One segment was established to circumvent a stream crossing, but this issue was subsequently addressed by deploying a portable bridge (location in Fig. 1). Since 2005, further trail development has been limited, with only 5 km (1%) of trails added. These tend to be concentrated near Barrow, and are shortcuts that avoid areas that have been damaged by excessive trail use. 5. Discussion Trails made before 1955 were primarily for the purpose of gas field development and maintenance. These trails are easily visible today due to the vegetation disturbance. In many places, there has been significant local thermokarst that resulted from the destruction of the organic mat with consequent changes to the surface albedo and surface insulation provided by the organic layer. The near-surface sediments over much of the study area are especially sensitive to thaw subsidence since these marine silts are oversaturated with respect to ice; in addition to ice wedges, the upper 10 m is estimated to exceed 80% ice by volume (Sellmann et al., 1975; Brown, 1968). Thaw of the upper permafrost and subsequent ground subsidence beneath trails often results in linear surface ponds, as shown in Fig. 2a. In some cases, it appears that the trail was plowed with a bulldozer, as evidenced by the remnants of soil that form berms on either side of remnant trail (Fig. 2b). To a large degree, these trails were developed without consideration of landscape elements, except that they tend to go around lakes. They cross marshes, ridges, streams and drained lake basins without deviating from their course since these features do not impede the progress of tracked vehicles. Those trail sections demonstrating significant thermokarst are not used for travel today, but other sections of the old trail system are still in use. These tend to be areas of higher and dryer terrain. Near Barrow, the old trail system provides the three most common routes of egress from the village to the interior, primarily because they are an extension of the existing road system in Barrow that allow travelers to take a groomed surface as far as possible onto the tundra. The first is near Fresh Water Lake (Imaiqsaun Lake, “A” on Fig. 1), which is the route hunters take if they intend to access the western part of the peninsula. A second route (“B”) extends south from an existing well head and pumping station several km south of Barrow (Fig. 3b), and is used by those accessing the central part of the peninsula or to reach distant sites on the Inaru River. The western region is accessed by traveling on a groomed road (Cakeater Road, “C”) eastward for 5 km, and using the road as a jumping-off point. There appears to be very little new trail development over the next 25 years (1955–1979), and that which did occur was near Barrow. During this period, snow machines became the primary means of travel across the tundra in winter as machines became more reliable, and oil revenues provided dividend funds to residents that made large purchases possible. ATVs subsequently became more common, and there was significant trail expansion after 1979 reflecting increasing access to summer hunting grounds and cabins. The main goal was to reach the destination quickly and safely. Travel by ATV in summer is constrained by landscape elements including lakes, marshes or drained lake basins containing shallow water, streams, and high-centered polygon terrain. Lakes occupy about 22% of the Barrow peninsula (Hinkel et al., 2003), and must be circumvented. Drained lake basins cover around 50% of the landscape, and most are filled with shallow water that must be avoided by ATVs. Numerous streams cross the tundra. Although narrow in some places, streams often occupy ice-wedge troughs that have progressively deepened from thermal ablation of the underlying wedge ice. Thus, streams
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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Fig. 3. (a) Narrow beach along the Chukchi Sea near Barrow with ATV tracks, and (b) one of the three egress trails from Barrow, labeled “B” on Fig. 1.
are narrow but deep, and safe fording places are not common (Fig. 4a). Finally, regions of high-centered polygons are avoided because they must be traversed slowly and carefully. The relief between the troughs and polygons is typically 1–2 m, with polygons 2–10 m in diameter (Fig. 4b). Traveling in the troughs is not feasible owing to the wet and soft conditions, and the steep-sided troughs makes it difficult and dangerous to ascend the polygon rims for fear of tipping sideways or backward. For these reasons, most new ATV trails follow dry, flat, smooth surfaces; the dominant geomorphic setting is the margins of recently drained lake basins. Here, complete or partial draining has exposed the former lake shelve to stabilization by vegetation and soil freezing. Since the basins drained relatively recently, near-surface ice enrichment and ice-wedge polygon development has been limited, especially in sand-rich sediments. Ice-wedge networks and trough development is usually apparent but at an early stage, so ice-wedge troughs are but minor depressions on the landscape and there are no polygon rims. These lake margins are often found between the modern lake or marshy basin, and the older high-centered polygon terrain further away from the lake (Fig. 4c). They represent the path of least resistance for ATV travelers who wish to quickly and safely traverse the area. They track nearly north–south, preferentially following the shores of large partially-drained oriented lakes, and are concentrated near the center of the peninsula. 5.1. Modern ‘superhighways” Modern tundra trails traveled by ATVs are heavily utilized, especially in the northern reaches near Barrow. Trails generally following the
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smooth lake margins for long stretches, with east-west connectors at the northern and southern end of large lakes or basins that often traverse rough terrain. The primary obstacles are streams, water tracks, and ice-wedge troughs. These require that the traveler slow down to a near stop. Crossing an ice-wedge trough can be hazardous since the heavy machine sinks into the thawed sediment in the trough, which often contains standing water. To get out of the trough requires that the operator accelerate, and the deeply-ribbed tires can quickly destroy the protective organic mat. Over a summer, the trail becomes soft within the trough and near the trough margin, requiring that travelers shift their route to either side of the path. This effectively widens the path at the trough, as shown in Fig. 4d, to create a “braided” trail. Abandonment of a trough crossing does promote recovery of the tundra. Once the organic mat is disturbed or destroyed, the surface albedo is reduced and heat transfer to depth is enhanced. As permafrost thaws and ice is converted to water, the surface subsides. The depression becomes filled with water from precipitation, surface flow, and from lateral flow of soil water above the impermeable permafrost. The thermal conductivity of the saturated soil increases, which promotes further heat transfer to depth and greater thaw subsidence in a positive feedback. Permafrost degradation will continue for years since revegetation is a slow process (Williams et al., 2013; Jorgenson et al., 2010). Over time, as the primary path shifts to accommodate ground destabilization concentrated at ice-wedge troughs and water tracks, the severely impacted zone widens to create a barrier. A similar trail widening process also occurs on the open tundra between ice-wedge troughs. Heavy traffic creates shallow linear depressions on the surface which hold water and, in time, the trampled ground is wet and soft. ATV travelers then shift their path, effectively widening the trail until it becomes a broadly traveled corridor, or “superhighway.” These major ATV transportation trails are easily visible on satellite images since they are 30–100 m wide. Because travel is dispersed over a broad region, visible thermokarst is largely limited to icewedge troughs and water tracks. They account for about one-quarter of the entire length of the modern tundra trail network. Regular trail users, such as indigenous hunters and researchers, claim that major trails have become noticeably wider over the past decade, with discernable impacts on the tundra surface and vegetation (Fig. 2b). 5.2. Field studies of local thermokarst To quantify the impact of superhighways on permafrost stability, measurements of maximum thaw depth were made at three sites near the end of the thaw season in mid-August 2010. The thaw depth in the impacted area was not significantly different from the thaw depth in the undisturbed tundra, and averaged 35 cm. However, as can be seen in Fig. 5a & b, the trail forms a depression owing to ground subsidence following degradation of the upper permafrost, and this displacement is not captured by thaw depth measurements since the ground surface is the datum. This problem is compounded by compression of the organic layer by the heavy ATVs. In and near ice-wedge troughs, it is not uncommon for the fibrous organic mat to be completely destroyed by the ATV tires, leaving an organic slurry residue that is difficult to measure with standard assessment methods. A GPR survey was conducted along one transect the following winter (April 2011) to assess the depth to the permafrost table beneath the trail, and to determine whether the top of the ice-rich upper permafrost has been depressed with respect to the surrounding undisturbed area (J.A. Doolittle, report on Ground-penetrating radar (GPR) study of icewedge networks in Barrow, 9 August 2011). Fig. 5c shows the radar record, with all scales in meters. The depth scale is estimated using a constant propagation velocity of 0.13 m/ns (Hinkel et al., 2001), and has been vertically exaggerated for the purpose of display. The highamplitude, dark-colored, horizontal, planar reflector near the top of the record represents the snow-soil interface. The interpreted depth to this interface ranged from about 0.1 cm in the undisturbed tundra to
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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Fig. 4. (a) Convergence of trails at stream fording place showing evidence of heavy traffic; (b) trail along a drained lake margin at the base of high-centered polygon terrain; (c) as in (b) but with ATVs visible; and (d) heavily traveled braided trail crossing ice-wedge troughs.
81 cm in the trail depression, and averaged 40 cm. In the near subsurface beneath the trail, there appear to be altered patterns or additional radar reflections that are found 10–20 cm deeper than the undisturbed tundra, suggesting a depressed permafrost table.
However, there is too much ambiguity in this interpretation process, and faster and more reliable methods are required if thermokarst effects beneath trails are to be assessed over a large region. High-tech alternatives include acquiring a very high resolution DEM with accuracy and
Fig. 5. Heavily traveled trail near “A” on Fig. 1 showing (a) ground depression and ponding along abandoned older trail, with subsequent development of new trail. The two stakes in (b) mark the end points for the winter GPR survey shown in (c). All units are in m.
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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precision appropriate to the scale of the disturbance (Kääb, 2008), or use of a terrestrial LiDAR scanner at select locations to measure absolute surface displacement over time (e.g. Barnhart and Crosby, 2013; Bodin et al., 2008). Low-tech solutions entail installing frost–defended stakes at both ends of a short transect across the trail. A cable is tightly stretched between the stake tops to create a relative datum from which the distance to the ground surface and frost table can be measured along the transect cable by repeated surveys. This method is currently being applied in the field at sites on the Barrow Peninsula because it is cost-effective, easy to implement, and yields reliable results. Of course, the cable must be removed between field sampling periods. 5.3. Remedial efforts Interviews with indigenous people demonstrate that they are well aware of the damage being done to the tundra, and have taken some preventative and remedial steps to mitigate against further damage (Eisner et al., 2008). A bridge was constructed and deployed across a major stream crossing located stream some 15 km south of Barrow along a primary trail. Built and deployed by Roy Nageak Sr., around 1995, the intent was to limit damage to the stream bank from fording and deepening of the stream bed from heavy ATV traffic (Figs. 1 & 6a). Mr. Nageak subsequently expressed regret for installing the bridge since it focuses traffic on the bridge approaches and concentrates damage on the nearby tundra. A Google Earth image from 2014 shows that the bridge is still at the same location, where it remains a convergence point for all nearby trails. Other remedial efforts involve deploying high-density polyethylene ground stabilizing mesh mats as a protective cover (Schneider, 2011), as shown in Fig. 6b. These molded sections are linked together to create a “hardened trail.” The open cell design permits water and solar radiation to penetrate the mesh to the ground surface, thus allowing grass growth or regeneration while protecting the roots and organic mat. Short test sections have been deployed by the North Slope Borough near Barrow (http://catalog.northslope.org/catalogs/3009-hardened-trail). They have also been used in state and national parks and preserves (Allen et al., 2000), where trails can cause unintended damage to waterways that impact hydrologic processes at the watershed scale (Arp and Simmons, 2012). The use of these and other geotextiles is not a panacea. As noted by the Alaska Department of Natural Resources (2008), trail hardening is expensive; $25,000 to $100,000 per linear mile for a standard ATV trail. Furthermore, when deployed across disturbed ice-wedges, the mats often buckle or separate as the trough continues to subside and deepen over time. Once the sides of the trough become too steep to safely cross for fear of flipping the machine, travelers leave the hardened trail in search of another crossing.
Fig. 6. (a) Stream crossing at bridge deployed around 1995 (location in Fig. 1), with heavy trail damage at approaches, and (b) road hardening with geotextile mats installed along a section of trail at point “A” near Barrow.
6. Conclusions 5.4. Methodological limitations There are several caveats associated with the methodology used in this study. First, there are significant temporal gaps in the imagery coverage, so trail detection indicates only that it was formed prior to the image acquisition year. Although few trails appear to have been added over the 24-year period 1955–1979 (8%), trail expansion during the 25-year period 1979–2004 (21%) was significant. Given that a similar level of expansion also occurred in 2005 (19%), we suspect that the trail network expanded most rapidly after 2000. A second caveat is introduced by cloud cover, which can obscure a portion of the study area in any given year. Third, the horizontal resolution of the imagery varies, and it is sometimes difficult to detect narrow, poorly defined trails, especially if there is some cloud cover. Finally, the GPR transect is time consuming and yields ambiguous interpretations. New methodologies must be developed to assess the impact of concentrated tundra disturbance on local ground subsidence and thermokarst over a large region.
Time series analysis of high-resolution aerial and satellite imagery over the period 1955–2014 reveals that there are 507 km of off-road trails on the Barrow Peninsula. About half of the total trail length was developed before 1955 in association with resource extraction, and an additional 40% were formed between 1979 and 2005 by ATVs with some of the modern trails (superhighways) up to 100 m in width. Damage to the tundra is especially pronounced in wet areas, such as icewedge troughs. Repeated use of tundra trails using heavy ATVs with deep-tread tires destroys the protective vegetative mat. Degradation of the underlying permafrost follows, with consequent ground subsidence and ponding. Temporal gaps in the imagery prevents further refinement in determining when trail network expansion occurred. Furthermore, since registration of ATV s and snow machines is not required, it is not possible to correlate the period of trail expansion to the proliferation of ATVs on the North Slope. However, distribution of oil revenues to residents since the 1970s appears to have contributed to increased purchase and use of
Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013
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ATVs in native communities since that time, and observations by both indigenous people and the authors support this view. In the presence of a slight topographic gradient, low-centered icewedge polygons can evolve into high-center polygon terrain. Preferential pooling of water in troughs will cause deepening as the ice wedges ablate from above, and trough pools eventually coalesce. As the surface hydrology integrates over time and a stream network develops, the pools drain and the high-centered polygons are left as elevated remnants. Human activities can accelerate this process, as observed along the tundra trails. Local trough deepening is hastened by physical destruction of the vegetation and organic mat, and the consequent impact to the surface energy balance and subsurface soil thermal properties. Trail braiding expands the affected area so that noticeable trough deepening occurs in several decades. This is a unidirectional process since the tundra does not heal from this level of disturbance. In time, deep troughs may create an impassable barrier to ATV travel along lake margins that are constricted by the lake on one side and older, highcentered polygon terrain on the other. Lake margins are the optimal surface for rapid ATV travel. These sites have already been developed, and alternative routes are far less favorable for ATV travel. The best approach seems to be preserving existing routes, but taking steps to minimize further damage and promote restoration. One mitigating or preventative approach might be to deploy hardened trails in especially sensitive trail sections. For example, one strategy would be to use protective mats at several undisturbed locations across a single ice-wedge trough to prevent damage before it occurs, and to encourage traffic diffusion across a wider area. Similar trail development has occurred in other villages on the North Slope of Alaska. In Atqasuk, a much smaller community of about 250 people 100 km south of Barrow, a network of newer ATV trails is apparent on recent high-resolution imagery. Local hunters who were interviewed (Eisner et al., 2008) contend that their greater isolation makes them more reliant on subsistence hunting and fishing compared to residents of Barrow, and therefore a higher proportion of villagers use ATVs. The village of Anaktuvuk Pass, on the northern edge of the Brooks Range, has had a more contentious history of limiting ATV use. The village boundaries lie within the Gates of the Arctic National Park. Armed with studies documenting the damage done by ATVs and ATGOs (Racine and Johnson, 1988), the 6-wheeled vehicles with large balloon tires that are the vehicle of choice in this upland terrain, the National Park Service attempted to restrict ATV traffic during the 1980s. The resulting dispute lasted over 10 years and necessitated Congressional redistricting of designated Wilderness areas, Federal lands and tribal lands in order to allow community right-of-way (Norris, 2002). It is clear that best practice is to mitigate tundra damage since it takes an Act of Congress to repudiate the right to use ATVs for subsistence hunting.
Acknowledgements This work was supported by NSF AON grant number AON-1107607. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. We are grateful to the Inupiat Corporation and CPS for administrative and logistic assistance. We thank Jim Doolittle of the USDA for the GPR analysis.
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Please cite this article as: Hinkel, K.M., et al., Detection of tundra trail damage near Barrow, Alaska using remote imagery, Geomorphology (2016), http://dx.doi.org/10.1016/j.geomorph.2016.09.013