Topographic Obstacles Research Articles

Gravity–capillary flows over obstacles for the fifth-order forced Korteweg–de Vries equation

The aim of this work is to investigate gravity–capillary waves resonantly excited by two topographic obstacles in a shallow water channel. By considering the weakly non-linear regime the forced fifth-order Korteweg–de Vries equation arises as a model for the free-surface displacement. The water surface is initially taken at rest over a uniform flow and the initial value problem for this equation is computed numerically using a pseudospectral method. We study nearly-resonant flows with intermediate capillary effects. Details of the wave interactions are analysed for obstacles with different sizes. Our numerical results indicate that the flow is not necessarily governed by the larger obstacle.

Near‐Inertial Wave Propagation in the Wake of Super Typhoon Mangkhut: Measurements From a Profiling Float Array

AbstractNear‐inertial internal waves (NIWs) are generated by inertially‐rotating winds under tropical cyclones (TC). Since NIWs are mostly horizontal, their vertical propagation out of the mixed layer is slow. However, mesoscale vorticity and shear increase vertical group speed by increasing near‐inertial frequency and horizontal wavenumber. To assess NIW propagation, a profiling float array under Super Typhoon Mangkhut in September 2018 made broad and persistent measurements in space and time of density in the upper 200 m and depth‐mean velocity. The TC wake was a region of positive vorticity on its southern side, displayed elevated shear, and thereby enhanced downward propagation of NIWs. The vertical energy flux is estimated as 0.04–0.11 W m−2, which is about 1%–4% (3%–8%) of the mean total (near‐inertial) wind work of 3.0 (1.3) W m−2 calculated from a high‐resolution TC model. Considerable uncertainties arise in the (a) estimated group speed based on wavelength, shear, and frequency and (b) energy density based on depth‐varying density and the NIW polarization relations, which are sensitive to frequency. Following the TC's passage, NIWs propagated southward and horizontal wavelengths decreased from 1,000 to 500 km, as time progressed. Also, we identify an interfacial wave at the mixed layer base, which displaces isopycnals vertically. The following process is suggested. As the North Equatorial Current flows over these displacements, which act as topographic obstacles, secondary NIWs propagate up‐/downward into the mixed layer/thermocline. These waves are 180° out of phase in the mixed layer and thermocline, which can enhance shear at the mixed layer base.

Transport and accumulation of plastic litter in submarine canyons—The role of gravity flows

Abstract Manned submersible dives discovered plastic litter accumulations in a submarine canyon located in the northwestern South China Sea, ∼150 km from the nearest coast. These plastic-dominated litter accumulations were mostly concentrated in two large scours in the steeper middle reach of the canyon. Plastic particles and fragments generally occurred on the upstream-facing sides of large boulders and other topographic obstacles, indicating obstruction during down-valley transportation. Most of the litter accumulations were distributed in the up-valley dipping slopes downstream of the scour centers. This pattern is tentatively linked to turbidity currents, which accelerated down the steep upstream slopes of the scours and underwent a hydraulic jump toward the scour centers before decelerating on the upstream-facing flank. Associated seabed sediment consisted of clayey and sandy silts, with unimodal or bimodal grain-size distributions, which are typical for turbidites. The focused distribution of the litter accumulations is therefore linked to turbidity currents that episodically flush the canyon. Our findings provide evidence that litter dispersion in the deep sea may initially be governed by gravity flows, and that turbidity currents efficiently transfer plastic litter to the deeper ocean floor.

Bathymetric Influences on Antarctic Ice‐Shelf Melt Rates

AbstractOcean bathymetry exerts a strong control on ice sheet‐ocean interactions within Antarctic ice‐shelf cavities, where it can limit the access of warm, dense water at depth to the underside of floating ice shelves. However, ocean bathymetry is challenging to measure within or close to ice‐shelf cavities. It remains unclear how uncertainty in existing bathymetry datasets affect simulated sub‐ice‐shelf melt rates. Here we infer linear sensitivities of ice‐shelf melt rates to bathymetric shape with grid‐scale detail by means of the adjoint of an ocean general circulation model. Both idealized and realistic‐geometry experiments of sub‐ice‐shelf cavities in West Antarctica reveal that bathymetry has a strong impact on melt in localized regions such as topographic obstacles to flow. Moreover, response of melt to bathymetric perturbation is found to be non‐monotonic, with deepening leading to either increased or decreased melt depending on location. Our computational approach provides a comprehensive way of identifying regions, where refined knowledge of bathymetry is most impactful, and also where bathymetric errors have relatively little effect on modeled ice sheet‐ocean interactions.

Tree‐Branching‐Based Enhancement of Kinetic Energy Models for Reproducing Channelization Processes of Pyroclastic Density Currents

AbstractKinetic energy models, also called kinetic models, are simple tools able to provide a fast estimate of the inundation area of pyroclastic density currents (PDCs). They are based on the calculation of the PDC front kinetic energy as a function of the distance from a source point. On a three‐dimensional topography, the PDC runout distance is estimated by comparing the flow kinetic energy with the potential energy associated with the topographic obstacles encountered by the PDC. Since kinetic models do not consider the occurrence of channelization processes, the modeled inundation areas can be significantly different from those observed in real deposits. To address this point, we present a new strategy that allows improving kinetic models by considering flow channelization processes, and consists in the inclusion of secondary source points in the expected channelization zones, adopting a tree branch‐like structure. This strategy is based on the redistribution of a key physical variable, such as the flow energy or mass depending on the considered kinetic model, and requires the adoption of appropriate equations for setting the characteristics of the secondary sources. Two models were modified by applying this strategy: the energy cone and the box model. We tested these branching models by comparing their results with those derived from their traditional formulations and from a two‐dimensional depth‐averaged model, considering two specific volcanoes (Chaitén and Citlaltépetl). Thereby, we show the capability of this strategy of improving the accuracy of kinetic models and considering flow channelization processes without including additional, unconstrained input parameters.

Pulsating flow dynamics of sustained, forced pyroclastic density currents: insights from a facies analysis of the Campo de la Piedra Pómez ignimbrite, southern Puna, Argentina

The Quaternary Campo de la Piedra Pomez ignimbrite (CPPI) is a superbly exposed, partially indurated, rhyolitic ignimbrite emplaced on the southern Puna of Argentina. It is characterized by a variety of facies that record in unprecedented detail the flow dynamics of the parent pyroclastic density currents (PDCs). Detailed facies analysis and internal architecture defined using a sequential stratigraphy approach reveal that CPPI was formed by sustained PDCs, generated from a low fountain eruptive style (boiling over). The PDCs had overall flow conditions characterized by high particle concentration and limited capability to surmount topographic obstacles. The mobility of the PDCs was largely controlled by high pore pressure and the continuous supply at the source (sustained forced convection-dominated PDCs). The successive forestepping-backstepping stacking patterns identified in the CPPI reflect a marked unsteadiness of its parent PDCs due to a pulsating discharge rate at the source. The lateral facies variations in the CPPI record the non-uniform character of its parent PDCs as they flowed outward from the base of the collapsing fountain. Proximal-medial areas characterized by steady aggradation rates pass through to distal braided thalwegs with highly variable aggradation rates, to frontal edges characterized by the step aggradations of secondary decoupled pumice-rich lobes. Collectively, the facies variations in the CPPI record the complex dynamics of sustained PDCs that include pulsating discharge rate at the source, progressive modification of the original topography, and lateral flow transformations.

Discovery of a giant cold-water coral mound province along the northern Argentine margin and its link to the regional Contourite Depositional System and oceanographic setting

In the southern oceans, only a few cold-water coral mound sites have so far been reported compared to the extensively surveyed North Atlantic. Here, we document for the first time the widespread occurrence of exposed and buried coral mounds at the northern Argentine margin named Northern Argentine Mound Province (NAMP). The integration of multibeam bathymetry, sediment echosounder, Acoustic Doppler Current Profiler (ADCP), and CTD data allowed the characterisation of its morphosedimentary, stratigraphic, and hydrodynamic setting.The NAMP, covering at least 2000 km2, represents the largest coral mound province in terms of areal extent reported so far and a major site in the southern oceans. The coral mound distribution is directly linked to morphosedimentary features of the regional Contourite Depositional System (CDS) influenced by the Antarctic Intermediate Water (AAIW), the Upper Circumpolar Deep Water (UCDW), and their interface, forming a complex bottom-current-controlled environment here termed Coral-Contourite System. Coral mounds are most abundant in the lower AAIW between 900 and 1050 m, just above the Ewing Terrace Moat, which has been shaped by high-velocity currents associated with the AAIW-UCDW boundary. This clustering appears to be the result of an increased concentration of suspended sediment and food particles, transported along the AAIW-UCDW density gradient and delivered upward by turbulent hydrodynamic processes, which are enhanced by the irregular moat topography. Within the Ewing Terrace Moat, coral mounds are restricted to topographic obstacles, modifying the otherwise highly erosive, destructive hydrodynamic regime and thereby locally creating favourable conditions for coral growth. The low-velocity currents of the UCDW, currently influencing the Ewing Terrace, promote extensive contourite deposition, which has led to a progressive demise and burial of pre-existing mounds. The presence of these buried mounds suggests past mound aggradation 100 to 150 m deeper likely due to a downward shift of the AAIW during glacials as a consequence of sea-level lowering. Overall, the mound formation in the NAMP seems to be mainly controlled by the availability of suspended particles, in combination with a sufficient bottom-current strength, keeping material in suspension while preventing destructive erosion and excessive sedimentation.Our study highlights (1) the importance of high-energy processes at water-mass boundaries for the delivery of sediment and food particles to corals, (2) the role of the AAIW as a crucial water mass for coral mound development, (3) the variability of mounds in response to glacial-interglacial cycles, and (4) the potential of CDSs in general and the Argentine margin in particular as hotspots for coral mounds and, thus, ideal site for future studies.

Heat Content Anomaly and Decay of Warm‐Core Rings: the Case of the Gulf of Mexico

AbstractIn this study, we harness the 25‐year satellite‐altimeter record, in concert with a vast array of in situ measurements, to estimate the heat content anomaly of 32 warm‐core rings in the Gulf of Mexico (GoM). The decay rate of these mesoscale eddies is studied in detail, and it is shown that they release the majority of their heat as they drift in the central GoM (away from topographic obstacles). The surface heat fluxes from the eddies are shown to be small in comparison to the total rate of heat loss from the eddies, suggesting that heat is primarily released toward the surrounding water masses. Integrating the total heat evolution equation over the warm‐core rings yields an estimate of their effective lateral diffusivity coefficient. The long‐term impact of warm‐core rings on heat and salt balance in the GoM is also discussed.

Subglacial topography and thickness of ice caps on the Argentine Islands

AbstractThis study presents the first subglacial topography and ice thickness models of the largest ice caps of the Argentine Islands, Wilhelm Archipelago, West Antarctica. During this study, ground-penetrating radar was used to map the thickness and inner structure of the ice caps. Digital surface models of all studied islands were created from aerial images obtained with a small-sized unmanned aerial vehicle and used for the construction of subglacial topography models. Ice caps of the Argentine Islands cover ~50% of the land surface of the islands on average. The maximum thickness of only two islands (Galindez and Skua) exceeds 30 m, while the average thickness of all islands is only ~5 m. The maximum ice thickness reaches 35.3 m on Galindez Island. The ice thickness and glacier distribution are mainly governed by prevailing wind direction from the north. This has created the prominent narrow ice ridges on Uruguay and Irizar islands, which are not supported by topographic obstacles, as well as the elongated shape of other ice caps. The subglacial topography of the ice caps is undulated and mainly dependent on the geological structure and composition of magmatic rocks.

The Entrainment Rate of Non‐Boussinesq Hazardous Geophysical Gas‐Particle Flows: An Experimental Model With Application to Pyroclastic Density Currents

AbstractThe entrainment rate of pyroclastic density currents is investigated by large‐scale experiments. The ground flows are initiated by the impact on the terrain of a dense gas‐particle fountain issuing from a cylindrical conduit, similarly to natural volcanic events. On impact, the excess density with respect to the surrounding atmosphere was up to 11.6 kg/m3, making the currents non‐Boussinesq. A power law model of the entrainment rate is developed, which is similar to that proposed for snow avalanches by Ancey (2004, https://doi.org/10.1029/2003JF000052) and is verified for the Richardson's number range between 0.25 and 5.95. Rapid changes of the entrainment are caused by (i) strong accelerations at the fountain impact on the ground; (ii) break in slope; and (iii) topographic obstacles. Such changes, together with the sedimentation rate, influence flow mobility. The use of the power law is suggested for modeling the motion of unsteady hazardous geophysical mass flows such as pyroclastic density currents and snow avalanches.

Wind regime for long-ridge yardangs in the Qaidam Basin, Northwest China

Yardangs are typical aeolian erosion landforms, which are attracting more and more attention of geomorphologists and geologists for their various morphology and enigmatic formation mechanisms. In order to clarify the aeolian environments that influence the development of long-ridge yardangs in the northwestern Qaidam Basin of China, the present research investigated the winds by installing wind observation tower in the field. We found that the sand-driving winds mainly blow from the north-northwest, northwest and north, and occur the most frequent in summer, because the high temperature increases atmospheric instability and leads to downward momentum transfer and active local convection during these months. The annual drift potential and the ratio of resultant drift potential indicate that the study area pertains to a high-energy wind environment and a narrow unimodal wind regime. The wind energy decreases from northwest to southeast in the Qaidam Basin, with the northerly winds in the northwestern basin changing to more westerly in the southeastern basin. The strong and unidirectional wind regime for the long-ridge yardangs in the northwestern Qaidam Basin results from the combined effects of topographic obstacles such as the Altun Mountains and of the interaction between the air stream and the yardang bodies. Present study suggests that yardang evolution needs such strong and unidirectional winds in high- or intermediate-energy wind environments. This differs from sandy deserts or sandy lands, which usually develop at low- or intermediate-energy wind environments. Present study clarifies the wind regime corresponding to the long-ridge yardangs’ development, and lays firm foundation to put forward the formation mechanisms for yardangs in the Qaidam Basin.

Seismic evidence of bottom current controlled sedimentation in the Ceará Rise region (central Atlantic)

This study is based on the seismoacoustic data (3.5 kHz) collected during cruise 9209 of the RV Maurice Ewing (1992), organized within the framework of preparation for ODP drilling – leg 154. The analysis of the acquired data helped distinguish major acoustic facies and their spatial distribution in the study area. It was found out that almost the entire northern segment of the Ceará Rise is covered by a vast field of contourite sediment waves. The distribution of sediment wave heights is suggested to be controlled by geostrophic currents of AABW and NADW as well as by processes on the boundaries between water masses. Contourite erosional features are not widespread and generally resulted from the interaction of the bottom current with prominent topographic obstacles like western scarp of the Ceará Rise or basement outcrops.

Sediment-starved trenches and rough subducting plates are conducive to tsunami earthquakes

Tsunami earthquakes generate large tsunamis but only moderate ground shaking. The discrepancy between the magnitude of the surface wave and the resulting tsunami height makes them a serious threat to coastal communities and a problem for tsunami early warning. Although at least 13 tsunami earthquakes took place since 1896, there is little consent on what controls their genesis. It remains unclear if they are tied to distinct subduction-zone structures or geometries or if they can occur along all active margins. To help shed light on the genesis of these unusual earthquakes, I combine marine acoustic data from subduction-zone segments that experienced tsunami earthquakes in order to search for structural similarities in the nature of the subducting oceanic plate. The structural comparison indicates that tsunami earthquakes preferentially occur in regions where the subducting plate is characterized by excess topography which is not blanketed by trench sediment. While subducted, the topographic obstacles likely cause fracturing along their track. This contributes to the development of a thick, structurally and lithological complex, and fragmented plate-boundary shear-zone, which may be prone to fail at a low velocity during a tsunami earthquake. The results suggest that when assessing the risk of a tsunami earthquake, special focus should be placed on the combination of lower-plate topography and trench sediment thickness.

Patch use in the arctic ground squirrel: effects of micro-topography and shrub encroachment in the Arctic Circle

We investigated the roles of vegetation structure, micro-topographic relief, and predator activity patterns (time of day) on the perception of predatory risk of arctic ground squirrels (Urocitellus parryii), an abundant pan-Arctic omnivore, in Arctic Circle tundra on the North Slope of Alaska, where tundra vegetation structure has been predicted to change in response to climate. We quantified foraging intensity by measuring the giving-up densities (GUDs) of the arctic ground squirrels in experimental foraging patches along a heath-graminoid-shrub moist tundra gradient. We hypothesized that foraging intensity of arctic ground squirrels would be greatest and GUDs lowest, where low-stature vegetation or raised micro-topography improves sightlines for predator detection. Furthermore, GUDs should vary with time of day and reflect 24-h cycles of varying predation risk. Foraging intensity varied temporally, being highest in the afternoon and lowest overnight. During the morning, foraging intensity was inversely correlated with the normalized difference vegetation index (NDVI), a proxy for vegetation productivity and cover. Foraging was additionally measured within landscapes of fear, confirming that vegetative and topographic obstructions of sightlines reduces foraging intensity and increases GUDs. We conclude that arctic ground squirrels may affect Arctic Circle vegetation of tundra ecosystems, but these effects will vary spatially and temporally.

Proximity to the risk and landscape features modulate female red deer movement patterns over several days after drive hunts

Hunting, including drive hunts, can be considered as a predation process. Although drive hunts are spatially and temporally well defined, the scale at which the hunting risk triggers anti-predator responses of prey remains poorly documented. The present study aims at 1) characterizing the delayed movement responses of female red deer Cervus elaphus to hunters and 2) understanding the main environmental determinants modulating these post-disturbance responses at the individual level. We hypothesized that red deer alter their movement behaviour for several days after a drive hunt by increasing their speed and exploring more space. Then, we predicted that individuals close to the hunting area would exhibit stronger responses (i.e. higher speed and larger home range size) than individuals located further away. We also expected topographic obstacles and vegetation cover to reduce the magnitude of the post-disturbance responses. We used GPS data from a large dataset collected on 91 female red deer in the National Hunting and Wildlife Reserve of La Petite Pierre (France), between 2004 and 2018. Each year, drive hunts are conducted by about 43 hunters and 6 dogs, once a week from October to February. Our results suggest that, over a 72-h period after a drive hunt, female red deer could show two contrasting strategies: 1) flight-type movements (i.e. higher maximum speed) associated with more space explored due to a temporary avoidance of the disturbed area and 2) lower movements to avoid detection and contacts with hunters. These two delayed responses are situation-specific, depending on risk proximity, landscape features, hunting conditions and individual characteristics. This study highlights that ungulate management using a ‘hunting for fear’ approach may be adjusted considering spatial and temporal scales of the effects of drive hunts on red deer movement patterns.

Hydrodynamic moraine-breach modeling and outburst flood routing - A hazard assessment of the South Lhonak lake, Sikkim

The presence of glacial lakes in the Himalaya makes it a potential mountain hazard, as catastrophic failure of such waterbodies may lead to high-magnitude glacial lake outburst flood (GLOF) events that can cause significant damage to the low-lying areas. The present study evaluates the hazard potential of the South Lhonak lake located in the state of Sikkim, using both one and two-dimensional hydrodynamic modeling approaches. Different breach parameters were calculated based on the lake bathymetry and moraine dimensions. The worst-case GLOF scenario is revealed during an overtopping failure of the moraine, producing a peak flood of 6064.6 m3 s−1 and releasing a total water volume of 25.7 × 106 m3. The GLOF hydrograph is routed to calculate peak flood (m3 s−1), inundation depth (m) and flow velocity (ms−1) along the main flow channel. The interaction of the flood wave with a major topographic obstruction located 15.6 km downstream of the lake, shows a significant reduction of the flow energy leading to a minimization of the South Lhonak GLOF impact. The flood wave reaches the nearest town Lachen, located at a distance of 46 km downstream from the lake, at 3 h 38 min after the initiation of the breach, with a peak flood of 3928.16 m3 s−1 and a maximum flow velocity of 13.6 ms−1. At Chungthang town, located at a distance of 62.35 km from South Lhonak lake, the flood wave potentially inundates settlements along the bank of the flow channel, where a peak flood of 3828.08 m3 s−1 is reached after 4 h of the initial dam breach event. The study also incorporates modeling of a framework to propose a potential flood remediation measure of the South Lhonak lake GLOF by demonstrating the effect of a lateral inline structure along the flow channel, to check the flow of the potential flood wave.

Wind velocity and wind curl variability over the Black Sea from QuikScat and ASCAT satellite measurements

Measurements of QuikScat for 2000–2009 and ASCAT scatterometers for 2009–2016 are used to describe the variability of the Black Sea wind velocity and wind curl. High-resolution (12.5 km) scatterometry data provide a possibility to investigate several small-scale features (“hot points”) of the wind field in the basin related to the topographic effects. They include: the gap winds near the Bosphorus, the Kerch Strait, and Tuapse; tip jets at the south and west of the Crimean peninsula, several capes of the Anatolian coast; the Kolkheti valley winds; wind shadow zones at the eastern Turkey coast and the western Crimean coast. Driving factors and directional variability of wind in these “hot points” is discussed.QuikScat and ASCAT data are used to describe seasonal and interannual variability of wind characteristics in 2000–2016. Comparison with in-situ data shows that QuikScat noticeably overestimates wind magnitude during low winds conditions (<2 m/s). ASCAT winds are generally lower than QuikScat, but the ASCAT wind curl is higher. The amount of low winds values (<2 m/s) in the ASCAT data is 6% of the total data, and it is 1% in the QuikScat data. Overestimation of low winds decreases wind gradients near the wind shadows, which is the most possible reason of the underestimation of the wind curl in the QuikScat dataset.Wind curl has a different sign and a completely different interannual variability in the western and eastern parts of the sea. Near the eastern coast of the basin, high-resolution satellite measurements reveal three powerful small-scales maximums of cyclonic vorticity. These maximums are related to the wind jets flowing around topographic obstacles in the vicinity of the Kerch Strait, in the mountain gap near Tuapse, and from the Kolkheti valley. They are observed throughout the whole year, but are largest in winter. The contribution of these maximums to the overall cyclonic vorticity is essential (30% in the QuikScat data and 50% in the ASCAT data). In the western part of the basin, the wind curl is negative (anticyclonic) on average, with the highest anticyclonic curl observed in summer. Winds are decreasing in 2000–2016 in agreement with the data on the 20 century. At the same time the basin-average wind curl rises due to its increase in the eastern Black Sea. We speculate that these trends are associated with the displacement of the Siberian Anticyclone to the west. The related rise of pressure over the eastern Europe correlates significantly with the wind curl variability on interannual time scales.

Lava flow morphology at an erupting andesitic stratovolcano: A satellite perspective on El Reventador, Ecuador

Lava flows pose a significant hazard to infrastructure and property located close to volcanoes, and understanding how flows advance is necessary to manage volcanic hazard during eruptions. Compared to low-silica basaltic flows, flows of andesitic composition are infrequently erupted and so relatively few studies of their characteristics and behaviour exist. We use El Reventador, Ecuador as a target to investigate andesitic lava flow properties during a 4.5 year period of extrusive eruption between February 2012 and August 2016. We use satellite radar to map the dimensions of 43 lava flows and look at variations in their emplacement behaviour over time. We find that flows descended the north and south flanks of El Reventador, and were mostly emplaced over durations shorter than the satellite repeat interval of 24 days. Flows ranged in length from 0.3 to 1.7 km, and the length of flows generally decreased over the observation period. We measure a decrease in flow volume with time that is correlated with a long-term exponential decrease in effusion rate, and propose that this behaviour is caused by temporary magma storage in the conduit acting as magma capacitor between the magma reservoir and the surface. We use the dimensions of flow levées and widths to estimate the flow yield strengths. We observe that some flows were diverted by topographic obstacles, and compare measurements of decreased channel width and increased flow thickness at the obstacles with observations from laboratory experiments. Radar observations, such as those presented here, could be used to map and measure properties of evolving lava flow fields at other remote or difficult to monitor volcanoes.

Energy Grade Line Analysis of Tsunami run-up on the Sendai Plain after the 2011 Tohoku earthquake

Energy Grade Line Analysis (EGLA) of tsunami run-up has been proposed as a simple tool to predict inundation heights and tsunami loads. A large database of more than 500 inundation measurements collected on the Sendai Plain on the east coast of Japan after the Tohoku earthquake of 2011 provided the writers the opportunity to evaluate the accuracy of the general EGLA method and to make adjustments to fit the model to the Sendai Plain. Refinements to the EGLA model included a correction for the influence of large linear topographic obstructions on tsunami flows in order to achieve better agreement with measured inundation heights. The study illustrates that inundation height predictions are sensitive to the choice of Froude number parameter and land use roughness coefficients. An important outcome of this study is a site-specific tool for probabilistic prediction of inundation heights over the Sendai Plain that is of value for tsunami counter measure planning including design of tsunami resistant structures.

Determining topographic shielding from digital elevation models for cosmogenic nuclide analysis: a GIS model for discrete sample sites

Topographic shielding (TS) is an important factor in cosmogenic nuclide surface exposure dating. The development of geographic information systems (GIS) and the availability of digital elevation models (DEMs) make it possible to derive this factor directly from a DEM. Most available GIS models derive the TS factors for an area (all cells in a DEM) without the consideration of surface conditions of individual sites, such as the strike, dip, and height above ground, into the calculation. This paper presents a new GIS model to derive the TS factors for discrete sample sites. This model uses the Skyline and Skyline Graph functions in ArcGIS to extract the set of azimuth and elevation angles of topographic obstructions around each site from a DEM (considering the sample height above ground) and then incorporates the strike and dip information of the sample surface to derive the TS factor. All processing tools and steps are streamlined in ArcGIS modelbuilder and this model can be run like a standard ArcGIS geoprocessing tool. It provides an easy and user-friendly means to derive the TS factors for discrete samples based on a DEM and the measured strike, dip and sample height for each site.