Geomorphological Evolution of the Navua River,Fiji

The Navua River on the main island of Fiji displays anomalous drainage features and associated fluvial forms. The River course makes a right-angled bend approximately halfway along its course, beyond which it flows through a narrow gorge cut into hard volcanic rocks. Several tributaries enter the gorge as high cascades from hanging valleys. It is proposed that the former course of the palaeo-Navua River flowed east in a straight line along the modern Waidina River, and eventually discharged into Fiji's Rewa River system. Evidence for this is the presence of a spectacular dry gorge (a wind gap) at the head of the Waidina River. Previous ideas of river capture fail to explain the evolution of the local fluvial geomorphology. Instead, tectonic uplift probably dammed the palaeo-Navua and diverted it to its present short course to the sea. This new hypothesis provides a more plausible, simultaneous account for all the major fluvial geomorphic features, their geographical distribution and their relationship with geology in the Navua region.     

Road-cut slope topography and stability relationships in St Lucia,West Indies

Slope topography is shown to be a significant factor in controlling the porewater pressures in road-cut slopes in the residual volcanic soils of St Lucia, West Indies. It is shown that this control diminishes as the permeability of the material decreases. This finding is confirmed by stability envelopes in which ‘stable’ and failed slopes are examined as a function of slope topography and material properties.     

Factors influencing the formation of shallow landslides in the Boston Mountains of northwest Arkansas,USA

Slope failures cause billions of dollars of damage annually and put human lives at risk. This study employed field measurements and observations to provide the framework for laboratory simulations to investigate the effects of environmental characteristics on slope stability in the highly fractured bedrock region of the Boston Mountains, northwest Arkansas. Field measurements, to determine characteristics and possible controls of 10 shallow slope failures along an interstate highway, revealed that slope failures occurred within a relatively narrow range of slope angles (17–36°) and in loamy soils. Based on field observations, flume experiments were conducted to investigate the relationships between soil texture, slope angle, bedrock fractures, soil compaction, and slope instability. Time to failure differed (p < 0.05) among treatment combinations. Generally, slopes composed of loam were more stable than slopes composed of sand. Time to failure decreased more on slopes of 15–20° than on slopes of 20–25°. Flume slopes with sod cover never failed. This study provided a methodology for using field analyses of slope failures to guide laboratory experiments and demonstrated that complex interactions among environmental factors work to stabilize or compromise steep (>20°) slopes.     

Trends and Spatial Patterns of Drought Incidence in the Omo‐Ghibe River Basin,Ethiopia

The objective of this paper is to evaluate trends and spatial patterns of drought incidence across the Omo‐Ghibe River Basin using monthly rainfall data from eight stations for the period 1972–2007. It also aims to estimate the probability of drought episodes for a 100‐year period. Drought indices were generated using the Standard Precipitation Index (SPI) computed at 3‐, 6‐, 12‐ and 24‐month time‐steps for three intensity classes: moderate, severe and extreme drought events. The Mann–Kendall's trend test and Sen's slope estimator were employed to detect temporal changes. The results show complex spatial patterns on the frequency and magnitude of drought events across the study area for all timescales and intensity classes. However, the total number of drought events for the three intensity classes for all timescales were larger in the southern lowlands, where there exists a serious water scarcity for the rain‐fed pastoral system, than in the northeastern part (around Wolaita Sodo area). In contrast to this, the longest and most extreme (SPI < ?4.0) drought events for all timescales were observed at Wolaita Sodo station. In a 100‐year period one could expect 57–69 drought events with 3 months' duration, 19–34 events with 6 months' duration, 9–16 events with 12 months' duration and 5–9 events with 24 months' duration. The SPI values show negative rainfall anomalies in the 1980s while positive anomalies have occurred in the 1990s and 2000s, which implies tendency towards decreasing drought events. The Mann–Kendall's trend test for the 12‐ and 24‐month timescales and for seasonal events also confirms this general trend.     

The role of soil processes in determining mechanisms of slope failure and hillslope development in a humid-tropical forest eastern Puerto Rico

Translational failures, with associated downslope earthflow components and shallow slides, appear to be the primary mechanism of hillslope denudation in the humid tropical forests of the mountains of eastern Puerto Rico. In-situ weathering of quartz diorite and marine-deposited volcaniclastics produces residual soil (saprolite; up to 21 m deep) / weathered rock profiles. Discontinuous zones of contrasting density and permeability particularly in quartz-diorite slopes at 0.5 m, and between 3 and 7 m, create both pathways and impedances for water that can result in excess pore pressures and, ultimately, aid in determining the location of failure planes and magnitudes of slope failures. In combination with relict fractures which create planes of weakness within the saprolite, and the potential significance of tensile stresses in the upper zone of saprolite (hypothesized to be caused by subsurface soil creep), shear failure can then occur during or after periods of heavy rainfall.Results of in-situ shear-strength testing show negative y-intercepts on the derived Mohr-Coulomb failure envelopes (approximately 50% of all tests) that are interpreted as apparent tensile stresses. Observation of tension cracks 1–2 m deep support the test data. Subsurface soil creep can cause extension of the soil and the development of tensile stresses along upper-slope segments. Shear-strength data support this hypothesis for both geologic types. Apparent values of maximum and mean tensile stress are greatest along upper slopes (16.5 and 6.29 kPa). Previously documented maximum rates of downslope movement coincided with local minima of shear strength, and the shear-strength minimum for all tests was located near 0.5 m below land surface, the shallow zone of contrasting permeabilities. These results indicate that subsurface soil creep, a slow semi-continuous process, may exert a profound influence on rapid, shallow slope failures in saprolitic soils.Data indicate that cove slopes in quartz diorite tend to be the most unstable when saturation levels reach 75%. Deep failures (7 m deep) appear the most critical but not the most frequent because pore pressure build-up will occur more rapidly in the upper perched zone of translocated clays before reaching the lower zone between 3 and 7 m. Frequent shallow failures could reduce the probability of deeper failures by removing overburden and reducing shear stress at depth. Deep failures are more likely to result from storm events of great duration and intensity.Sixty-six ‘naturally occurring’ and more than 100 ‘road-related’ landslides were mapped. Forest elevations exceed 1000 m, but the majority of these failures were found between 600 and 800 m in elevation. This appears to be the area where there is sufficient concentration of subsurface water to result in excess pore pressures. The high percentage of slope failures in the 600–800-m range, relative to the percentage at higher elevations, suggests that differences in soil-water processes are responsible for the form of these mountain slopes. Steep linear segments are maintained at higher elevations. Slope angles are reduced in the 600–800-m range by frequent shallow slides, creating a largely concave surface. In combination, slope segments above 800 m, and those between 600 and 800 m, produce the characteristic form of the mountains of eastern Puerto Rico.     

The landslide response of alpine basins to post-Little Ice Age glacial thinning and retreat in southwestern British Columbia

The role of post-Little Ice Age (LIA) Neoglacial retreat on landslide activity is investigated in 19 alpine basins along the upper Lillooet River Valley, British Columbia. We examine how Neoglacial scouring and glacial recession have modified hillslope form and slope stability, and construct a decision-making flowchart to identify landslide hazards associated with glacial retreat. This work is based on field mapping, GIS analysis, statistical associations between landslides and terrain attributes, and a comparison between Neoglaciated and non-Neoglaciated terrain within each basin.The bedrock landslide response to glacial retreat varies appreciably according to lithology and the extent of glacial scour below the LIA trimline. Valleys carved in weak Quaternary volcanics show significant erosional oversteepening and contain deep-seated slope movement features, active rock fall, rock slides, and rock avalanches near glacial trimlines. Basins in stronger granitic rock rarely show increased bedrock instability resulting from post-LIA retreat, except for shallow-seated rock slides along some trimlines and failures on previously unstable slopes. In surficial materials, landslides associated with post-LIA retreat originate in till or colluvium, as debris slides or debris avalanches, and are concentrated along lateral moraines or glacial trimlines.Significant spatial association was also observed between recent catastrophic failures, gravitational slope deformation, and slopes that were oversteepened then debuttressed by glacial erosion. Eight out of nine catastrophic rock slope failures occurred just above glacial trimlines and all occurred in areas with a previous history of deep-seated gravitational slope movement, implying that this type of deformation is a precursor to catastrophic detachment.     

Structure and evolution of mass transport deposits in the South Caspian Basin,Azerbaijan

The Quaternary to late Pliocene sedimentary succession along the margin of the South Caspian Basin contains numerous kilometre‐scale submarine slope failures, which were sourced along the basin slope and from the inclined flanks of contemporaneous anticlines. This study uses three‐dimensional (3D) seismic reflection data to visualise the internal structure of 27 mass transport deposits and catalogues the syndepositional structures contained within them. These are used to interpret emplacement processes occurring during submarine slope failure. The deposits consist of three linked structural domains: extensional, translational and compressive, each containing characteristic structures. Novel features are present within the mass transport deposits: (1) a diverging retrogression of the headwall scarp; (2) the absence of a conventional headwall scarp around growth stratal pinch outs; (3) restraining bends in the lateral margin; (4) a downslope increase in the throw of thrust faults. The results of this study shed light on the deformation that occurred during submarine slope failure, and highlight an important geological process in the evolution of the South Caspian Basin margin.     

黄土高原典型小流域道路特征及影响因素

通过GPS野外实测黄土高原纸坊沟小流域的道路特征,在GIS空间分析等方法支持下研究了流域内各级道路的分布规律。研究表明:黄土高原道路可分为4级,2级以下的土路是小流域的代表路型。各级别道路总长度都随级别增大而增加且累计总长度与道路级别线性相关,道路平均长度和密度则随级别增大而减小。道路网系和流域水系在结构规律上具有一定的相似性。不同级别道路分布范围受地形特征影响不同。爬坡道路坡度与地表坡度之间存在线性回归关系,且次级道路分布受主路控制;典型爬坡路在大于25°坡度范围内多呈"之"字形弯曲,随坡度的减小道路弯曲跨度和与等高线夹角增大。研究结果有助于定量模拟黄土高原地区道路分布特征,并为土壤侵蚀模型中道路影响因子的确定提供参考。     

Geomorphic Effects of the Extreme Rainfall Event of 20–21 July, 2004 in the Latnjavagge Catchment, Northern Swedish Lapland

Mass transfers triggered by a rare rainfall event on 20–21 July, 2004, with 58.4 mm of rain within 24 h and 71.7 mm of rain within 48 h in the Latnjavagge catchment (9 km² , 950–1440 m a.s.l.; 68°20'N, 18°30'E) in the higher Abisko mountain region (Swedish Lapland), are quantified and analysed in direct comparison with mean annual mass transfers in this drainage basin. In years without rare rainfall events the Latnjavagge catchment is characterized by restricted sediment availability resulting in low mechanical denudation and mass transfers. During the rare rainfall event of 20–21 July, 2004, major stability thresholds on the slope systems (triggering debris flows and slides) and in the channel systems (break‐up of channel debris pavements and step–pool systems) in the Latnjavagge catchment were passed and mass transfers by debris flows, slides and fluvial debris transport in creeks and channels were several times higher than the mean annual mass transfers in Latnjavagge. In the calculation of longer‐term mass transfers and sediment budgets, rare events like the 20–21 July, 2004 rainfall event have to be considered as essential components. A reliable estimation of the recurrence intervals of such rare events is especially problematic. The general problem of defining an adequate length of process monitoring programmes is pointed out.     

REVISITING THE TOPIA ROAD: WALKING IN THE FOOTSTEPS OF WEST AND PARSONS*

ABSTRACT. In 1940, Berkeley graduate‐student geographers Robert West and James Parsons traveled to Mexico's Sierra Madre Occidental to retrace the Topia Road, colonial Mexico's main trans‐Sierran trail linking isolated mountain mining hamlets with the Pacific Coast and the world beyond, a journey chronicled in a 1941 Geographical Review article. Almost sixty years later, we document an attempt to retrace West and Parsons's route. Based on field observations, interviews with local informants, replication of Parsons's photographs, and his field notes, we evaluate landscape alteration in what West and Parsons referred to as some of the most isolated settlements in Mexico. We assess changes in the still‐remote communities along the route in terms of three influences: mining, migration, and drug trafficking.     

Holocene mass‐wasting events in Lago Fagnano,Tierra del Fuego (54°S): implications for paleoseismicity of the Magallanes‐Fagnano transform fault

High‐resolution seismic imaging and coring in Lago Fagnano, located along a plate boundary in Tierra del Fuego, have revealed a dated sequence of Holocene mass‐wasting events. These structures are interpreted as sediment mobilizations resulting from loading of the slope‐adjacent lake floor during mass‐flow deposition. More than 19 mass‐flow deposits have been identified, combining results from 800 km of gridded seismic profiles used to site sediment cores. Successions of up to 6‐m thick mass‐flow deposits, pond atop the basin floor and spread eastward and westward following the main axis of the eastern sub‐basin of Lago Fagnano. We developed an age model, on the basis of information from previous studies and from new AMS‐¹⁴C ages on cored sediments, which allows us to establish a well‐constrained chronologic mass‐wasting event‐catalogue covering the last ～12 000 years. Simultaneously triggered, basin‐wide lateral slope failure and the formation of multiple debris flow and postulated megaturbidite deposits are interpreted as the fingerprint of paleo‐seismic activity along the Magallanes‐Fagnano transform fault that runs along the entire lake basin. The slope failures and megaturbidites are interpreted as recording large earthquakes occurring along the transform fault since the early Holocene. The results from this study provide new data about the frequency and possible magnitude of Holocene earthquakes in Tierra del Fuego, which can be applied in the context of seismic hazard assessment in southernmost Patagonia.     

MASS WASTING IN THE SWAUK WATERSHED,WASHINGTON

Mass wasting evidence is common along the margins of the Columbia River Basalts. I identified, mapped, dated, and assessed the environment of nearly 160 discrete slope failures (excluding rockfall) along the margins of the Columbia River Basalts in the Swauk watershed of central Washington. Rotational slides, translational slides, flows, and complex slide-flows were identified via topographic map, airphoto, and field analysis. Geographic information systems analysis revealed that these features cover 38% of the watershed. Translational slides are the most numerous of the slope failures, whereas complex slide-flows cover the most area. I placed each slope failure into a relative age category (active, inactive-young, inactive mature, and inactive-old) based on the characteristics of the main scarp, lateral flanks, internal morphology, vegetation cover, and toe relationships. Most Swauk watershed slope failures are inactive-mature. Organic sediments from an inactive-mature sag pond formed ～6880 ¹⁴C yr BP, whereas inactive-young sediments dated at ～5930 ¹⁴C yr BP. Inactive slope failures are often associated with steep slopes, inclined beds, incompetent geologic units, or streamcuts. Streamcuts, roadcuts, or clearcuts typically accompany active slope failures. Rain-on-snow events and associated mass wasting in winter 1996 provide a plausible trigger analog for inactive mass wasting. Rockfall deposits cover ～29% of the watershed, range from inactive to active in age, and occur atop pre-existing slope failures in well-jointed Columbia River Basalts. Mass wasting has played a key role in shaping the topographic and hydrologic patterns of the watershed. [Key words: mass wasting, watershed, Washington state, Columbia River Basalts, rain-onsnow.]     

暴雨内涝影响下的城市道路交通拥挤特征识别

近年来,暴雨内涝频繁发生,常引发严重的城市交通拥堵问题。本文利用自主开发的宏观交通模拟工具,模拟了上海市中心城区50年一遇和100年一遇暴雨强度情景下每条路段的小时交通量,通过计算道路饱和度,研究了不同强度暴雨内涝对中心城区高架出入口和重要道路拥挤程度的影响。结果表明：①100年一遇暴雨内涝对上海市中心城区道路交通服务能力影响显著,可导致7个高架道路出入口关闭,部分出入口严重拥堵;②暴雨内涝对道路拥堵状况影响的差异性明显,变拥挤路段占道路总里程的13.35%,其中一级道路的拥挤程度变化最为明显,如：大连路、武宁路,周家嘴路和长寿路等主要路段服务水平下降。     

On Squatters,Settlers and Early Surveyors: historical development of country road reserves in southern New South Wales

Country roads are an important social, economic and ecological component of agricultural landscapes in many parts of New South Wales. Using the Lockhart Shire Council area as a case study, this paper describes how present-day rural road networks are a historical vestige of past land-use decisions; a collection of Travelling Stock Routes, pastoral boundaries of early squatters, rail reserves, county and parish boundaries, and an ad hoc collection of mostly 1 chain (20.12 m) roads surveyed during the late 1800s. Road reserves were allocated across the landscape at the discretion of early surveyors, so new land titles could access vital water resources. Nineteenth-century surveyors faced many difficulties in surveying the extent of NSW, sometimes resulting in misalignments which today give ‘character’ to many country roads. Road reserves were often surveyed 2 or 3 chains wide to allow for predicted usage and provide materials for road construction. In the 1870s, Travelling Stock Routes of varying widths from 1 to 80 chains wide (one mile) were gazetted, which often form an integral part of many country road networks. In this way, present-day conservation values of many road reserves is a direct legacy of land policies, administrators and surveyors on the landscape today.     

Multi‐scale spatiotemporal analyses of moose–vehicle collisions: a case study in northern Vermont

Moose–vehicle collisions (MVCs) pose a serious safety and environmental concern in many regions of Europe and North America. For example, in the state of Vermont, one‐third of all reported MVCs resulted in motorist injury or fatality while collisions have increased from two in 1982 to 164 in 2002. Our work used a MVC dataset from 1983 to 1999 in the Northeastern Highlands of Vermont (four major roads) to perform space, time and spatiotemporal analyses and guide future mitigation strategies. An adapted kernel density estimator was implemented for exploratory analyses to detect high density collision hotspots on roads. The kernel in space showed seven major density peaks which varied in magnitude and spread between roads. The kernel estimator in time for all roads showed an exponentially increasing trend with annual periodicity and a seasonal cyclic component, where the majority of collisions occurred from May to October. Spatiotemporal kernel estimation exhibited discontinuous density hotspots in time and space suggesting changing animal movement patterns across roads. We used an adapted Ripley's K‐function to test the hypothesis that MVCs clustering occurred at multiple scales in space, in time and in space–time combined. Statistically significant spatial clustering was evident on all roads at spatial scales from 2 to 10 km. A more consistent clustering in time occurred on all roads at a scale distance of 5 years. Similar to the kernel estimation, annual periodicity was also evident. Positive space–time clustering was present at small spatial (5 km) and temporal scales (2 years) indicating that where MVCs occur is also influenced by when they occur. In retrospect, using multiple road lengths, and the combined kernel estimation and Ripley's K‐function in time and space, provided a powerful methodology to study varying spatiotemporal patterns of wildlife collisions along roads. This can greatly assist transportation planners in identifying optimal mitigation strategies along specific roads, such as deciding on location and spatial length for permanent and expensive measures (e.g. crossing structures and associated fencing) versus less permanent and inexpensive structures (e.g. wildlife signage and reduced speed limits).     

Gully retreat in a semi-urban catchment in Saskatoon, Saskatchewan

Slope instability has been an on-going problem along the South Saskatchewan River within the city of Saskatoon, and has adversely affected private property, bridge foundations, and municipal roads and services. Wooded declivities were present prior to urban de velopment, but changes in drainage patterns associated with construction of an urban park about 30 years ago have resulted in pronounced gullying. Morphometric measurements of two gullies conducted from 1994 to 2000 showed that their headwalls have retreated approximately 7 and 19 m, respectively, with accompanying widening and deepening of the features. The process of retreat is driven largely by the seasonal precipitation regime; it mainly occurs after heavy summer rainstorms, although freeze–thaw activity during snowmelt and subsurface flow are also implicated. Retreat is further controlled by the inherent strength of the root mat that binds the surface soils until the undercut trees finally collapse. Surface runoff through the gullies recently has been redirected through a storm sewer that is designed to mitigate the erosion problem.     

Spatial distribution of surface conditions and runoff generation in small arid watersheds, Zin Valley Badlands, Israel

High rill density may be regarded as indicative of frequent and integrated runoff along a drainage network. However, field observations of soil development and rill geometry in small, first-order catchments (0.1 to 1 ha) of the Zin Valley Badlands, northern Negev, Israel, suggest a pattern of partial area contribution and frequent flow discontinuities along hillslopes and channels. Changing soil properties, associated with an increase of slope angle and slope length, appear to be responsible for high infiltration on the slopes, leading to nonuniform runoff generation within small drainage basins. Runoff observation, sprinkling tests, and soil analysis along ridges and sideslopes were carried out to test this hypothesis. The results confirmed that infiltration capacity on the sideslopes is significantly higher than on the ridges. Under current rainfall conditions, only extreme rainfall intensities are sufficient to generate runoff along entire slopes. The discontinuous nature of most runoff events causes erosion on ridges and deposition on slopes, which enhances soil development on the valley sideslopes, creating a positive feedback on infiltration rate and depth. This demonstrates that the links between within-storm rainfall conditions and spatial distribution of soil characteristics have to be incorporated into our understanding of landscape development in badlands.     

基于GIS的北川县地震次生滑坡灾害空间预测

自然灾害的预测预报被认为是主动减灾防灾研究中较为经济有效的方式,其中,滑坡空间预测是滑坡灾害研究的基础工作。以汶川地震重灾区北川县为研究区,选取坡度、高程、岩石类型、地震烈度、水系、道路等6个重要滑坡影响因素作为评价因子,全面分析了地震滑坡分布与各影响因子之间的统计相关性,分别采用多元回归模型与神经网络模型计算滑坡灾害敏感性指数,并进行分级和制图。结果表明,极高和高敏感区主要分布在曲山、陈家坝等乡镇,主要沿着龙门山断裂带周边地区的河流和道路呈带状分布。其中,回归模型的预测精度为73.7%,神经网络模型的预测精度为81.28%,在本区域内,神经网络模型在滑坡灾害空间预测方面更具优势。     

MAPPING KENTUCKY'S FRONTIER TRAILS THROUGH GEOGRAPHICAL INFORMATION AND CARTOGRAPHIC APPLICATIONS*

Historical first‐generation frontier roads in America's trans‐Appalachian West often evolved from buffalo and Indian trails into pioneer routeways such as Daniel Boone's Trace and, eventually, into twentieth‐century hard‐surface highways. Period cartographers found these routes difficult to document accurately, and present‐day scholars often depict them only on small‐scale maps, which simply illustrate connections between origin and destination points. Accurately mapping Kentucky's first‐generation roads at large scale requires detailed site and contextual topographic information over long distances, but historical maps, diaries, surveyors' reports, and other period documents often lack sufficient detail for route‐related sites to support mapping. Use of gis software enables positioning historical routes onto U.S. Geological Survey contour‐ and hill‐shaded base maps by mapping verifiable locations and linking them through interpretation of best‐choice routes that consider frontier migrants' transportation priorities, such as direction, distance, gradient, and land‐surface character.     

LANDSLIDE‐GLACIER INTERACTION IN A NEOPARAGLACIAL SETTING AT TVERRBYTNEDE,JOTUNHEIMEN, SOUTHERN NORWAY

A tongue‐like, boulder‐dominated deposit in Tverrbytnede, upper Visdalen, Jotunheimen, southern Norway, is interpreted as the product of a rock avalanche (landslide) due to its angular to subangular boulders, surface morphology with longitudinal ridges, down‐feature coarsening, and cross‐cutting relationship to ‘Little Ice Age’ moraines. The rock avalanche fell onto glacier ice, probably channelled along a furrow between two glaciers, and stopped on the glacier foreland, resulting in its elongated shape and long runout distance. Its distal margin may have become remobilized as a rock glacier, but a rock glacier origin for the entire landform is discounted due to lack of source debris, presence of matrix, lack of transverse ridges, and sparcity of melt‐out collapse pits. Lichenometric dating of the deposit indicates an approximate emplacement age of ad 1900. Analysis highlights the interaction of rock‐slope failures and glaciers during deglacierization in a neoparaglacial setting, with reduced slope stability due to debuttressing and permafrost degradation, and enhanced landslide mobility due to flow over a glacier and topographic channelling. Implications for the differentiation of relict landslides, moraines and rock glaciers are discussed and interrelationships between these landforms are considered in terms of an ice‐debris process continuum.