Leo Jones

Landslide Laser Scanning: a new look at an old problem

An essential part of the study of any landslide problem is the development of a 3-D conceptual model of the landslide and its surroundings. Traditionally this has been done by pen and paper during a walk-over geomorphological survey (Forster 1989) ideally with pits and boreholes to give subsurface information. More recently aerial photographs and aerial ortho-photographs have aided the surface assessment and geophysical surveys have added to the understanding of the third dimension. A more detailed quantitative model could be achieved by accurate ground surveying by EDM, theodolite and level. However, this is time consuming and a high degree of surveying expertise is necessary for an accurate result where small movements might need to be monitored. More recently ‘total station’ surveying equipment has increased the speed at which this process can be completed. However, the advent of affordable semi-automated laser scanners and accurate differential GPS technology has opened up a new opportunity in rapid landslide modelling. The British Geological Survey is currently using and developing laser scanning for the characterization, modelling and monitoring of landslides, essentially a terrestrial LIDAR technique (Hobbs et al. 2002). A small part of that work has been looking at two landslides in the Cotswolds, in association with the School of Earth and Environmental Sciences of the University of Portsmouth, which has a major research interest in studying the landslides in the Jurassic strata near the village of Broadway in …

Monitoring landslides in hazardous terrain using terrestrial LiDAR: an example from Montserrat

It is important to monitor unstable slopes to determine where adverse changes in morphology are indicating the likelihood of failure and a consequent risk to life and property. However, where highly unstable slopes need to be monitored, their very nature precludes the use of direct measurement by surveyors and remote means must be used to protect the safety of the survey team. The use of LiDAR (Light Detection and Ranging) enables the accurate location of a network of points that can be used to create a detailed 3D terrain model, or DEM (Digital Elevation Model), of greater coverage and accuracy than conventional methods, with almost complete safety of the operators. There can be few more hazardous situation than that of monitoring a volcanic andesite lava dome for signs of an impending collapse. Partial collapse of a lava dome generates hot, fast-moving pyroclastic density currents, which are categorized as pyroclastic surge or block-and-ash flow deposits based on sedimentary structures. Monitoring in such circumstances requires that measurements are taken from a distance that minimizes the threat from eruption and from asphyxiation by volcanic gases. The method also needs to be rapid to minimize the time spent by the monitoring team in the hazardous zone. The techniques described here are applicable to the monitoring of any unstable slopes and active landslide where hazardous conditions threaten the survey team. Montserrat is a volcanic island of the Leeward Islands in the Caribbean Sea situated 43 km SW of Antigua. The survey was carried out from Perches Mountain, on the margin of the Soufriere Hills volcanic crater (Fig. 1). The survey covered …

Fracturing, block faulting, and moulin development associated with progressive collapse and retreat of a maritime glacier: Falljökull, SE Iceland

Since 2007, Falljokull in southeast Iceland has been undergoing passive downwasting, providing an ideal opportunity to study a range deformation structures developed in response to ice-marginal collapse and retreat. An integrated terrestrial lidar, Ground Penetrating Radar, and glaciological structural study of the clean, debris-free ice at the margin of Falljokull has allowed a detailed model of the surface and subsurface 3D structure to be developed. Collapse of the glacier margin takes the form of a multiple rotational failure controlled by large-scale, down-ice dipping normal faults. As the fault-bound blocks of ice are displaced downslope, they rotate leading to localized compression and the formation of down-faulted graben-like structures. Moulins present within the marginal zone of Falljokull are closely associated with the zones of relatively more intense brittle deformation which crosscut the glacier. A model is proposed where the moulins have formed in response to the progressive collapse of englacial drainage channels located along down-ice dipping normal faults. The preferential development of the moulins and englacial drainage channels along the normal faults weakens the ice along these structures, promoting or even accelerating further collapse of the ice margin. The complex pattern of surface lowering within the marginal zone of Falljokull has also been shown to be directly related to movement on the main faults controlling the collapse of the ice margin. This evidence suggests that structurally controlled collapse may, in some instances, have a profound effect on glacier surface lowering and geodetic mass balance measurements.

Structural evolution triggers a dynamic reduction in active glacier length during rapid retreat: Evidence from Falljökull, SE Iceland

Over the past two decades Icelands glaciers have been undergoing a phase of accelerated retreat set against a backdrop of warmer summers and milder winters. This paper demonstrates how the dynamics of a steep outlet glacier in maritime SE Iceland have changed as it adjusts to recent significant changes in mass balance. Geomorphological evidence from Falljokull, a high-mass turnover temperate glacier, clearly shows that between 1990 and 2004 the ice front was undergoing active retreat resulting in seasonal oscillations of its margin. However, in 2004–2006 this glacier crossed an important dynamic threshold and effectively reduced its active length by abandoning its lower reaches to passive retreat processes. A combination of ice surface structural measurements with radar, lidar, and differential Global Navigation Satellite Systems data are used to show that the upper active section of Falljokull is still flowing forward but has become detached from and is being thrust over its stagnant lower section. The reduction in the active length of Falljokull over the last several years has allowed it to rapidly reequilibrate to regional snowline rise in SE Iceland over the past two decades. It is possible that other steep, mountain glaciers around the world may respond in a similar way to significant changes in their mass balance, rapidly adjusting their active length in response to recent atmospheric warming.

Modelling Volume Change Potential in the London Clay

Abstract The London Clay Formation is particularly susceptible to shrink–swell behaviour that has resulted in a long history of foundation damage owing to ground movement across the outcrop. Damage has cost up to £500 million in a single year. Underlying most of the Greater London area, the London Clay Formation is of major engineering importance as it is on and within this formation that the majority of the citys infrastructure, buildings and underground services are constructed. The Volume Change Potential (VCP) of a soil is the relative change in volume to be expected with changes in soil moisture content, and the subsequent shrinkage or swelling can cause major damage to structures above or below ground. Detailed statistical and spatial analyses of data across the London Clay outcrop have revealed a significant geographical trend in the VCP of this deposit, confirming an overall increase from west to east, but also showing subtle trends with depth. This paper describes how this analysis was carried out and shows how such assessments can yield valuable information about shrink–swell behaviour not only of the London Clay but also of similar shrink–swell-prone argillaceous formations elsewhere.

Monitoring coastal change using terrestrial LiDAR

Abstract The paper describes recent applications by the British Geological Survey (BGS) of the technique of mobile terrestrial Light Detection And Ranging (LiDAR) surveying to monitor various geomorphological changes on English coasts and estuaries. These include cliff recession, landslides and flood defences, and are usually sited at remote locations undergoing dynamic processes with no fixed reference points. Advantages, disadvantages and some practical problems are discussed. The role of GPS in laser scanning is described.

Testing the utility of structure-from-motion photogrammetry reconstructions using small unmanned aerial vehicles and ground photography to estimate the extent of upland soil erosion

Quantifying the extent of soil erosion at a fine spatial resolution can be time consuming and costly; however, proximal remote sensing approaches to collect topographic data present an emerging alternative for quantifying soil volumes lost via erosion. Herein we compare terrestrial laser scanning (TLS), and both unmanned aerial vehicle (UAV) and ground photography (GP) structure-from-motion (SfM) derived topography. We compare the cost-effectiveness and accuracy of both SfM techniques to TLS for erosion gully surveying in upland landscapes, treating TLS as a benchmark. Further, we quantify volumetric soil loss estimates from upland gullies using digital surface models derived by each technique and subtracted from an interpolated pre-erosion surface. Soil loss estimates from UAV and GP SfM reconstructions were comparable to those from TLS, whereby the slopes of the relationship between all three techniques were not significantly different from 1:1 line. Only for the TLS to GP comparison was the intercept significantly different from zero, showing that GP is more capable of measuring the volumes of very small erosion features. In terms of cost-effectiveness in data collection and processing time, both UAV and GP were comparable with the TLS on a per-site basis (13.4 and 8.2 person-hours versus 13.4 for TLS); however, GP was less suitable for surveying larger areas (127 person-hours per ha(-1) versus 4.5 for UAV and 3.9 for TLS). Annual repeat surveys using GP were capable of detecting mean vertical erosion change on peaty soils. These first published estimates of whole gully erosion rates (0.077 m a(-1)) suggest that combined erosion rates on gully floors and walls are around three times the value of previous estimates, which largely characterize wind and rainsplash erosion of gully walls. Copyright (c) 2017 John Wiley & Sons, Ltd.

Concentrated, ‘pulsed’ axial glacier flow: structural glaciological evidence from Kvíárjökull in SE Iceland

A detailed structural glaciological study carried out on Kviarjokull in SE Iceland reveals that recent flow within this maritime glacier is concentrated within a narrow corridor located along its central axis. This active corridor is responsible for feeding ice from the accumulation zone on the south-eastern side of Oraefajokull to the lower reaches of the glacier and resulted in a c. 200 m advance during the winter of 2013–2014 and the formation of a push-moraine. The corridor comprises a series of lobes linked by a laterally continuous zone of highly fractured ice characterised by prominent flow-parallel crevasses, separated by shear zones. The lobes form highly crevassed topographic highs on the glacier surface and occur immediately down-ice of marked constrictions caused by prominent bedrock outcrops located on the northern side of the glacier. Close to the frontal margin of Kviarjokull, the southern side of the glacier is relatively smooth and pock-marked by a number of large moulins. The boundary between this slow moving ice and the active corridor is marked by a number of ice flow-parallel strike-slip faults and a prominent dextral shear zone which resulted in the clockwise rotation and dissection of an ice-cored esker exposed on the glacier surface. It is suggested that this concentrated style of glacier flow identified within Kviarjokull has affinities with the individual flow units which operate within pulsing or surging glaciers

The occurrence of dinoflagellate cysts in calcareous/siliceous microfossil preparations from the Eocene of southeast England

INTRODUCTION During routine analysis of some calcareous and siliceous microbiotas from the London Clay Formation of southeast England (Wilkinson, 2004a, b), some extremely well-preserved, dinoflagellate cysts were observed. These are all large, chorate (spine-bearing) forms, the overwhelming majority of which are referable to Cordosphaeridium gracile (Eisenack, 1954) Davey & Williams, 1966 (Fig. 1). This observation confirms that palynomorphs can be extracted effectively from clay-rich samples using a combination of clay deflocculation and sieving, as described recently by Riding & Kyffin-Hughes (2004). PROCESSING The London Clay Formation from 25 localities in southeast England was processed for calcareous and siliceous microfossils (diatoms, foraminifera, ostracods and radiolaria) (Wilkinson, 2004a, b). The standard British Geological Survey (BGS) procedure for the extraction of calcareous/siliceous microfossils from poorly- or non-indurated sediments was used. Samples were disaggregated by soaking in a mixture of warm water and detergent and agitated periodically. The majority of the clay fraction was removed using a 72 μm sieve. The residue was then boiled in sodium hexametaphosphate [(NaPO3)6] before being sieved again and dried. The sieving will have removed the majority of the palynomorphs, except the larger dinoflagellate cysts. Virtually all pollen and spores and most dinoflagellate cysts of this age are less than 72 μm in at least one dimension. MICROFOSSIL ASSEMBLAGES Specimens of Cordosphaeridium gracile were recovered from three clay quarries. These are Brambledown Pit, Kent [TQ 967 715], Bull’s Lodge Pit, Chelmsford, Essex [TL 7470 0850] and Heckfordbridge Pit, Colchester, Essex [TL 9550 2260], although only two are discussed here …

The geomorphology of Svínafellsjökull and Virkisjökull-Falljökull glacier forelands, southeast Iceland

ABSTRACT A detailed, 1:10,500-scale, surficial geology and glacial geomorphology map of Svínafellsjökull and Virkisjökull-Falljökull glacier forelands in southeast Iceland depicts the landsystem imprint of Holocene glacier fluctuations, volcanogenic outburst floods and recent (post-1990) climate-induced rapid ice-front retreat. The map is based on field survey data in combination with 2012 airborne LiDAR data, 2009–2012 terrestrial LiDAR data and 2007 colour aerial photography. The base digital elevation model (DEM) is compiled from an ice-cap wide airborne LiDAR dataset. The mapped glacial landforms are dominated by sequences of recessional moraines laid down in the mid-Holocene, the Little Ice Age, and the last ∼100 years; the state of landform preservation generally decreasing with age. Interspersed with glaciofluvial sedimentation associated with typical ice-marginal retreat sequences is key geomorphological evidence of high-magnitude volcanogenic outburst floods (jökulhlaups) associated with the eruptions of Öraefajökull in 1362 and 1727 CE. Ice-front retreat has accelerated since c.2005 leaving a rapidly evolving buried-ice landscape in front of Virkisjökull-Falljökull – including an ice-cored esker, a large ice-floored (supraglacial) lake, and numerous actively forming kettle holes and ice caverns. This map could act as a ‘reference frame’ for geomorphologists studying the temporal evolution of glacial landform-sediment assemblages undergoing rapid change.