Centrifuge modelling of reverse fault–foundation interaction

The propagation of reverse faults through soil to the ground surface has been observed to cause damage to surface infrastructure. However, the interaction between a fault propagating through a sand layer and a shallow foundation can be beneficial for heavily loaded foundations by causing deviation of the fault away from the foundation. This was studied in a series of centrifuge model tests in which reverse faults of dip angle 60° (at bedrock level) were initiated through a sand layer, close to shallow foundations. The tests revealed subtle interaction between the fault and the shallow foundation so that the foundation and soil response depend on the foundation loading, position, breadth and flexibility. Heavily loaded rigid foundations appeared best able to deviate fault rupture away from the foundation but this deviation could be associated with significant foundation rotations. However, a lightly loaded foundation was unable to deviate a reverse fault and the fault emerged beneath the foundation. This led to gapping beneath the foundation as well as significant rotations and may cause severe structural distress. As well as providing insight into the mechanisms of behaviour, the data from the tests is used to validate finite element analyses in a separate article.     

Influence of sea surface wind wave turbulence upon wind-induced circulation, tide–surge interaction and bed stress

A three-dimensional finite volume unstructured mesh model of the west coast of Britain, with high resolution in the coastal regions, is used to investigate the role of wind wave turbulence and wind and tide forced currents in producing maximum bed stress in the eastern Irish Sea. The spatial distribution of the maximum bed stress, which is important in sediment transport problems, is determined, together with how it is modified by the direction of wind forced currents, tide–surge interaction and a surface source of wind wave turbulence associated with wave breaking. Initial calculations show that to first order the distribution of maximum bed stress is determined by the tide. However, since maximum sediment transport occurs at times of episodic events, such as storm surges, their effects upon maximum bed stresses are examined for the case of strong northerly, southerly and westerly wind forcing. Calculations show that due to tide–surge interaction both the tidal distribution and the surge are modified by non-linear effects. Consequently, the magnitude and spatial distribution of maximum bed stress during major wind events depends upon wind direction. In addition calculations show that a surface source of turbulence due to wind wave breaking in shallow water can influence the maximum bed stress. In turn, this influences the wind forced flow and hence the movement of suspended sediment. Calculations of the spatial variability of maximum bed stress indicate the level of measurements required for model validation.     

Compartmentalisation and groundwater–surface water interactions in a prospective shale gas basin: Assessment using variance analysis and multivariate statistics on water quality data

An environmental concern with hydraulic fracturing for shale gas is the risk of groundwater and surface water contamination. Assessing this risk partly involves the identification and understanding of groundwater–surface water interactions because potentially contaminating fluids could move from one water body to the other along hydraulic pathways. In this study, we use water quality data from a prospective shale gas basin to determine: if surface water sampling could identify groundwater compartmentalisation by low-permeability faults; and if surface waters interact with groundwater in underlying bedrock formations, thereby indicating hydraulic pathways. Variance analysis showed that bedrock geology was a significant factor influencing surface water quality, indicating regional-scale groundwater–surface water interactions despite the presence of an overlying region-wide layer of superficial deposits averaging 30–40 m thickness. We propose that surface waters interact with a weathered bedrock layer through the complex distribution of glaciofluvial sands and gravels. Principal component analysis showed that surface water compositions were constrained within groundwater end-member compositions. Surface water quality data showed no relationship with groundwater compartmentalisation known to be caused by a major basin fault. Therefore, there was no chemical evidence to suggest that deeper groundwater in this particular area of the prospective basin was reaching the surface in response to compartmentalisation. Consequently, in this case compartmentalisation does not appear to increase the risk of fracking-related contaminants reaching surface waters, although this may differ under different hydrogeological scenarios.     

Considering river structure and stability in the light of evolution: feedbacks between riparian vegetation and hydrogeomorphology

River ecological functioning can be conceptualized according to a four‐dimensional framework, based on the responses of aquatic and riparian communities to hydrogeomorphic constraints along the longitudinal, transverse, vertical and temporal dimensions of rivers. Contemporary riparian vegetation responds to river dynamics at ecological timescales, but riparian vegetation, in one form or another, has existed on Earth since at least the Middle Ordovician (c. 450 Ma) and has been a significant controlling factor on river geomorphology since the Late Silurian (c. 420 Ma). On such evolutionary timescales, plant adaptations to the fluvial environment and the subsequent effects of these adaptations on fluvial sediment and landform dynamics resulted in the emergence, from the Silurian to the Carboniferous, of a variety of contrasted fluvial biogeomorphic types where water flow, morphodynamics and vegetation interacted to different degrees. Here we identify several of these types and describe the consequences for biogeomorphic structure and stability (i.e. resistance and resilience), along the four river dimensions, of feedbacks between riparian plants and hydrogeomorphic processes on contrasting ecological and evolutionary timescales. Copyright © 2014 John Wiley & Sons, Ltd.     

Storm surge computations in estuarine and near-coastal regions: the Mersey estuary and Irish Sea area

An unstructured grid storm surge model of the west coast of Britain, incorporating a high-resolution representation of the Mersey estuary is used to examine storm surge dynamics in the region. The focus of the study is the major surge that occurred during the period 11–14 November 1977, which has been investigated previously using uniform grid finite difference models and a finite element model of the west coast of Britain. However, none of these models included the Mersey estuary. Comparison of solutions in the eastern Irish Sea with those computed with these earlier models showed that, away from the Liverpool Bay region, the inclusion of the Mersey estuary had little effect. However, at the entrance to the Mersey, its inclusion did influence the solution. By including a detailed representation of the Mersey estuary within the model, it was possible to conduct a detailed study of storm surge propagation in the Mersey, which had never previously been performed. This detailed study showed for the first time that the surge’s temporal variability within the estuary is influenced by surge elevation at its entrance. This varies with time as a function of spatial and temporal variations of wind stress over the west coast of Britain. Within the Mersey, calculations show that the spatial variability is mainly determined by changes in bottom topography, which had not been included in earlier finite difference models. However, since water depth is influenced by variations in tidal elevation, this, together with tide surge interaction through bottom friction and momentum advection, influences the surge. The ability of the finite element model to vary the mesh in near-shore regions to such an extent that it can resolve the Mersey and hence the impact of the Mersey estuary upon the Liverpool Bay circulation shows that it has distinct advantages over earlier finite difference models. In the absence of detailed measurements within the Mersey at the time of the surge, it was not possible to validate predicted surge elevations within the Mersey. However, significant insight into physical processes influencing the surge propagation down the estuary, its reflection and spatial/temporal variability could be gained.     

On the sensitivity of computed surges to open-boundary formulation

A coarse-grid (resolution of order 7 km) model of the west coast of Britain is used to examine the sensitivity of computed storm-surge elevations and currents to a range of open-boundary conditions. The storm-surge period 1 to 26 March 1994 is used for this comparison, as it is a time of significant wind activity. Also current measurements in the North Channel of the Irish Sea together with coastal elevation measurements are available for model validation. Elevations and currents previously computed with a coarse-grid shelf-wide model can also be incorporated into the open-boundary condition to examine the influence of far-field effects. Initial model calculations with no far-field input show the importance of including shelf-wide effects from either the external shelf model, or by using observations from coastal gauges interpolated along the open boundary of the west-coast model. Provided the west-coast models open boundary is taken sufficiently far away from the region of interest, in this case the Irish Sea, then either a radiation condition or an elevation-specified condition is appropriate provided far-field effects are taken into account. If these are not included, then neither boundary condition is successful. For the radiation condition it is necessary to include both elevations and currents from a far-field model in order to reproduce the surge. In the case of an elevation-specified boundary condition far-field effects can be incorporated in hindcast calculations by including observed sea-level changes. In a storm-surge prediction calculation the radiation condition with a far-field model is required. Calculations show that computed elevations are spatially more coherent than currents, with flows through the western Irish Sea showing the greatest sensitivity to open-boundary formulation during storm events.Responsible Editor: Phil Dyke     

Stormflow dynamics and loads of Escherichia coli in a large mixed land use catchment

Storm events are major transporters of faecal microbial contaminants, but few studies have reported storm loads or concentration dynamics in relation to discharge or other pollutants, notably fine sediment. Episodically, high loads of faecal contamination during storm flows impact downstream uses of water bodies, particularly contact recreation and shellfish harvesting. We examined the storm dynamics of Escherichia coli, turbidity and discharge in the mixed land use Motueka catchment (2047 km²; 60% forest and 19% pasture) to gain insights into E. coli sources and transport. We also explored different approaches for calculating E. coli loads. Discharge and field turbidity were recorded continuously, and E. coli concentrations were sampled during events, over a 13‐month period near the mouth of the Motueka River. E. coli loads were estimated by interpolation, averaging estimators and by using linear regression with smearing correction of the log‐transformed variables: discharge, turbidity, and both turbidity and discharge. The annual E. coli load was dominated (～98%) by export during events. Comparison of monthly monitoring with the intensive storm monitoring campaign suggests that simple stratification of the sampling into storm and baseflow would greatly improve export estimates. E. coli peak concentrations always preceded discharge and turbidity peaks (which had similar timing). Turbidity can be a useful surrogate for faecal microbes in smaller catchments, but in the Motueka turbidity was no better for predicting E. coli concentration than discharge. Runoff from grazed pasture and direct deposition from livestock are probably the ultimate E. coli sources in the Motueka catchment. However, in‐channel stores seem to dominate E. coli dynamics during events and account for the typical feature of bacterial concentrations peaking ahead of discharge and turbidity. This study demonstrates the importance of storm events to faecal microbial loads and shows that E. coli concentration dynamics may contrast with those of turbidity. Copyright © 2009 John Wiley & Sons, Ltd.     

A large landslide event in a post‐glacial landscape: rethinking glacial legacy

Threlkeld Knotts (c. 500 m above sea level) in the English Lake District has hitherto been considered to be a glacially‐modified intrusion of microgranite. However, its surface features are incompatible with glacial modification; neither can these nor the subsurface structures revealed by ground‐penetrating radar (GPR) be explained by post‐glacial subaerial processes acting on a glacially‐modified microgranite intrusion. Here we re‐interpret Threlkeld Knotts as a very large post‐glacial landslide involving the microgranite, with an estimated volume of about 4 × 10⁷ m³. This interpretation is tested against published and recent information on the geology of the site, the glacial geomorphic history of the area and newly‐acquired GPR data. More than 60 large post‐Last Glacial Maximum (LGM) rock–slope failures have significantly modified the glaciated landscape of the Lake District; this is one of the largest. Recognition of this major landslide deposit in such a well‐studied environment highlights the need to continuously re‐examine landscapes in the light of increasing knowledge of geomorphic processes and with available technology in currently active or de‐glaciating environments. Copyright © 2012 John Wiley & Sons, Ltd.