No description available.

No description available.

Between 1985 and 1990 a computerised database was assembled that contains information concerning the distribution and nature of 9000 landslides in Great Britain. The South West accounts for 1,700 of the listed slope failures. Problems have arisen because in many areas the database merely serves as an archive of previous studies and is not a true representation of landsliding in an area. The aim of this research was to identify landslide environments correlated with landslide frequency and character and through this develop a landslide susceptibility map. Analyses were undertaken so that distinct inland and coastal landslide environments could be defined. These landslide environments characterised the likely extent and type of landslides that would occur in each land system. In order to compile data over such a large region a land systems approach was taken to characterise areas where the geological and geomorphological conditions were similar. The methods used for primary data collection were aerial photograph interpretation, airborne thematic mapper data, terrain evaluation and limited field mapping for ground truthing purposes. This landslide susceptibility map used probability to assess the relative importance of parameters important to slope stability. The landslide susceptibility map, along with the landslide environments, highlighted areas were landsliding was more prevalent as well as identifying areas where landsliding is less likely to occur. Two landslide environments were identified as being particularly susceptible to landsliding, and this was primarily related to geological conditions. It was found that within environments changes in topography also acted as a strong control on the nature and extent of landsliding. It is anticipated that the method used for assessing landslide susceptibility could have widespread application in all GIS based landslide studies and can be used to develop the potential of the complete UK landslide database, once the rest of the original data have been updated.

In this thesis the relationship between beach morphodynamics and recreational hazards was investigated for the first time within the United Kingdom (UK). Four field experiments, conducted during 2006-2008 provided new insights into the spatio-temporal dynamics of UK beach types and their associated hazard signatures. The extent of data collection ranged from national (UK beach classification) to regional (temporal morphologic variation) to site specific (macrotidal rip current dynamics). Detailed morphodynamic characteristics of 98 beaches within the UK were collected. Twelve distinct beach groups were identified through cluster analysis, each having a unique morphodynamic signature. Conceptualisation within a relative two-dimensional framework using the dimensionless fall velocity (Q) and the relative tide range (RTR) required an absolute wave energy flux threshold to differentiate between intermediate beaches with (H2T > 5) and without (H2T < 5) three-dimensional bar morphology. The role of geologic control, sediment abundance and drainage characteristics in constraining beach morphodynamics was shown to be significant within the sites studied. Rip currents were responsible for 68% of all recorded incidents between 2005-2007 throughout all 76 beaches patrolled by the Royal National Lifeboat Institution (RNLI). Hazard type and severity varied between morphodynamic beach types. Intermediate beaches with low-tide bar/rip morphology (Q = 2-5 and RTR < 7), including Low-Tide Terrace and Rip (LTT+R) and Low-tide Bar/Rip (LTBR) beaches, presented the greatest risk to the insea beach user. These high risk beaches, representing 59% of the west coast beaches in Devon and Cornwall, also attracted the greatest visitor populations. Seasonal monitoring of hydrodynamics and morphology at LTT+R and LTBR beaches in Devon and Cornwall (annual Hsio% = 3-4 m; mean spring tidal range = 4.2-8.6 m) identified key mechanisms controlling the temporal hazard signature (THS), a term used here to describe the spatio-temporal variation in type and severity o f bathing hazard within a specified region both in the alongshore as well as in the cross-shore (significant in macrotidal environments). The morphological template controlled the presence, extent and intensity of beach rip current systems, where the development of low/tide transverse and inter-tidal bar/rip systems during summer presented the greatest morphological hazard. Typical summer wave forcing by relatively small, long period swell {H, - 0.5-1 m; Tp ~ 6-10 s) over this morphology provided conditions conducive to hazardous rip currents. Under these conditions hazard exposure was increased due to the accessibility of the relatively low energy surf zone. Both spring/neap and semi-diurnal tidal variations were identified as key controls on the THS. Variable tidal excursion modulates rip current activity, and tidal translation rates control the rate of change of the THS. The 'optimum' combination of these mechanisms results in the 'switching' on and off of rip currents during spring low tides and the subsequent rapid alongshore migration of rip channel/hazard location as the surf zone inundates the landward inter-tidal bar system. In conjunction with high insea population, these 'optimum hazard scenarios' drove the high risk, coast-wide 'mass rescue' events identified in the incident records. This work provides a scientific, standardised basis for a beach risk assessment model and lifeguard training programs within the RNLI. Improved understanding of macrotidal rip currents has initiated new field and modelling efforts to further general quantitative understanding of these systems, vital to the improvement of beach safety services.

Morphodynamics of beaches protected by detached breakwaters have been investigated in this thesis at timescales ranging from days to years. A site consisting of nine breakwaters on a coast open to energetic wave conditions in a meso-tidal environment is studied. Two phases of breakwaters were built at the site: phase one comprising of four fully emergent, 200m long structures, which have caused the formation of tidal tombolos; and phase two, five 160m long breakwaters, submerged at high tide, behind which salients have formed. The site allows for comparison of response at beaches protected by different breakwater designs under similar wave and tidal forcing. Three analysis strands are presented: video derived intertidal changes; differential global positioning system (DGPS) surveys; and numerical modelling of the breakwater scheme. Together, the three strands allow for determination of important changes to beach morphology and their relation to forcing. A dataset of weekly mean sea level contours and a set of pre- and post- storm inter-tidal surveys were extracted from the video data. DGPS surveys were conducted of the beach and the inshore bathymetry at monthly intervals; these allowed definition of supra- and sub-tidal changes not achievable from the video data. A 'state of the art' numerical model, MIKE21, was used to model waves, hydrodynamics and sediment transport about the scheme for prevailing boundary conditions. The numerical modelling gave insight into the sediment transport pathways and the relative importance of waves and tides as driving forces for morphological change. The beaches protected by the breakwaters were found to be eroding at an average rate of 0.5m yr'' (vertical elevationc hange),a rate comparablet o unprotectedb eachesT. hus it seemst he detached breakwater scheme is failing to substantially reduce the local erosion problem. The dynamic natureo f the beachese, speciallyt he tombolos and salients,m eanst hat an equilibrium shorelinec annotb e establishedT. he breakwaterp rotectedb eachesw ere less variable than the unprotectedb eachesd, isplaying 65-75% of the standardd eviation of vertical change.H owever, morphological changesw ere still considerable:r angei n cross-shorem ovemento f the means ea level contour was between 6m in the bay centres and 80m on the tombolo horns. Empirical orthogonal function analysis of the DGPS and mean sea level contour datasets allowed for determination of three main modes of change which were similar for both phases. The changes were: a general erosion and accretion (46-75% of the dataset variance), a longshore movement of the salients and tombolos (6-27% of the dataset variance), and cross-shore profile changes (-10% of the dataset variance). Correlation analysis and consideration of the numerical modelling results and storm induced morphological change provided relation of the observed changes to wave and tidal forcing. Differences in morphological response were noted for the different breakwater designs. Patterns of response behind the larger breakwaters were better defined due to higher gradients in wave energy between sheltered and unsheltered regions. Cross-shore changes displayed additional three-dimensionality for the smaller breakwaters which led to an increase and decrease in the amplitude of salient sinuosity. An additional mode of change, sub-tidal erosion and accretion of the bay floors, was observed for the larger breakwaters. Temporal signals showed more seasonality for the phase two breakwaters. In summer the beaches protected by the smaller breakwaters were wider, higher and salient amplitudes were greater. The changes behind the phase one breakwaters exhibited greater dependence on specific wave and surge events. The general erosion and accretion showed an eroding trend with erosion exacerbated by higher water levels for beaches protected by both sizes of breakwater. The longshore movement of the tombolos and salients was forced by obliquely incident waves. For the tombolos, numerical modelling also demonstrated the importance of tidally induced movement. Magnitude of longshore movement (typically -'25m) depended strongly on the antecedent morphological conditions. Cross-shore profile changes were storm induced for the beaches protected by the larger structures: storms reducing profile gradients rapidly and subsequent poststorm recovery. For the beaches protected by the smaller structures, storm induced gradient changes were also important but an additional correlation with wave period was noted. The sub-tidal changes to the phase one bay floors were forced by storm and surge conditions.

No description available.

This thesis uses a combination of industrially acquired seismic reflection data, along with supplementary bathymetry and geotechnical borehole data, to investigate the architecture, structural elements and evolution of submarine mass wasting deposits. The primary study area is the mid-continental margin of Norway and the Levant margin, east-Mediterranean Sea serves as a secondary study area. The principal aim is to gain an improved understanding of the evolution of submarine mass wasting deposits and the processes involved. To this end this thesis consists of three core research chapters which present the results of investigations into the mechanism responsible for the formation of a non-typical example of a submarine mass wasting deposit, the spectrum of geological features that can be found associated with submarine mass wasting deposits, and an example of a frontally confined submarine mass wasting deposit. In the first core chapter, an example of a non-typical submarine mass wasting deposit from the Norwegian continental margin was investigated using three- dimensional (3D) seismic data. The mass wasting deposit, informally named the 'South Voring Slide' (SVS) was found to differ from previously documented examples in that it showed significant thinning and volume loss which could not be accounted for by the transfer of material downslope over a basal shear surface. Analysis of the deformation and geometry of the SVS suggests that it developed due to the mobilisation of an approximately 40 m thick interval at the lower part of the SVS, and its removal from beneath a thin overburden which subsequently underwent extensional fragmentation. Submarine mass wasting deposits exhibit many different types of kinematic indicators from which information relating to the dynamic emplacement of the event can be deduced. The second core research chapter presents a classification scheme and comprehensive compilation of all of the various kinematic indicator types, fully illustrated using best examples taken from the 3D seismic database available from the Norwegian and Levant margins. For each kinematic indicator, a definition and seismic recognition criteria are presented along with discussion and worked examples of the kinematic information which they provide. It was shown that the application of 3D seismic data to the study of submarine mass wasting deposits can yield much information pertaining to the direction and magnitude of transport, mode of emplacement, dominant mass wasting process and rheology. In particular, it was shown that 3D seismic data allows swift and confident evaluation of the direction of translation, and in many cases also allows the degree of translation of the displaced material to be constrained. The final core research chapter documents and describes an example of a large frontally confined submarine mass wasting deposit from the Norwegian margin. The mass wasting deposit, informally named the 'Confined Storegga Slide (CSS)', is unique in that it detaches along a deep-seated basal horizon at a depth of 640 m below the seabed and exhibits spectacular contractional deformation in its distal region. Using a combination of 2D and 3D seismic data and bathymetry data, it has been possible to fully delimit the extent of the CSS, which has a section length of 135 km and involves some 1,227 km3 of material. The results of this study show that the CSS developed as part of the large, multi-phase Holocene Storegga Slide, and demonstrates how subsequent phases Storegga Slide development induced later deformation and volumetric changes which have not previously been recognised.

Two 3D seismic reflection datasets from the West African and Brazilian continental margins were analysed to determine their architectural elements and to further the understanding of the sedimentary processes that control their morphology. The results suggest a strong influence of local slope variations on the sedimentary processes and depositional and erosional products within these complex deep water settings. The Niger Delta dataset is characterised by large channel-levee systems and thrust-related folds. The folds degrade by channel erosion and slope failure, which creates laterally discontinuous erosional surfaces on the crests and flanks of the anticlines and chaotic deposits at their bases. The type of slope failure depends on the length and morphology of the local slope, sediment properties and the presence of anisotropics, such as faults. The location and morphology of the channel-levee systems on the Niger Delta are affected by topographical effects associated with thrust-related folding. Fold-induced local changes in gradient cause turbidity currents to deposit sediment upstream of the folds and erode the seafloor downstream of them. This results in the formation of knickpoints along the present- day thalweg of a channel-levee system. A model for the formation and evolution of the knickpoints predicts that they migrate upstream and leave internal erosion surfaces and terraces with coarse sediments in the sedimentary record. They may be an important process by which channels cut through uplifting fold belts. The Espirito Santo Basin dataset is characterised by salt diapirs, slope failures, channels and canyons. Interaction of turbidity currents with variations in topography has led to the formation of large depressions, which occur above abrupt breaks in slope and in trails that follow underlying erosional channels. They are inferred to form by Froude-supercritical currents that become unstable as they encounter topographical irregularities, such as scarps and knickpoints. This leads to the formation of erosional scours and deposits similar to sediment waves, which, when confined within channels, appear as roughly circular depressions.

Vegetation plays an important role in changing the flow characteristics and flooding of natural channels. In this study a series of laboratory programmes were undertaken to investigate the effects of emergent and submerged vegetation on the flow structure in compound, simple and wide channels. The vegetation consisted of rigid rods replicating tree vegetation. In a laboratory flume, wooden rods were used to replicate this with three density states and one non-vegetated state being used to compare their effects on the flow characteristics in the flume. Velocities were measured using 3-D Acoustic Doppler Velocimeters (ADV), with one downward facing and one upward facing probe. The results showed that the velocities decreased within and near the vegetation zones, and the flow accelerated and transferred towards the upper part of the rods, or to the non-vegetated zone, along the flume. The vegetation density was shown to control the magnitude of these effects. Also, in the transition zones between the floodplain and the main channel in the compound channel, or in transition zones between the vegetated and non-vegetation zones, or near the top of the vegetation in the simple and wide channels, the velocity fluctuations and Reynolds stresses were observed to have relatively high values. The magnitude of the velocity fluctuations and the Reynolds stresses were shown to be highly depended upon the vegetation density. Vegetation reduces the energy and increases the flow depth, with a reduction in the velocities. It also protects the channel bed against erosion due to resistance of the flow and enhances settlement of sediment. Behind the rods the longitudinal velocity was observed to be very low, but other velocities (transverse and vertical) were high. Also velocity fluctuations and Reynolds stress components were higher behind the rods. However, as the distance from the rods increased, then the longitudinal velocity increased and the transverse and vertical velocities and Reynolds stresses all decreased. For many conditions in vegetated open channel flows, rods may produce transverse waves due to vortex shedding. Some experiments in the wide channel, of 1200 mm width, 10 m length, and 24 mm rods diameter were undertaken using different rod arrangements and densities to investigate the frequency and amplitude of these waves. From these experiments, and a subsequent analysis of the results, a new equation was developed for calculating the Strouhal number for water and two new equations were established for calculating the wave amplitude for different rod arrangements and densities. These equations were experimentally proved to better match the data variations than the conventional equation.

This thesis uses industry 3D seismic reflection datasets to investigate the Silverpit structure, a proposed impact crater located in the southern North Sea, UK. The principal aim of this thesis is to investigate the origin of the Silverpit structure. Research has focused on constraining the age of the Silverpit structure, investigation into regional magmatic activity in the southern North Sea study area and structural analysis of the Silverpit structure. The Silverpit structure is a multi-ringed circular structure 20 km in diameter found within Cretaceous and Eocene age marine sediments. The Silverpit structure is composed of a 3 km diameter excavated cavity is surrounded by a series of concentric listric faults. The outer rings of the structure are composed of extensional grabens and concentric folds. Within the excavated cavity, a series of localised uplifted reflections, termed the central uplift, can be identified. A boundary marks a common upper limit of deformation. Undeformed reflections above this boundary have parallel onlapping geometry onto the underlying cavity and are an indication of instantaneous creation of accommodation space. This boundary between undeformed and faulted reflections has been interpreted to be the crater floor and has been dated to be Middle Eocene in age. A tertiary dyke swarm, 54 Ma old, has been identified and mapped 20 km to the North of the Silverpit structure. The dykes are characterised by a linear seismic disturbance and linear coalesced depressions at the upper limit of the seismic disturbance. The depressions above the dyke tips formed during the release of volatiles from the intruding magma. The dykes and coalesced depressions are new Earth analogues for Martian pit chain craters. No magnetic anomaly can be identified over the Silverpit structure, ruling out an igneous origin. The age of the Silverpit structure is older than the onset of regional folding, therefore ruling out a folding/salt withdrawal origin. The circular Silverpit structure is unrelated to the underlying elongate salt geometry. Any fault growth associated with salt movement would trace the underlying salt body we would therefore expect any faults related to salt movement to be elongate, not circular. The morphology of the Silverpit structure is characteristic of an impact crater. The features mapped, central uplift, excavated crater, multi rings and folds, are all diagnostic features of an impact crater. Bolide impact is the most likely origin for the Silverpit structure as all the alternative origins can be ruled out. Importantly further diagnostic evidence is still needed to confirm the Silverpit structure as an impact crater. Until such evidence is found the Silverpit structure is classified as a probable impact crater.