This thesis investigates the characteristics and impacts of large wood accumulations within river reaches of different size and style. Four reaches were studied: (i) a wide, braided, headwater reach, characterised by dead wood (Tagliamento River, Italy); (ii) a lower gradient, wide, braided reach, characterised by resprouting wood (Tagliamento River, Italy); (iii) a low gradient, single thread reach with a natural supply of dead wood (Highland Water, UK), and (iv) a low gradient, single thread reach that has been restored by felling trees into the river (River Bure, UK). In each reach, quantities of wood, types of accumulation and their association with sediment retention, landform and propagule bank development were investigated, generating four main findings: 1. There were marked differences in the size and character of large wood accumulations among the four reaches. 2. Retention of fine sediment and organic matter by wood was observed on all four reaches, giving rise to notable spatial heterogeneity in surface sediments. 3. Sediment retention resulted in the development of different landforms among the four reaches. In the two multi-thread reaches, accretion of finer sediment around large wood led to island development. In the naturally-functioning single-thread reach, wood jams spanned the river channel, accumulating sediment and organic matter to produce unvegetated wood jams, and inducing other landforms, notably pools and bars. Such geomorphic heterogeneity was anticipated in the restored reach, but to date this has not significantly occurred. 4. Spatio-temporal variations were observed in propagule abundance and species richness within different wood-related mesohabitats. Higher abundance and species richness were associated with finer, more organic sediments retained within wood accumulations and related mesohabitats. In the restored reach such associations were not statistically significant, further indicating that responses to wood emplacement take longer than the 4 years since restoration. iv Overall, this research has strengthened the evidence concerning the differing nature of wood accumulations in rivers of different size and style, and it has demonstrated the importance of large wood for retaining organic matter and plant propagules, resources essential for riparian vegetation succession and for the success of river restoration efforts.

Climate change predictions indicate an increase in the frequency and duration of flood events along with longer dry antecedent conditions , which could alter patterns of trace metal release from contaminated river bank sediments. This study took a laboratory mesocosm approach. Chemical analysis of water and sediment samples allowed the patterns of Pb and Zn release and key mechanisms controlling Pb and Zn mobility to be determined. Trace metal contaminants Pb and Zn were released throughout flooded periods, however the highest concentrations of dissolved Pb were observed at the end of the longest flood period and high concentrations of dissolved Zn were released at the start of a flood. These concentrations were found to exceeded environmental quality standards. Key mechanisms controlling mobility were (i) anglesite solubility control of dissolved Pb, (iii) evaporation, precipitation and dissolution of Zn sulphate salts, (iii) oxidation of galena and sphalerite, (iv) reductive dissolution Mn/ Fe hydroxides and co precipitation/adsorption with Zn. In light of climate change predictions these results indicate future scenarios may include larger or more frequent transient 'pulses' of dissolved Pb and Zn released to river systems. These short lived pollution episodes could act as a significant barrier to achieving the EU Water Framework Directive objectives.

A series of detailed laboratory experiments have been carried out to investigate the processes of needle-ice growth and the mechanisms by which needle ice incorporates and transports sediment. The use of laboratory techniques has made it possible to control and monitor the environmental conditions at the soil surface closely, and thus isolate the elements that are important for needle ice growth. Two types of fine-grained soil sample were used: disturbed (remoulded) and undisturbed. These were taken from sites where needle ice was seen to grow naturally. Remoulding the soil sample affected the growth of needle ice and therefore the amount of sediment uplifted by the ice. Several types of needle ice were observed: clear, multitiered, crystals with dispersed sediment, soil caps and soil aggregates. Each type was produced under different conditions of soil-surface temperature and moisture. Soil-moisture availability was particularly important in controlling the type and rate of crystal growth. An algorithm has been developed with which to predict the type of crystal that will grow in a given freezing cycle. It is suggested that sediment becomes incorporated into the crystals when there is a disturbance in the environment of needle-ice growth. This disturbance is a result of instabilities in the balance of heat at the freezing front caused when either soil-surface temperature or soil-moisture content fall below a minimum threshold. Typical sediment yields ranged from 0.002 to 2.5 g cm\(^-\)\(^2\). The sediment incorporated into the needle-ice crystals was coarser than the bulk soil from which it was lifted. The transport of sediment by needle ice was also investigated. It was found that the distance of sediment transport is dependent on the slope angle, length of ice crystal, process of crystal melt, and type of marker particle and soil sample. A series of simple, statistical models is presented that attempts to predict the growth and morphological effects of needle ice.

Suction is an important stress-state variable of unsaturated soils. The magnitude of suction affects the shear strength, the hydraulic conductivity, and the volume change behaviour of unsaturated soils. The measurement of soil suction is a prerequisite for the characterisation of unsaturated soils. Soil suction can be determined either by adopting direct or indirect measurement techniques. Despite several techniques available currently for measuring and controlling matric and total suctions of soils in the laboratory, several aspects related to various suction measurement techniques, such as the water phase continuity in null- type tests and compatibility of test results from various measuring techniques are yet to be explored in detail. Similarly, studies concerning determination of air-entry values (AEVs) and residual suctions of soils that exhibit volume change during the drying process are limited. Suctions of two soils from Libya (a silty sand and an inorganic clay with intermediate plasticity) were experimentally measured using null- type axis-translation, filter paper, and chilled-mirror dew-point techniques. Axis-translation and vapour equilibrium techniques were used for establishing the drying and wetting suction-water content soil-water characteristic curves (SWCCs) of the soils. Compacted soil specimens were prepared by varying moulding water content, dry density, compaction type, and compaction effort in order to investigate the influence of initial compaction conditions on measured suctions and SWCCs of the soils. The water content-void ratio relationships (shrinkage curves) of the soils from Clod tests were used in conjunction with the drying suction-water content SWCCs to establish the suction-degree of saturation SWCCs that enabled determination of the air-entry values (AEVs) and residual suctions of the soils. Initially saturated slurried specimens of the soils were also considered for comparing with the test results of compacted soil specimens. The test results from the investigation showed that the influence of compaction conditions on SWCCs of the soils was distinct only at a low suction range, whereas their impact was insignificant at higher suctions. The volume change of the soils during the drying process had significant impact on the AEVs and residual suctions. For initially saturated slurried specimens, the AEVs and the residual suctions of the soils determined form the suction-water content SWCCs were found to be distinctly lower than their counterparts determine from the suction-degree of saturation SWCCs. Suctions corresponding to the plastic limits of the soils agreed well with those determined from suction-degree of saturation SWCCs, whereas suctions corresponding the shrinkage limits overestimated the AEVs. An increase in the chamber air pressure soon after the null-type tests were completed clearly indicated that the water phase continuity between the water in the soil specimens, the water in the ceramic disk, and the water in the compartment below the ceramic disk was lacking for all specimens tested. Soil specimens with higher water contents created better continuity in the water phase. At high suction range, the test results from the techniques based on vapour equilibrium (i.e., non contact filter paper, salt solution and chilled-mirror dew-point tests) showed very good compatibility, whereas differences were noted between the test results at low suction range from the techniques that are based on liquid phase equilibrium (i.e., pressure plate and null-type tests).

In this work, different thermo-poro mechanical models for large scale landslides were developed to predict the transition between the creep and the catastrophic phases of a landslide. First, a refinement was made of an existing thermo-poro-mechanical landslide model, including a realistic formulation for the dynamics of the moving slide. The model equations were then solved by using an unconditionally stable numerical scheme and the results were compared with a similar existing model and data available from landslide case studies. It was found that the refined model gave different predictions for the slide’s acceleration and velocity which however were only marginally so in the time window of realistic run-out distances. Secondly, a thermo-poro-mechanical model for landslides was developed based on rate process theory. The model was initially used in an attempt to back-analyse a real landslide case. It was subsequently used to explore factors that influence the transition from an initial phase of creep to a final catastrophic phase. It was found that a threshold initial velocity separates the creep and collapse regimes, beyond which frictional heating leads to the final failure. A different thermo-poro-mechanical model for landslides based on a constitutive theory that combines a thermo-plasticity model with a creep model for soils was also developed. The model was initially used in an attempt to back analyse a real landslide. It is able to predict a transition between the creep and collapse phases. Thermal diffusivity of the shear band material plays a major role on the predicted duration of the creep phase. Finally a landslide model based on Perzyna’s visco-plasticity theory was also developed. It was found that the extremely low velocity predicted during the creep phase leads to insignificant heat dissipation inside the shearband making the prediction of thermal pressurization and collapse impossible with this model.

Conventional morphodynamic models are typically based on a coupled system of hydrodynamic equations, a bed-update equation, and a sediment-transport equation. However, the sediment-transport equation is almost invariably empirical, with numerous options available in the literature. Bed morphological evolution predicted by a conventional model can be very sensitive to the choice of sediment-transport formula. This thesis presents a physics-based model, where the shallow water-sediment-mixture flow is idealised as being divided into two layers of variable (in time and space) densities: the lower layer concerned with bedload transport, and the upper layer representing sediment in suspension. The model is referred to as a Quasi-2-Layer (Q2L) model in order to distinguish it from typical 2-Layer models representing stratified flow by two layers of different but constant and uniform densities. The present model, which does not require the selection of a particular empirical formula for sediment transport rates, is satisfactorily validated against widely used empirical expressions for bedload and total transport rates. Analytical solutions to the model are derived for steady uniform flow over an erodible bed. Case studies show that the Q2L model, in contrast to conventional morphodynamic approaches, yields more realistic results by inherently including the influence of the bed slope on the sediment transport. This conclusion is validated against experimental data from a steep sloping duct. An analytical study using the Q2L model investigates the influence of bed-slope on bedload transport; the resulting expressions are in turn used to modify empirical sediment transport formulae (derived for horizontal beds) in order to render them applicable to arbitrary stream-wise slopes. The Q2L model provides an alternative approach to studying sediment-transport phenomena, whose adequate analysis cannot be undertaken following coniv ventional approaches without further increasing their degree of empiricism. The Q2L model can also lead to the enhancement of conventional morphodynamic models. For coarse sediments and/or relatively low flow velocities, bedload transport is usually responsible for most sediment transport. Bedload transport consists of a combination of particles rolling, sliding and saltating (hopping) along the bed. Hence, saltation models provide considerable insight into near-bed sediment transport. This thesis also presents an analysis of the statistics and mechanics of a saltating particle model. For this purpose, a mathematically simple, computationally efficient, stochastic Lagrangian model has been derived. This model is validated satisfactorily against previously published experimental data on saltation. The model is then employed to derive two criteria aimed at ensuring that statistically convergent results are achieved when similar saltation models are employed. According to the first criterion, 103 hops should be simulated, whilst 104 hops ought to be considered according to the second criterion. This finding is relevant given that previous studies report results after only a few hundred, or less, particle hops have been simulated. The model also investigates sensitivity to the lift force formula, the friction coefficient, and the collision line level. A method is proposed by which to estimate the bedload sediment concentration and transport rate from particle saltation characteristics. This method yields very satisfactory results when compared against widely used empirical expressions for bedload transport, especially when contrasted against previously published saltation-based expressions.

The importance of high-magnitude, short-lived flood events in controlling the evolution of bedrock landscapes is not well understood. During such events, erosion processes can shift from one regime to another upon the passing of thresholds, resulting in abrupt landscape changes that can have a long lasting legacy on landscape morphology. Geomorphological mapping and topographic analysis document the evidence for, and impact of, extreme flood events within the Jökulsárgljúfur canyon (North-East Iceland). Surface exposure dating using cosmogenic 3He of fluvially sculpted bedrock surfaces determines the timing of the floods that eroded the canyon and helps constrain the mechanisms of bedrock erosion during these events. Once a threshold flow depth has been exceeded, the dominant erosion mechanism becomes the toppling and transportation of basalt lava columns and erosion occurs through the upstream migration of knickpoints. Surface exposure ages allow identification of three periods of rapid canyon cutting during erosive flood events about 9, 5 and 2 ka ago, when multiple active knickpoints retreated large distances (> 2 km), each leading to catastrophic landscape change within the canyon. A single flood event ~9 ka ago formed, and then abandoned, Ásbyrgi canyon, eroding 0.14 km3 of rock. Flood events ~5 and ~2 ka ago eroded the upper 5 km of the Jökulsárgljúfur canyon through the upstream migration of vertical knickpoints such as Selfoss, Dettifoss and Hafragilsfoss. Despite sustained high discharge of sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1); there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive flood: the vertical knickpoints have not diffused over time and there is no evidence of incision into the canyon floor. The erosive signature of the extreme events is maintained in this landscape due to the nature of the bedrock, the discharge of the river, large knickpoints and associated plunge pools. The influence of these controls on the dynamics of knickpoint migration and morphology are explored using an experimental study. The retreat rate of knickpoints is independent of both mean discharge, and temporal variability in the hydrograph. The dominant control on knickpoint retreat is the knickpoint form which is set by the ratio of channel flow depth to knickpoint height. Where the knickpoint height is five times greater than the flow depth, the knickpoints developed undercutting plunge pools, accelerating the removal of material from the knickpoint base and the overall retreat rate. Smaller knickpoints relative to the flow depth were more likely to diffuse from a vertical step into a steepened reach or completely as the knickpoint retreated up the channel. These experiments challenge the established assumption in models of landscape evolution that a simple relationship exists between knickpoint retreat and discharge/drainage area. In order to fully understand how bedrock channels, and thus landscapes, respond and recover to transient forcing, further detailed study of the mechanics of erosion processes at knickpoints is required.

The quality of water within an estuary is inseparable from the component parts; suspended particulate matter (SPM) and the balance of macro-nutrients. Long-term temporal variations and the horizontal advection of both SPM and macro-nutrient concentrations are well-constrained, but the vertical fluxes associated with micro-scale processes, such as turbulence and flocculation, are poorly constrained. The importance of three micro-scale processes on the interactions between SPM and inorganic macro-nutrients, nitrate (NO–3 ), ammonium (NH+4) and phosphate (PO3 –4 ), are examined in four field campaigns and five laboratory experiments. Field campaigns were conducted in two turbid estuaries. One field campaign was conducted in the Seine estuary, France, and three campaigns in the Tamar estuary, U.K., in order to consider the effects of seasonal variations (spring, summer and autumn). Physical conditions measured included current velocity, turbidity, turbulence and particle size, were recorded using a suite of oceanographic instrumentation. Five different laboratory studies were conducted using the same mini annular flume, with different background conditions. Inorganic macro-nutrients were measured spectrophotometrically on a continuous flow analyser (for NO – 3 and PO3 –4) and fluorimetry (NH +4). Three hypotheses are presented as potential mechanisms controlling the release and uptake of macro-nutrients from sediments. Mechanism One (M1) described an exchange process between inorganic macro-nutrients and flocculation/disaggregating particles. It was hypothesised that flocculating particles would decrease water column macro-nutrient concentrations, and vice versa. In this study, flocculation was observed in both field sites, but there was no significant relationship between flocculation and macro-nutrient concentration. Similarly, the five laboratory studies demonstrated no statistically significant relationships between flocculation and macro-nutrient concentrations. Mechanism Two (M2) hypothesised that turbulence would enhance the release portion of the exchange processes described in M1. Furthermore, it was proposed that increased turbulence would break bonds between macro-nutrients and the surface of particle faces. Turbulence was observed to limit the floc size in all experiments (both field and laboratory), but this study determined that it did not promote a significant release mechanism for inorganic macro-nutrients. However, this study observed that turbulence played a key role in the vertical distribution of PO 3 – 4 and NH +4. In both the Seine and Tamar estuaries, surface and near-bed concentrations were observed to be statistically significantly different (p = < 0.05). Finally, Mechanism Three (M3) hypothesised that increased salinity provides additional salt water cations that would enhance flocculation and M1. This study measured an increase in floc size with increasing salinity in but did not enhance the proposed M1. Instead, as with turbulence, differences in water density as a result of the salinity affected the vertical distribution of NO –3. This research concluded that micro-scale processes have no significant impact on the water-column concentration of inorganic macronutrients. Instead, it was observed that two of the three micro-scale processes, turbulence and salinity, play a key role in the vertical distribution of inorganic macro-nutrients in the Tamar and Seine estuaries.

This thesis investigates the development of regional drainage patterns within intra-continental rift basins using remote sensing data, field studies and numerical models. By examining modern-day extensional settings, such as the Basin and Range, USA, and the East African Rift System (EARS), this study elucidates the controls upon regional source-to-sink systems and assesses the findings in relation to existing, conceptual tectono-stratigraphic and drainage models. Rift basins are generally well studied and facies models well established. However, there is a tendency to overlook the regional perspective. Many drainage evolution studies and tectono-stratigraphic models focus upon the development of individual basin-bounding faults and half-grabens, often overlooking the influence of regional-scale drainage evolution upon landscape and stratigraphic development. On a regional scale, extensional basins are segmented into numerous sub-basins, which: (i) exist at different elevations; (ii) subside at different rates; (iii) vary in their degree of fluvial connectivity; and, (iv) may experience significant shifts between erosional and depositional regimes as drainage networks evolve. Through observations of the Basin and Range, and three-dimensional numerical modelling, it is shown how sub-basins with identical tectonic and climatic boundary conditions can exhibit vastly different stratigraphic fills depending on the degree of fluvial connectivity and their relationship to adjacent sub-basins. In addition, drainage integration is recognised as an overlooked, yet important process in the source-to-sink evolution of rift basins. Drainage integration between sub-basins of varying elevations is shown to cause widespread erosion and sediment bypass in the upstream basin, while contemporaneously increasing sediment supply to the downstream basin. A case study of the Tecopa Basin shows drainage integration as a powerful driver of base level fall and landscape transience in the absence of significant tectonic or eustatic controls. The differential gradients created by base level fall cause further upstream drainage rearrangements. To understand the evolution of regional hinterland drainages, two rift segments of varying maturity are compared. The Okavango Rift Zone represents a rift initiation phase segment while the Albertine Rift represents a rift climax phase segment. Through analysis of the contributing drainage patterns a model is proposed where early rift drainage is dominated by antecedent directions, with large, low relief, low slope catchments. As rifts mature catchments increase in relief and slope but reduce in asymmetry as the influence of antecedent drainage direction is reduced via tectonic tilting, drainage reversals and ponding. The thesis concludes that current tectono-stratigraphic models of sub-aerial rift settings commonly overlook the role of inter-basin erosion, under-appreciate the influence of antecedent drainage direction in hinterland input, and over-emphasise the role of axial rivers. Future tectono-stratigraphic models should acknowledge distinctions based upon the degree of fluvial connectivity (isolated or integrated sub-basins) and the regional position relative to adjacent sub-basins (upstream and terminal and sub-basins).

This study was designed to evaluate the status, distribution, spatial variability, controlling factors, storage, and change in the levels of soil organic carbon (SOC) in two major alluviums of Bangladesh. The two alluviums the Brahmaputra and the Ganges were selected because they occupy a large area of Bangladesh with a wide diversity of agro-ecosystems. SOC levels were studied across the four sub-sites in the aforementioned alluviums at 0-30 cm depths to evaluate their spatial and temporal variability. The sub-sites, Delduar and Melandah, are in the Brahmaputra alluvium. The other two sub-sites, Mirpur and Fultala, are in the Ganges alluvium. Additionally, SOC and total nitrogen (TN) distribution were studied across eight soil profiles (0-120 cm depths) under the two alluviums. The results revealed that the SOC contents were very low in all the sites. The classical statistics showed that the variability of the SOC was moderate across the four sub-sites. The SOC distribution was positively skewed across all the sub-sites except Fultala. A semivariogram model showed there was generally a weak spatial correlation (R2 < 0.5) of SOC in the study sites. A relatively large sampling grid (1600m) and intensive soil management were perhaps responsible for the observed weak spatial dependency. SOC variability is lower across the highland (HL) and medium highland (MHL) sites than the medium lowland (MLL) and lowland (LL) sites. Changes in land use and land cover were also more intensive in the HL and MHL sites than the MLL and LL sites. The reason for low SOC in the HL and MHL sites may be due to their lower inundation level, e.g., land levels in relation to flooding depths, together with greater intensity of use. Temporal variability of SOC datasets revealed that SOC has declined across all the sites during the last 20-25 years due to the intensive land use with little or no crop residue inputs. It is plausible that SOC has declined to an equilibrium level, and further decline may not occur unless land use intensity changes further. The findings show that SOC is positively related to the TN and clay contents in the soils. This is not surprising as SOC is a major pool of TN, and soil clay fraction is known to protect SOC degradation. SOC and TN storage is higher in the surface soil horizon (0-20 cm) than the sub surface soils. Topsoil horizon is tilled and receives greater crop residue inputs which are subsequently mineralized resulting in higher accumulation of SOC and TN. It appears that inundation land types and land management practices may be the major driving factors of SOC storage and distribution across the study sites.