Rivers are essential components of the earth surface. The world’s largest rivers have been studied much less than to smaller rivers. They are dominated by meandering channels – whether these are individual km-wide meander bends or ‘accessory’ meandering channels in an anabranching system. Large rivers, specifically ones with laterally migrating bends, can build a variety of floodplain elements that are represented by a complex surface topography, the dynamics and characteristics of which are not yet fully understood. This research brings a greater understanding to, and quantification of, the floodplain topography and geomorphology of large rivers. The project uses remote sensing imagery of the World’s largest rivers, LiDAR datasets of meandering scroll bar topography, and global coverage elevation data. Novel analytical methods are created, involving image manipulation and GIS processing, to quantify these landforms in a way that was not possible until recent technological and computational advancements. A new hierarchical classification schema of meandering floodplain deposits is presented and applied to quantify meandering deposits for two large rivers, the Amazon and the Ob. Both floodplains show similar downstream morphological changes to their floodplains despite their different sizes and locations. The new classification schema works well to describe meandering floodplain deposits. The geomorphology of scroll bars is investigated for the Mississippi River, revealing the heterogeneity of these deposits and that local meander bend conditions are important in determining scroll bar formation and preservation on the floodplain. The periodicity of scroll bars from a range of rivers is investigated and it is shown that scroll bars are intrinsically linked with the width of the adjacent river channel and respond to local planform changes. Floodplains of large rivers have complex overbank sedimentation processes that create spillage sedimentation phenomena. Spillage sedimentation was quantified down a 1700 km reach of the Amazon River and a 1400 km reach for the River Ob, revealing spatial discontinuities in spillage phenomena. Spillage dominance depends on diverse sediment loadings, hydrological sequencing, and morphological opportunity. Understanding spillage dynamics is important in quantifying overbank sedimentation rates and the spatial distribution of fine-grained deposits. The findings of this thesis highlight that despite the incredible complexity and heterogeneity of large river floodplains, order can be inferred via classification schemas and fundamental relationships identified. The thesis uses novel methods and conceptual models to bring a greater understanding and quantification to this complex floodplain geomorphology.

Climate change, population growth, and water resources crossing political boundaries are the main issues threating water allocation for agricultural, industrial and domestic uses in developing countries. Integrated water resources management developed in a sustainable manner is essential to allow future generations to meet their water needs. A lack of data in developing countries is the biggest problem that can hinder developing necessary understandings. The Tigris river basin is a prime example, not only because it is located in a developing country, but also due to its long history of armed conflicts and breakdown of law and care. Unstable situation makes data collection difficult, available data poor in quality and the measuring tools and methods rudimental. The insufficient data lead to the impact of the climate change on water resources to be not conclusively detrained. This study shows the climate change impacts through investigate the evapotranspiration (ET) changes over the years. Five potential evapotranspiration models have been studied and classified according to the complexity in terms of the number of variables. Choosing the most suitable ET model helped to fill and reconstruct gaps in historical data sets. The statistical downscaling model SDSM was used to predict the evapotranspiration changes for the next 100 years. Google Earth and 3DRoutBuilder helped to produce an entire river profile with a simple, good quality representation of river networks. That aid the run of the hydrodynamic model (ISIS -1D) which has been utilised to produce water levels and water flow information to establish a robust river losses and water balance assessment for a river. Planning of alternative water resources schemes on river basins located in Arid-Semi Arid region needs an assessment of the hydrologic/hydraulic behaviours of that river. In view of this, the thesis further explores the sustainability of water quantities of rivers based on generated climate scenarios and population increases.

Der Bericht informiert über Methodik und Ergebnisse der Regionalisierung von Niedrigwasserkennwerten sächsischer Fließgewässer. Diese können im »Wasserhaushaltsportal Sachsen« per Internet abgefragt werden. Ausgegeben wird jeweils der Niedrigwasserkennwert für das Gesamtjahr sowie Sommer- und Winterhalbjahr an einem ausgewählten Gewässerquerschnitt. Die Kennwerte sind eine wichtige Eingangsgröße bei der Festlegung bzw. Überprüfung von Mindestwasserführungen in Fließgewässern nach § 33 WHG. Die Publikation richtet sich vorrangig an Wasserwirtschaftsbehörden und Gewässernutzer in Sachsen.

Nach Abschluss des KliWES-Teilprojektes »Säule B – ArcEGMO« liegen sachsenweite Wasserhaushaltsdaten für den Ist-Zustand (1961–2010) und ausgewählte Zukunftsszenarien (2011–2100) des Klimas und der Landnutzung vor. Der vorliegende Bericht ergänzt die als interaktive Karten erfolgte Veröffentlichung von Projektergebnissen im Wasserhaushaltsportal Sachsen. Diese sollen vorrangig Umweltverwaltungen, wissenschaftliche Einrichtungen sowie Ingenieur- und Planungsbüros bei der Bearbeitung regionaler Fragestellungen der Anpassung an Klimawandelfolgen unterstützen.

Maintaining river health is of vital importance to the human populations that depend on them for drinking water, and for the income generated from industry and leisure activities. The key to a clear understanding of the current state of the river environment lies in assimilating the various data that are available for a particular river catchment. As a result of the large expense involved in extensive data collection programmes, measurements are often only taken at a handful of monitoring locations, resulting in large portions of a river network remaining unmonitored and rendering it difficult to assess the health of the river as a whole. Interpreting observations associated with a particular response variable pivots on understanding many other variables whose underlying relationships are often highly complex and which may not be routinely measured. Cutting-edge statistical methods can play a crucial role in the interpretation of such data, particularly when faced with small sample sizes and the presence of latent processes. In particular, developing models for environmental data that relax the assumption of simple linear dependencies between response and covariate is a core theme of this thesis, which can enable powerful descriptions of such complex systems. This approach adopts and promotes modern flexible regression techniques based on penalised splines, which are motivated and summarised in Chapter 2; these permit regression relationships to assume a wide variety of non-linear shapes, without requiring the modeller to impose a priori structure. This thesis aims to address two related, but distinct regression problems for data collected within a river catchment. Firstly, the relationship between rainfall data collected at a rain gauge and subsequent river flow rates collected at a point downstream is tackled in Chapter 3. In this application, it is of particular interest to understand the degree, duration and time-lag of the influence of a rainfall event on a measurable increase in river flow rates at a downstream location. This relationship is complex because it is governed by attributes of the surrounding river environment that may not be readily available, such as soil composition, land use and ground strata. However, rainfall and flow data are frequently collected at a high temporal resolution, and Chapter 3 develops models that exploits this feature that are able to express complex lagged dependence structures between a sequence of flow rates and a rainfall time series. The chapter illustrates how the resulting model enables insight into the sensitivity of the river to additional rainfall, and provides a mechanism for obtaining predictions of future flow rates, without recourse to traditional computationally intensive deterministic modelling. This thesis also tackles the problem of constructing appropriate models for the spatial structure of variables that are carried by water along the channels of the river network. This problem cannot be approached using traditional spatial modelling tools due to the presence of the different volumes of water that mix at confluence points, often causing sudden changes in the levels of the measured variable. Very little literature is available for this type of spatial problem, and none has been developed that is appropriate for the large data sets that are becoming increasingly common in many environmental settings. Chapters 4 and 5 develop new regression models that can incorporate spatial variation on a stream network that respects the presence of confluences, flow rates and direction, while including non-linear functional representations for the influence of covariates. These different model components are constructed using the same modern flexible regression framework as used in Chapter 3, and the computational benefits of adopting this approach are highlighted. Chapter 4 illustrates the utility of the new models by applying them to a large set of dissolved nitrate concentrations collected over a Scottish river network. The application reveals strong trends in both space and time, and evidence of a subtle interaction between temporal trend and the location in space; both conclusions would have been difficult to reach using other techniques.

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Recent research has indicated that variability of antecedent flows is a fundamental control on the entrainment and transport of sediment in river systems. Specifically, the low flows between successive floods appear to have a far greater influence on the stability of a river bed than previously assumed. Increased durations of low flows increase sand-gravel bed stability so as to delay entrainment and significantly reduce transport. Although a degree of quantification of “memory stress” effects has been attempted by previous researchers, their applied methodology precludes development of appropriate mathematical relationships implicit to correcting existing sediment transport equations. The overall aim of this thesis is therefore to address this deficiency via robust physical and mathematical modelling. In total, 84 flume experiments were carried out in a flume. Two poorly sorted (g ≥1.6) sand-gravel mixtures of unimodal and bimodal distribution were compared and contrasted for sensitivity of modality to memory effects upon bedload and entrainment threshold. Five memory timescales (10, 30, 60, 120 and 240 minutes) were tested and contrasted with baseline data obtained for runs performed without any memory. Experiments employed a stepped discharge hydrograph covering sub-threshold to fully mobile conditions. A reference transport based approach was employed to determine entrainment threshold, and to develop mathematical descriptors of memory effects. Results show that increasing memory timescales up to 240 minutes increases entrainment thresholds ( ) by up to 49% whilst subsequent transport decreases by up to 97%. The memory effect prevails non-linearly for the range of low flows of non-dimensional transport between 10-6 to 10-1. Using these flume data, novel mathematical functions for bedload are developed to account for the influence of memory timescales. Here, memory is described via rising exponents of the function to quantify degree of non-linearity of transport to shear stress, and changes in the structure of the bed due to memory are represented within a lumped coefficient. Trends in the suite of exponents and coefficients indicate that changes in bed structure are of greater importance than the shift in non-linearity of bedload. Hence, the first framework for correcting existing graded sediment formulae for memory stress has been effectively developed using a scaling of the granular scale roughness parameter, An. Predicted results are calibrated and validated against available memory stress datasets from both field and laboratory based studies. Results show that without memory correction, over 80% of estimates fail to predict measured bedload effectively; once An based correction is applied, 100% of data are predicted effectively.

To comply with the European Union Water Framework Directive (2000), National Governments are required to achieve good chemical and ecological status of freshwater bodies. Fine sediment has been shown to be a major cause of the degradation of lakes and rivers, and as a result research in geomorphology has been directed towards the understanding of fine sediment dynamics. It was identified by a review of published literature that at present a paucity of information on sediment dynamics existed for the East Midlands, UK. The use of tracers within a sediment fingerprinting framework has recently become a heavily used technique to investigate the sources of fine sediment pressures. However, uncertainties associated with tracer behaviour have been cited as major potential limitations to sediment fingerprinting methodologies. At present few studies have quantified the uncertainties associated with tracer use, or the exact reasons why different tracers are producing different sediment provenance results. This thesis had two aims based on these gaps in published literature. First, to assess the impact of sediment sampling methodology, tracer selection, particle size corrections and organic enrichment corrections on a fine sediment fingerprinting study. Secondly, to develop a partial sediment budget for the Upper Nene river basin and its major tributaries. The results of this thesis were presented in two parts. The first part investigated Aim 1 when fingerprinting; historically deposited sediment, suspended sediment and recently deposited sediment. The second part investigated Aim 2 by constructing a fine sediment budget for the Nene river basin, consisting of; sediment yield, sediment provenance, floodplain sediment accumulation and channel bed sediment storage. A mean difference of 24.1% between the predicted contributions of sediment originating from channel banks was found when using nine different tracer groups to fingerprint the river sediment samples. When fingerprinting contributions from urban street dusts mean differences between tracer group predictions were lower, at between 8% and 11%. There was little indication that organic matter content and / or particle size caused differences between tracer group predictions. Within-source variability in tracer concentrations, and small contrasts between the tracer concentrations of the sediment source groups, were identified as probable causes of inherent uncertainty present in the fingerprinting analysis. It was determined that the ratio of the percentage difference between median tracer concentrations in the source groups and the average within-source tracer concentration coefficient of variation could indicate the likely uncertainty in model predictions prior to tracer use. When fingerprinting historically deposited sediment, a reservoir core was fingerprinted with the least uncertainty, with tracer group provenance predictions ~28% apart and with consistent down-core trends. When fingerprinting an on-line lake core and four floodplain cores, differences between tracer group predictions were as large as 100%; the down-core trends in changing sediment provenance were also different. The differences between tracer group predictions could be attributed to the organic matter content and particle size of the sediment. There was also evidence of the in-growth of bacterially derived magnetite and chemical dissolution affecting the preservation of tracer signatures. Despite the prior indications that organic matter and particle size were causing tracer non-conservatism in historical sediment cores, data corrections were found to often be ineffective at reducing the differences between tracer group predictions. The corrections were found to either have no effect on, or increase the mean differences between, tracer group predictions when fingerprinting river sediment. The sediment budget identified that the annual sediment yield of 13 - 19 t km-2 yr-1 for the Nene is low in comparison to other UK catchments. Channel banks were found to be the dominant sediment source in the Nene, typically contributing between 60% and 100% of the sediment. Rates of sediment accumulation on the Nene’s floodplain was found to be highly variable (920– 7,200 t km-2 yr-1); the presence of flood defences were likely to be a cause of this variability, and have caused a reduction in the accumulation rate since 1963. It was found that large quantities of sediment accumulated on channel beds during periods of low flows (~ 28% of the annual sediment yield), which was flushed from the bed by a series of flood events (leaving <1% of the annual sediment yield in temporary storage). An original contribution to research was made by quantifying the uncertainties associated with tracer use in a fine sediment fingerprinting investigation, as well as identifying the probable causes of the observed uncertainty. The fine sediment dynamics of the Nene basin were also investigated for the first time, and it was identified that the high contributions from channel banks in the Nene were highly a-typical for UK catchments.

This study focuses upon flow hydraulics, sediment transport and riffle-pool maintenance on the River Rede, Northumberland, UK. Analysis of bed structure indicate pools to be coarser than riffles, suggesting these to be zones of maximum tractive force at high flow. Tractive force reversal can be demonstrated using a combination of velocity, shear stress and gravel tracer data, and is therefore advocated as a mechanism for maintaining the riffle-pool form. Three dimensional flow structures are likely to increase the likelihood of reversal in pools situated on bends, which may not always be detected using one-dimensional measures of flow hydraulics. Magnetic tracing and basket trapping techniques were used to provide an insight into rates of movement, accumulation, initial motion criteria and routing, of sand. Sand is transported selectively and is mobilised at between 11-22 Nm⁻². Deposition of sub 2mm material is prevalent on morphological high points (bars/riffle margins), although greatest quantities were routed through morphological lows. Freeze core evidence shows limited intragravel storage. Gravel tracer movements showed evidence of size selective entrainment overall, however hiding effects were also found to be evident at two scales; 30-50mm and 110-140mm (for riffles) and 20- 90mm and 11O-140mm for pool. Slope exponents for log-log relations between scaled grain size (D/D₅₀) versus dimensionless shear stress (Өc) of ≈-0.9 suggest that hiding strongly influences sediment transport. Stream power estimates from pgQs demonstrate a higher threshold for motion for gravel in pools (132 Wm⁻²) compared with riffles (127 Wm⁻²). Differences in initial motion criteria (8e) between riffles and pools were found to be significant (p<0.05), indicating pool sediments to be less mobile than riffle, despite pool sediments being less compact. Reduced mobility of pool bedload sediment results from clasts being sheltered by immobile lag gravel found in the pool. It appears therefore that mobility differences between riffles and pools, related to bed structure, does not explain riffle-pool maintenance on the Rede. Scaled travel distance (L/L₅₀s) for tracers in the reach as a whole showed a convex-up relationship with scaled grain size (D/D₅₀s), demonstrating that for tracer grains progressively coarser than the surrounding D₅₀ surface grains, travel distance drops off rapidly, whereas grains progressively finer than the surrounding clasts, travel further but at a less rapid rate. Furthermore, virtual velocity (V*) of tracer grains showed a positive dependence upon D/D₅₀s. Gravel tracer movement provided important insights into riffle-pool maintenance. Transfer of material through the Rede riffle-pool sequence appeared to be influenced by flow magnitude and duration. For low magnitude high frequency flows below 25% bankfull, intra-unit movement was found to predominate. Medium magnitude and frequency flows (up to 50% bankfull) appeared capable of inter-unit transport; scour from pool troughs and deposition on pool exit slopes I riffle heads, movement of material from riffles to bar edges and from bar to bar. For higher magnitude low frequency flows up to bankfull, there was less scour from pools, and a dominance of bar-to-bar sediment transfer. Limited evidence of sediment routing and deposition in pools suggest these to be scour / sediment source zones only, with supply originating from the bed and outer bank. These data demonstrate the importance of different flow magnitude and frequency in creating / maintaining different areas of the riffle-pool structure.

Baroclinic tides result from the interaction of barotropic tides with topography in stratified oceans. They play an important role in driving deep ocean mixing. In this research, investigations of the dynamics of baroclinic tides and internal solitary waves (ISWs) in the northern South China Sea (SCS) are conducted, mainly by means of the Massachusetts Institute of Technology general circulation model (MITgcm). Firstly, simulations of internal wave generation at the Luzon Strait (LS) are carried out. By conducting three-dimensional (3D), high-resolution experiments, it was found that the generated wave field features a multi-modal structure: large, pronounced ISWs of first mode (amplitude ~120 m) and second mode (amplitude ~120 m) were reproduced. The two north-south aligned ridges in the LS contribute together to the generation of the second mode ISWs, whereas the easternmost ridge of the two is responsible for the first mode ISWs. It was found that multiple generation mechanisms of internal waves could occur in this region, and overall it belongs to a mixed lee wave regime. A specific type of short internal waves arose during the 3D simulation. These ride on a second mode ISW with similar phase speed, trailing a first mode ISW. The short waves possess wavelengths of ~1.5 km and amplitudes of ~20 m, and only show up in the upper layer up to a depth of ~500 m. Scrutiny of the generation process showed that these short waves appear in two distinct regions and are produced due to two mechanisms, namely, the disintegration of an inclined baroclinic bore near the LS, and the overtaking of a second mode ISW in the deep water by a faster first mode ISW. Robust evidence has been sought from satellite imagery and by solving the theoretical Taylor-Goldstein Equation to verify their existence. The effects of superposition of multiple tidal harmonics (diurnal and semidiurnal) on the resultant ISW generation were investigated. It was first found that, by analyzing historical observational data, the occurrence of ISWs in the far-field always follow strong semidiurnal barotropic tidal peaks in the LS, regardless of whether it is the maximum for the diurnal or total tidal strength. However, modelling results of MITgcm and a linear internal tide generation model demonstrate that the diurnal tidal harmonics modulate the arrival time and amplitude of the propagating ISWs. Specifically, it leads to the emergence of the so-called A and B type ISWs and an alternation and transition between the two. Secondly, the shoaling process of ISWs in the northern SCS slope-shelf area is investigated. A series of two-dimensional (2D) experiments are set up to study the shoaling of a large-amplitude second mode concave ISW over a linear slope that resembles the SCS slope. Modelling results show that a strong transformation of the wave profile starts to take place when the wave is approaching the shelf break. A convex type wave is born at the trailing edge of the incident wave and gradually disintegrates into a group of ISWs due to the steepening of the rear wave profile. The frontal face of the wave gets flatter when travelling on the slope, but forms a steep structure right above the shelf break. However, this steep structure shows no tendency to evolve into an ISW: instead, it gets increasingly flat again while evolving on the shelf. The trailing convex wave packet travels faster and merges with the frontal concave wave. Finally, a wave packet with rank-ordered convex ISWs moves forward steadily on the shelf. Energy transfer to the ambient modes is evident, as both first mode and higher modes are clearly seen during and after the shoaling process. First mode ISW evolution is studied too by performing 3D, high-resolution experiments over the wide northern SCS slope and shelf area. It was found that the wave profiles change drastically near the shelf break and the Dongsha Atoll. In agreement with satellite imagery, the wavefront of the leading ISW becomes more spatially oblique with respect to its original orientation as it progresses westward due to the inclination of the slope in the topography. Wave disintegration is prominent in the shallow water zone, and wave polarity reverses near the turning point (at the 130 m isobath), which is consistent with the predictions of weakly nonlinear theory. A series of 2D experiments were set up to inspect the effects of rotation on the shoaling ISW. The results indicate that under the rotation, upon reaching the continental shelf, one shoaling ISW could disintegrate into one ISW packet and one secondary solibore that contains a number of rank-ordered waves with much shorter wavelength than an ISW. The secondary solibore is very pronounced in the northern portion of the northern SCS slope and shelf, but could hardly be discerned in the southern portion, which is consistent with the outcome of 3D simulations.