The study considers the seasonal distribution of non-tidal peak flows on a large number of rivers draining varied catchments across Scotland and Northumberland. Peaks over threshold (POT) flood series from 156 gauging stations are used, and are subject to two quality control measures. Firstly, threshold values are standardised to give 45 peaks over a ten year period and secondly, records are adjusted to compensate for non-stationarity in the sampled data. The database assembled consists of 3458 station-years of record. A comprehensive description of the seasonal patterns found is presented, based on these quality-controlled data and utilising a number of methods of characterisation. Directional statistics are employed to indicate the central tendency of time-of-year values for each station, a six-season analysis gives more detailed information, and the seasonality of large peaks is compared with that of full POT series. Finally, a classification analysis is used to summarise these patterns. These patterns are related to five catchment characteristics: the seasonality of rainstorms; soil moisture deficit lengths; catchment size; lake storage and snowmelt, although the effect of the last of these is unclear as suitable data were not available for analysis. A discriminant analysis is employed to relate the five physical factors to flood seasonality. The study concludes with a discussion on the implications of its findings. A method of assessing seasonal flood risk using POT series is presented, offering an accurate means of relating flood magnitude to recurrence interval for any period of less than one year. The implications of seasonal heterogeneity, both within and between flood records, are also discussed. The suitability of the exponential model for use with POT records is questioned and it is suggested that explicit recognition of the seasonality of flooding may be necessary in order to make accurate design flood estimates.

Over fifty peaks-over-threshold flood records from across Scotland are analysed for temporal and spatial variability over two standard periods, 1954-92 and 1964-1992, using a number of time series statistical techniques. The results of this analysis demonstrate both clear temporal and spatial patterns in the frequency and magnitude of floods. The period 1964-73 is characterised by decreasing flood frequencies, with the early 1970s standing out as being "flood poor", particularly in the eastern regions of Scotland, whilst the 1950s and more recently, the late 1980s and early 1990s can be characterised as "flood rich". A broadly similar pattern is evident in many flood magnitude series. The influence of climatic variability upon flood records is also examined over the same periods of time, using a regional synoptic classification of daily weather types. The Westerly, Cyclonic and South-Westerly weather types are identified as important mechanisms in initiating Scottish flood events. These weather types also show a strong link with catchment location, with the Westerly type being important in the more westerly catchments, the South-Westerly type being important in the south-west and west whilst flood events in the more eastern catchments are often associated with the incidence of the Cyclonic weather type. The annual frequencies of these three key weather types also show clear patterns of temporal variability. However, a seasonal split of weather type frequencies also reveals some contrasting seasonal trends which are masked within the amalgamated annual series. The most dramatic variability is evident within the time series of the South-Westerly weather type, where frequencies have been increasing steadily since the mid 1970s in all seasons. Recent increases in the incidence of the most frequently occurring weather type - the Westerly - appear to be confined to the winter months (December to February). The relationship between temporal variability within flood series and the climate are tentatively explored through a simple comparison exercise using the two sets of time series plots, linked by information obtained on trigger weather types and dominant seasons of flooding. This process suggests that the relationship between flood series and the climate is a complex one which cannot be adequately explained using the results of this research. More likely other factors, such as precipitation variability, need to be introduced into the equation before a more complete picture is acquired. The possible consequences of the variability detected within flood series are highlighted with reference to flood frequency analysis, a technique which makes the assumption that flood records, and ultimately climatic records, are stationary through time. By splitting individual flood records into hydrologically similar sub-periods, it is revealed that frequency-magnitude relationships may vary considerably, depending upon the period of record used. It is recommended that, in future, hydrologists consider splitting flood series (into hydrologically similar sub-periods or in terms of the weather types linked to each flood) in order to examine the range of frequency-magnitude relationships which exist.

Meandering rivers across the globe present a striking similarity in their alignment, and this pattern has intrigued scientific curiosity for almost a century. However, still very little is known about ‘unconventional’ river meanders with a “zigzagging” planform appearance, but which occur in rivers from the temperate to the tropic regions. In order to compare the characteristics of flow, morphology and morphodynamics of conventional (‘round’ and ‘fat’) and unconventional (‘sharp’ and ‘angular’) river bends, I studied the Prut (Romania/Moldova), one of the last remaining freely meandering lowland rivers of Europe. The Prut is a clay-bed river that meanders for about 585 km of channel length in its lower section, with an average discharge of c. 90 m3 s-1 and channel width ranging from 39-84 m. For the analysis of long-term channel dynamics (chapter 2), bend morphometry, morphodynamics and temporal trajectories were determined for a section of Prut that included multiple replicates of each bend type. Data were extracted from historical maps and imagery for a c. 250 km long section of the Prut River spread over a 90-year period (1915-2005). For that, I used a geographical information system (GIS) and state-of-the-art software PyRIS to derive measures of bend-scale morphometry and migration features for a total of 118 simple and back-traceable river bends, after exclusion of compound bends and bends that cut-off during those 90 years. Normalized bend curvatures (CmaxB) ranged from 0.01 to 1.44 (as for 2005). Sharp bends were distinguished from round bends and angular bends from fat bend geometries by width normalized bend curvature, CmaxB threshold (round < 0.5 and sharp > 0.5) and the maximum to mean bend curvature ratio, Cr threshold (fat < 3 and angular > 3), respectively. Over the 90 year period, studied bends (N=118) displayed a gradual narrowing and homogenization of channel width (median= 72 m and interquartile range (IQR)= 63-86 m in 1915; median = 52 m and IQR = 50-57 m in 2005), a reduction of migration rates (mean= 0.038 widths/yr, SD=0.028 in 1915-1960 to mean= 0.015 widths/yr, SD = 0.011 in 1980-2005) and a slightly increase of sinuosity (from 1.41 to 1.63). Sharp and angular bends tended to stabilize over longer periods (20-90 years), and concomitantly displayed more unchanged shape transitions (for sharp bends: 1915-1960= 49%; 1960-1980= 61%; 1980-2005= 57%, N=49) compared to their conventional counterparts (for round bends: 1915-1960= 37%; 1960-1980= 22%; 1980-2005= 22%, N=49). These results provide new insights into the formation and maintenance of unconventional bends (sharp and angular), which on the Prut are mainly due to autogenic processes. In unconventional bends, special patterns of erosion and deposition create features like deeper pools or zones of flow separation, with fallen trees possibly acting as additional allogenic factor. These interrelationships among bend form, flow and river bed features in unconventional meanders were then investigated in detail fora set of 14 bends (chapter 3). Boat operated ADCP (Acoustic Doppler Current Profiler) field surveys enabled detailed records of 3-D velocity and depth distribution along these bends.This dataset enabled for the first time to systematically compare flow characteristics of unconventional with that in conventional meanders under field conditions. The studied set of bends comprised at least 2 bends of each simple bend shape types (see chapter 2) i.e. angular, sharp, round and fat bend shapes. A significant linear increase in the lateral extent of the Inner-Bank Flow Separation zone (IBFS) at the bend apex with increasing bend curvature was recorded (IBFS = 6.1+ 45.8 CmaxB + 8.2; R2= 0.55; p < 0.001; N=14). In angular bends, hydraulics was influenced by a locally eroded point bar and a steep sloping upstream riffle to pool transition. This was reflected by the significant increase in depth ratio (ratio of apex pool depth to upstream riffle depth) with stronger bend angularity, as defined by curvature ratio (ratio of maximum to mean of bend curvature) (Hratio = 0.83 + 0.41 Cr; R2 = 0.44; p < 0.005; N=14). These morphological features influenced the extent of the horizontal recirculation occurring within the IBFS zones at the bend apex, and were further associated with the formation of two separate IBFS zones in fat bends compared to a singular zone in angular bends. Pool depth (Hp) displayed an increasing trend with higher curvature but then to stabilize at bend curvatures greater than c. 0.5, suggesting the existence of a negative autogenic feedback at high bend curvature, as pool depth is also weakly related to the IBFS size (IBFS = -14+ 10.3 Hp + 10.2; R2= 0.30; p<0.02; N=14). The observed interdependence between flow patterns and planform shape in angular and fat bends partially explains their different morphodynamics, too. While fat bends are prone to develop multilobing or a delayed cut-offs, angular bends may display increased temporal stability and even inward migration. Hence, this thesis demonstrates for the first time the different characteristics of conventional and unconventional meanders in terms of their morphometry and hydraulic geometry based on the study of bends from a real meandering river, and complemented by documentation of the long-term morphological trajectories of these meander types during a nearly centennial period. Thus, this thesis provides new insights into the so far poorly explored linkages between forms and processes of river bends, and indicates field-based explanations on the formation and relative stability of unconventional bend forms over extended time periods.

Rivers and streams are increasingly recognised as important components in the global carbon cycle, and act as net sources of CO2 and CH4 to the atmosphere. However, the origins and controls over the fate of these greenhouse gases are still poorly constrained. This thesis firstly explores the production of CO2 and CH4 in the sediment of several rivers over a geological gradient (chalk, sand and clay), to investigate the magnitude and controls on production. It was found that, whilst there are some general patterns due to geology, variables such as organic carbon content are much better predictors of production of CO2 and CH4 and these can vary widely within a single reach. The response of production to temperature was found to be very constant across rivers and in both summer and winter, suggesting a uniform increase in production of both CO2 and CH4 with projected future climate change. However, production of CH4 was much more responsive to temperature change than was CO2, meaning a greater proportion of carbon is mineralized as CH4 under warmer conditions and indicating a positive feedback with global warming. In addition, the amount of CO2 and CH4 out-gassed from the rivers to the atmosphere was measured. It was found that the amount out-gassed could not be explained by local sediment respiration alone: CO2 out-gassing was consistently higher (and CH4 consistently lower) than that produced by the riverbed. Instead, CO2 out-gassing was under hydrological control, and was correlated with rainfall. The source of this was ingress from the surrounding catchment, with higher out-gassing during periods of high rainfall. This strong hydrological controls on CO2 emissions were however modulated by biological processes, as lower emissions were measured during the day than night; and the difference could be accounted for by local in-stream GPP.

The first objective of this investigation was to derive and compare scour depth equations in a long contraction using the most widely used sediment transport equations and a variety of other equations. The second objective was to determine a procedure to find an equivalent rectangular section which would convey the same water discharge and sediment load at same slope as an irregular, natural channel in order to simplify numerical computations of scour depth and to allow appropriate application of long contraction scour theory. Some of the transport equations were manipulated algebraically to develop equations for scour depth and slope in a long contraction; others were manipulated using computer programs written especially for each equation, thus deriving scour depth equations. A computer program was written to compare characteristics of a non-rectangular section with rectangular sections of different widths in order to derive a procedure to find an equivalent rectangular transport section (a triangular section was used in this investigation) but the procedure is equally valid for any irregular, natural section. This investigation indicated that depth in the contraction is greater than in the wider approach channel. How much greater depended on which sediment-transport equation was used. Most of the derived scour equations, based on the different sediment transport equations, predicted that the y₂/y₁ ratio decreases as slope, velocity, c, and τₒ'/τ(c) or √τ/ρ)/ω increase; a few do not. Most of the analysis predicted S₂ < S₁, but a few do not. Field and experimental data provided extra evidence that the depth in the contracted section is greater than in the approach reach and how much greater. The evidence that the slope is flatter is not sufficient to be completely convincing. The equivalent rectangular transport section which can carry the same water and sediment discharge at the same slope as the natural section has a depth which is a large fraction of the deepest part of the original section, and the width is considerably narrower than the top width of the original section. Results of the investigation also indicated that the slope, velocity, sediment concentration, and sediment size have little effect on the geometry of the equivalent rectangular section.

toc / Geography and Geology / Master / Master of Philosophy

Thesis (M.S.)--State University of New York at Binghamton, Geological Sciences & Environmental Studies Department, 2007. / Includes bibliographical references.

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