The form, flow and dynamic character of meanders in a lowland river

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.