Braided rivers: an exploratory study combining flume experiments and the analysis of remotely-sensed data

Abstract

Braided rivers exhibit extremely complex and dynamic morphologies as their multiple channels are constantly re-worked. The research reported in this thesis explored a number of properties of braided river form and dynamics and some controlling factors through three individual but complementary research elements. The first research element was concerned with some of the controls on the transition between single thread and multi-thread channel patterns. Twenty-seven different flume experiments were conducted, supported by fourteen replicates. In these experiments, channel confinement (maximum possible channel width) and formative discharge were varied in a 25 x 2.9 m flume of constant slope (1%) and bed material (D50 = 1mm) with sediment supply constrained to match sediment output. As the maximum potential channel width increased, the channel pattern changed from a single channel with alternate bars, to the formation of mid-channel bars, and finally to a multi-thread braided pattern. Bed elevation frequency distributions showed distinct changes in their median, standard deviation, skewness and kurtosis as channel width and discharge increased, indicating the consequences of confining braided channels and regulating discharge on their bed elevation and morphology. The second and third parts of the research use remotely sensed data sets to explored (i) the degree to which a real river shows similar characteristics to those generated in the flume experiments and (ii) the variety in braiding patterns that are found in association with different boundary conditions of slope, width, discharge, and riparian vegetation. For the second research element, a Lidar survey of a 36 km reach of the lower Tagliamento river, Italy, was investigated. Within this reach, the river shows only small variations in slope and bed material size and is subject to the same flood flows. Analysis focused on thirty-six 1 km sub-reaches and demonstrated clear associations among the median, standard deviation, kurtosis and skewness of the bed and also clear downstream trends. Measures of vegetation cover showed statistically-significant associations with the median, standard deviation, kurtosis and skewness of the bed, particularly when only the 32 truly braided reaches were analysed. The measures of vegetation cover also showed downstream trends that corresponded with the trends in bed morphology. Abstract 5 Overall, variations in bed morphology showed similar characteristics to those observed in the laboratory flume, but also they showed correspondence with riparian vegetation cover, indicating a topographic signature of vegetation on the bed morphology. The downstream trends appear to be associated with the changing vigour of the riparian vegetation and possibly variations in river baseflow characteristics associated with varying groundwater levels in the alluvial aquifer. The most mature patches of vegetation within the braid plain of the most downstream part of the 36 km reach appear to occur on remnants of braid plain isolated by river bed incision. The third and final research element considered the morphology of six European braided rivers of different slope, width, discharge and riparian vegetation type. Information extracted from Google Earth and other aerial imagery, and gauged river flow data supported an analysis of changes in braided river characteristics through time, and among the six European river sites. Four traditional planform indices were used to characterise the braiding pattern (Bi – braiding index, Ai and Ai2 –anastomosing indices; Si – main channel sinuosity) were combined with measures of stream power and its component variables (width, Q10, and slope). Robust data for bed material calibre was not available. Statistical analysis of the entire data set revealed a potential influence of riparian vegetation type on the relationship between unit stream power and braid channel width; and a trend of increasing Bi, Ai, Ai2, and Si with decreasing unit stream power. However, a larger and more complete data set is needed to confirm these general trends and to fully explore transitional rivers. This research has illustrated the morphological consequences of confining braided rivers and the dependence of the braiding pattern on stream power. It has also illustrated the role of vegetation in contributing to the morphological complexity of braided rivers and the potential role of riparian vegetation in constraining the relationship between stream power and braided river width.