Floodplain Geomorphology and Topography in Large Rivers

Abstract

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.

Date of Award	Jul 2016
Original language	English
Awarding Institution	University of Brighton