Sediment dynamics of the Himalayan foreland basin from the Neogene to present times
Creators: name: Quick, Laura
Publisher: name: The University of Edinburgh
Year: 2022
Subjects: grain size
sediment recycling
foreland basin
sediment transport
hyperconcentrated flow
terraces
paleochannles
Indo-Gangetic basin

description: The large Himalayan rivers of the Indo-Gangetic Plain support 10% of the world’s population. Most of the world’s most poorest and vulnerable communities live downstream of the Himalayan mountain front, where Himalayan rivers dominate the landscape. However, most of these large river systems are also the source of devastating floods. About 10 – 30 km downstream of the Himalayan Mountain front, the rivers of the Indo-Gangetic Plain undergo a dramatic change in sediment characteristics where they switch from depositing coarse gravel, to sand. This geomorphic boundary is termed the gravel-sand transition. In the Indo-Gangetic plain the gravel-sand transition is also associated with a change in channel behaviour and flood dynamics. Upstream of the transition, in the gravel portion of the river, channel movement is characterised by sudden channel switching (avulsion), where the river abandons its current path forming a new one, or reoccupies a previously abandoned channel (paleochannel). Such avulsion events occur at various scales and from nodes at varying positions along the river course. Downstream of the transition, the channel laterally migrates tens to hundreds of kilometres across the fan surface, via incremental bank erosion and bar formation, often eroding and reworking arable farmland in the distal sandy plains. The gravel-sand transition therefore marks an important boundary in river behaviour and associated flood risk. The overall position of the gravel-sand transition in the Himalayan foreland is largely controlled by long-term (millennial) subsidence rates. However, short-term (sub-millennial) controls on coarse sediment supply and characteristics (size, shape, and flux) delivered to the plains could drive changes in the position of the gravel front and internal channel dynamics (i.e. channel avulsion/migration and incision/aggradation). Understanding the controls on coarse sediment characteristics (size, shape, and flux) delivered to the Indo-Gangetic Plains and subsequent river behaviour is fundamental in determining how these river systems may respond to projected climate change and extreme events, and is crucial for improving flood risk mitigation and practise in the region. This thesis examines the controls on coarse sediment supply and characteristics delivered to the Indo-Gangetic Plains by using a combination of field data collected from present-day Himalayan rivers and the long-term fluvial sediment record of the Miocene Siwalik Group, which was formed by similar rivers found on the Indo-Gangetic plain today. Specifically this thesis explores the controls on the characteristics of the coarse sediment exported by the larger trans-Himalayan rivers (size, shape, and flux), and the mechanisms of coarse sediment transport and subsequent river behaviour through time. The first part of this thesis examines the controls on coarse sediment characteristics delivered to the Indo-Gangetic Plains. Along strike of the Himalayan mountain range, Himalayan rivers recycle differing amounts of quartzite-rich Siwalik conglomerate. I demonstrate that despite the conglomerates forming less than 1% of the catchment geology, recycling the conglomerate can contribute up to 100% of the exported coarse bedload for some catchments. I also find that Himalayan rivers which have a larger portion of recycled conglomerate in their catchments exhibit quartzite-rich gravel bars and grain size distributions similar to that of the Upper Siwalik conglomerate. Furthermore, pebble roundness is greater in catchments with a recycled conglomerate component. Secondly this thesis explores whether coarse sediment can be transported beyond the gravel-sand transition. The Miocene Siwalik succession of the Himalayan foothills reveals punctuated episodes of coarse gravel sedimentation below the Middle – Upper Siwalik boundary (which represents the gravel-sand transition in vertical succession). These coarse conglomerate beds represent the temporary transport of gravels far into the distal Miocene Gangetic Plains. Sedimentological characteristics of the conglomerate beds combined with modified sediment entrainment calculations suggest that these gravel deposits were formed by rare hyperconcentrated flood events. Calculated return intervals suggest these conglomerate beds represent rare hyperconcentrated floods with a return interval of 500 to a 1,000-years. Finally this thesis aims to build an understanding of the chronology of aggradation, incision, and avulsion at the head of a major fan system in the Indo-Gangetic Plain, and link these processes to changes in environmental forcings (e.g. tectonics and climate). For this chapter I used OSL dating techniques to date terraces and paloechannels associated with the the Karnali fan, in western Nepal. OSL dates suggest that there is a weak correlation of aggradation and incision to periods of increased monsoon intensity. Paleochannels dated on the Karnali fan surface correlate to a period of decreasing monsoon strength. However, due to the wide scatter of terrace and paleochannel ages and associated errors with OSL dating, it is difficult to attribute periods of channel aggradation, incision and avulsion on the Karnali fan to one specific environmental forcing.