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Lesson 4 River Channel Migration

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GNR 624 : River basin management Lesson 4 River channel migration Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management River channel migration is the geomorphological process due to lateral migration of an alluvial river channel across its floodplain. This process is mainly driven by the combination of bank erosion of and point bar deposition over time. River channel migration, refers to meandering streams whereas In braided streams, channel change is driven by sediment transport. Physical Processes involved: Bank Erosion As flow enters the bank of an alluvial river, the centrifugal force created by the bend instigates helicoidal flow, a corkscrew like pattern of flow, which drives the hydraulic action acting on the opposing bank. This is where the primary process in river channel migration of bank erosion occurs. Often the bank is undercut, another result of the helicoidal flow, which leads to the creation of cut banks. Factors that limit the rate of bank erosion include the rate of deposition of the point bar, stream power, and the critical shear stress of the stream bed. Point Bar Deposition The sediment taken from the bank during the process of bank erosion is deposited on the opposing side of the channel fueling the process called point bar deposition. The helicoidal flow also plays a role in this process by acting as a cross channel component that moves the sediment to the other side. The processes of point bar deposition and bank erosion are intertwined and in most cases the erosion rate of cut banks is equal to the deposition rate of point bars. In addition, point bars act as topographic obstructions once formed that further drive flow into the opposite bank, creating a positive feedback loop. This leads to the meanders of an alluvial river becoming more well defined over time Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Incised meanders • A meander that lies at the bottom of a steep walled canyon • Most often occurs at an existing meander after the rejuvenation of a river. • There is then severe downwards erosion creating a steep walled canyon. • Two types of incised meander: • Entrenched: have a symmetrical cross section resulting from very rapid incision by the river of the valley sides being made of hard, resistant rock. • Ingrown: formed when the incision or uplift is less rapid and the river may shift laterally, producing an asymmetrical cross section shape. Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Oxbow Lakes - Meander neck becomes smaller new course of the river. When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation. Old Stage - Final stage of a river is reached when the river is flowing on more or less flat surface. The speed so low that no net erosion occurs any more. Erosion on the slopes is balanced by deposition on the floodplains. Peneplain -a flat and relatively featureless landscape with minimal relief; considered to be the end product of the geomorphic cycle. rivers just sweep over the floodplain and rework the sediment, creating various floodplain features. Isolated remnants of resistant bedrock may rise over the Peneplain surface, the so-called monad nocks or inselbergs. Rejuvenation - Old or mature stage landforms may be uplifted due to climate change or tectonic activity Entrenched meander: a winding, sinuous stream valley with abrupt sides. In a fashion, the landscape reverts to an earlier stage in its evolution (e.g. from intermediate/mature to the initial/young Because the river is moving on a flat surface, deposits sediments at the bed often act as barriers to free flow of water making it to form meanders. Point Bar is a depositional feature of streams found in abundance in mature or meandering streams. Point bars are composed of sediments that are well stored and typically reflects the overall capacity of the stream. Point bar deposits on the inner meander bend where there is low energy Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management MEANDERS • Sudden bends in the course of a river. • Form when areas of alternating pools, deep water and riffles (Shallow water) develop at equally spaced intervals along a stretch of a river. • distance between pools is usually 5-6 times the width of the river bed. • Because water is deeper in pools, the river is more efficient when passing over them. • The energy and erosive power is therefore increased when passing over these areas. River is less efficient when passing over riffles as there is more friction, causing the river to lose energy. • River gaining and losing efficiency at different intervals causes the river’s flow to become uneven, and maximum flow is concentrated on one side of the river. Turbulence increases in and around pools. • This leads to corkscrew-like currents in the river called helicoidal flow. • These helicoidal deepening of the pools – river cliff. • Increased amount of eroded material being deposited on the inside of the immediate bend, where the river loses energy- slip off slope. • Combination of erosion and deposition exaggerated bends until large meanders are formed. give the meanders an asymmetric cross-section. Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Avulsion is the sudden abandonment of a part or the whole of a meander belt by a stream for some new course. Channels may avulse into an abandoned channel or create a new channel depending on the pre-existing boundary conditions that initiate the avulsion. Anastomosing channel - A type of alluvial channel with multiple, interconnected, coexisting channels. Anastomosing channels have vegetated islands between channels, whereas braided channels have bare bars. Two processes occur simultaneously in anastomosing channels: (1) avulsion, which creates a pattern of multiple channels; and (2) lateral migration of the individual channels that exist within the anastomosing pattern (i.e., individual meander belts). Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Brahmaputra river course migration Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Channel Migration Zone (CMZ) : Historical Migration Zone (HMZ)—the collective area the channel occupied in the historical record Avulsion Hazard Zone (AHZ)—the area not included inthe HMZ that is at risk of avulsion over the timeline Erosion Hazard Area (EHA)—the area not included in the HMZ or the AHZ that is at risk of bank erosion from stream flow or mass wasting over the timeline. EHA has two components: Erosion Setback (ES) -is the area at risk of future bank erosion by stream flow; Geotechnical Setback (GS)- defined by channel and terrace banks that are at risk of mass wasting (due to erosion of the toe. ES at a side slope angle that forms a stable bank configuration, thereby accounting for mass wasting processes that will promote a stable angle of repose Disconnected Migration Area (DMA)—the portion of the CMZ where man-made structures physically eliminate channel migration CMZ = HMZ + AHZ + EHA – DMA (EHA = ES + GS) DMA by field mapping and assessment of surficial geology, fluvial landforms, geotechnical characteristics and current physical conditions of the given area. Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Ganges – Padma River system at confluence Braided river course Brahmaputra Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Prof. R. Nagarajan, CSRE , IIT Bombay 10.1 GNR 624 : River basin management Thank you Prof. R. Nagarajan, CSRE , IIT Bombay 10.1.
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