Sediment dynamics across gravel-sand transitions : implications for river stability and floodplain recycling
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The gravel-sand transition (GST) is commonly observed along rivers. It is characterized by an abrupt reduction in median grain size, from gravel- to sand-size sediment, and by a shift in sand transport mode from wash load–dominated to suspended bed material load. We documented changes in channel stability, suspended sediment concentration, flux, and grain size across the GST of the Karnali River, Nepal. Upstream of the GST, gravel-bed channels are stable over hundred- to thousand-year time scales. Downstream, floodplain sediment is reworked by lateral bank erosion, particularly during monsoon discharges. Suspended sediment concentration, grain size, and flux reveal counterintuitive increases downstream of the GST. The results demonstrate a dramatic change in channel dynamics across the GST, from relatively fixed, steep gravel-bed rivers with infrequent avulsion to lower-gradient, relatively mobile sand-bed channels. The increase in sediment concentration and near-bed suspended grain size may be caused by enhanced channel mobility, which facilitates exchange between bed and bank material. These results bring new constraints on channel stability at mountain fronts and indicate that temporally and spatially limited sediment flux measurements downstream of GSTs are more indicative of flow stage and floodplain recycling than of continental-scale sediment flux and denudation rate estimates.
Dingle , E , Sinclair , H , Venditti , J , Attal , M , Kinnaird , T C , Creed , M , Quick , L , Nittrouer , J & Gautam , D 2020 , ' Sediment dynamics across gravel-sand transitions : implications for river stability and floodplain recycling ' , Geology , vol. 48 , no. 5 , pp. 468-472 . https://doi.org/10.1130/G46909.1
Copyright © 2020 Geological Society of America. This work has been made available online in accordance with publisher policies or with permission. Permission for further reuse of this content should be sought from the publisher or the rights holder. This is the author created accepted manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at https://doi.org/10.1130/G46909.1
DescriptionData collection was funded through a NERC GCRF Building Resilience grant (NE/P015905/1) to H.D.S. and an EPSRC GCRF Institutional grant (EP/P510944/1) to M.A. Sampling equipment was lent by Ed Tipper and Bob Hilton. The RTK-GPS was loaned from the NERC Geophysical Equipment Facility.
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