Abrasion-set limits on Himalayan gravel flux

Abstract

Rivers sourced in the Himalayan mountain range carry some of the largest sediment loads on the planet, yet coarse gravel in these rivers vanishes within approximately 10-40 kilometres on entering the Ganga Plain (the part of the North Indian River Plain containing the Ganges River). Understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms. Rapid increase in gravel flux and subsequent channel bed aggradation (that is, sediment deposition by a river) following the 1999 Chi-Chi and 2008 Wenchuan earthquakes reduced channel capacity and increased flood inundation. Here we present an analysis of fan geometry, sediment grain size and lithology in the Ganga Basin. We find that the gravel fluxes from rivers draining the central Himalayan mountains, with upstream catchment areas ranging from about 350 to 50,000 square kilometres, are comparable. Our results show that abrasion of gravel during fluvial transport can explain this observation; most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the Ganga Plain. These findings indicate that earthquake-induced sediment pulses sourced from the Greater Himalayas, such as that following the 2015 Gorkha earthquake, are unlikely to drive increased gravel aggradation at the mountain front. Instead, we suggest that the sediment influx should result in an elevated sand flux, leading to distinct patterns of aggradation and flood risk in the densely populated, low-relief Ganga Plain.

Possible threshold controls on sediment grain properties of Peruvian coastal river basins

“…This in turn could result in larger clasts in these streams and could potentially cause gravel fronts to shift towards more distal sites (Dingle et al, 2017), thereby coarsening the sediment caliber at our sampling sites. These basins have recently been analysed for denudation rates averaged by mean 10 Bebased catchments and annual water fluxes (please see Reber et al, 2017, and information presented above).…”

“…The size and shape of gravels bear crucial information about (i) the transport dynamics of mountain rivers (Hjulström, 1935;Shields, 1936;Blissenbach, 1952;Koiter et al, 2013;Duller et al, 2012;Attal et al, 2015), (ii) the mechanisms of sediment supply and provenance (Parker, 1991;Paola et al, 1992a, b;Attal and Lavé, 2006), and (iii) environmental conditions such as uplift and precipitation (Heller and Paola, 1992;Robinson and Slingerland, 1998;Foreman et al, 2012;Allen et al, 2013;Foreman, 2014). The mechanisms by which grain size and shape change from source to sink have often been studied with flume experiments (e.g.…”

Alluvial cover controlling the width, slope and sinuosity of bedrock channels

“…The bedload fraction tends to decrease with increasing drainage area (e.g. Turowski et al, 2010), possibly even to the extent that bedload supply Q s is independent of drainage area (see Dingle et al, 2017). There are additional complications that arise from non-linear averaging of sediment supply both with varying floods and stochastically varying bedload supply.…”

Advance, Retreat, and Halt of Abrupt Gravel‐Sand Transitions in Alluvial Rivers

“…An increase of the characteristic flow rate reduces the gravel and sand reach slopes (Blom et al, 2016), which enhances GST advance (Marr et al, 2000) as the relative effect of the sand reach slope on GST migration is almost negligible compared to the gravel reach slope. If the stable GST position is known, equation (2) can aid in constraining the gravel flux (Dingle et al, 2017).…”

“…A GST can halt in systems governed by subsidence, delta progradation, or base level rise (Parker, 2004) due to the creation of accommodation space for the gravel supply in the gravel reach (Dingle et al, 2017;Marr et al, 2000;Paola, Heller, et al, 1992).…”