Sediment dynamics across gravel-sand transitions: Implications for river stability and floodplain recycling

Abstract

<p>The gravel-sand transition is observed along most rivers. It is characterized by an abrupt reduction in median bed 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 document changes in channel stability, suspended sediment concentrations, flux and grain size across the gravel-sand transition of the Karnali River, Nepal. Upstream of the gravel-sand transition, gravel-bed channels are stable over hundred to thousand-year timescales. 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 gravel-sand transition. The results demonstrate a dramatic change in channel dynamics across the transition, from relatively fixed, steep gravel-bed rivers with infrequent avulsion to lower gradient, relatively mobile sand-bed channels. The increase in sediment concentrations and near-bed suspended grain size may be caused by enhanced channel mobility, which facilitates exchange between bed and bank materials.   These results bring new constraints on channel stability at mountain fronts, and indicate that temporally and spatially limited sediment flux measurements downstream of gravel-sand transitions are more indicative of flow stage and floodplain recycling than of continental-scale sediment flux and denudation rate estimates.</p>

Grain Size and Beach Face Slope on Paraglacial Beaches of New England, USA

“…These gravel to sand transitions (GST) often migrate over time (Ferguson, 2003;Marr et al, 2000;Robinson and Slingerland, 1998), resulting in a depositional sequence that is as a whole distinctly bimodal (Marr et al, 2000;Paola et al, 1992). No universal theory currently exists to explain GST (Dingle et al, 2020), although the phenomena is well documented as well as a resultant 1-10 mm gap in grain size similar to that observed in Fig. 4 (e.g.…”

“…Dade and Friend, 1998;Wolcott, 1988). Possible explanations for this grain size gap include grain-size dependent changes in particle breakdown or comminution (Jerolmack and Brzinski, 2010), nonlinearities in bedload transport (Ferguson, 2003), transitions from viscousto-turbulent dependent sediment suspension thresholds (Lamb and Venditti, 2016), a switch from washload to suspended/bedload transport of sands (Dingle et al, 2020), and shifts from gravel beds to cohesive channel banks as the predominant control on channel geometries for gravel and sandy channels (Dunne and Jerolmack, 2018).…”

Dynamic flood topographies in the Terai region of Nepal

“…At ~5 km downstream, the channel bifurcates into two branches. The gradient of the gravel reaches, which extend to the gravel–sand transition at ~40 km downstream in each branch, is 0.001–0.002 m m −1 (Dingle et al ., 2020). The gravel–sand transition occurs over a distance of ~2–3 km, downstream of which the channel bed is exclusively sand, and the two branches of the Karnali River rejoin.…”

“…River systems downstream of the Himalayan mountain front are typically described as shallow, aggrading alluvial systems (e.g. Sinha et al ., 2005; Tandon et al ., 2006; Dingle et al ., 2016), which are characterized by large sediment loads (Sinha and Friend, 1994; Lupker et al ., 2012) and high rates of lateral channel migration and avulsion (Chakraborty et al ., 2010; Dingle et al ., 2020). Sediment accumulation in channels may be caused by simple natural variability in sediment flux from upstream parts of the catchment, but also by changes in land use, engineering works (e.g.…”

Times associated with source-to-sink propagation of environmental signals during landscape transience

“…The second group of parameters focuses on differences in the characteristics of grains within a certain sediment package, such as grain size distributions (Duller et al, 2010(Duller et al, , 2019Whittaker et al, 2010Whittaker et al, , 2011Foreman et al, 2012;Parsons et al, 2012;Foreman, 2014;D'Arcy et al, 2017), median or other characteristic grain sizes (D50, D84, sortable silt; McCave and Hall, 2006;Schlunegger and Norton, 2015;Chen et al, 2018;McCave and Andrews, 2019;Watkins et al, 2020), the location of the gravelsand transition in alluvial fans and river systems (Allen et al, 2015;Dubille and Lavé, 2015;Blom et al, 2017;Dingle et al, 2017Dingle et al, , 2020Armitage et al, 2018a), sorting and related textural characteristics (e.g., in glacio-marine sediments: Anderson et al, 1980;D'Orsay and Van De Poll, 1985;Pudsey, 1992;Helland et al, 1997;Passchier et al, 2019), or grain shape (Stanley and De Deckker, 2002;Kalińska and Nartišs, 2014).…”