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Geometry of Meandering and Braided Gravel-Bed Threads from the Bayanbulak Grassland, Tianshan, P

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Geometry of Meandering and Braided Gravel-Bed Threads from the Bayanbulak Grassland, Tianshan, P Earth Surf. Dynam., 4, 273–283, 2016 www.earth-surf-dynam.net/4/273/2016/ doi:10.5194/esurf-4-273-2016 © Author(s) 2016. CC Attribution 3.0 License. Geometry of meandering and braided gravel-bed threads from the Bayanbulak Grassland, Tianshan, P. R. China François Métivier1, Olivier Devauchelle1, Hugo Chauvet1, Eric Lajeunesse1, Patrick Meunier2, Koen Blanckaert3, Peter Ashmore4, Zhi Zhang5, Yuting Fan6, Youcun Liu7, Zhibao Dong5, and Baisheng Ye5,† 1Institut de physique du globe de Paris – Sorbonne Paris Cité, Université Paris Diderot, CNRS – UMR7154, 1 rue Jussieu, 75238 Paris CEDEX 05, France 2Département de Géologie, CNRS – UMR8538, Ecole Normale Supérieure, 24 rue Lhomond, 75005 Paris, France 3Research center for Eco-environmental sciences, Chinese Academy of sciences, Beijing, P. R. China 4Department of Geography, Social Science Centre Rm 2322, Western University, London, Ontario, Canada 5The States Key laboratory of Cryospheric Science, Cold and Arid Region Environmental and Engineering and Research Institute, Chinese Academy of Sciences, 260 Donggang west road, Lanzhou, China 6Xinjiang Institute of Ecology and geography, Chinese Academy of Sciences, Urumqi, P. R. China 7Key Laboratory of Water Environment and Resource, Tianjin Normal University, 393 Binshui west road, Tianjin 300387, China †deceased Correspondence to: François Métivier ([email protected]) Received: 12 October 2015 – Published in Earth Surf. Dynam. Discuss.: 13 November 2015 Revised: 15 February 2016 – Accepted: 9 March 2016 – Published: 22 March 2016 Abstract. The Bayanbulak Grassland, Tianshan, P. R. China, is located in an intramontane sedimentary basin where meandering and braided gravel-bed rivers coexist under the same climatic and geological settings. We report and compare measurements of the discharge, width, depth, slope and grain size of individual threads from these braided and meandering rivers. Both types of threads share statistically indistinguishable regime relations. Their depths and slopes compare well with the threshold theory, but they are wider than predicted by this theory. These findings are reminiscent of previous observations from similar gravel-bed rivers. Using the scaling laws of the threshold theory, we detrend our data with respect to discharge to produce a homogeneous statistical ensemble of width, depth and slope measurements. The statistical distributions of these dimensionless quantities are similar for braided and meandering threads. This suggests that a braided river is a collection of intertwined threads, which individually resemble those of meandering rivers. Given the environmental conditions in Bayanbulak, we furthermore hypothesize that bedload transport causes the threads to be wider than predicted by the threshold theory. 1 Introduction son, 1987; Ashmore, 1991; Schumm, 2005; Kleinhans and van den Berg, 2011). Linear stability analyses, supported by laboratory exper- The morphology of alluvial rivers extends between two end- iments, explain how bedload transport generates bars, and members: in meandering rivers, the flow of water and sed- favors the formation of meandering or braided patterns iments is confined in a single thread, whereas in braided (Parker, 1976; Fredsøe, 1978; Fujita and Muramoto, 1985; rivers the flow is distributed into intertwined threads sepa- Devauchelle et al., 2007; Ashmore, 1991; Zolezzi et al., rated by bars (Fig.1; Leopold and Wolman, 1957; Fergu- Published by Copernicus Publications on behalf of the European Geosciences Union. 274 F. Métivier et al.: Braided and meandering threads Channel Bars Kizik Yulduz 43°N 2a Channel Thread Floodplain 2b 42°30'N Thread 3 84°E 84°30'E Figure 2. Landsat 5 mosaic image of the Bayanbulak Grassland. Figure 1. Definitions involved in the morphology of meandering Red (meandering) and blue (braided) markers indicate measurement (left) and braided (right) rivers (Métivier and Barrier, 2012). sites. White rectangles correspond to Figs.4 and5. 2012). This mechanism proves more efficient in wide and 350 2006 shallow channels. Field measurements indicate that the bank- 300 Sampling period full aspect ratio (ratio of width to depth) of braided rivers 2007 is usually much larger than that of meandering ones, thus –3 250 3 suggesting that the bar instability is indeed responsible for m s 200 braiding (Parker, 1976; Fredsøe, 1978; Fujita and Muramoto, 1985; Devauchelle et al., 2007; Ashmore, 1991; Zolezzi 150 et al., 2012). What exactly controls the aspect ratio of an alluvial river remains an open question, although sediment Discharge 100 discharge and riparian vegetation seem significant in this 50 respect: high sediment load and weak vegetation both fa- vor wider and shallower channels, and often induce braid- 0 Jul ing (Smith and Smith, 1984; Gran and Paola, 2001; Tal and Jan Feb Mar Apr May Jun Aug Sep Oct Nov Dec Paola, 2007, 2010; Brauderick et al., 2009; Dijk et al., 2012; Month Métivier and Barrier, 2012). Figure 3. Water discharge of the Kaidu River at the Dashankou sta- In a fully developed braided channel, emerged bars sepa- tion, downstream of the grassland (monthly average). Source: Xin- rate the threads from each other (Fig.1), and the very defi- jiang Institute for Ecology and Geography (unpublished data). nition of bankfull conditions becomes ambiguous. Most au- thors treat the channel as a whole by defining lumped quan- tities, such as the total channel width or the average water dispersed in size (Glover and Florey, 1951; Henderson, 1963; depth (Métivier and Barrier, 2012). Conversely, few studies Seizilles et al., 2013; Gaurav et al., 2015). focus on the morphology of braided and meandering chan- nels at the level of individual threads (Church and Gilbert, 2 Field site 1975; Mosley, 1983; Ashmore, 2013; Gaurav et al., 2015). In sand-bed rivers, the geometry of braided threads ap- The Bayanbulak Grassland is an intramontane sedimentary pears to be indistinguishable from that of meandering ones. basin standing at an elevation of about 2500 m in the Tian- This observation accords with recent laboratory experiments shan Mountains (Fig.2). Two main wetlands, the Qong Yul- (Seizilles et al., 2013; Reitz et al., 2014). To our knowledge, duz basin (known as the Swan Lake in Chinese), and the this similarity has not been fully investigated in gravel-bed Kizik Yulduz basin, are distributed around the main Kaidu rivers. River. They are immediately surrounded by sloping mead- Here, we report on measurements in the Bayanbulak ows (slope S ∼ 0.01), themselves enclosed with the Tian- Grassland, Tianshan Mountains, P. R. China, where tens of shan Mountains which provide water to the Kaidu River meandering and braided gravel-bed rivers develop in the (Zhang et al., 2002). The hydrology of the basins is con- same environment. After comparison with other data sets trolled by snowmelt and summer orographic precipitations from the literature, we compare the morphology of braided (Zhang et al., 2002; Yang and Cui, 2005). Snow accumulates and meandering threads in our data set. Finally, we rescale from November to March, and starts melting in April, in- our measurements based on the threshold theory to generate ducing the water discharge to rise in all rivers (Zhang et al., and analyze a single statistical ensemble from rivers highly 2007). Orographic precipitation takes over in summer (be- Earth Surf. Dynam., 4, 273–283, 2016 www.earth-surf-dynam.net/4/273/2016/ F. Métivier et al.: Braided and meandering threads 275 Figure 4. (a) Meandering and (b) braided rivers in the Bayanbulak Grassland. Left panels: field picture; right panels: satellite image (Google Earth). The corresponding locations also appear in Fig.2. Figure 5. Satellite and panoramic view of a metamorphosis from braided to meandering (Bayanbulak Grassland, 84.578◦ E, 42.721◦ N, Google Earth). Marker on the satellite image indicates the viewpoint of the panoramic image. Its location also appears in Fig.2. tween 260 and 290 mm), and the discharge continues to rise 2012). Finally, most rivers flow over gravel, whose size dis- until August (Fig.3). tribution does not vary significantly over the basin (Fig.6). The morphology of the Bayanbulak rivers varies between All these features combine to make the Bayanbulak Grass- highly meandering (sinuosity above 1.3 to 1.5) and braided, land an ideal field site to investigate the morphology of and the same river often switches from one to the other along gravel-bed rivers. its course (Figs.4 and5). The rivers span about 4 orders of magnitude in discharge, and about 2 in width (Fig.6). Al- though a variety of grass species grow in the basin, their 3 Method influence on the channel morphology is probably moderate (Zhang et al., 2002; Andrews, 1984; Métivier and Barrier, We carried out two field campaigns in July 2012 and July 2013, during the high-flow season to compare the geom- www.earth-surf-dynam.net/4/273/2016/ Earth Surf. Dynam., 4, 273–283, 2016 276 F. Métivier et al.: Braided and meandering threads 0.6 1.4 2.5 1.2 0.5 2.0 1.0 0.4 1.5 0.8 0.3 PDF 0.6 1.0 0.2 0.4 0.5 0.1 0.2 0.0 2 0 2 4 0.0 0 1 2 3 0.0 2 1 10− 10 10 10 10 10 10 10 10− 10− 3 –3 Discharge (m s ) Width (m) Grainsize (d50 in m) Figure 6. Normalized histograms (probability density function) of water discharge, width and grain size. Blue: this study; gray: GBR (gravel bed river) data set (Church and Rood, 1983; Parker et al., 2007; King et al., 2004; Ashmore, 2013). etry of braided and meandering threads (Fig.3). We treated and Abt, 2001). We extracted the median grain size d50 and the threads of braided rivers individually, based on the wet- the size of the 90th percentile d90 from these distributions. ted area at the time of measurement (Fig.1). We measured Finally, the sinuosity of the threads was measured using the cross section geometry, the discharge, the grain-size dis- the topographic profiles when available.
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