Using bedform migration and orientation to infer sediment transport pathways in a sandy braided river
Using bedform migration and orientation to infer sediment transport pathways in a sandy braided river
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DOI：
10.1201/9781315644479-180
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发表时间：
2016
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通讯作者：
C. Unsworth;A. Nicholas;P. Ashworth;C. Simpson;J. Best;S. Lane;D. Parsons;G. Sambrook
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文献类型：
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作者：
C. Unsworth;A. Nicholas;P. Ashworth;C. Simpson;J. Best;S. Lane;D. Parsons;G. Sambrook
沙质辫状河的形态动力学因河床形态的存在而变得复杂，河床改变了动量、剪切力和沉积物输送的空间分布。这些效应还没有被很好地理解，并且通常在形态动力学模型中被简化。本文介绍了加拿大南萨斯喀彻温省沙质辫状河 600 m 河段的声学多普勒电流剖面仪 (aDcp) 水流测量和无人机 (UAV) 摄影测量。研究了冲积层床形对不同水流方向、床坡和沉积物可用性的响应，并讨论了使用床形脊线跟踪来估计沉积物输送方向。床型波峰方向在空间上也与附近床型的迁移率相关。因此，波峰迁移速率的跨河道梯度可能会导致迁移较慢的波峰线向下游延伸（例如，在坝前）。此外，沉积物输送和床形迁移的速率可能受到与供应限制输送相关的空间滞后效应的影响。这对于辫状河来说可能很重要，因为辫状河的特点是沉积物丰富和缺乏的区域混合在一起，例如，由于沉积物沉积在坝滑动面上。在此，我们展示了一项研究结果，该研究结合了加拿大南萨斯喀彻温河的高分辨率无人机和 aDcp 现场数据测量（参见 Parker 等人，2013），该研究旨在量化和了解地形和水流如何相互作用以控制辫状河道内沉积物的路线。 2 现场数据收集 2015 年，在河流 600 m x 100 m 范围内收集了量化河道形态、流量和沉积物输送的数据（图 1a）。研究河段的特点是具有一系列河床形态规模，沙丘高达 0.3 m，长度为 3 m，单位沙坝高达 0.9 m，宽度 > 50 m（图 1b）。研究范围以右侧的一个紧急坝和左侧的植被泛滥平原为界。通过在相距 30 m 的横截面处重复测量来获取平均 3D 流速数据，以使用 SonTek M9 aDcp 得出平均横截面流动结构（图 2a）。通过重复的无人机调查来量化地床形态的空间分布和方向，并使用 Pix4D 软件使用运动摄影测量结构对图像进行校正和合并（图 1-4）。通过对 aDcp 深度探测进行插值生成低分辨率 (1 m x 0.5 m) DEM，并用于估计平均河床坡度（图 3）。
The morphodynamics of sandy braided rivers are complicated by the presence of bedforms, which alter the spatial distribution of momentum, shear and sediment transport. These effects are not understood well and are typically simplified in morphodynamic models. This paper presents acoustic Doppler current profiler (aDcp) measurements of flow, and unmanned aerial vehicle (UAV) photogrammetric surveys of a 600 m reach of the sandy braided South Saskatchewan River, Canada. The response of alluvial bedforms to varying flow direction, bed-slope and sediment availability is examined, and the use of bedform crestline tracking to estimate sediment transport direction is discussed. Bedform crest orientation is also spatially linked to the migration rate of nearby bedforms. Thus, a crosschannel gradient in crest migration rate can cause slower migrating crestlines to be stretched downstream (e.g., at bar fronts). In addition, rates of sediment transport and bedform migration are likely to be affected by spatial lag effects associated with supply limited transport. This may be important in braided rivers that are characterized by a mixture of sediment rich and starved areas, for example, due to deposition of sediment onto bar slip faces. Herein, we present results from a study using a combination of high-resolution UAV and aDcp field data measurements of the South Saskatchewan River, Canada (cf. Parker et al., 2013), that seeks to quantify and understand how topography and flow interact to control the routing of sediment within braided river channels. 2 FIELD DATA COLLECTION Data quantifying channel morphology, flow and sediment transport were collected within a 600 m by 100 m reach of the river in 2015 (Figure 1a). The study reach is characterized by a range of bedform scales, with dunes up to 0.3 m high and 3 m in length, and unit bars up to 0.9 m high and > 50 m wide (Figure 1b). The study reach was bounded by an emergent bar on the true right and by vegetated floodplain on the true left. Mean 3D flow velocity data were acquired by repeated measurement at crosssections 30 m apart to derive an average cross section flow structure using a SonTek M9 aDcp (Figure 2a). The spatial distribution and orientation of bedforms was quantified with repeated UAV surveys, whereby images were rectified and merged using Structure from Motion photogrammetry using the software Pix4D (Figures 1-4). A low resolution (1 m x 0.5 m) DEM was produced by interpolating the aDcp depth soundings and used to estimate the mean bed slope (Figure 3).