Riverine Sediment Changes and Channel Pattern of a Gravel-Bed Mountain Torrent

The alluvial channel of the Langgriesgraben (Austria) is a highly active Geomorphic riverine subcatchment of the Johnsbach River with intermittent discharge and braided river structures. The High Sediment yield Entails both issues and opportunities. For decades, the riverbed was exploited as a gravel pit. Today, as part of the Gesäuse National Park and after Renaturation, the sediment yield endangers a locally important bridge located at the outlet of the subcatchment. High-resolution geospatial investigations are vital for the quantification of sediment redistribution, which is relevant in terms of river management. Based on unmanned aerial system (UAS) surveys in 2015 (July, September, and October) and 2019 (August and October), high-resolution digital elevation models (DEMs) were generated, which enable us to quantify intra- and multiannual sediment changes. As surface runoff at the subcatchment occurs on only a few days per year with flash floods and debris flows that are not predictable and thus hardly observable, the subsurface water conditions were assessed on electrical resistivity tomography (ERT) measurements, which were conducted in 2019 (November) and 2020 (May, June). Results of the UAS-based surveys showed that, considering the data quality, intra-annual sediment changes affected only small subareas, whereas multiannual changes occurred in the entire study area and amount to net sediment deposition of & 8776;0.30.4 m3m & 8722;2, depending on the channel section. In addition, the elevation differences for both intra-annual surveys revealed linear patterns that can be interpreted as braided river channels. As in both survey periods the same areas were affected by changes, it can be concluded that the channel mainly affected by reshaping persisted within the 4-year observation period. The subsurface investigations showed that although both near-surface and groundwater conditions changed, near-surface sediments are mostly dry with a thickness of several meters during the observations.位于奥地利的兰格里斯格拉本河（Langgriesgraben）冲积河道，是约翰斯巴赫河（Johnsbach River）一处活动性极强的地貌型河流子汇水区，兼具间歇性径流与辫状河地貌结构。高泥沙产出既伴随诸多挑战，也蕴藏着发展机遇。数十年来，该河道河床曾被作为砾石矿场进行开采。如今，作为格绍伊国家公园（Gesäuse National Park）的组成部分并完成生态复育后，高泥沙产出已对该子汇水区出口处的一座当地重要桥梁构成安全威胁。高分辨率地理空间调查对于量化泥沙再分布过程至关重要，相关工作对河流管理具有重要指导意义。基于2015年（7月、9月、10月）与2019年（8月、10月）开展的无人机系统（unmanned aerial system, UAS）调查，本研究生成了高分辨率数字高程模型（digital elevation models, DEMs），可用于量化年内及多年际的泥沙变化量。由于该子汇水区的地表径流每年仅出现数日，且伴随不可预测、难以观测的山洪与泥石流，研究团队通过电阻率层析成像（electrical resistivity tomography, ERT）测量评估了地下水文条件，相关测量于2019年11月与2020年5月、6月完成。基于无人机系统的调查结果显示，考虑数据质量的前提下，年内泥沙变化仅影响小型子区域；而多年际泥沙变化则覆盖整个研究区域，其净泥沙沉积量为0.3~0.4 立方米每平方米（0.3~0.4 m³·m⁻²），具体数值取决于河道段的位置。此外，两次年内调查的高程差数据呈现出线性分布特征，可被解读为辫状河道的分布形态。由于两次调查周期内受变化影响的区域一致，可得出结论：在4年的观测周期内，主要受河道重塑作用影响的河段始终保持稳定。地下调查结果显示，尽管近地表与地下水文条件均发生了变化，但观测期间近地表沉积物大多处于干燥状态，其厚度可达数米。