Velocity profiles and basal stresses in natural debris flows     

Georg Nagl  Johannes Hübl  Roland Kaitna 《地球表面变化过程与地形》2020,45(8):1764-1776

The internal deformation within debris flows holds essential information on dynamics and flow resistance of such mass-wasting processes. Systematic measurements of velocity profiles in real-scale debris flows are not yet available. Additionally, data on basal stresses of the solid and the fluid phase are rare. Here, we present and analyse measurements of vertical velocity profiles in two debris flows naturally occurring in the Gadria Creek, Italy. The method is based on cross-correlation of paired conductivity signals from an array of sensors installed on a fin-shaped wall located in the middle of the channel. Additionally, we measure normal stress and pore fluid pressure by two force plates with integrated pressure transducers. We find internal deformation throughout the flows. Only at the very front was some en-bloc movement observed. Velocity profiles varied from front to tail and between flows. For one debris flow, pore fluid pressure close to normal stress was measured, whereas the other flow was less liquefied. The median shear rates were mostly less than 5 s⁻¹ and Savage numbers at the basal layer ranged from 0.01 to 1. Our results highlight the variable nature of debris flows and provide quantitative data on shear rate and basal stress distribution to help guide model development for hazard assessment and landscape evolution. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

岩体体裂隙率野外测量及计算方法的研究     

张杨  宁立波  尹峰  赵国红  白冰珂  朱晛亭 《工程地质学报》2020,28(1):10-18

裂隙率作为表征岩体裂隙特征的一项重要参数,有线裂隙率、面裂隙率、体裂隙率3种表征方法。目前常用测线法、统计窗法获取线、面裂隙率仅能从一维或二维反映裂隙发育情况,其方法还存在测量结果随机性、应用受出露条件所限等不足;现有体裂隙率测量方法也存在着实际应用性较差、成本高等问题,尚缺乏简便易操作的野外测量方法。为此本文提出了一种新的野外测量岩体体裂隙率的工作方法--球体法。由于构造、成岩裂隙发育的组系性、方向性,球体法根据各组裂隙产状从不同方向进行观测,通过多次测量统计岩体出露面上裂隙的发育特征,推断岩体内部裂隙的发育状况,其值能更好地反映局部区域岩体内部裂隙发育程度。此外,在测量时利用专利仪器可以得到沿裂隙面法线方向上隙宽以及隙间距数据真实值,测量条件对出露面平整情况要求不高,测量结果受露头平整性影响不大,具有更好的实用性。本文以安庆大龙山采石场研究区为例应用球体法,野外现场测量表明该方法可以在较为方便操作的基础上实现快速测量;研究区体裂隙率均值为2. 286%,集中分布在1%~4%,结合研究区实际情况以及误差分析结果,球体法可以很好地反映研究区各个测量点的裂隙发育程度。  相似文献   

CG-5型重力仪漂移特性与分时段平差处理     

梁霄  徐如刚  林承灏  储飞  肖伟鹏  王飞 《大地测量与地球动力学》2020,40(6):655-660

详细分析了4台CG-5型重力仪的静、动态漂移特性，结果发现，单次的静态测试结果中，CG-5型重力仪静态漂移呈很好的线性上升趋势，但划分不同时间尺度，每个时间段的仪器漂移率仍存在一定差异。同时，静态漂移率也存在长期变化的特征，不同测期CG-5型重力仪的漂移率差异较大：刚出厂时，CG-5型重力仪的漂移率变化幅度较大；随着时间变化，幅度逐渐减小，最终在一定区间内震荡。4台仪器的动态漂移率与仪器空间变化关系不密切，但仪器间的漂移特性在静态漂移率的长期变化、上下山的动态实验和单个测期各测线的动态漂移率变化中都表现出较为一致的变化趋势。基于以上认识，采用分时段平差的方法对观测数据进行分段漂移改正，改正后各台仪器和总体的联测精度指标均有所提高。  相似文献   

1860年以来博斯腾湖碳沉积过程演变   总被引：1，自引：1，他引：0  

于志同  李广宇  张恩楼  刘兴起 《湖泊科学》2019,31(1):293-304

选取博斯腾湖3个不同点位岩芯，在²¹⁰Pb和¹³⁷Cs年代序列基础上，利用沉积物中的各理化指标，分析了该湖碳沉积的时空变化特征，结合各指标的相关性、沉积速率、C/N、同位素特征等，探讨了该湖1860年以来碳沉积环境的变化过程，为干旱区湖泊碳埋藏研究提供了依据.结果表明：1860-1910年，沉积速率相对较小，受人类活动影响较小，磁化率、中值粒径、总无机碳（TIC）较为稳定，总有机碳（TOC）含量相对较低，此时该湖有较多陆源有机质的输入；1910-1950年，湖泊西部浅水域沉积速率明显高于东部深水区，西北湖区水域有大量外源物质的输入，而湖泊初级生产力较低，内源贡献相对较小；1950-1980年，全湖TOC、TIC含量均呈现升高的趋势，尤其是西北近黄水沟水域升高最快，湖泊内源贡献在增加，陆源组分的输入相对前一阶段要少；1980-2002年，沉积速率快速升高，尤其湖泊东部水域最为明显，TOC含量均呈现升高的趋势，湖泊西部水域初级生产力要高于东部深水区，湖面蒸发较强，气候比较温暖；2002年以来，全湖沉积速率相对较高，外源有机质贡献较小.过去150年博斯腾湖沉积物碳累积速率整体上呈现出升高的趋势，尤其是近50年来快速升高，东部湖区碳累积速率比西部湖区高.  相似文献   

Dynamic buckling of braces in concentrically braced frames     

Sina Kazemzadeh Azad  Cem Topkaya  Milad Bybordiani 《地震工程与结构动力学》2018,47(3):613-633

Axially loaded members might experience compressive forces above their static buckling capacity because of dynamic buckling under rapid shortening. Although the subject is studied in the context of engineering mechanics, it has not been thoroughly investigated in the field of earthquake engineering. Such dynamic overshoots in the compressive capacity can also be observed for braces of concentrically braced frames (CBFs) during earthquakes. Consequently, a comprehensive investigation is conducted in this study regarding the effects of dynamic buckling of braces on the seismic behavior of steel CBFs. After providing a theoretical background, recent dynamic experiments on braces and CBFs are simulated and discussed to investigate the occurrence of dynamic overshoot during these tests. Eight archetype CBFs are then designed, modeled, and subjected to a large set of ground motions to provide a quantified insight on the frequency and anticipated level of dynamic overshoot in the compressive capacity of braces during earthquakes. Results of a total of 1600 nonlinear time history analyses revealed that dynamic overshoots occur frequently in braces and affect the behavior of CBFs notably. Considerable increases are recorded in forces transmitted to other members of CBFs as a consequence of such dynamic overshoots. Importance of incorporating these dynamic overshoots in the capacity design procedure of columns, beams, and gusset plates is highlighted. Furthermore, results of a parametric study are presented and summarized in the form of a simple formula that can be used as a guide for estimating the level of dynamic overshoot.  相似文献   

20 Ma以来Mohns洋中脊的非对称扩张速率与地壳结构   总被引：1，自引：1，他引：0       下载免费PDF全文

张涛  高金耀  王威  吴招才  沈中延  杨春国 《地球物理学报》2018,61(8):3263-3277

超慢速扩张的Mohns洋中脊共轭两侧的地球物理场与地壳结构具有显著的非对称性.利用我国第五次北极科学考察采集的水深、重力与磁力数据，结合历史资料，我们计算了14条垂直Mohns洋中脊剖面的扩张速率、剩余水深、剩余地幔布格重力异常（RMBA）、地壳厚度和非均衡地形.对洋中脊共轭两侧以上计算结果的进一步对比发现，Mohns洋中脊两侧整体（下文均指同一地质时刻各剖面的平均值）的非对称性呈现明显的两段性：20~10.5 Ma，相比Mohns洋中脊东侧，西侧的扩张速率更慢、地壳更厚、非均衡地形更低；10.5~0 Ma，扩张速率、地壳厚度和非均衡地形的非对称的极性与20~10.5 Ma期间完全相反.后一阶段，整体扩张速率在西侧更快、剩余水深更浅，但是对应更薄的地壳和更高的非均衡地形.我们推断前者为冰岛沿Kolbeinsey洋中脊的作用增厚了Mohns洋中脊西侧地壳并使得洋中脊向西侧跳动，而后一阶段反映了岩浆供给减少后西侧集中的构造活动导致的更多的拉伸与隆升.沿各剖面上，10.5~0 Ma期间构造活动集中的洋中脊西侧均具有薄地壳和高非均衡地形，但构造拉伸的增加并不总是对应增快的扩张速率.岩浆在浅部更多地向东侧的分配以及洋中脊向西侧的跳动可能使得东西两侧具有相近的扩张速率.  相似文献   

Investigating young water fractions in a small Mediterranean mountain catchment: Both precipitation forcing and sampling frequency matter     

Francesc Gallart  María Valiente  Pilar Llorens  Carles Cayuela  Matthias Sprenger  Jérôme Latron 《水文研究》2020,34(17):3618-3634

The proportion of water younger than 2–3 months (young water fraction, F_yw) has become increasingly investigated in catchment hydrology. F_yw is typically estimated by comparing seasonal tracer cycles in precipitation and streamflow, through water sampling. However, some open research questions remain, such as: (i) whether part of the summer precipitation should be discarded because the high evapotranspiration demand, (ii) how well F_yw serves as a metric to compare catchments, and (iii) how sampling frequency affects F_yw estimates. To address these questions, we investigated F_yw in soil-, ground- and stream waters for the small Mediterranean Can Vila catchment. Rainfall was sampled at 5-mm intervals. Mobile soil water and groundwater were sampled fortnightly. Stream water was sampled depending on flow at variable time intervals (30 min to 1 week). Over 58 months, this sampling provided 1,529 δ¹⁸O determinations. Isotopic analyses results led us to include summer precipitation in the input signal. We found the highest F_yw in mobile soil waters (34%), while this was almost zero for groundwater except during wet periods. For stream waters, F_yw depended on the discharge variations, so that the flow-weighted young water fraction () was 22.6%, whereas the time-weighted F_yw was just 6.2%. Both and its discharge sensitivity (S_d) varied when different 12-month sampling periods were investigated. The young water fraction that would be obtained from a virtual thorough sampling () was estimated from the S_d and the observed stream flow. This showed an underestimation of by 25% for the frequent dynamic sampling and 66% for weekly sampling, due to missing high flows. Our results confirm that F_yw and its discharge sensitivity are metrics very sensitive to meteorological forcing during the analysed period. Thus, comparisons between catchments need long-term mean annual values and their variability. Our findings also support the dependence of F_yw estimates on the sampling rate and show the advantages of flow-weighted analysis. Finally, catchment water turnover investigations should be accompanied by the analysis of flow duration curves.  相似文献   

How fast does tufa grow? Very high-resolution measurement of the tufa growth rate on artificial substrates by the development of a contactless image-based modelling device     

Ivan Marić  Ante Šiljeg  Neven Cukrov  Vlatko Roland  Fran Domazetović 《地球表面变化过程与地形》2020,45(10):2331-2349

Accurate determination of the tufa growth rate (TGR) is required to answer the fundamental geomorphological question of tufa evolution. The TGR has been measured by various direct and indirect methods. One of the most popular direct methods uses modified micro-erosion meter (MEM), which has several drawbacks. Here, we present for the first time a coordinate measuring macro-photogrammetry device (CMD) for monitoring the TGR in a contactless manner. The CMD was applied on 28 limestone plates at 14 locations within the Skradinski buk area, Croatia, and measurements were performed in the laboratory. The TGR was derived from digital tufa high-resolution models (DTHRMs). The accuracy of the device was evaluated using state-of-the-art three-dimensional (3D) scanners and error calculation at checkpoints. Moreover, the precision was evaluated with the split test (n = 5). A total of 74 DTHRMs with a spatial resolution of 0.0236 mm were created. The TGR ranged from 0.327 to 19.302 mm a⁻¹, with an average of 5.771 mm a⁻¹. A higher TGR was observed on the limestone plates near mosses, located in fast and turbulent water rather than in stagnant water. We found that specific micro-environmental factors (e.g. proximity to moss) positively affected tufa growth. Erosion events were observed, as well as the presence of aquatic insect larvae (Simuliidae and Chironomidae), which positively affected tufa growth. The CMD is a precise and accurate device that does not suffer from the drawbacks of the MEM method and has many other advantages. It has a high capability of tufa erosion detection, enables the identification of macroinvertebrates, and multispectral or hyperspectral cameras can be mounted on the device for spectral reflectance analysis of the tufa surface. The CMD can be applied in any study requiring a sub-millimetre data quality and involving the comparison of consecutive 3D models and derivation of various parameters of smaller objects. © 2020 John Wiley & Sons, Ltd.  相似文献   

Evaluating debris-flow and anthropogenic disturbance on 10Be concentration in mountain drainage basins: implications for functional connectivity and denudation rates across time scales     

Francesco Brardinoni  Reto Grischott  Florian Kober  Corrado Morelli  Marcus Christl 《地球表面变化过程与地形》2020,45(15):3955-3974

We examine the sensitivity of ¹⁰Be concentrations (and derived denudation rates), to debris-flow and anthropogenic perturbations in steep settings of the Eastern Alps, and explore possible relations with structural geomorphic connectivity. Using cosmogenic ¹⁰Be as a tracer for functional geomorphic connectivity, we conduct sampling replications across four seasons in Gadria, Strimm and Allitz Creek. Sampling sites encompass a range of structural connectivity configurations, including the conditioning of a sackung, all assessed through a geomorphometric index (IC). By combining information on contemporary depth of erosion and sediment yield, disturbance history and post-LGM (Last Glacial Maximum) sedimentation rates, we constrain the effects of debris-flow disturbance on ¹⁰Be concentrations at the Gadria sites. Here, we argue that bedrock weakening imparted by the sackung promotes high depth of erosion. Consequently, debris flows recruit sediment beyond the critical depth of spallogenic production (e.g., >3 m), which in turn, episodically, due to predominantly muogenic production pathways, lowers ¹⁰Be concentration by a factor of 4, for at least 2 years. In contrast, steady erosion in Strimm Creek yields very stable ¹⁰Be concentrations through time. In Allitz Creek, we observe two- to fourfold seasonal fluctuations in ¹⁰Be concentrations, which we explain as the combined effects of water diversion and hydraulic structures on sediment mixing. We further show that ¹⁰Be concentration correlates inversely with the IC index, where sub-basins characterized by high concentrations (long residence times) exhibit low IC values (structurally disconnected) and vice versa, implying that, over millennial time scales a direct relation exists between functional and structural connectivity, and that the IC index performed as a suitable metric for structural connectivity. The index performs comparably better than other metrics (i.e., mean slope and mean normalized channel steepness index) previously used to assess topographic controls on denudation rates in active unglaciated ranges. In terms of landscape evolution, we argue that the sackung, by favouring intense debris-flow activity across the Holocene, has aided rapid postglacial reshaping of the Gadria basin, which currently exhibits a topographic signature characteristic of unglaciated debris-flow systems. © 2020 John Wiley & Sons, Ltd.  相似文献   

Erosion characteristics of loess tunnels on the Loess Plateau: A field investigation and experimental study     

Xi-An Li  Li Wang  Bo Hong  Lin-Cui Li  Jia Liu  Hao-nan Lei 《地球表面变化过程与地形》2020,45(9):1945-1958

Loess tunnels are a common geo-hazard in the Loess Plateau and not only cause considerable soil and water loss, but also aggravate and even induce the occurrence and development of other disasters such as ground fissures, mudflows, collapses, and landslides. To date, research on the hydrological characteristics and erosion behaviour of loess tunnel systems has focused on field investigation data and limited river basin observation data, whereas field test information and data are very scarce. In this study, field surveys, observations, field scouring experiments, and laboratory-based sediment percentage tests were conducted to analyse the erosion characteristics, spatial distribution, and hydrological characteristics of a large-scale loess tunnel system in the hilly Loess Plateau southeast region of northern China. The results showed that the loess tunnel erosion exhibited periodicity. Tunnel erosion in each period shows a similar erosion process, that is, thin-layer water flow erosion and lateral expansion, tunnel wall collapse and deposit due to the loss of support from the lateral erosion, and erosion and transport of deposits by water flow. Waterfall erosion, lateral erosion, headward erosion, and the resulting collapses were the main forms of tunnel erosion. Besides this, the base level of erosion significantly affects the erosion characteristics of the loess tunnel. The hydrological characteristics during field scouring experiments exhibited three different stages: a lag effect, attributed to the temporary loss of water velocity in the first stage; small water flow fluctuations in the second stage; and an increase in total seepage loss with increased water injection flow in the third stage. The erosion rate was positively correlated with the flow quantity. The results of this study not only provide valuable reference data for research on the mechanism and velocity of erosion events in loess, but also provide a theoretical basis for the prevention of loess tunnel disasters in engineering construction. © 2020 John Wiley & Sons, Ltd.  相似文献