Gravel sediment routing from widespread,low- intensity landscape disturbance,Current River Basin,Missouri

During the last 160 years, land-use changes in the Ozarks have had the potential to cause widespread, low-intensity delivery of excess amounts of gravel-sized sediment to stream channels. Previous studies have indicated that this excess gravel bedload is moving in wave-like forms through Ozarks drainage basins. The longitudinal, areal distribution of gravel bars along 160 km of the Current River, Missouri, was evaluated to determine the relative effects of valley-scale controls, tributary basin characteristics, and lagged sediment transport in creating areas of gravel accumulations. The longitudinal distribution of gravel-bar area shows a broad scale wave-like form with increases in gravel-bar area weakly associated with tributary junctions. Secondary peaks of gravel area with 1·8–4·1 km spacing (disturbance reaches) are superimposed on the broad form. Variations in valley width explain some, but not all, of the short-spacing variation in gravel-bar area. Among variables describing tributary drainage basin morphometry, present-day land use and geologic characteristics, only drainage area and road density relate even weakly to gravel-bar areal inventories. A simple, channel network-based sediment routing model shows that many of the features of the observed longitudinal gravel distribution can be replicated by uniform transport of sediment from widespread disturbances through a channel network. These results indicate that lagged sediment transport may have a dominant effect on the synoptic spatial distribution of gravel in Ozarks streams; present-day land uses are only weakly associated with present-day gravel inventories; and valley-scale characteristics have secondary controls on gravel accumulations in disturbance reaches. Copyright © 1999 John Wiley & Sons, Ltd.     

无定河流域不同水土流失区底栖动物生物完整性指数构建与健康评价

无定河流域位于黄土高原与毛乌素沙地过渡带,水土流失严重,生态环境具有脆弱性和波动性。于2021年春季(4月)和秋季(10月)对无定河流域上、中、下游及其6条支流和流域内的3个淤地坝开展水生态系统调查,旨在厘清无定河流域底栖动物群落特征,构建底栖动物生物完整性指数并开展健康评价。无定河流域春季共采集到底栖动物105种,平均密度为181 ind./m²,平均生物量为0.760 g/m²,秋季共采集到底栖动物67种,平均密度为94 ind./m²,平均生物量为0.454 g/m²。通过对两季度研究区域内底栖动物27个生物参数开展分布范围检验、判别能力分析和相关性分析,构建无定河流域底栖动物生物完整性指数,对全流域40个样点(6个参照点和34个受损点)进行B-IBI健康评价。评价结果表明,总体上无定河流域底栖动物生物完整性较好,40个样点中春季有19个处于健康或亚健康状态,秋季有23个处于健康或亚健康状态,其中无定河上、中游干支流大都以健康和亚健康为主,无定河下游干支流以及3个淤地坝水体健康状况较差。在不同水土流失类型区域,底栖动物群落特征和生物完整性评价具有显著性差异。本研究结果可为无定河流域河流健康评估提供科学依据。     

A DEVICE FOR MEASURING GULLY HEADWALL MORPHOLOGY

This paper describes a low-cost device for measuring the three-dimensional morphology of a gully headwall. The device was designed to operate in a gully system with the following characteristics: overhanging banks caused by a thick, dense root mat; retreat of the underlying unconsolidated sediments through small slab failures, leading to a considerable variation in retreat rate at each point on the headwall; and changes in the orientation of the headwall owing to changes in sediment properties and the topographical and hydrological controls of gully growth. The device is used to measure a series of closely spaced vertical profiles of the headwall, and the collected data are combined to draw a contour map showing the distance from the plane of the instrument to the headwall. Comparing maps for sequential times enables retreat rates for the diffferent proportions of the headwall to be quantified.     

THE RELATIVE CONTRIBUTION OF SOIL TILLAGE AND OVERLAND FLOW EROSION TO SOIL REDISTRIBUTION ON AGRICULTURAL LAND

This study uses evidence for the long-term (35 years) pattern of soil redistribution within two agricultural fields in the UK to identify the relative importance of tillage and overland flow erosion. Spatially distributed long-term total soil redistribution data for the fields (Dalicott Farm and Rufford Forest Farm) were obtained using the caesium-137 (¹³⁷Cs) technique. These data were compared with predicted patterns of soil redistribution. Recent studies have demonstrated that the redistribution of soil by tillage may be described as a diffusive process. A two-component model was, therefore, developed which accounts for soil redistribution by both overland flow and diffusive processes. Comparison of the predicted patterns of overland flow erosion alone with the observed (¹³⁷Cs-derived) data indicated a poor agreement (r² = 0.17 and 0.11). In contrast, a good agreement exists between the predicted pattern of diffusive redistribution and the observed data (r² = 0.43 and 0.41). These results give a clear indication that diffusive processes are dominant in soil redistribution within these fields. Possible diffusive processes include splash erosion, soil creep and tillage. However, the magnitude of the diffusion coefficients for the optimum predicted pattern (c. 350–400 kg m⁻¹ a⁻¹) demonstrates that tillage is the only process capable of explaining the very significant soil redistribution which is indicated by the ¹³⁷Cs data. Consideration is given to the implications of these results for both soil erosion prediction and landscape interpretation.     

ESTIMATING RATES OF DENUDATION USING COSMOGENIC ISOTOPE ABUNDANCES IN SEDIMENT

We propose, as a testable hypothesis, a basin-scale approach for interpreting the abundance of in situ produced cosmogenic isotopes, an approach which considers explicitly both the isotope and sediment flux through a drainage basin. Unlike most existing models, which are appropriate for evaluating in situproduced cosmogenic isotope abundance at discrete points on Earth's surface, our model is designed for interpreting isotope abundance in sediment. Because sediment is a mixture of materials, in favourable cases derived from throughout a drainage basin, we suggest that measured isotope abundances may reflect spatially averaged rates of erosion. We investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed.     

三峡截流以来长江洪季潮区界变动河段冲刷地貌

潮区界河段河势演变对三峡工程的响应是长江经济带建设中的重要问题。然而受观测手段所限,对三峡截流以来潮区界变动范围及其地貌演变的客观认识亟待探讨。对大通站洪季水位资料进行频谱分析,初步判断了近期长江洪季潮区界位置;对比1998年和2013年水下地形资料,分析了三峡大坝截流以来该河段河槽的冲淤演变特征;利用多波束测深系统对冲刷明显河段的微地貌进行了高分辨率观测。结果显示:（1）1998-2013年潮区界变动河段河槽整体冲刷5 649.7万m3。其中,上段全面冲刷,太白、太阳两洲并岸,铜陵沙被冲开,主槽刷深达5.6 m;中段主泓摆动,天然洲南冲北淤,黑沙洲中水道淤死,南水道左岸最大冲深达8.9 m;下段近岸冲刷强烈,北岸最大冲深达15.4 m;（2）该河段近期处于剧烈的冲刷环境,左岸冲刷尤为显著;（3）冲刷深槽分布在顺直河段,深达5.4~12.6 m;冲刷坑分布在分汊河段平面形态突变处,最大冲深达28.1~30.5 m;水下侵蚀陡坡分布在近岸侵蚀严重的顺直河段,坡度为0.59~0.62。     

江苏中部淤泥质海岸岸线变化遥感监测研究

海岸线监测是了解海岸冲淤变化的基础。针对淤泥质海岸潮间带坡度平缓的特点,考虑到潮汐对遥感海岸线监测的影响,基于多潮位站插值校正的水边线离散点潮位赋值及坡度计算对水边线方法进行了改进,并结合潮间带实测坡度资料校正,推算遥感海岸线。选择江苏中部冲淤变化频繁、自然岸线保有率较高的扁担河口至川东港岸段开展海岸线变迁遥感监测研究。结果表明,研究区坡度主要在0.001~0.002之间,潮间带宽度由北向南越来越宽。北部扁担河口至射阳河口岸段处于冲刷环境中,大量以养殖塘围堤为主的人工岸线不断被侵蚀后退;射阳河口至四卯酉河口岸段以海岸线在自然状态下的动态变化为主,2010-2015年平均冲淤速率小于10 m/a,变化幅度较小;南部四卯酉河口至川东港岸段,自然岸线淤长明显,同时人工围垦导致岸线不断向海推进。根据监测结果,认为新洋港至斗龙港岸段应为研究区由北部侵蚀转向南部淤长的过渡带。     

射阳港区双导堤建设前后冲淤变化及成因研究

基于近年来射阳港区水下地形数据和定点全潮水文观测资料,分析了双导堤建设前后冲淤变化特征,并对其成因进行了初步探讨。研究结果表明,研究区近6年来总体上以淤积为主,净淤积量约为65.23×106m3,年均净淤积速率约为7.9 cm/a;淤积区域主要分布于导堤两侧浅水区,侵蚀区域主要分布于导堤口附近。2008-2013年间研究区冲刷、淤积和相对稳定区域面积分别占15%、55%和30%。堤口东侧的SSW02站实测最大流速为1.54 m/s,北堤北部的SSW01站为1.4 m/s,无论是涨潮还是落潮,堤口均大于导堤北侧;导堤产生了沿堤水流和口门回流,改变了导堤两侧附近海域潮流方向并增大了口门处的沉积动力。研究区泥沙运动方式主要表现为波浪掀沙、潮流输沙;区域海流泥沙的沿程落淤和导堤挡流效应导致导堤两侧出现淤积,导堤产生的回流促使堤口冲刷和堤内泥沙回淤。导堤引起的局部流场变化是海底冲淤格局重新分布的主因。     

盐城滨海港区防波堤建设对水下地形的影响研究

利用双频测深仪和声学多普勒流速仪对滨海港区附近海域进行了区域测深和定点全潮水文观测,通过与以往调查研究资料的对比,分析了防波堤建成前后海底冲淤变化特征。研究结果表明:近20年以来水下岸坡为侵蚀最剧烈的区域,海底平原则处于相对稳定状态;受自然侵蚀的影响,水下岸坡继续向岸内移,目前最强侵蚀区已内移至–8~–12 m水深的水下岸坡;近年来自然侵蚀明显减弱,年均侵蚀速率约为前10年的1/4;受防波堤挡流效应的影响,口门南侧已由往复流转变为旋转流;目前已出现三大淤积区域,其中以口门处淤积最严重,最大淤积厚达3.5 m,已造成口门处水深不足–10 m;防波堤对周边海域直接影响范围约13.4 km~2,最远可达废黄河口。分析认为,防波堤建设是引起港区海底冲淤格局发生变化的主因。该研究为防波堤的后续维护及滨海港区30万t级航道工程论证提供了基础资料。     

The prediction of traveling jet trenching in stiff clay based on the erosion failure mechanism

The performance of traveling jet trenching in stiff clay was tested. All the key parameters varied in the experiment, including the jet velocity, the shear strength of clay, the diameter of the nozzle, and the translational velocity of the nozzle. The jet–soil interface was found to be an arc. An equation was given to describe the interface. On the basis of the erosion failure mechanism, a new method is presented for predicting jet trenching depth. We first calculated the trajectory of the jet–soil interface and then predicted the distribution of the boundary layer shear stress along the jet–soil interface. The point where the wall shear stress was equal to the critical shear strength of clay was the ultimate trenching depth. The predictive results were found to fit the experimental results well.