利用涡旋定性分析弯曲河道的剥蚀和增生

观察发现弯曲河道的内堤会横向增生而外堤则渐渐被剥蚀。这一现象是由于河道转弯引发了横截面上产生一个次生旋流。尽管有数值模拟和野外测量来研究相应过程,笔者在这里用涡旋动力学进行简化的分析来清楚地解释次生旋流的起源。笔者阐明速度的垂向差异产生的涡旋会保持旋度守恒从而在下游产生一个河道向旋度分量。这一新的旋度分量产生了横截面上的次生旋流。分析表明涡旋提供了一个有效工具来研究流动和剥蚀问题。     

Scales of boundary resistance in coarse-grained channels: turbulent velocity profiles and implications

Gravel-bed surfaces are characterized by morphological features occurring at different roughness scales. The total shear stress generated by the flow above such surfaces is balanced by the sum of friction drag (grain stress) and form drag components (created by bed forms). To facilitate a better understanding of total resistance and bed load transport processes, there is a need to mathematically separate shear stress into its component parts. One way to do so is to examine the properties of vertical velocity profiles above such surfaces. These profiles are characterized by an inner layer that reflects grain resistance and an outer layer that reflects total resistance. A flume-based project was conducted to address these concerns through systematically comparing different roughness scales to ascertain how increased roughness affects the properties of vertical velocity profiles. Great care was taken to create natural roughness features and to obtain flow data at a high spatial and temporal resolution using an Acoustic Doppler Velocimeter.Average vertical velocity profiles above each roughness scale were clearly segmented. The vertical extent of the inner flow region was directly related to the scale of roughness present on the bed (and independent of flow depth), increasing with increased roughness. On a rough but rather uniform “plane” bed made of heterogeneous coarse sediments (with no bed forms), the shape of the velocity profile was clearly dominated by the local variations in grain characteristics. When pebble clusters were superimposed, the average shear stress in the outer flow region increased by 100% from the plane bed conditions. The ratio of inner grain shear stress to outer total shear stress for this pebble cluster experiment was 0.18 under shallow flow conditions and 0.3 under deep flow conditions. The grain stress component that should be used in bed load transport equations therefore appears to vary in these experiments between 15% and 30% of the total channel stress, increasing with decreased resistance. Roughness height (K_s/D₅₀) values at the grain scale for the plane bed and pebble cluster experiments were 0.73 and 0.63, respectively. These are values that should be used in flow resistance equations to predict grain resistance and grain stress for bed load transport modeling.     

戈壁风蚀面与植被覆盖面地表性质粗糙度长度的确定

以Monin-Obukhov相似性理论为基础, 利用量纲分析法分别推导出不同层结稳定度下确定戈壁风蚀面与植被覆盖面空气动力学参数的物理模型, 并利用该模型研究了粗糙度长度与粗糙元性质, 流经近地层流体特征以及大气层结稳定度之间的关系。得出以下结论: 戈壁风蚀面上空气动力学粗糙度长度与砾石粒径、高度、覆盖度、自由风速、摩擦速度以及大气层结稳定度有关; 植被覆盖面空气动力学粗糙度长度取决于植被类型、植被高度、覆盖度、零平面位移高度、自由风速、摩擦速度以及大气层结稳定度。     

Woody vegetation and channel morphogenesis in low-gradient, gravel-bed streams in the Ozark Plateaus, Missouri and Arkansas

Woody vegetation affects channel morphogenesis in Ozark streams of Missouri and Arkansas by increasing local roughness, increasing bank strength, providing sedimentation sites, and creating obstructions to flow. Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area. In upstream reaches, streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions. At drainage areas of greater than 100–200 kM², Ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates, referred to as disturbance reaches and stable reaches, respectively. Whereas stable reaches can develop stable forested floodplains (if they are not farmed), disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes.Disturbance reaches can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectional lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload-transporting events. The same floods caused up to 20 m of erosion of adjacent cutbanks, scoured non-vegetated areas between vegetation bands, and increased thalweg depth and definition. In high-energy (or riffle) zones, channel movement is dominantly by avulsion. In these zones, vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them. Woody vegetation on islands creates steep, root-defended banks that contribute to narrow channels with high velocities.Calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age, mainly through decreasing stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age.     

沙丘背风侧气流的变化特征及其意义

对腾格里沙漠东南缘格沙丘主、副梁和新月形形沙丘表面气流的野外发现分离流、附着未偏向流和附着偏向流等３种背风坡次生气流。前者以弱的反向流为特征，多发生在横向气流条件下，坡度较陡的背风坡上；后二者具有相对高的风速，多发生在坡度缓和的背风坡上，其方向在横向气流条件下保持原来的方向，在斜向气流作用下发生偏转，且其强度为原始风入射角的余统函数，根据３种次生气流强度，方向等特征，阐述了其相应的风成沉积过程和可能产生的层理类型，并对利用风成交错层理恢复古气流环境中的有关问题作了初步探讨。     

直立植物防沙措施粗糙特征的模拟实验

通过风洞模拟实验研究了直立植物防沙措施的粗糙特征及其影响因素。实验结果表明,在各种一定风速条件下,对数风速廓线中的粗糙度Ｚ０ＳＦ值随植物密度的增加呈幂函数增加,幂函数的指数〈１;而在各种一定植物密度条件下,Ｚ０随风速的增大而减溃,遵循单分子增长风线的指数函数。根据实验结果建立的包含风速和植物密度两个因子的Ｚ０的双因子综合模型对Ｚ０的预测值与实验值吻合得比较好。对数风速郭线中的Ｚ０是一个反映近地表气     

确定坡面径流过程曼宁糙率系数的实验方法研究

曼宁糙率系数是用水动力学方法进行流速计算的关键参数。坡面流曼宁糙率系数与明渠流的不同。为确定坡面径流过程的曼宁糙率系数,自行研发了一种包括供水系统、实验水槽和数据观测记录系统的室内可变糙率坡面实验系统。通过87场预实验验证了供水系统的稳定性和准确性。以坡度、实测流量、实测水深、不同糙率板上河砂的平均直径和地表粗糙度为自变量,以曼宁糙率系数为因变量,选用均方根误差(RMSE)和决定系数(R ²)为评价指标,对166种实验场景进行了支持向量机(Support Vector Machines, SVM)训练与预测,发现：① 紊流的训练结果难以预测层流和过渡流的曼宁糙率系数,说明流态不同时,实验因素对水流的影响机制不同;② 若要较为准确地预测曼宁糙率系数,至少需要包括实测水深在内的3种因素;③ 当同时考虑4种及更多种因素时,紊流状态下均可对曼宁糙率系数进行较为准确的预测。     

Response of unconfined turbidity current to relay‐ramp topography: insights from process‐based numerical modelling

This natural‐scale experimental study combines structural modelling of soft‐linked normal‐fault relays with a CFD (computational fluid dynamics) numerical simulation of a range of unconfined turbidity currents overrunning the relay‐system topography. The flow, released from an upslope inlet gate 2000‐m wide and 50‐m to 100‐m high, rapidly expands and adjusts its thickness, velocity and sediment load to the substrate slope of 1.5°. A lower initial sediment concentration or smaller thickness renders the quasi‐steady flow slower and its sediment‐transport capacity lower. A 3D pattern of large interfering Kelvin‐Helmholtz waves causes fluctuations of the local flow velocity magnitude and sediment concentration. Four zones of preferential sediment deposition are recognized: a near‐gate zone of abrupt flow expansion and self‐regulation; a flow‐transverse zone on the counter‐slope of fault footwall edges; a flow‐transverse zone at the fault‐scarp toes and a similar transverse zone near the crest of the hanging wall counter‐slopes. The sand deposited on the counter‐slope tends to be re‐entrained and fed back to the current by a secondary reverse underflow. The spatial extent and sediment accumulation capacity of depozones depend upon the released current volume. The impact of relay system on an overrunning current depends upon the fault separation distance and stage of tectonic evolution. An early‐stage relay system, with small vertical displacement and little overlap of faults, is bypassed by the current with minimum flow disturbance and no pronounced deposition. An advanced‐stage system, with greater fault displacement and overlap, gives a similar hydraulic effect as a single fault segment if the fault separation is small. If the separation is relatively large, the flow tends to be internally redirected sideways from the ramp into the hanging wall synclinal depressions. Since normal‐fault relays are common features in extensional basins, the study bears important implications for turbiditic slope‐fan models and for the spatial sand prediction in subsurface exploration of faulted submarine slopes.     

植被覆盖地表空气动力学粗糙度与零平面位移高度的模拟分析

 空气动力学粗糙度、零平面位移高度是植被覆盖地表的两个重要的空气动力学特征参数,利用数值计算的方法,运用Matlab软件编程,对内蒙古四子王旗草地地表的空气动学粗糙度、零平面位移高度进行模拟计算、绘图及分析。得到了在8种植被密度和6种植被高度情况下摩阻速度、空气动学粗糙度、零平面位移高度与风速的关系,并与野外实验数据进行对比分析,发现模拟值可以很好的反映空气动力学参数的性质。进而分别得到了包括植被密度的风速与摩阻速度、空气动学粗糙度的关系式和包括植被高度的风速与摩阻速度、空气动学粗糙度的关系式,这有助于进一步研究该地区草地风沙运动机理,以及床面与近地层气流相互作用的力学性质。     

榆靖高速公路防护体系近地面流场特征

通过野外调查及风场观测,研究了在主导风向为NW-SE时,榆靖高速公路K404处上风向植被防护体系的地面流场变化特征。结果表明:（1）风速和地形存在较好的相关性。在流动沙丘和固定沙丘迎风坡,风速放大率均为正,背风坡为负,背风坡没有产生回流,风速整体减小。风速放大率计算结果表明,愈接近地面风速的变化程度越大,风速放大率绝对值与垂直高度呈指数函数关系,风速放大率的大小与沙丘类型无关。（2）接近公路的贴地面风速显著下降,现有植被防护体系能够缓解流沙危害,植被的防风效果较好。（3）流动、半固定、固定沙丘的迎风坡地表风速廓线基本符合对数律,丘顶和背风坡风速廓线偏离对数曲线,固定沙丘粗糙度高于流动沙丘。     

排导结构中泥石流的流动形态

根据能量守恒原理，推导出泥石流在排导槽中的能量方程。根据此方程，可求出泥石流在排导槽中任意横断面的平均流速。对方程涉及的各参数进行了深入的讨论，首次引入泥石流对一般水体的修正系数K。并把该成果应用于平川泥石流的治理。     

沙粒跃移云及Magnus力对床面有效粗糙度的影响

建立了考虑Magnus效应的、吹过平坦沙面的稳定风场中风与跃移沙粒相互耦合的风沙跃移云模型,并通过数值模拟研究了风沙跃移运动对床面粗糙度的影响。结果表明,跃移沙粒对风速的影响类似于床面粗糙度的影响,有效粗糙度随摩阻速度的变化关系可用二次函数表征;Magnus力对有效粗糙度有明显的影响,但这种影响并不改变有效粗糙度与摩阻风速之间的二次函数关系。     

Modeling forced pool–riffle hydraulics in a boulder-bed stream, southern California

The mechanisms which control the formation and maintenance of pool–riffles are fundamental aspects of channel form and process. Most of the previous investigations on pool–riffle sequences have focused on alluvial rivers, and relatively few exist on the maintenance of these bedforms in boulder-bed channels. Here, we use a high-resolution two-dimensional flow model to investigate the interactions among large roughness elements, channel hydraulics, and the maintenance of a forced pool–riffle sequence in a boulder-bed stream. Model output indicates that at low discharge, a peak zone of shear stress and velocity occurs over the riffle. At or near bankfull discharge, the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements. The strength of flow convergence is enhanced during model simulations of bankfull flow, resulting in a narrow, high velocity core that is translated through the pool head and pool center. The jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder. The extent of flow convergence and divergence is quantified by identifying the effective width, defined here as the width which conveys 90% of the highest modeled velocities. At low flow, the ratio of effective width between the pool and riffle is roughly 1:1, indicating little flow convergence or divergence. At bankfull discharge, the ratio of effective width is approximately 1:3 between the pool and downstream riffle, illustrating the strong flow convergence at the pool head. The effective width tends to equalize again with a ratio of 1:1 between the pool and riffle during a modeled discharge of a five-year flood, as the large roughness elements above the pool become drowned out. Results suggest that forced pool–riffle sequences in boulder-bed streams are maintained by flows at or near bankfull discharge because of stage-dependent variability in depth-averaged velocity and tractive force.     

Field determination for roughness length above the different non-erodible surfaces

Non-erodible elements, for its disturbance to the near-surface airflow, have been widely used in arid and semi-arid regions to protect the surface from wind erosion. Roughness length was usually used to evaluate the protection effect of non-erodible elements from wind erosion. In this study, the wind profiles above five types of non-erodible surfaces including gravel, wheat straw checkerboard barriers, cotton stem checkerboard barriers, shrubs, and herbs were measured and analyzed. The wind velocities above these surfaces increased with height approximately in logarithmic functions. The roughness length of different non-erodible surfaces was calculated by the functions of wind profiles. The results reveal that:(1) Roughness length increased with wind velocity in given wind velocity ranges. (2) On vegetative surfaces, wind did not effectively bend the stems. The threshold wind velocity for bending the stems of Achnatherum splendens was 4 m/s, 10 m/s for Agropyron cristatum, and for Artemisia ordosica, no obvious bending of stems even for wind velocity reaching 12 m/s. (3) Correlation analysis results show that the vegetation''s coverage and frontal area affect the roughness length more significantly than the other parameters. (4) The protective results of these non-erodible elements were evaluated. The checkerboard sand barriers made of cotton stem could provide more effective protection than that made of wheat straw. In the same coverage conditions, vegetation could provide more effective protection from wind erosion than gravel, and the blending of different non-erodible elements especially the combination of blending of vegeation and checkerboard sand barriers could provide more effective protection to the surface.     

Flow structure at an asymmetrical stream confluence

Measurements of downstream and cross-stream velocities at a small, asymmetrical stream confluence show that the structure of low-stage flows is influenced by the tributary/main stem momentum flux ratio, the total discharge of the incoming flows, and the bed morphology. Flow accelerates through the confluence during all three measured events. This acceleration is associated with a downstream reduction in channel capacity caused in part by the presence of a large bar along the inner bank of the downstream channel. As the momentum ratio increases, flow from the lateral tributary increasingly deflects flow from the main stream toward the outer channel bank within the confluence. As a result, the mixing interface between the converging flows also shifts outward. The large bar in the downstream channel deflects flow along the inner bank toward the adjacent scour hole, enhancing flow convergence downstream of the confluence and producing a region of flow separation adjacent to, or in the lee of the bar. The loci of maximum topographic deflection and flow separation vary with momentum ratio and total discharge.Secondary circulation within the downstream channel is characterized by a single large helical cell when the momentum ratio exceeds one, and weak surface-convergent helical cells on opposite sides of the mixing interface when the momentum ratio is less than one. Curvature of the flow, and thus the strength of helical motion, is greatest on the tributary side of the mixing interface. Although the flow events measured in this study did not exceed the threshold for sediment movement, the bed morphology at the confluence can be explained by the flow structure observed during these low-stage events, suggesting that formative flows may have similar downstream and cross-stream velocity fields.     

Controls on the height and spacing of eolian ripples and transverse dunes: A numerical modeling investigation

Ripples and transverse dunes in areas of abundant sand supply increase in height and spacing as a function of time, grain size, and excess shear velocity. How and why each of these factors influence ripple and transverse dune size, however, is not precisely known. In this paper, the controls on the height and spacing of ripples and transverse dunes in areas of abundant sand supply are investigated using a numerical model for the formation of eolian bedforms from an initially flat surface. This bedform evolution model combines the basic elements of Werner's [Werner, B.T., 1995. Eolian dunes: Computer simulations and attractor interpretation. Geology 23, 1107–1110.] cellular automaton model of dune formation with a model for boundary layer flow over complex topography. Particular attention is paid to the relationship between bed shear stress and slope on the windward (stoss) side of evolving bedforms. Nonlinear boundary layer model results indicate that bed shear stresses on stoss slopes increase with increasing slope angle up to approximately 20°, then decrease with increasing slope angle as backpressure effects become limiting. In the bedform evolution model, the linear boundary layer flow model of Jackson and Hunt [Jackson, P.S., Hunt, J.C.R., 1975. Turbulent wind flow over a low hill. Quarterly Journal of the Royal Meteorological Society 101, 929–955.], generalized to 3D, is modified to include the nonlinear relationship between bed shear stress and slope. Bed shear stresses predicted by the modified Jackson and Hunt flow model are then used to predict rates of erosion and deposition iteratively through time within a mass-conservative framework similar to Werner [Werner, B.T., 1995. Eolian dunes: Computer simulations and attractor interpretation. Geology 23, 1107–1110.]. Beginning with a flat bed, the model forms ripples that grow in height and spacing until a dynamic steady-state condition is achieved in which bedforms migrate downwind without further growth. The steady-state ripple spacing predicted by this model is approximately 3000 times greater than the aerodynamic roughness length of the initially flat surface, which is a function of grain size and excess shear velocity. Once steady-state ripples form, they become the dominant roughness element of the surface. The increase in roughness associated with ripple formation triggers the same bedform instability that created ripples, causing dunes to form at a larger scale. In this way, the numerical model of this paper suggests that ripples and dunes are genetically linked. Transverse dunes in this model have a steady-state height and spacing that is controlled by the effective roughness length of the rippled surface, which is shown to be on the order of 500 times greater than the original roughness length, but varies significantly with the details of ripple morphology. The model predictions for ripple and dune spacing and their controlling variables are consistent with field measurements from the published literature. The model of this paper provides a preliminary process-based understanding of the granulometric control of ripples and dunes in areas of abundant sand supply and unidirectional prevailing winds, and it argues for a genetic linkage between ripples and dunes via a scaling relationship between eolian bedform size and the aerodynamic roughness length.     

直立植被粗糙度和阻力分解的风洞实验研究

直立植被覆盖的床面上,由于直立植被消耗风动量,裸露床面上的剪切力因之降低,从而抑制风蚀。在风洞中用圆柱形木棒模拟直立植被,研究了地表总阻力系数C_D、直立植被阻力系数C_f和粗糙度z₀及其影响因素。实验结果表明,直立植被高度和密度对地表总阻力系数C_D和粗糙度z₀都有影响,但是程度不同。地表总剪切力分解为作用在直立植被上的剪切力和作用在裸露地表上的剪切力,表达式为τ=F_r/S+τ_s·S'/S。随着植被密度和高度增加,作用在植被上的剪切力增加,而作用在裸露地表上的剪切力减少。当侧影盖度L_c≥0.03时,τ_s可以忽略不计,用C_D=2u_*²/ u_z²+C_fL_c计算植被阻力系数C_f。计算出的直立植被阻力系数C_f与1/√L_c之间成线性关系,而与AR无关。这与Marshall在风洞中通过直接测量粗糙元的阻力计算出的粗糙元阻力系数遵循类似的规律。地表总阻力系数C_D与粗糙度z₀之间较好的相关性说明,只要采用适当的测量方法,用曲线拟合方法获得直立植被覆盖地表总阻力系数C_D和粗糙度z₀是可靠的。     

域外国家与南海周边地缘经济联系强度演化与态势评估

随着全球经济重心逐渐向亚太地区转移以及区域地缘经济联系日趋紧密,南海战略地位日益凸显,南海周边已成为全球重要战略区域和亚洲最重要的国际地缘问题区域。运用社会网络分析法、修正引力模型等方法,从贸易、投资、旅游等视角切入探究域外国家与南海周边区域的地缘经济联系演变及态势。结果表明：① 2005—2019年域外国家与南海周边区域的贸易联系日益紧密但贸易合作的非均衡化现象较为突出,域内出口贸易网络呈现以中国为核心,老挝、文莱和柬埔寨为边缘地带的相对稳定网络结构特征。② 域外国家对南海周边区域的投资规模显著提升但投资规模的两极化差异态势较为明显,投资区位呈现由高度集中向渐进式扩散转变态势。③ 域外国家入境旅游流持续增长,旅游网络由中国为单一核心演变为中国和泰国为双核心的网络结构,旅游目的地日趋多元化。④ 域外国家与南海周边区域地缘经济联系强度有所降低,但与东盟9国地缘经济联系强度整体上升;虽然以美日为代表的域外国家逐渐减弱与中国的地缘经济联系,但地缘经济联系网络整体仍然呈现以中国为核心,以泰国和新加坡为次核心的网络结构。     

草原区植被对土壤风蚀影响的风洞模拟试验研究

在前期研究的基础上,选择内蒙古乌兰察布荒漠草原区为研究区域,通过野外移动风洞模拟试验,开展荒漠草原植被对土壤风力侵蚀影响的定量化分析研究,旨在探讨不同植被盖度下空气动力学粗糙度的变化、不同植被盖度下的风沙流结构特征及植被盖度与风蚀输沙率的定量关系,从而为水土流失机理研究提供理论依据。试验研究表明：空气动力学粗糙度随着地表植被盖度的增加呈三次函数关系增加,拟合函数为Z0=ax3+bx2+cx+d;随着距地高度的增加,各层收集到的风蚀量呈不同程度降低,随着地表植被盖度的增加,各层输沙量也均降低;不同风速下植被盖度与风蚀输沙率之间呈幂函数相关。