汊口滩沉积特征及沉积模式——以鄱阳湖赣江三角洲汊口滩为例

随着我国许多油田进入高含水开发阶段,剩余油挖潜难度增大,复合分流河道带和单河道的划分已不能满足生产要求,迫切需要进行分流河道内部有利砂体的识别。现代沉积研究是认识分流河道内砂体发育特征的有效手段。通过对鄱阳湖赣江三角洲的现场考察,发现汊口滩是三角洲分流河道中发育的重要砂体类型,以发育在分流河道的分汊口处为典型特征。根据水动力条件、沉积物组合、沉积构造等特征,将汊口滩划分为滩头、滩中、滩尾3个沉积单元。从滩头到滩尾具有水动力条件减弱、沉积物层理规模减小、单砂层厚度减小、粒度变细、泥质夹层增多的特点。汊口滩主要是由于在分汊口处,水流受到汊口的顶托作用流速降低,沉积物按粒度分异堆积形成;堆积方式主要以向上游方向的逆流加积为主。与水道砂体相比,汊口滩发育的层理类型多,而且内部夹层发育,非均质性更强;由于夹层的遮挡作用,砂体不易发生水淹,有利于形成剩余油的富集区。     

利用FY-2F快速扫描资料分析对流初生阶段的云顶物理量特征

基于FY-2F静止气象卫星提供的2015年5—9月的高分辨率数据,通过温度阈值法识别出深、浅对流后,分析和比较了深、浅对流在对流初生(convective initiation,CI)至发展阶段中云顶高度、云顶快速降温率(cloud top cooling rate,CTC)以及多通道差值等云顶物理量特征的变化异同。结果表明:深、浅对流在CI阶段的云顶物理量特征具有相似变化特征,即云顶高度均在短时间内快速上升,CTC值均先减小后增大;深、浅对流差异表现为深(浅)对流云顶上升高度能(不能)超越水汽层高度;深对流CTC最低值较浅对流CTC最低值更低。基于CI阶段深、浅对流的CTC最低值的差异,通过个例验证,表明利用深、浅对流CTC最低值的差异,可以在识别出CI的基础,判断出CI是否发展成为深对流,从而能提前做出预警。     

白天和夜间通用的卫星遥感沙尘判识算法构建与验证分析

Himawari-8是日本发射的新一代静止气象卫星,与前一代的MTSAT-2相比,在时间、空间分辨率上都有了很大提升,特别是红外通道数量从4个增至10个,为红外遥感沙尘提供了新的观测数据。本研究利用Himawari-8的红外观测数据,发展了仅用红外通道的沙尘全天候判识算法,可以实现对白天和夜间的连续监测。算法在前人基础上去除了可见光通道,同时引入更多红外通道来进行云检测和沙尘判识。由于一日之中,地表温度发生变化,因此针对白天和夜间设置了两套不同的判别阈值,来保证算法的全天适用性。最后通过两次沙尘事件对沙尘判别结果的分析和检验表明,遥感判识结果与地面气象站和PM_(10)观测较为一致,说明了只用红外通道全天候判识沙尘的可行性。     

MULTIVARIATE GEOMORPHIC ANALYSIS OF FOREST STREAMS: IMPLICATIONS FOR ASSESSMENT OF LAND USE IMPACTS ON CHANNEL CONDITION

Multivariate statistical analyses of geomorphic variables from 23 forest stream reaches in southeast Alaska result in successful discrimination between pristine streams and those disturbed by land management, specifically timber harvesting and associated road building. Results of discriminant function analysis indicate that a three-variable model discriminates 10 disturbed from 13 undisturbed reaches with 90 per cent and 92 per cent correct classification respectively. These variables are the total number of pools per reach, the ratio of mean residual pool depth to mean bankfull depth, and the ratio of critical shear stress of the median surface grain size to bankfull shear stress. The last variable can be dropped without a decrease in rate of correct classification; however, the resulting two-variable model may be less robust. Analysis of the distribution of channel units, including pool types, can also be used to discriminate disturbed from undisturbed reaches and is particularly useful for assessment of aquatic habitat condition. However, channel unit classification and inventory can be subject to considerable error and observer bias. Abundance of pool-related large woody debris is highly correlated with pool frequency and is an important factor determining channel morphology. Results of this study yield a much needed, objective, geomorphic discrimination of pristine and disturbed channel conditions, providing a reference standard for channel assessment and restoration efforts.     

ANABRANCHING RIVERS: THEIR CAUSE,CHARACTER AND CLASSIFICATION

Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1–3 are lower energy and types 4–6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low w/d ratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges.     

IDENTIFICATION AND ANALYSIS OF NATURAL CHANNEL NETWORKS FROM DIGITAL ELEVATION MODELS

The identification and analysis of natural channel networks from digital elevation models are discussed from the point of view of their environmental applications. An interactive, graphical software package that implements some of the most widely used techniques for the automatic recognition of channel networks and for the computation of some useful geomorphologic indices and functions is presented.     

EXPERIMENTAL INVESTIGATION AND ANALYSIS OF HEC-6 RIVER MORPHOLOGICAL MODEL

Only comparatively few experimental studies have been carried out to investigate the performance of the HEC-6 river morphological model. The model was developed by the Hydrologic Engineering Center of the US Army Corps of Engineers. In this study, experiments were carried out in a 20 m long concrete flume 0.6 m wide with varying rectangular cross-sections. The channel bed is paved with uniform sand of D₅₀ = 0.9 mm and D₉₀ = 1.2 mm within the test reach of 12 m. Two types of experiments were carried out with sediment transport, one under steady uniform flow and another under steady non-uniform flow conditions. Nine steady uniform flow experiments were carried out to compare the measured equilibrium relationship of flow and sediment transport rate with two bedload formulae, namely, Du Boys and Meyer–Peter and Muller, and with three total load formulae, namely, Toffaleti, Laursen and Yang. It was found that even though the sediment transport consists of a certain portion of bedload, the total load formulae give satisfactory results and better agreement than the two bedload formulae. Five steady non-uniform flow experiments were carried out under various conditions of varying bed profile and channel width and also with sediment addition and withdrawal. The measured transient water surface and bed profiles are compared with the computed results from the HEC-6 model. It was found that the Toffaleti and Yang total load formulae used in the HEC-6 model give the most satisfactory prediction of actual bed profiles under various conditions of non-uniform flow and sediment transport. The effects of Manning's n, variations of sediment inflow, various sediment transport formulae, sediment grain size and the model numerical parameters, i.e. distance interval Δx and numerical weighting factor, on the computed water surface and bed profiles were determined. It was found that the selection of the sediment transport formulae has the most significant effect on the computed results. It can be concluded that the HEC-6 model can predict satisfactorily a long-term average pattern of local scour and deposition along a channel with either a small abrupt change in geometry or gradually varying cross-sections. However, the accuracy of the model prediction is reduced in the regions where highly non-uniform flow occurs.     

不同水沙条件下黄河下游二级悬河的发展过程

以大量实测横断面资料为基础,研究了1973~1997年不同水沙过程对黄河下游二级悬河形成过程的影响。发现花园口至高村河段二级悬河集中形成于1985~1997年的枯水阶段。高村以下河段二级悬河的形成可分两个阶段,1973~1985年大水条件下是冲刷主槽和淤积滩地的时期,但是由于泥沙主要堆积在靠近滩唇的部位,造成河床横比降增大;1985~1997年枯水条件下泥沙在主槽中大量淤积,导致二级悬河的形势更加严重。生产堤限制了泥沙的堆积范围,导致生产堤内滩地平均高程增长速度明显快于生产堤外滩地平均高程增长速度,生产堤距较窄的河段生产堤内滩地平均高程增长速度较快,显示了生产堤对二级悬河的发展有促进作用。     

海底输油管道底砂床冲刷机理研究

设计了海底输油管道水槽冲刷试验模型,研究了海底输油管道与砂床处于不同相对位置情况下床砂起动流速的变化,采用理想流体映射定理对其进行了理论分析,探讨起动流速变化规律。结合有限元数值模拟对试验进行细化分析,研究了海底管道底砂床砂粒起动的产生机理,根据研究结果将冲刷过程划分为五个阶段。阐明了海底管道暴露冲刷的危害性和实时监测的重要性。     

渤海海峡跨海通道建设前后区际城市群物流网络结构的时空演变

随着网络社会不断崛起,城市空间内涵发生了明显改变,已经成为人文地理学研究的新的重要内容。通过改进重力模型,基于有向加权网络分析方法以及BRIM算法,探讨了渤海海峡跨海通道建设前后山东半岛城市群与辽中南城市群物流网络结构的演变过程,研究发现：①渤海通道建设主要影响城市低等级物流联系,对高等级网络的影响局部集中在辽中南南部和山东半岛东部且自身物流规模较大的城市之间,其中大连、沈阳以及青岛、烟台、潍坊、济南等少数重点城市之间垄断了大量的物流资源;② 通道建设促进了“双通道双向合成流”格局形成,在各自城市群内部,物流结构将出现陆、海方向上的分离性变化,大连、烟台作为通道门户城市的地位逐渐确立;③ 首位离心流、向心流格局在演变过程中均表现出渤海通道指向性,仅有沈阳⇄济南、沈阳⇄淄博、大连⇄青岛、鞍山⇄潍坊等少数城市之间保持了常态化互惠关系,但互惠程度并不对称,辽中南地区城市的物流输出规模普遍高于具有互惠关系的山东半岛城市,前者表现为典型的“双核-边缘”结构,后者具有多极网络化特征;④ 通道建设前后山东半岛城市群的物流城市类型保持稳定,辽中南地区的向心流主导型城市则不断向铁岭、抚顺、本溪等东北部城市蔓延;⑤ 大连在渤海通道建设后首次与烟台、青岛等山东半岛城市群东部城市实现了同一子群结构衔接,总体上优化了山东半岛与辽中南城市群的物流空间格局。