Analysis of rill step–pool morphology and its comparison with stream case

In this paper, the morphology of step–pool features is analysed using rill measurements and literature data for streams. Close-range photogrammetry was used to carry out ground measurements on rills with step–pool units, shaped on a plot having slope equal to 14, 15, 22, 24 and 26%. Data were used to compare the relationships between H/L, in which H is the step height and L is the step length, and the mean gradient of the step–pool sequence, S_m, for streams or the slope of the step–pool unit, S, for rills. The relationship of H/L against S_m is widely used to test the occurrence of the maximum flow resistance condition in streams, which is associated with the range 1 ≤ (H/L)/S_m ≤ 2. Further analyses were carried out to compare both the formation process and the profile of the pool in rills with those related to streams. Moreover, for a single rill channel, an analysis of flow characteristics expressed in terms of Darcy–Weisbach friction factor and Froude number was developed. The results allowed us to state: (i) the relationships of H/L versus S_m and S are quite similar and the steepness ratio for streams, (H/L)/S_m, and for rills, (H/L)/S, generally ranges from 1 to 2; (ii) the formation process and the profile of the pool in rills are not consistent with those occurring in streams; (iii) in the rills, the longitudinal size of the pool is dominant with respect to the maximum scour depth; (iv) the presence of a sequence of step–pool units within a rill segment noticeably increases flow resistance compared to segments with a flat bed; (v) the Froude number of the flow over the sequence of step–pool units in rills is slightly below the range of 0.8–1 corresponding to the maximum flow resistance in step–pool units.     

Spatial variations in surface sediment structure in riffle–pool sequences: a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)

Riffle–pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in situ grain‐scale field measurements from a riffle–pool sequence to parameterize a physically‐based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilizes pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a two‐dimensional computational fluid dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle–pool sequences. This finding has implications for the prediction of sediment transport and the morphological evolution of gravel‐bed rivers. Copyright © 2012 John Wiley & Sons, Ltd.     

基于HYDRUS-2D的渗滤池回灌地下水数值模拟研究

人工回灌是提高水资源利用水平的重要工程措施。深入研究渗滤池回灌过程机理,基于Hydrus-2D,结合具体渗滤池回灌算例,建立渗滤池回灌的二维饱和-非饱和渗流数值模型。模拟结果表明：回灌水入渗形成的＂湿润峰＂逐渐向饱和带移动,同时饱和带的浸润线逐渐上升,形成不断扩大的饱和区,直到形成一个连续的饱和带。回灌水出流速率经过了迅速增大到一峰值后快速减小,然后又缓慢变小的过程,回灌水入渗速率随时间变化的这一特征与Green-Ampt入渗模型的结论是一致的。     

2019年沈阳一次凌晨气温骤升过程成因精细分析

2019年12月2日01时沈阳国家基本气象站(沈阳站)气温1 h内异常回升了6.5℃,利用常规和加密地面观测站实况、NCEP再分析及沈阳站逐小时风廓线雷达资料,从系统配置、热力学诊断、城市热岛效应等方面对此次异常升温事件成因进行精细分析。结果表明:沈阳站异常升温阶段,超低空到近地面暖脊过境造成辽宁大部分地区出现气温回升,暖脊过境是引发沈阳站午夜时分异常升温的原因之一;沈阳站位于沈阳市的东南部,当地面风为西北风时,风从温度相对较高的城区吹向郊区,造成沈阳南部郊区各站气温回升,与北部郊区对比得出升温幅度约为3—4℃。城市热岛中心向下风方漂移是沈阳站气温骤升又一原因。值得注意的是,气温回升前的温度基础值对气温回升幅度存在一定程度影响,回升前的基础温度越低,则回升后造成的升温幅度越大。     

海气交换与海气边界层观测研究进展

半个世纪以来，气象学家和海洋学家一直关注着海洋对于气候变化的作用。然而，要找到一条研究海洋对于气候影响的成功之路并不是一件易事。本文将综述海气边界层观测研究以及海气相互作用对气候影响研究的进展。     

Distribution of large volume image data using the mixed mode of Java Servlet and COM on Web

This paper reports some researches on distribution of large volume image data using techniques of the Mixed Mode of Java Servlet and COM on Web. The architecture and key technologies are discussed in detail. The web distribution system of image is implemented and the system is tested by the application instances. At last, the advantages and disadvantages for this web image distribution mode are analyzed.     

Recent improvements to the physical model of the Bohai Sea,the Yellow Sea and the East China Sea Operational Oceano-graphy Forecasting System

In order to satisfy the increasing demand for the marine forecasting capacity, the Bohai Sea, the Yellow Sea and the East China Sea Operational Oceanography Forecasting System (BYEOFS) has been upgraded and improved to Version 2.0. Based on the Regional Ocean Modeling System (ROMS), a series of comparative experiments were conducted during the improvement process, including correcting topography, changing sea surface atmospheric forcing mode, adjusting open boundary conditions, and considering atmospheric pressure correction. (1) After the topography correction, the volume transport and meridional velocity maximum of Yellow Sea Warm Current increase obviously and the unreasonable bending of its axis around 36.1°N, 123.5°E disappears. (2) After the change of sea surface forcing mode, an effective negative feedback mechanism is formed between predicted sea surface temperature (SST) by the ocean model and sea surface radiation fluxes fields. The simulation errors of SST decreased significantly, and the annual average of root-mean-square error (RMSE) decreased by about 18％. (3) The change of the eastern lateral boundary condition of baroclinic velocity from mixed Radiation-Nudging to Clamped makes the unreasonable westward current in Tsushima Strait disappear. (4) The adding of mean sea level pressure correction option which forms the mean sea level gradient from the Bohai Sea and the Yellow Sea to the western Pacific in winter and autumn is helpful to increasing the fluctuation of SLA and outflow of the Yellow Sea when the cold high air pressure system controls the Yellow Sea area.     

乌东德水电站水垫塘边坡雾化雨入渗数值分析

高坝泄洪雾化雨入渗是影响其水垫塘边坡稳定的关键因素之一。为分析乌东德水电站水垫塘边坡受泄洪雾化雨影响程度,选取水垫塘边坡典型断面,建立二维非饱和、非稳定渗流计算模型,采用有限元法模拟泄洪雾化雨入渗过程,分析雾化雨入渗条件下坡体饱和区范围及压力水头的变化规律,同时对比研究坡面防护措施的效果。结果表明,对于坡体内部渗透性上强、下弱的部位,随着入渗补给量的持续增加,在阻水部位压力水头会逐渐升高;当坡面采取弱透水材料防护后,可显著减少坡面雾雨入渗量,从而显著降低雾化雨入渗对坡体渗流场的影响     

西太平洋暖池核心区晚第四纪颗石藻属种变化及对环境演化的响应

对西太平洋暖池核心区KX97321-2站位380ka以来的颗石藻化石进行了属种鉴定及百分含量分析。结果显示,380ka以来Florisphaera profunda百分含量升高,对应营养跃层加深,初级生产力降低,Gephyrocapsa spp.的百分含量降低;在中布容期,Gephyrocapsa caribbeanica是优势种,是对中布容期特殊的环境条件适应演化的结果;对380ka以来F.profunda换算的初级生产力进行频谱分析,发现有100ka周期和30ka周期,而分段分析发现,0～80ka和320～380ka 2万年周期明显,中间80～320ka 4万年周期明显。     

Northward expansion of the western Pacific Warm Pool in late 1990s and early 2000s

Based on 48-year (1958-2006) ocean reanalysis data of Simple Ocean Data Assimilation and 23-year (1984-2006) global ocean-surface heat flux products developed by the Objectively Analyzed Air-Sea Heat Flux Project, meridional variation of the western Pacific Warm Pool (WPWP) is addressed. The results show that there is a significant expansion of the northern edge of the WPWP in the late 1990s and early 2000s. This variation is mainly within 120°E-160°E by 8°N-20°N, we define this region (120°E-160°E by 8°N-20°N) as the core region. Furthermore, analyses on upper ocean heat budget show that the short wave radiation plays a key role in the northward expansion of the northern edge of the WPWP in the core region. It is proved that the northward expansion may be caused by the change of the mixed layer which became shallower in 1994-2006 compared with 1984-1993 in the study region. The short wave radiation flux distribution within the shallower mixed layer leads to a positive anomaly in seawater temperature, promoting the northward expansion of the WPWP.