3D float tracking: in situ floodplain roughness estimation

This paper presents a novel technique to quantify in situ hydrodynamic roughness of submerged floodplain vegetation: 3D float tracking. This method uses a custom‐built floating tripod that is released on the inundated floodplain and tracked from shore by a robotic total station. Simultaneously, an acoustic Doppler current profiler (ADCP) collects flow velocity profiles and water depth data. Roughness values are derived from two methods based on (1) run‐averaged values of water depth, slope and flow velocity to compute the roughness based on the Chézy equation, assuming uniform flow, (2) the equation for one‐dimensional free surface flow in a moving window. A sensitivity analysis using synthetic data proved that the median value of the roughness, derived using method 2, is independent of (1) the noise in water levels, up to 9 mm, (2) bottom surface slope, and (3) topographic undulations. The window size should be at least 40 m for a typical lowland river setup. Field measurements were carried out on two floodplain sections with an average vegetation height of 0·030 (Arnhem) and 0·043 m (Dreumel). Method 1 resulted in a Nikuradse roughness length of 0·08 m for both locations. Method 2 gave 0·12 m for Arnhem and 0·19 m for Dreumel. In Arnhem, a spatial pattern of roughness values was present, which might be related to fractional vegetation cover or vegetation density during the flood peak. 3D float tracking proved a flexible and detailed method for roughness determination in the absence of waves, and provided an unrestricted view from shore. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermal lenses and internal solitons in Chapala Lake, Mexico

INTRODUCTIONThelargestlakeinMexicoandthirdlargestinLatinAmerica,ChapalaLake(approximately2 0°N) ,is 1 50 0mabovesealevel,measures 75× 2 5km ,hasaveragedepthof6m (maximumdepthcloseto 1 1m)andamountainchainalongitsnorthernandsouthernshores (Fig .1a) .TheLermaRiverflowsintoit;theSantiagoRiverflowsoutofit.Thebeautifullandscape,andwarmclimateconducivetobreezecirculationalongitsshore ;absenceofabrupttemperaturevariations,andthemoderateairhumidityat tractmanytouriststothelake .Manyretirees…     

象山港电厂温排水的实测和数值模拟研究

为了研究象山港电厂温排水的热污染问题,对其附近海域的潮流和水温扩散进行了实测研究,给出了该海域潮流及水温扩散的特征。同时利用近海潮流模式(ECOMSED)建立了象山港潮流三维数值模型,对研究海域的潮流和水温扩散进行了模拟,得到了大小潮涨、落憩时刻的温升场的扩散范围,对比分析发现数模结果与实测结果符合较好。     

An evaluation of two alongshore transport equations with field measurements

The performance of two well-known equations to predict the depth-averaged alongshore suspended sediment flux [Van Rijn, L.C., 1984. Sediment transport, part II: suspended load transport. Journal of Hydraulic Engineering 110, 1613–1641; and Bailard, J.A., 1981. An energetics total load sediment transport model for a plane sloping beach. Journal of Geophysical Research 86, 10938–10954] was assessed by comparing predictions with 2306 field estimates based on a vertical stack of three optical backscatter sensors and a single electromagnetic flow meter. The observations were collected at four cross-shore positions on the intertidal beach of Egmond aan Zee, the Netherlands, during calm to storm conditions, with the offshore significant wave height peaking at 3.7 m. Measured hydrodynamics were employed in the computations of both models. Also, default parameter values were used without calibration to the data. We found that both models underpredicted the observations. Overall, the Van Rijn model outperformed the Bailard model, with about 70% of the model prediction lying between 1/5 to 5 of the observations under energetic conditions. For the Bailard model this was only about 20%. The performance of the Van Rijn model is, however, sensitive to the wave-related roughness, one of its highly uncertain free parameters. This may allow for an easy calibration when estimates of the depth-averaged alongshore sediment flux are available but may lead to serious errors in situations without data to constrain the predictions. We suspect that the discrepancy between the observations and model predictions is due to an overestimation of the observed fluxes (high turbidity, air bubbles) and an underestimation of the modeled fluxes because of missing physics related primarily to breaking waves.     

一种三角高程测量新方法

三角高程测量是目前常用的一种高程测量方法,但在实际工作中,由于过程麻烦,要求实地施测数据较多,因而测量结果误差较大,存在明显的不足。根据作业中的实际,通过对三角高程原理的分析,提出一种三角高程测量的新方法,分析了其测量原理、使用过程以及优势,并应用于实践。     

Asymmetric tidal straining across an inlet: Lateral inversion and variability over a tidal cycle

Tidal straining is a phenomenon of temporal variations in stratification and mixing resulting from the interaction of a longitudinal salinity gradient with the vertical shear of the horizontal tidal velocity. As a result, the theory predicts stronger and weaker stratification during ebb/low tide and flood/high tide, respectively. In contrast to this well-known temporal asymmetry, in this study, we document in situ measurements demonstrating a lateral asymmetry and lateral inversion of tidal straining at Barataria Pass, a narrow (∼600 m wide) tidal inlet of Barataria Bay in southeastern Louisiana. During flood, the eastern side of the channel had strong stratification of 4 PSU salinity change over a 1.5 m thin layer while the western side had a 2 PSU change over a 12 m water column. This strong lateral difference decreased as flood continued until near the end of the flood when it reached vertically well-mixed condition across the channel. During ebb it was just the opposite such that the western side became stratified while the eastern end was well-mixed. This resulted to a small correlation coefficient of −0.05 for stratification between the west and east sides, although the central channel and east side have a high correlation coefficient of 0.88. The tidally averaged salinity was higher on the western end than the eastern end except in a narrow boundary layer close to the eastern shore. This is an apparent contradiction to what the Coriolis effect would produce in classical estuarine dynamics. Our hypothesis for the observed difference arises from the influence of the river water coming out of the Mississippi River through the Southwest Pass of the Birdfoot Delta. This water mass may have played a role in the observed, complicated lateral inversion of the tidal straining. This study underlines the complexity of estuarine dynamics proximal to large deltaic systems and we anticipate that these results will underscore the need for a modeling study to further investigate this dynamic process.     

Observational and numerical modeling methods for quantifying coastal ocean turbulence and mixing

In this review paper, state-of-the-art observational and numerical modeling methods for small scale turbulence and mixing with applications to coastal oceans are presented in one context. Unresolved dynamics and remaining problems of field observations and numerical simulations are reviewed on the basis of the approach that modern process-oriented studies should be based on both observations and models. First of all, the basic dynamics of surface and bottom boundary layers as well as intermediate stratified regimes including the interaction of turbulence and internal waves are briefly discussed. Then, an overview is given on just established or recently emerging mechanical, acoustic and optical observational techniques. Microstructure shear probes although developed already in the 1970s have only recently become reliable commercial products. Specifically under surface waves turbulence measurements are difficult due to the necessary decomposition of waves and turbulence. The methods to apply Acoustic Doppler Current Profilers (ADCPs) for estimations of Reynolds stresses, turbulence kinetic energy and dissipation rates are under further development. Finally, applications of well-established turbulence resolving particle image velocimetry (PIV) to the dynamics of the bottom boundary layer are presented. As counterpart to the field methods the state-of-the-art in numerical modeling in coastal seas is presented. This includes the application of the Large Eddy Simulation (LES) method to shallow water Langmuir Circulation (LC) and to stratified flow over a topographic obstacle. Furthermore, statistical turbulence closure methods as well as empirical turbulence parameterizations and their applicability to coastal ocean turbulence and mixing are discussed. Specific problems related to the combined wave-current bottom boundary layer are discussed. Finally, two coastal modeling sensitivity studies are presented as applications, a two-dimensional study of upwelling and downwelling and a three-dimensional study for a marginal sea scenario (Baltic Sea). It is concluded that the discussed methods need further refinements specifically to account for the complex dynamics associated with the presence of surface and internal waves.     

高分五号可见短波红外高光谱相机在轨辐射性能评估

对遥感器及其图像的在轨辐射性能(信噪比、相对辐射定标精度、绝对辐射定标精度和动态范围)评估是检验遥感器定量应用能力的重要任务之一。本文结合高分五号卫星可见短波红外高光谱相机AHSI (the Advanced Hyperspectral Imager)在轨性能评估任务,依据卫星发射以来的在轨测试数据,介绍了AHSI相机在轨辐射性能评估的原理、方法及相应测试情况。测试结果表明:AHSI信噪比可见近红外最高达到近700,短波红外最高达到近500;相对辐射定标精度误差小于0.5%,星上绝对辐射定标不确定度小于3%,场地绝对辐射定标真实性检验精度误差小于5%;动态范围在轨可见短波均可以256档增益精细调整。AHSI在轨辐射性能良好且稳定,能够有效支持后续定量化应用需求。     

反演地表质量变化的附有空间约束的三维加速度点质量模型法

重力卫星可以在相同误差尺度下对全球质量变化进行连续重复观测,并在近十余年来取得了巨大成功,探索重力卫星数据精化处理方法和相关应用研究具有重要意义.本文基于三维加速度点质量模型法的基本原理,进一步发展建立了时变重力场模型球谐位系数的变化和地面点质量变化的关系,可有效考虑地表质量变化导致的负荷形变的影响;引入等权形式、线性形式、指数形式和高斯形式的空间约束方法处理南北条带噪声和向下延拓导致的病态问题,并与零阶Tikhonov正则化方法进行对比分析.采用模拟数据和一个月的实测GRACE时变重力场模型计算全球质量变化,对三维加速度点质量模型法和几种空间约束方法进行对比分析验证.计算结果表明,对于3°等面积的全球格网质量点,高斯和指数形式空间约束方法的最优相关距离约为500km,等权和线性形式空间约束方法的最优相关距离约为600km,各方法均可有效处理条带噪声的影响,四种空间约束方法的计算效果优于零阶Tikhonov正则化方法,本文的相关方法为进一步利用三维加速度点质量模型法监测全球质量变化提供了借鉴.     

Absorbed dose,equivalent dose,measured dose rates,and implications for OSL age estimates: Introducing the Average Dose Model

Luminescence ages are calculated by dividing an absorbed dose by the dose rate to which the natural dosimeter has been exposed. In practice, one measures an equivalent dose, D_e; in the absence of an alpha dose contribution, this should be indistinguishable from the dose absorbed in nature. Here we first review the relationship between absorbed dose, equivalent dose and dose rate, and the measurements that lead to their estimation; we restate that, in contrast to recent suggestions, an equivalent dose is not a physically different quantity from a beta or gamma dose absorbed by quartz grains. Statistical analysis of OSL data is of great importance when dealing with single grain data, since such data commonly exhibit significant scatter. However, dose rate measurements provide an arithmetic mean of dose rates absorbed by individual grains; in this article, we propose a new model to estimate the average dose absorbed by the grains. We thus introduce a new model for OSL age estimates: the Average Dose Model (ADM). We argue that ADM ages should be more accurate than Central Age Model (CAM) based ages, and we provide experimental evidence supporting this expectation. We also argue that the use of the Finite Mixture Model should be avoided. Finally, we discuss the implications for multi-grain age estimates derived from well-bleached samples.