Estimation of longitudinal dispersion coefficient in rivers

The longitudinal dispersion coefficient is a crucial parameter for 1D water quality analyzing in natural rivers, and different types of empirical equations have been presented in the literature. To evaluate the precision of those commonly used equations, 116 sets of measured data for rivers in U.S. and UK have been collected for comparison. Firstly, the precisions of selected ten empirical equations under different aspect ratio (water surface width B/water depth H) have been compared, and calculation shows that most of the equations have underestimated the longitudinal dispersion when 20 < B/H < 100, in which most of the natural rivers located. The regression analysis on the collected data sets proved that the product of water depth H and the cross-sectional averaged velocity U has a higher linear correlation with the longitudinal dispersion coefficient than the product of H and shear velocity u1, and then a new expression of longitudinal dispersion coefficient, which is a combination of the product of HU and other two nondimensional hydraulic and geometric parameters, was deduced and the exponents were determined by the regression analysis. The comparison between the measured data and the predicted results shows that the presented equation has the highest precision for the studied natural rivers. To further evaluate the precision of the empirical formulae to artificial open channels, comparison was made between laboratory measuring data and empirical equation prediction, and the results have shown that the newly presented model is effective at predicting longitudinal dispersion in trapezoidal artificial channels too.     

基于流速幂级数解的明渠纵向离散系数计算方法

针对纵向离散系数计算多直接采用经验公式或在流速分布的求解中引入经验公式,不能完全反映离散物理机制的问题,基于矩形断面一维水流控制方程,利用幂级数求解流速分布的理论解,提出了矩形断面明渠纵向离散系数的计算方法,并进一步将其推广至天然河流。利用该方法计算了104条天然河流的纵向离散系数,结果表明计算值与实测值处于同一量级,计算结果在可接受范围内。     

Improvement on the Existing Equations for Predicting Longitudinal Dispersion Coefficient

Accurate prediction of longitudinal dispersion coefficient (K) is a key element in studying of pollutant transport in rivers when the full cross sectional mixing has occurred. In this regard, several research studies have been carried out and different equations have been proposed. The predicted values of K obtained by different equations showed a great amount of uncertainty due to the complexity of the phenomenon. Therefore, there is still a need to make an improvement on the existing predictive models. In this study, a multi-objective particle swarm optimization (PSO) technique was used to derive new equations for predicting longitudinal dispersion coefficient in natural rivers. To do this, extensive field data, including hydraulic and geometrical characteristics of different rivers were applied. The results of this study were compared with those of the previous studies using the statistical error measures. The comparison revealed that the proposed model is superior to the previous ones. According to this study, PSO algorithm can be applied to improve the performance of the predictive equations by finding optimum values of the coefficients. The proposed model can be successfully applied to estimate the longitudinal dispersion coefficient for a wide range of rivers’ characteristics.     

遗传门限自回归模型在感潮河段水位预测中的应用

应用门限自回归(TAR)模型建立了同时受潮汐和径流双重影响的长江下游感潮河段高桥水文站月水位TAR预测模型,建模过程中运用遗传算法来实现模型参数的优化.计算结果显示,门限自回归模型可以拟合感潮河段的非线性特性,拟合及预测精度均满足水文预报规范要求,遗传算法的引入简化了建模过程,提高了模型的预测精度并保证了其预测性能的稳定性.研究结果表明用遗传门限自回归模型预测感潮河段的水位是可行的,该模型在感潮河段其他水文要素的非线性时序预测中也具有广泛的实用价值.     

Pareto Optimal Multigene Genetic Programming for Prediction of Longitudinal Dispersion Coefficient

Water Resources Management - The longitudinal dispersion coefficient (Kx) is fundamental to modeling of pollutant and sediment transport in natural rivers, but a general expression for Kx, with...     

Man's Impact on Water Resources

ABSTRACT

The SSARR model is applied to simulate flood levels in the Mekong river delta. The Mekong and Bassac rivers which are two main rivers and the Vaico river are interlaced together with canals and floodplains with storage. The flow exchanges between these rivers, and the tidal influences in their estuarial reaches make it necessary to utilize the backwater mode of the SSARR model in the simulation. In addition to the backwater mode, the diversion algorithm of the SSARR model is also used to take into account the over bank flow conditions.

Since the river storages during the flood periods are considerable, the rivers have been divided into several storage reaches or reservoirs. The flow between these storage reaches depends on the water level differences between each pair. Moreover, the various flood plain storages are also represented by a network of interconnected storage reaches or reservoirs. The upstream boundary condition is the discharge in the Mekong river at Phnom Penh, and the downstream boundary conditions are the tides at the mouths of the three rivers. Rainfall over the study area is also considered as input to the model.

In general, very close agreement is obtained in the calibration and verification except at thte stations Can Tho and My Thuan where tidal influences are significant. However, good results are obtained for the envelope of high tides at these two stations. Sensitivity analysis is made to determine the effects of the overbank flow parameters, and the exchange flow parameters on the flow in the rivers. More data collection of delta discharges and water levels is recommended for the future to improve the existing backwater relationship for better simulation of tidal fluctuation in the estuarial reaches of the delta.     

上海市青松片水功能区水质保护措施

对上海市青松片水功能区的划分及确定的水质保护目标进行了回顾和分析,提出了存在的问题。根据实测和数学模型分析的成果,针对性地提出了改善和保持青松片水功能区水质的保护措施:①削减、截流污染源,在排污量中,约80%的COD,约67%的BOD5和约90%的NH3-N是分散禽畜排泄的,因此保护措施的重点是削减面源污染;②引清冲淡,利用黄浦江潮位变化高潮引水和低潮排水或利用泵站引排水,可以适当改善青松片内部分河道的水质;③底泥疏浚。     

波浪作用下污染物的混合和离散

本文建立了基于Sigma坐标的对流扩散数学模型，提出了通过数值模拟结果计算波浪场中纵向离散系数的方法，并通过数值模拟对波浪作用下污染物的混合和离散进行了研究。文中研究了水质点垂向振动和波浪参数对混合过程和纵向离散系数的影响；非各向同性扩散系数对纵向离散系数的影响；以及扩散系数的垂向分布对纵向离散系数的影响。研究结果表明，水质点的垂向速度对纵向离散系数有显著影响；若按波浪场中常用的湍流模型确定湍动扩散系数的分布规律，扩散系数的垂向分布对纵向离散系数的影响并不明显。     

Modified Saint-Venant equations for flow simulation in tidal rivers

Flow in tidal rivers periodically propagates upstream or downstream under tidal influence. Hydrodynamic models based on the Saint-Venant equations (the SVN model) are extensively used to model tidal rivers. A force-corrected term expressed as the combination of flow velocity and the change rate of the tidal level was developed to represent tidal effects in the SVN model. A momentum equation incorporating with the corrected term was derived based on Newton’s second law. By combing the modified momentum equation with the continuity equation, an improved SVN model for tidal rivers (the ISVN model) was constructed. The simulation of a tidal reach of the Qiantang River shows that the ISVN model performs better than the SVN model. It indicates that the corrected force derived for tidal effects is reasonable; the ISVN model provides an appropriate enhancement of the SVN model for flow simulation of tidal rivers.     

Dispersion coefficients of sewers from tracer experiments.

In this paper, 60 tracer experiments in 37 different sewer reaches have been analyzed for longitudinal dispersion under dry weather flow conditions. It was found that dispersion coefficients of sewers are two to three orders of magnitude smaller than those measured in rivers and do not differ much from system to system. Suitable equations were identified to predict reasonable dispersion coefficients in sewer reaches with uniform geometry and stable flow conditions. For engineering applications that require a high degree of accuracy the performance of tracer measurements is recommended.     

沿海地区洪潮遭遇分析

沿海地区直接入海的河流,河道水位由上游洪水和入海口的天文潮水位共同影响,如何考虑洪水与潮水的遭遇情况,对推求河道水面线起着至关重要的作用。文章以台山市东滘河为例介绍了河道洪水与外海潮水位遭遇的分析方法,由此确定了东滘河与外海的洪潮遭遇组合情况,为推求河道水面线及相关工程设计提供了依据。     

感潮河段航道整治设计理论与方法探讨

通过长江下游大量航道整治研究成果的总结提炼,探讨和论述感潮河段航道整治设计理论与方法。结果表明:根据不同的径流和潮汐影响,感潮河段中可划分为常年潮流段、季节性潮流段和常年径流段3个区段;根据河型特征和滩槽格局,浅滩类型有单级分汊型、多级分汊型、水下心滩型和水下沙洲型浅滩四种。应综合考虑航道整治影响因素及外部条件,确立航道整治总体原则,梳理得出不同区段设计通航水位及其与上下游衔接,以及整治参数(整治水位和整治线宽度)的确定方法;针对各类浅滩特点,采取相应的整治思路和整治工程措施,整治建筑物包括混凝土构件混合堤和新型生态守护结构。     

天然河流纵向离散系数确定方法的研究进展

对天然河流纵向离散系数的确定方法进行综述,详细分析了理论公式、示踪试验、经验公式这3种方法的研究现状和发展趋势。讨论了基于断面流速分布的理论公式的适用范围以及国内外学者对该公式使用的拓展;阐述了利用现场示踪试验数据计算纵向离散系数的矩量法、演算法、直线图解法和多种优化方法,并分析了它们各自的优缺点;列举了一些具有代表性的经验公式。探讨了弯道和死水区对天然河流纵向离散系数的影响;阐述了针对传统一维纵向离散方程的缺点提出的一些其他模型,并提出了天然河流纵向离散系数研究中一些有待深入研究的问题。     

再析海洋水电之潮汐电站投资效益因子——工程选址及增能降耗技术路线的探讨

海洋水电为海洋水动能发电,其中潮汐能电站50多年来经济效益指标一直居高不下,因此提高潮汐能投资效益指标已成为提高其电力市场竞争的关键点;借江河水电工程经验,剖析潮汐电站效益因子,提出其物理模拟实验及数值仿真模拟的主要任务目标,提出其增效因子潮差增量、波浪爬高、纳潮量等与其枢纽布置及效益之间的关联性。剖析潮汐电站可增效与可降耗因子的影响因素,提出其节能降耗研究技术路线;用潮汐能工程实例说明节能降耗研究的关键节点;结合潮汐工程前沿阔长特点,提出利用其开展海洋水力发电多能互补研究的建议。     

DEVELOPMENT OF COUPLED 1D-2D MATHEMATICAL MODELS FOR TIDAL RIVERS

Some coupled 1D-2D hydrodynamic and water quality models depicting tidal water bodies with complex topography were presented. For the coupled models, finite element method was used to solve the governing equations so as to study tidal rivers with complex topography. Since the 1D and 2D models were coupled, the principle of model coupling was proposed to account appropriately for the factors of water level, flow and pollutant flux and the related dynamical behavior was simulated. Specifically the models were used to probe quantitative pollution contribution of receiving water from neighboring Jiangsu and Zhejiang Provinces to the pollution in the Huangpu River passing through Shanghai City. Numerical examples indicated that the developed coupled 1D-2D models are applicable in tidal river network region of Shanghai.     

二维感潮河流污染计算的分裂特征线解法

本文提出的分裂特征线解法，主要用于二维感潮河段的污染计算，为水环境规划提出依据，污染计算实际上是对流扩散方程的数值解法，文中除介绍了分裂特征线解法的计算过程外，还介绍了收敛性，稳定性分析的结论，既有定量的研究，也有定性的讨论，文中所介绍的工程计算实例，验证了方法的准确性，编制的软件已用于我国长江，鸭绿江等河流的水质预测和规划。     

上海市引清调水改善水环境探讨

分析了引起上海市水污染的主要原因是大量污水排入河道 ,引入河道的清水量少以及没有合理调活水体等。阐明上海市引清调水的有利条件和必要条件为河网的潮汐特性、水利分片综合治理格局已基本形成、水源地水质较好以及调水试验的实践和研究。根据上海市的水利和水环境特点 ,在加强污染源综合整治的同时 ,以改善苏州河水质为重点 ,对上海市浦西地区 ,浦东新区及崇明、长兴、横沙岛提出了引清调水、改善水环境的构想     

大型潮汐河网水质模型的求解方法及其应用

用有限体积法对描述河流污染物运动规律的对流扩散方程积分，提出了一种处理潮汐河网汉道水质模拟的新方法；同时针对潮汐河网水质离散方程的特点建立了一种水质模型分级求解程式，实际应用表明：综合利用本文提出的汊道模拟方法及模型分级求解对于大型潮汐河网水质模拟不仅可以提高其模拟精度，而且具有极快的求解速度。     

Simulation of Water Quality under Different Reservoir Regulation Scenarios in the Tidal River

Reservoir regulation is one of the effective ways to improve water quality. However, when water quality requirement is considered, reservoir operation is complicated due to tidal regimes in the tidal rivers. The study was carried out in the tidal river reach between Dongjiang Hydro-Project and Shilong in Dongjiang River. To evaluate the effect of reservoir regulation on water quality, a two-dimensional hydrodynamic model combined with water quality has been developed. Under the scenarios of differrent flood discharge from the Shima River and different release discharge of Dongjiang Hydro-Project, water quality is simulated using the two-dimensional model. NH₃-N and COD concentrations are taken as the major indicators for water quality. Results indicate that (1) water quality is related to the tidal fluctuation, NH₃-N and COD concentrations increase at flood tide and decrease at ebb tide; (2) There is no obvious over-limit of COD concentration, NH₃-N concentration is found as the main pollution impacts water quality at water intake; (3) flood discharge from Shima River flows back to the upper reach of water intake during higher high tidal period, NH₃-N concentration non-compliance occurs, flood tide is the main factor which impacts water quality; (4) reservoir regulation of Dongjiang Hydro-Project causes a significant improvement on water quality, release discharge of Dongjiang Hydro-Project is determined to insure the water supply safety, which keeps tidal-induced backwater always flowing to lower reach of the water intake. The study provides a hydrodynamic perspective on water quality improvement due to reservoir regulation in the tidal river.     

上海市奉贤区近些年地表水水质变化趋势分析

奉贤区水域属长江流域,黄浦江水系,人工河道众多,水网密布,为典型的感潮河网地区。文章对上海市奉贤区1999年及2011年两次河道水质普查结果进行比较,并根据2000-2011年日常水质监测数据,采用有机污染综合指数法分析了近10余年间奉贤区骨干河道变化趋势,分析了河道水质污染的原因,提出了水环境改善的建议。