Pollutant mixing and transport process via diverse transverse release positions in a multi-anabranch river with three braid bars

A multi-anabranch river with three braid bars is a typical river pattern in nature, but no studies have been conducted to describe mixing characteristics of pollutants in the river. In this study, a physical model of a typical multi-anabranch river with three braid bars was established to explore the pollutant mixing characteristics in different branches. The multi-anabranch reach was separated into seven branches, B1, B2, B3, B4, B5, B6, and BT, by three braid bars. Five tracer release positions located 2.9 m upstream from the inlet section of the multi-anabranch reach were adopted, and the distances from the five positions to the left bank of the upstream main channel were 1/6B, 1/3B, 1/2B, 2/3B, and 5/6B （B is the width of the upstream main channel）, respectively. The longitudinal velocities and pollutant concentrations in the seven branches were measured. The planar flow field and mixing characteristics of pollutants from the bottom to the surface in the multi-anabranch river were obtained and analyzed. The results show that the pollutant release positions are the main influencing factors in the pollutant transport process, and the diversion points and pollutant release positions jointly influence the percentage ratios of the pollutant fluxes in branches B 1, B2, and B3 to the pollutant flux in the upstream main channel.

四岛型群汊河道二次流特性研究

开展四岛型群汊河道的二次流特性研究是河道整治、取水建筑物设计及污染物混合运移预测等工程建设和水环境保护实施的基础。选取天然河道中四岛型群汊河段,建立四岛型群汊河道物理模型试验系统,在四岛型群汊段布置26个测量断面,详细测量各断面7个不同水深处的三维水动力参数。通过探讨断面二次流方向、大小与分汊岛相对位置和进入汊道水流流速之间的关系,分析四岛型群汊河道的二次流分布特征。基于各汊道断面测点的横向、纵向流速,计算断面二次流强度值并对四岛型群汊河道各汊道二次流强度的特点进行分析。结果表明:四岛型群汊河道的二次流方向和大小由分汊岛之间的位置关系与进入汊道水流的平面流速分布共同决定,二次流强度较大区域集中在汇流区、分流区及汊道曲率变化较大处。

群汊河道出口汇流断面污染物混合特性研究

准确预报群汊河道中污染物扩散范围与混合过程,可为工业和市政排放工程的设计、取水口保护区范围的确定、水资源保护等提供科学依据。选取存在3~5个分汊岛的群汊河道,开展不同上游流量和排污位置工况的3种典型群汊河道污染物输移物理模型试验,在群汊段各汊道布置污染物浓度测量断面。选取代表断面并计算污染物混合偏差程度值,分析不同上游流量和排污位置条件下,污染物浓度分布在群汊段沿程变化情况。计算群汊河道出口汇流断面的污染物混合偏差程度值,分析上游流量和排污位置对出口汇流断面浓度分布的影响。结果表明,在相同流量、同一排污位置条件下,断面混合偏差程度值沿程减小;在同一排污位置条件下,随着分汊岛数量的增多,群汊段出口汇流断面混合偏差程度值越小。

五岛群汊河道污染物输移混合特性的试验研究

污染物在有多个分汊岛的群汊河道中的输运混合特性目前尚不清楚,建立存在5个分汊岛的群汊河道物理模型试验系统,通过试验研究了恒定点源排放下群汊河道中污染物扩散特性和汊道污染物混合机理,试验结果表明:来流流量的增加,使得污染物浓度整体下降,污染物混合均匀程度降低;污染物排污位置的改变,对污染物浓度的分布影响显著;支汊中,污染物沿程输运是一个逐渐混合均匀的过程,但是突扩型支汊例外,其混合均匀性有降低的现象。研究为掌握污染物在群汊河道中的输移混合特性提供了科学依据。

三岛型群汊河道污染物断面浓度分布影响因素研究

为了揭示污染物在三岛型群汊河道中的输移扩散机理,初步探求污染物断面浓度分布的影响因素,通过物理模型试验,对不同流量工况下浓度分布状态、浓度高值区的范围和位置、浓度值的大小等方面变化进行分析。不同流量工况下,汊道断面浓度分布的大致形态和趋势基本保持一致,小流量增大时浓度高值区范围略有缩小,位置稍向左岸移动,高值区浓度值增大。不同排污位置偏离测量断面中心线越远,断面的整体浓度值越小;随着排污位置从左到右移动,浓度高值区也由左岸向右岸移动;排污位置距离测量断面的中心线越近,高值区浓度值越大。