Abstract The interaction between flow and vegetation in constructed wetlands plays a major role in determining wetland performance. In this study, a two-dimensional depth-averaged model was used to simulate flow, mass transport and contaminant removal in a conceptual free-water surface (FWS) wetland with heterogeneous vegetation patterns. The main objectives of this study were (1) to quantify the effectiveness of FWS wetlands with different vegetation patterns in reducing pollutant load, and (2) to identify optimal vegetation distributions that maximize contaminant removal. Simulations were performed for different random vegetation fields characterized by imposed mean, variance and correlation lengths of stem density. The wetland was assumed to receive water from a stream and to deliver it back to the same stream according to an imposed head drop. The simulations show that the concentration reduction efficiency increases monotonically with average stem density, whereas mass removal has a peak for an intermediate value of average stem density. The ensemble average of the total mass removal decreases for increasing stem density variance and correlation length, because the presence of vegetation patches with significantly different stem density promotes preferential flow paths. Preferential flow paths parallel to the mean flow direction were found to reduce the hydraulic efficiency of wetlands by producing short-circuiting, whereas, for the same mean stem density, alternating stripes of stem density perpendicular to the flow direction provided higher concentration and mass reduction. By providing a quantitative understanding of the impact of spatial vegetation heterogeneity, the results provide useful guidelines for design and maintenance of constructed wetlands for wastewater treatment.

中文翻译：

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具有异质植被密度的自由水表面湿地污染物去除效率的变化

摘要 人工湿地中水流与植被之间的相互作用在决定湿地性能方面起着重要作用。在这项研究中，二维深度平均模型用于模拟具有异质植被模式的概念性自由水面 (FWS) 湿地中的流量、质量传输和污染物去除。本研究的主要目标是 (1) 量化具有不同植被模式的 FWS 湿地在减少污染物负荷方面的有效性，以及 (2) 确定可最大限度地去除污染物的最佳植被分布。对不同的随机植被场进行了模拟，其特征是茎密度的强加平均值、方差和相关长度。假设湿地从溪流中接收水，并根据强加的水头下降将其送回同一条溪流。模拟表明，浓度降低效率随平均茎密度单调增加，而质量去除在平均茎密度的中间值处具有峰值。总质量去除的集合平均值随着茎密度方差和相关长度的增加而降低，因为茎密度显着不同的植被斑块的存在促进了优先流动路径。发现平行于平均流动方向的优先流动路径会通过产生短路降低湿地的水力效率，而对于相同的平均茎密度，垂直于流动方向的茎密度交替条纹提供更高的浓度和质量减少。通过提供对空间植被异质性影响的定量理解，