Saline intrusion in partially mixed estuaries
Saline intrusion in partially mixed estuaries
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DOI：
10.1016/j.ecss.2003.10.001
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发表时间：
2004-03
影响因子：
2.8
通讯作者：
D. Prandle
中科院分区：
文献类型：
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作者：
D. Prandle
限制兴趣部分混合河口，潮汐平均线性化理论的垂直结构的盐度和速度（伴随盐水入侵）的早期研究扩展到考虑到潮汐应变和相关的对流翻转。这些理论的适用性进行评估参考的“单点”的数值模型，其中指定的随时间变化的周期的深度平均潮流的振幅，Sx，和（时间和垂直）恒定的盐梯度，Sx。该模型强调了对流翻转在抵消潮汐应变引入的不稳定密度结构中的重要性。在模型中忽略倾覆，结果与线性化理论推导结果吻合得很好。然而，加入翻转大大增加了潮汐平均的表面到床的差异为两个剩余电流，δu和盐度，δs。潮流的垂直结构是一个最大的，因此，潮汐应变的影响，在浅的大潮汐河口。在部分混合河口，潮高和潮流的传播对盐水入侵仍然不敏感。该模型的适用性通过Rippeth等人（J. Phys. Oceanogr. 31（2001）2458）。为了探索河口响应的一般性，该模型在盐水入侵长度L和水深D的广泛范围内运行。另外，还对λ和河床应力系数k的变化进行了敏感性分析。响应框架包括：δu、δs、势能异常φ、底摩擦功和内剪切功、盐水混合速率和效率以及扩散与翻转的相对混合比。通过将与垂向扩散有关的混合速率与河流流量Q相等，导出了盐水入侵长度L <$D2/k U <$Uo（河流流速）的表达式。这一提法同意早期推导水槽试验的基础上，并表现出合理的协议与观测值在六个河口（8例）。然而，在漏斗形河口，侵入体的轴向迁移引入了D，Uo和Uo的主要变化，从而使上述L表达式的应用复杂化。此外，在L调整到λ和Q的变化中所涉及的时间滞后可以解释观测中遇到的复杂性。
Restricting interest to partially mixed estuaries, earlier studies of tidally averaged linearised theories relating to the vertical structure of salinity and velocities (accompanying saline intrusion) are extended to take account of tidal straining and associated convective overturning. The applicability of these theories is evaluated by reference to a ‘single-point’ numerical model in which the time-varying cycle of depth-averaged tidal current amplitude, Û, and a (temporally and vertically) constant saline gradient, Sx, are specified. This model highlights the importance of convective overturning in counteracting unstable density structures introduced by tidal straining. By omitting overturning in the model, results agree closely with linearised theoretical derivations. However, incorporating overturning substantially increases tidally averaged surface-to-bed differences for both residual currents, δu, and salinity, δs. The vertical structure of tidal currents is a maximum, and hence the effect of tidal straining, in shallow macro-tidal estuaries. The propagation of tidal elevations and currents remains insensitive to saline intrusion in partially mixed estuaries. The applicability of the model was evaluated by simulation of recent measurements by Rippeth et al. (J. Phys. Oceanogr. 31 (2001) 2458). To explore the generality of estuarine responses, the model was run for a wide range of values of saline intrusion lengths, L, and water depths, D. Additional sensitivity analyses were made for changes in Û and bed stress coefficient, k. Response frameworks are shown for: δu, δs, potential energy anomaly φ, work done by bed friction and internal shear, rates and efficiency of saline mixing and ratios of relative mixing by diffusion to overturning. By equating the rate of mixing associated with vertical diffusion with river flow, Q, an expression for saline intrusion length L∝D2/k U ̂ Uo(Uoriver flow velocity) was derived. This formulation agrees with an earlier derivation based on flume tests and showed reasonable agreement with observed values in six estuaries (eight cases). However, in funnel-shaped estuaries, axial migration of the intrusion introduces major variations in D, Û and Uo, thereby complicating the application of the above expression for L. Moreover, the time lag involved in the adjustment of L to changes in Û and Q may explain much of the complexity encountered in the observations.