The Temporal Response of the Length of a Partially Stratified Estuary to Changes in River Flow and Tidal Amplitude.
The Temporal Response of the Length of a Partially Stratified Estuary to Changes in River Flow and Tidal Amplitude.
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DOI：
10.1175/2008jpo3933.1
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发表时间：
2009-04
影响因子：
3.5
通讯作者：
Ralston DK
中科院分区：
文献类型：
--
作者：
Lerczak JA;Geyer WR;Ralston DK
利用2004年春季和夏季沿哈德逊河口长度沿着收集的108天时间序列和Qf、UT和近地表盐度的长期（13.4年）记录，研究了部分混合河口长度对淡水流量Qf和潮汐振幅UT变化的时间响应。当Qf中等偏高时，河口的潮汐平均长度L5（此处定义为河口到河口上游垂直平均盐度为5 psu的位置的距离）在整个小潮周期中波动超过47 km，波动范围为28 km至>75 km。在低流量期间，L5变化很小，在春季小潮周期，并接近一个稳定的长度。的响应进行量化和比较，来自全球河口盐平衡的线性化模型的预测。该模型受Qf和UT相对于平均流量Qo和潮汐振幅UTo的波动影响，并预测了平均强迫的线性响应时间尺度τ和稳态长度Lo。考虑了两种垂直混合方案，其中a）混合与UT成比例，B）混合对层结的依赖性也被参数化。基于L5和模型预测的河口长度之间的最小二乘拟合，估计的τ从高平均流量时期（Qo = 750 m3 s −1，τ = 4.2天）到低流量时期（Qo = 170 m3 s −1，τ = 40.4天）变化一个数量级。在观测到的流量范围内，Lo = Qo−0.30±0.03，与河口向陆盐通量由河口垂直交换环流驱动的理论标度一致。估计的τ与排放平流时间尺度（LoA/Qo，其中A是河口的横截面积）成比例。然而，τ比理论预测大三到四倍。分层依赖混合的模型预测L5的变化具有更高的技能比与UT成比例的混合模型。该模型提供了深入了解的时间依赖性响应的部分分层河口强迫的变化，并解释了强烈的依赖性的振幅的春季小潮响应淡水流量。然而，线性模型的效用是有限的，因为它假设一个统一的渠道，因为基本的动态是非线性的，强迫，Qf和UT，可以经历大幅度的变化。特别是，河流流量在与河口响应时间尺度相当或更短的时间尺度上可能会发生一个数量级以上的变化。
The temporal response of the length of a partially-mixed estuary to changes in freshwater discharge, Qf, and tidal amplitude, UT, is studied using a 108 day time series collected along the length of the Hudson River estuary in the spring and summer of 2004 and a long-term (13.4 year) record of Qf, UT, and near-surface salinity. When Qf was moderately high, the tidally-averaged length of the estuary, L5, here defined as the distance from the mouth to the up-estuary location where the vertically-averaged salinity is five psu, fluctuated by more than 47 km over the spring-neap cycle, ranging from 28 km to >75 km. During low flow periods, L5 varied very little over the spring-neap cycle and approached a steady length. The response is quantified and compared to predictions of a linearized model derived from the global estuarine salt balance. The model is forced by fluctuations in Qf and UT relative to average discharge, Qo, and tidal amplitude, UTo, and predicts the linear response time scale, τ, and the steady-state length, Lo, for average forcing. Two vertical mixing schemes are considered, in which a) mixing is proportional to UT and b) dependence of mixing on stratification is also parameterized. Based on least-squares fits between L5 and estuary length predicted by the model, estimated τ varied by an order of magnitude from a period of high average discharge (Qo = 750 m3s−1, τ = 4.2 days) to a period of low discharge (Qo = 170 m3s−1, τ = 40.4 days). Over the range of observed discharge, Lo ∝ Qo−0.30±0.03, consistent with the theoretical scaling for an estuary whose landward salt flux is driven by vertical estuarine exchange circulation. Estimated τ was proportional to the discharge advection time scale (LoA/Qo, where A is the cross-sectional area of the estuary). However, τ was three to four times larger than the theoretical prediction. The model with stratification dependent mixing predicted variations in L5 with higher skill than the model with mixing proportional to UT. This model provides insight into the time dependent response of a partially-stratified estuary to changes in forcing and explains the strong dependence of the amplitude of the spring-neap response on freshwater discharge. However, the utility of the linear model is limited because it assumes a uniform channel and because the underlying dynamics are nonlinear and the forcing, Qf and UT, can undergo large amplitude variations. River discharge, in particular, can vary by over an order of magnitude over timescales comparable to or shorter than the response timescale of the estuary.