The impact of tidal straining on the cycle of turbulence in a partially stratified estuary

Simpson, John H.; Williams, Eirwen; Brasseur, L. H.; Brubaker, J. M.

doi:10.1016/j.csr.2004.08.003

Cited by 99 publications

(57 citation statements)

References 18 publications

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“…Data collected from a number of ROFIs and estuarine systems have found evidence for the tidal straining mechanism. Examples include Liverpool Bay (Simpson et al 1990), the Rhine ROFI (Fisher et al 2002) and the York estuary (Simpson et al 2005). Simpson and Souza (1995) studied the semi-diurnal switching of stratification in the Rhine ROFI and concluded that this semi-diurnal variation resulted from crossshore tidal straining.…”

Section: Introductionmentioning

confidence: 99%

On the vertical structure of the Rhine region of freshwater influence

2006

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An idealised three-dimensional numerical model of the Rhine region of fresh water influence (ROFI) was set up to explore the effect of stratification on the vertical structure of the tidal currents. Prandle's dynamic Ekman layer model, in the case of zero-depth-averaged, crossshore velocities, was first used to validate the response of the numerical model in the case of barotropic tidal flow. Prandle's model predicted rectilinear tidal currents with an ellipse veering of up to 2%. The behaviour of the Rhine ROFI in response to both a neap and a spring tide was then investigated. For the given numerical specifications, the Rhine plume region was well mixed over the vertical on spring tide and stratified on neap tide. During spring conditions, rectilinear tidal surface currents were found along the Dutch coast. In contrast, during neap conditions, significant cross-shore currents and tidal straining were observed. Prandle's model predicted ellipse veering of 50%, and was found to be a good indicator of ellipticity magnitude as a function of bulk vertical eddy viscosity. The modelled tidal ellipses showed that surface currents rotated anti-cyclonically whereas bottom currents rotated cyclonically. This caused a semi-diurnal cross-shore velocity shearing which was 90°out of phase with the alongshore currents. This cross-shore shear subsequently acted on the horizontal density gradient in the plume, thereby causing a semi-diurnal stratification pattern, with maximum stratification around high water. The same behaviour was exhibited in simulations of a complete spring-neap tidal cycle. This showed a pattern of recurring stratification on neaps and de-stratification on springs, in accordance with observations collected from field campaigns in the 1990's. To understand the increase in ellipticities to 30% during neaps and the precise shape of the vertical ellipse structure, stratification has to be taken into account. Here, a full threedimensional numerical model was employed, and was found to represent the effect of de-coupling of the upper and lower layers due to a reduction of mixing at the pycnocline.

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Section: Introductionmentioning

confidence: 99%

On the vertical structure of the Rhine region of freshwater influence

2006

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“…During the flood tide phase, the vertical structure becomes homogeneous. The modulation of increasing the stratification in the ebb period and the mixing during the flood phase is explained by the tidal straining (e.g., SIMPSON et al, 2004). Considering the oscillatory tidal flow, the water displacement on the surface will be higher due to the near bottom bed shear, resulting in slower near bed currents.…”

Section: Hydrological Regimementioning

confidence: 99%

Intra-tidal variation of stratification in a semi-arid estuary under the impact of flow regulation

2013

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A B S T R A C TThe Jaguaribe River is the largest semi-arid Brazilian river and represents an important water source for the state of Ceará. The Castanhão Weir, built to regulate the flow of the Jaguaribe, began operating successfully in the 1990s. As a result, the hydrodynamic regime of the estuary has undergone alteration and is no longer subject to the former annual climatological variability of the dry and rainy seasons. This paper introduces an assessment of the changes in the hydrological conditions resulting from the construction of the weir and investigates the structure of the density stratification in the estuary. A field experiment was conducted at the end of the dry season in November 2009, during which salinity and temperature were monitored at different levels for three tidal cycles. The temporal variations in salinity and temperature during the tidal cycles were 16 psu and 1.5°C, respectively. The salinity exhibited a vertical variation of four units, while the temperature variation was much lower. Several stratification indicators were calculated from the observed density: the stratification coefficient, the Brunt-Väisälä frequency and the potential energy anomaly. The results showed that the parameters behave similarly, differences being observed only in their relative magnitudes. The vertical structure of the estuary exhibited marked stratification during the ebb tide but vertical homogeneity during both flood and high tide. With the regularized flow existing under present conditions, the estuary exhibits hyposaline and a partially mixed structure for the period studied, despite the typical expectation of a hypersaline and well-mixed vertical structure. R E S U M OO rio Jaguaribe é o maior rio do semi-árido brasileiro, constituindo uma importante fonte de água para o estado do Ceará. Na década de 1990 entrou em operação o Açude do Castanhão com o objetivo de regularizar a vazão do rio Jaguaribe, o qual obteve grande sucesso. Como consequência foi alterado o regime hidrodinâmico do estuário, o qual atualmente não está mais sujeito a variabilidade climatológica anual de estações seca e chuvosa. O presente trabalho apresenta uma avaliação da mudança do regime hidrológico antes e após a construção do açude, e investiga a estrutura de estratificação da densidade nos estuários. Foi realizado um experimento de campo em novembro de 2009, no fim da estação seca, quando a salinidade e temperatura da água foram monitorados em diferentes níveis durante três ciclos de maré. A variação temporal da salinidade e temperatura durante os ciclos de maré foram de 16 psu e 1,5ºC, respectivamente. A salinidade chegou a apresentar variação vertical de 4 unidades, enquanto que a variação da temperatura foi bem inferior. Foram calculados parâmetros indicadores da estratificação a partir da densidade: coeficiente de estratificação; frequência de Brunt-Väisälä; e a anomalia da energia potencial. Os resultados mostraram que os parâmetros comportam-se da mesma maneira, apenas com diferença da magnitude relativa de seus valo...

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“…The fit of the net southward velocities between the simulation results and the conceptual model of the response outlined above is excellent (R 2 5 0.78, based on 65 vertical cells and model output every 12 h; n 5 3315), indicating that the circulation in the lake interior can be predominantly described as the superposition of a simple seiche and convective circulation. Substituting the peak convective circulation velocity into (5), we return an effective turbulent eddy viscosity of 1.6 3 10 3 m 2 s 21 , which appears to be reasonable based on observation during slack-water periods in estuaries (Lu and Lueck 1999;Simpson et al 2005) and numerical simulations in lakes (Wang 2003). It also allows us to compute a spin-up time T 5 h 2 /v for the circulation, which for a 100-m-deep layer and the above value of n gives a spin-up time of 70 d. This matches well with the approximate time to spin-up of the convective circulation determined visually from Fig.…”

Section: Ra~bmentioning

confidence: 99%

Differential cooling drives large‐scale convective circulation in Lake Tanganyika

Verburg

Antenucci

Hecky

2011

Limnology & Oceanography

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Field data, scaling analysis, and three-dimensional numerical experiments demonstrate that the dominant large-scale circulation pattern during the southeast trade winds in Lake Tanganyika (East Africa) is a downwind flow in the metalimnion, with a returning southward upwind flow in the upper region of the epilimnion. This is in the opposite direction to all prior literature available on the lake, which has assumed that the southeast trade winds must drive a northward surface flow due to momentum induced by the wind. The gradient in wind speed and humidity along the length of the lake and the resulting gradient in heat exchange with the atmosphere result in warm water accumulating at the northern end of the lake. The north basin of Lake Tanganyika, thus, acts as a heat sink while the south basin is a net source of heat to the atmosphere. The resulting variation in buoyancy flux is strong enough to drive a convective flow in the upper layer in the opposite direction to the wind. A large-scale convective circulation results in an annual exchange of 6.2 3 10 12 m 3 of water and an annual net transport of heat (averaging 0.2 terawatt) from the northern to the southern region of the lake. Currents in the upper 100-m layer, with a mean flow of 0.2 Sverdrup and flow speed of 0.08 m s 21 , can be described by the superposition of a parallel convective flow with the internal seiche motion.

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The impact of tidal straining on the cycle of turbulence in a partially stratified estuary

Cited by 99 publications

References 18 publications

On the vertical structure of the Rhine region of freshwater influence

On the vertical structure of the Rhine region of freshwater influence

Intra-tidal variation of stratification in a semi-arid estuary under the impact of flow regulation

Differential cooling drives large‐scale convective circulation in Lake Tanganyika

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