Rodolfo Bolaños

Tidal, Riverine, and Wind Influences on the Circulation of a Macrotidal Estuary

AbstractThe effect of tides, river, wind and Earth’s rotation on the three-dimensional circulation in the Dee, a macrotidal estuary, are investigated using a fine-resolution model. The interactions of the large tidal amplitude, currents, river, and wind-generated circulation require baroclinic and unsteady studies to properly understand the estuarine dynamics. Assessment of the model skill has been carried out by model–observation comparisons for salinity, which is the main control for density, surface elevation, current, and turbulence. Stationary nondimensional numbers were only partially able to characterize the dynamics in this (real) complex macrotidal estuary. At low water, tidal straining and constrained river flow cause stratification. Large spatial variability occurs in the current and residual patterns, with flood-dominated maximum values occurring within the tidal channels. The tides control residual circulation by modulating stratification through tidal straining and bathymetric constraint on ...

Extracting sea level residual in tidally dominated estuarine environments

Sea level comprises a mean level, tidal elevation and a residual elevation. Knowledge of what causes maximum water levels is often key in coastal management. However, different methods to extract deviations in water level (residuals) from modelled and observed elevation can give different results. The Dee Estuary, northwest England is a macrotidal estuary that undergoes periodic stratification. It is used here to demonstrate methods to extract the residual water level in response to the following interactive processes: tidal, river-induced stratification and flow, meteorology and waves. Using modelling techniques, the interaction and contribution of different physical processes are investigated. Classical harmonic tidal analysis, model simulations and filtering techniques have been used to “de-tide” the total elevation for short-term (approximately month long) records. Each technique gives a different result highlighting the need to select the correct method for a required study. Analysis of the residual components demonstrates that all processes inducing residuals interact with the tide generating a semi-diurnal residual component. It is suggested that modelling methods enable the full effect of tidal interaction to remain in the residual, whilst harmonic tidal analysis (partly) modify and filtering methods (fully) remove this component of the residual. The analysis methods presented and their influences on the resultant residual are applicable to other study sites. However, when applied specifically to the mouth of the Dee Estuary, the external surge is found to be the main contributor to the total residual, whilst local wind and stratification effects are of secondary importance.

Process Contribution to the Time-Varying Residual Circulation in Tidally Dominated Estuarine Environments

In tide-dominated environments, residual circulation is the comparatively weak net flow in addition to the oscillatory tidal current. Understanding the 3D structure of this circulation is of importance for coastal management as it impacts the net (longer term and event-scale) transport of suspended particles and the advection of tracer quantities. The Dee Estuary, northwest Britain, is used to understand which physical processes have an important contribution to the time-varying residual circulation. Model simulations are used to extract the time-varying contributions of tidal, riverine (baroclinicity and discharge), meteorological, external and wave processes, along with their interactions. Under hypertidal conditions, strong semi-diurnal interaction within the residual makes it difficult to clearly see the effect of a process without filtering. An approach to separate the residual into the isolated process contribution and the contribution due to interaction is described. Applying this method to two hypertidal estuarine channels, one tide dominant and one baroclinic dominant, reveals that process interaction can be as important as the sub-tidal residual process contributions themselves. The time variation of the residual circulation highlights the impact of different physical process components at the event scale of tidal conditions (neap and spring cycles) and offshore storms (wind, wave and surge influence). This gives insight into short-term deviation from the typical estuarine residual. Both channels are found to react differently to the same local conditions, with different short-term change in process dominance during events of high and low energy.

Controls on monthly estuarine residuals: Eulerian circulation and elevation

The Dee Estuary, at the NW English–Welsh border, is a major asset, supporting: one of the largest wildlife habitats in Europe, industrial importance along the Welsh coastline and residential and recreational usage along the English coast. Understanding of the residual elevation is important to determine the total water levels that inundate intertidal banks, especially during storms. Whereas, improved knowledge of the 3D residual circulation is important in determining particle transport pathways to manage water quality and morphological change. Using mooring data obtained in February–March 2008, a 3D modelling system has been previously validated against in situ salinity, velocity, elevation and wave observations, to investigate the barotropic–baroclinic wave interaction within this estuary under full realistic forcing. The system consists of a coupled circulation-wave-turbulence model (POLCOMS-WAM-GOTM). Using this modelling system the contribution of different processes and their interactions to the monthly residuals in both elevation and circulation is now assessed. By studying a tidally dominated estuary under wave influence, it is found that baroclinicity induced by a weak river flow has greater importance in generating a residual circulation than the waves, even at the estuary mouth. Although the monthly residual circulation is dominated by tidal and baroclinic processes, the residual estuarine surface elevation is primarily influenced by the seasonal external forcing to the region, with secondary influence from the local wind conditions. During storm conditions, 3D radiation stress becomes important for both elevation and circulation at the event scale but is found here to have little impact over monthly time scales.

Effects of Instrumented Bottom Tripods on Process Measurements

AbstractThe measurement and assessment of ocean bottom processes are important sources of information for understanding bedform evolution and sediment entrainment and for improving numerical models. Instrumented tripods have been used to investigate bottom boundary layer and sediment dynamics processes for several decades. In this paper, the effects of instrumented tripods on hydrodynamics and on the sea bed are investigated via numerical modeling and field data collected under moderate to strong tidal currents and mild surface waves. Under high currents, streamlines are modified and structure-induced vertical velocities are produced. To minimize this effect, a rotation of the three-dimensional current measurement under the frame is recommended. Acceleration of the flow under the frame is also significant (on the order of 10%–20%), which leads to an increase in bottom stress and can produce a large scour pit in energetic currents. Wave–structure interactions mainly increase turbulence near the frame. No s...

Measuring suspended sediment and its wave and turbulence forcing in the Dee estuary

Coastal areas support many human activities and represent a very important habitat for many marine and bird species. The Dee estuary, located in the eastern Irish Sea, is 20 km long, 8 km wide at the mouth and is characterized by a 10 m tidal range. Suspended and seabed sediments in the Dee contain a diverse assemblage of non-cohesive and cohesive sediments, and therefore the threshold of motion at the bed could be a complex process, dependent on several factors. In this paper, we present data collected during 2 deployments in the Dee, including both acoustic and optical instruments to study the link between the hydrodynamics, turbulence, and suspended sediments. Suspended sediment concentration is clearly controlled by tides following the flood/ ebb and spring/neap cycle. Moderate wave events were observed to increase sediment concentration, though mainly near the bed. High concentrations of silt and very fine sand were found that could support the flocculation processes during the flood and ebb cycle. Sediment concentrations at heights above 1 m from the bed do not present direct relation with bottom stress.

Measurement and modelling of suspended sediment size profiles above sandy rippled beds under waves

Measurements are presented of suspended sediment concentration size fractions with height above the bed. The data were collected over a rippled bed composed of medium sand under regular waves. The variation in the gradient of the suspension profiles was obtained and compared with a mixing length based on a simple exponential reduction in concentration with height above the bed. Using these mixing lengths, with the fall velocity for the different sediments size fractions, estimates for the sediment diffusivity were obtained. The variation in the magnitude of the sediment diffusivity with particle size was assessed and compared with empirical predictions.

On the impact of wind on the development of wave field during storm Britta

The observation of extreme waves at FINO 1 during storm Britta on the 1st November 2006 has initiated a series of research studies regarding the mechanisms behind. The roles of stability and the presence of the open cell structures have been previously investigated but not conclusive. To improve our understanding of these processes, which are essential for a good forecast of similarly important events offshore, this study revisits the development of storm Britta using an atmospheric and wave coupled modeling system, wind and wave measurements from ten stations across the North Sea, cloud images and Synthetic Aperture Radar (SAR) data. It is found here that a standard state-of-the-art model is capable of capturing the important characteristics of a major storm like Britta, including the storm path, storm peak wind speed, the open cells, and peak significant wave height (Hs) for open sea. It was also demonstrated that the impact of the open cells has negligible contribution to the development of extreme Hs observed at FINO 1. At the same time, stability alone is not sufficient in explaining the development of extreme Hs. The controlling conditions for the development of Britta extreme Hs observed at FINO 1 are the persistent strong winds and a long and undisturbed fetch over a long period.

The use of a wave boundary layer model in SWAN

A Wave Boundary Layer Model (WBLM) is implemented in the third-generation ocean wave model SWAN to improve the wind-input source function under idealized, fetch-limited condition. Accordingly, the white capping dissipation parameters are re-calibrated to fit the new wind-input source function to parametric growth curves. The performance of the new pair of wind-input and dissipation source functions is validated by numerical simulations of fetch-limited evolution of wind-driven waves. As a result, fetch-limited growth curves of significant wave height and peak frequency show close agreement with benchmark studies at all wind speeds (5 ∼ 60 ms−1) and fetches (1 ∼ 3000 km). The WBLM wind-input source function explicitly calculates the drag coefficient based on the momentum and kinetic energy conservation. The modeled drag coefficient using WBLM wind-input source function is in rather good agreement with field measurements. Thus, the new pair of wind-input and dissipation source functions not only improve the wave simulation but also have the potential of improving air-sea coupling systems by providing reliable momentum flux estimation at the air-sea interface. This article is protected by copyright. All rights reserved.

Erratum to: Controls on monthly estuarine residuals: Eulerian circulation and elevation

Brown et al. (2014) presented model simulations to identify the contribution of waves, tide, local meteorology, baroclinicity and external circulation to the residual circulation during a calm current-dominant period in Liverpool Bay. This period considers 15 days from the 6 – 21 February 2008, when wave heights were observed to be less than 0.7 m. This corrigendum is to correct Figure 6 published in Brown et al. (2014) following the recognition that Figure 7 had been duplicated. The correct Figure 6, as described in the full manuscript, is presented here. These results are for the Welsh Channel of the Dee Estuary that is dominated by hypertidal barotropic conditions. The clear dominance of the tide in creating a 2-layer horizontally sheared residual circulation is shown, while baroclinicity induced by the weak river flow has secondary importance.