Michael Hartnett

Modelling nutrient and chlorophyll_a dynamics in an Irish brackish waterbody

Abstract This paper presents aspects of a research investigation into nutrient fluxes into and out of an important Irish estuary, Wexford Harbour, located in the southeast of Ireland. Details of the methodology applied, the data collection and analysis and the models used are outlined. In particular, the paper addresses modelling of chlorophyll_a production in the brackish waterbody and its dependency on light attenuation. There are three distinct stages to the research carried out: (i) collection, collation and analysis of physical, chemical and biological water quality variables; (ii) development, validation and application of a chlorophyll_a production model; (iii) development of a Geographic Information System. The nutrient and chlorophyll_a production model simulates nine interactive water quality parameters; during the first stage these above parameters were measured to provide initial conditions for the model and for calibration purposes. From measurements, relationships were derived between the water quality variables and salinity, which were used to specify initial spatially varied values of the parameters to the model at different seasons. A relationship was also developed between light intensity, turbidity and concentrations of chlorophyll_a, based on field measurements. This relationship was incorporated into the model. The paper presents chlorophyll_a predictions using this relationship and illustrates how the enhanced model can be used as an accurate modelling tool for managing coastal ecosystems. The water quality model was incorporated into the ArcView GIS system providing an easy-to-use graphical user interface to run simulations and interrogate results.

HF Radar Activity in European Coastal Seas: Next Steps toward a Pan-European HF Radar Network

High Frequency radar (HFR) is a land-based remote sensing instrument offering a unique insight to coastal ocean variability, by providing synoptic, high frequency and high resolution data at the ocean atmosphere interface. HFRs have become invaluable tools in the field of operational oceanography for measuring surface currents, waves and winds, with direct applications in different sectors and an unprecedented potential for the integrated management of the coastal zone. In Europe, the number of HFR networks has been showing a significant growth over the past ten years, with over 50 HFRs currently deployed and a number in the planning stage. There is also a growing literature concerning the use of this technology in research and operational oceanography. A big effort is made in Europe towards a coordinated development of coastal HFR technology and its products within the framework of different European and international initiatives. One recent initiative has been to make an up-to-date inventory of the existing HFR operational systems in Europe, describing the characteristics of the systems, their operational products and applications. This paper offers a comprehensive review on the present status of European HFR network, and discusses the next steps towards the integration of HFR platforms as operational components of the European Ocean Observing System, designed to align and integrate Europe’s ocean observing capacity for a truly integrated end-to-end observing system for the European coasts.

Influence of seasonal circulation on flushing of the Irish Sea.

We applied a three-dimensional general ocean and coastal circulation model to the Irish Sea in order to determine water renewal time scales in the region. The model was forced with meteorological data for 1995, a year with relatively warm summer and when extensive hydrographic surveys were conducted in the Irish Sea. We investigated intra-annual variability in the rates of net flow through the Irish Sea and carried out several flushing simulations based on conservative tracer transport. The results indicate that the net northward flow of 2.50 km(3)/d is seasonally highly variable and under certain conditions is reversed to southward. The variability in obtained residence times is high; baroclinic effects are significant. Obtained results point at the importance of spatial and temporal consideration for transport of pollutants in the shelf seas. Implications for management are numerous and involve activities such as transport, fishing, use of resources, nature conservation, monitoring, tourism and recreation.

Modal analysis of an existing offshore platform

This paper presents details of the development and application of a numerical model, SPECMOD, for modal analysis of offshore lattice platforms, along with details of a structural monitoring programme carried out on such a platform. The model was developed using the finite element method. Three-dimensional beam elements were employed to represent structural members. The paper outlines how added mass, entrained water and marine growth on submerged piles are incorporated into the consistent mass matrix, having first linearized Morisons equation. The paper also outlines how the soil-structure interaction of foundation members is represented in the model. Kinsale Head Gas Production Platform Alpha, located off the south coast of Ireland, is sited in approximately 92 m of water and is often subjected to harsh environmental loadings. The structural monitoring programme was undertaken to determine some of the dynamic characteristics of the platform. In particular the project team sought to estimate the five fundamental natural frequencies of the platform. The results from this project are presented in the paper. The finite element model, SPECMOD, was used to develop a numerical model of the platform and to predict its natural frequencies. Details of this model application are presented along with a comparison between the estimated and numerically predicted natural frequencies. The model was then used to determine the sensitivity of the natural frequencies to hydrodynamic masses, results from these sensitivity analyses are presented.

Modelling the far field hydro-environmental impacts of tidal farms - A focus on tidal regime, inter-tidal zones and flushing

The introduction of tidal stream turbines into water bodies can have an impact on the environment due to changes in the hydrodynamic flow fields resulting from the extraction of energy by the tidal turbines. Water levels, tidal currents and flushing characteristics could potentially be significantly altered with the introduction of tidal turbine farms, which could lead to possible loss of habitat and a change in the tidal regime. Therefore, planning of tidal turbines field deployments must take into account possible hydro-environmental impacts. This paper describes research undertaken by the authors in the Shannon Estuary to predict changes in the tidal regime and flushing characteristics, with the introduction of tidal turbine farms of different array configurations. The model was simulated using a 2D hydrodynamic model that was modified to incorporate the effects of tidal turbine fields. Water levels are shown to have been affected with the inclusion of turbines, especially in areas upstream of the turbine farm where inter-tidal zones could become predominately inundated resulting in loss of habitat in the estuary. Flushing parameters were also shown to be altered with the inclusion of turbines, with residence time shown to be increased, which could change pollutant transport in the region.

Determination of flushing characteristics of the Irish Sea: A spatial approach

The Authors devised a novel generic approach to assessing the flushing of the Irish Sea through the determination of spatially distributed residence times and the development of flushing homogeneity curves. Results indicate that flushing of the Irish Sea is both spatially and temporally highly variable. Average residence times of the material introduced in winter may be up to 28% higher than the material introduced in summer, and the aerial flushing deviation index may reach up to 470 day. The spatial approach to flushing is an extremely useful complement to classical flushing analysis considering significant implications for management of water quality.

Parameterizing suspended canopy effects in a three-dimensional hydrodynamic model

ABSTRACT This paper presents an amendment of an existing three-dimensional hydro-environmental model (Environmental Fluid Dynamics Code) to incorporate effects of suspended canopies on the vertical flow structure. Five different modelling approaches are investigated, encompassing hydrodynamic form drag imparted by the suspended canopy, an amended two-equation turbulence scheme representing turbulence generated locally by elements within the canopy, and three separate formulations for vertical profiles of drag coefficients. Data from laboratory experiments with rigid cylinders are used to validate the calculations of velocity and shear stresses. The results show that the most accurate reproduction of the canopy flow was obtained using a vertically varying drag coefficient along with a two-equation turbulence closure scheme that includes additional turbulence production and dissipation terms. The numerical model reproduced velocity profiles accurately, but the shear stresses are slightly overestimated.

Mathematical modelling of flow, pesticide and nutrient transport for fish-farm planning and management

Abstract In recent years, there has been a rapid growth in fish farming, particularly salmon farming, in UK and Eire coastal waters. This increase in production has also led to an awareness by the fish-farming industry that over-production could have a deleterious effect on the hydro-ecology of coastal waters, which would inhibit the maintenance of current production farming levels. In planning a fish-farm configuration for coastal basins, complex deterministic mathematical models can provide invaluable information for farm optimisation, with the models predicting the tidal current structure and solute levels in the basin. Details are given herein of a refined mathematical model for predicting tidal current and Nuvan, biochemical oxygen demand (BOD) and nitrogen levels for a proposed fish-farm configuration in a bay off the Eire coastline. The model accurately predicts field-measured velocities at two sites within the bay, and further predicts Nuvan, BOD and nitrogen levels that are well below those levels which would be known to affect adversely the hydro-ecology of the bay.

Sea level rise in the Severn Estuary and Bristol Channel and impacts of a Severn Barrage

Many research projects in recent years have focused on marine renewable energy devices and structures due to the growing interest in marine renewable energy. These devices and structures have very different life spans. Schemes such as the Severn Barrage in the UK, as originally proposed by the Severn Tidal Power Group (STPG), would be the largest tidal renewable energy generation project in the world and would be operational for well over a century if built. Due to the long working life of some of these marine renewable energy schemes, it is important to study the impacts of climate change on such schemes, and particularly sea level rise. This study focuses on investigating the impacts of sea level rise due to climate change on the largest macro-tidal estuary in the UK, namely the Severn Estuary and Bristol Channel, and the alterations of the impacts and the performance of the Severn Barrage as a result of climate change. A hierarchy of computer models was implemented to identify the more localised impacts of climate change in the region of the study. Moreover, the potential benefits of the barrage on reducing flood risk, as well as the impact of climate change and the barrage on intertidal mudflats were investigated. The model predictions showed that the barrage would reduce flood risk due to the sea level rise. Furthermore, annual power output and the initial reduction in flood risk of the barrage would not be affected by sea level rise. Sea level rise at the outer Bristol Channel predicted using a nested global and regional models.Extent of sea level rise within the Severn Estuary and Bristol Channel was predicted by implementing a coastal model.Impacts of sea level rise on intertidal mudflats in the estuary was studied.Benefits of a proposed Severn Tidal Barrage in reducing flood risk due to the sea level rise are demonstrated.Impact of the sea level rise on the performance of the barrage was investigated.

Forecasting of Surface Currents via Correcting Wind Stress with Assimilation of High-Frequency Radar Data in a Three-Dimensional Model

This paper details work in assessing the capability of a hydrodynamic model to forecast surface currents and in applying data assimilation techniques to improve model forecasts. A three-dimensional model Environment Fluid Dynamics Code (EFDC) was forced with tidal boundary data and onshore wind data, and so forth. Surface current data from a high-frequency (HF) radar system in Galway Bay were used for model intercomparisons and as a source for data assimilation. The impact of bottom roughness was also investigated. Having developed a “good” water circulation model the authors sought to improve its forecasting ability through correcting wind shear stress boundary conditions. The differences in surface velocity components between HF radar measurements and model output were calculated and used to correct surface shear stresses. Moreover, data assimilation cycle lengths were examined to extend the improvements of surface current’s patterns during forecasting period, especially for north-south velocity component. The influence of data assimilation in model forecasting was assessed using a Data Assimilation Skill Score (DASS). Positive magnitude of DASS indicated that both velocity components were considerably improved during forecasting period. Additionally, the improvements of RMSE for vector direction over domain were significant compared with the “free run.”