The Scottish Shelf Model. Part 2: Pentland Firth and Orkney Waters Sub-Domain

Part 2 of the hydrodynamic model developed for Scottish waters.

6 Summary and Conclusions

6.1 Introduction

The aim of this report is to describe the setting up, calibration and application of a three dimensional hydrodynamic and a wave model of the Pentland Firth and Orkney Waters ( PFOW). These models have been developed so that they can become community models for further development and application by Marine Scotland and other partners. In the present study, the models have been calibrated against available data; they have been used to simulate one full year (hydrodynamic model run for a full year with climatological forcing, while the wave model is run for a representative year) and they have been used to provide estimates of tidal and wave energy resources in the PFOW.

The FVCOM model has been used for this study for both the hydrodynamic and wave models. This model was chosen because of its capabilities as well as it being freely available, which then fulfils the aim for them to become community models.

Due to the exposed nature of the area around the north of Scotland and the Orkney Waters this is an ideal area for wave energy extraction. Similarly due to the nature of the high current speeds flowing through the Pentland Firth and the islands that make up the Orkneys, a lot of focus has been put on the potential for energy extraction from these strong currents. These models provide estimates of the energy available from these two sources as well as a means to determine the net effect of future deployments of such devices.

6.2 Hydrodynamic model

The PFOW hydrodynamic model was setup using bathymetry taken from a number of sources, from the freely available but coarser EMODnet/ NOOS data, to the UKHO and Marine Scotland higher resolution datasets. Where data from these sources was not readily available, Admiralty Charts were digitised (with permission from the Hydrographic office) to fill in any gaps. All bathymetry was reduced to mean sea level as the common datum.

The model mesh was initially setup using the mesh generator which forms part of the MIKE by DHI suite, although this was later converted into an SMS format mesh so that the quality of the mesh could be adjusted to fit in with the requirements of FVCOM. The mesh used spherical coordinate system (latitude and longitude). The model was run with 10 vertical sigma layers.

An analysis of the data available for forcing the hydrodynamic ( HD) model provided three main periods for calibration and validation. These were in 2001, 2009 and 2012 which aligned with suitable data for comparison. The simulation in 2001 was aimed at calibrating the hydrodynamic part of the model, whereas the 2009 was for comparison of the baroclinic version of the model with all forcing/inputs available. Datasets existed for calibration and validation of the model in the form of timeseries of water levels and current speeds as well as transects recording currents across either end of the Pentland Firth. Additionally temperature and salinity profiles were available for comparisons with the model.

Boundary conditions for water levels, depth-averaged currents, temperature and salinity were taken from the Atlantic Margin Model ( AMM) developed by NOC-L. Water levels and currents were provided at hourly intervals, whereas the temperature and salinity were provided at daily intervals for each of the 40 layers in the AMM. Meteorological forcing was provided by NOC-L and derived from the Met Office model. The heating input was calculated internally by FVCOM rather than provided externally. This was found to provide the best results for sea surface temperature. River flow data was provided by CEH from their Grid to Grid model. Salinity was set at 0 psu, and temperature at 7 degrees Celsius which was felt appropriate for the observed sea water temperatures.

The model was initially driven by water level boundaries alone, however it proved to be very difficult to get a stable model when temperature and salinity were included as well as the 10 layers required. After experimentation, a nested boundary approach was used which applies current speed at the boundaries in addition to the water level, temperature and salinity, this proved to make the model much more stable and usable.

Comparisons between the model results and measurements of water level and current speeds showed generally good agreement. Comparisons of the 10 layer baroclinic model showed that salinity comparisons with data were generally within the 1 psu in line with our target. Temperature was within 1 degree Celsius, although our target was to be within 0.5 degrees, however much of this difference was due to the AMM derived boundary conditions which also exhibit temperatures that are too high.

One requirement of this study was to produce a one-year climatic run based upon climatological forcing to represent a typical annual cycle. Mean boundary forcing for water levels (mean yearly tides), currents, temperature and salinity were taken from the Scottish Waters Shelf Model climatology results. An efficient method was developed to interpolate the forcing data onto the nested boundary nodes and elements. River climatology was also provided by CEH and used for this study following analysis by NOC-L. Meteorological forcing was derived by NOC-L from ECMWF (ERA-Interim) data to provide monthly mean wind-stress, pressures, heating and evaporation minus precipitation from the period 1981-2010.

Average monthly temperature and salinity simulated by the model were compared against sea surface temperature and salinity climatological datasets and residual currents for the months of February and August; the results compared well with this data.

6.3 Wave model

The objective for the wave model was to construct a calibrated and validated wave model for the Pentland Firth and Orkney Waters, and subsequently use the model to carry out simulation for an idealised year and determine wave energy resources. After discussion with Marine Scotland and analysis of available data, it was decided that a representative year approach should be used for the idealised year.

The model mesh was derived from the one used for the hydrodynamic model, although the resolution was reduced in order to bring model run times to practical limits. Boundary wave data at four locations were obtained from the UK Met Office wave models for the period 2000-2012 whilst wind data was obtained from the UK Met Office forecast models that had been archived at NOC-L.

The wave model was calibrated against measured wave data at Scapa Flow, Dounreay and Holm Sound; the model results are satisfactory at Scapa Flow and Holm Sound, but generally under-predicts wave height at Dounreay (negative bias). At all three locations, the correlation is greater than 0.85. Suggestions are made in the summary and further work section on how to reduce the negative bias and improve the correlation.

Analysis of wave energy flux was used to determine a representative year (2005) upon which wave simulation for the entire year were carried out. The results from this simulation were used to calculate wave power and compared against results from previous studies. Additionally the effect of wave-current interaction was also assessed which showed that the effect accounted for less than 10% effect upon the wave height.

Suggestions are also included in this report on how to include energy extraction devices (waves and currents) into the FVCOM model.


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