Scottish Shelf Model. Part 3: St Magnus Bay Sub-Domain

2 Available data for model development

2.1 Introduction

In order to carry out the numerical modelling works for the St Magnus Bay area ( SMB), the following data have been collated and/or identified:

• Bathymetry data, required for creating the bathymetry for the numerical model.
• Forcing data, required for specifying the forcing conditions in the numerical wave and flow models.
• Calibration and validation data, required for calibrating and validating the numerical models.

This section of the report describes the data collated/identified for the SMB model area. Where appropriate, reference is made to the overall project data review report (Halcrow, 2012) and the Pentland Firth and Orkney Waters modelling report (Price et al. 2015). As the SMB model is set within the PFOW model area, there are common datasets being used by both models as time was spent during the PFOW model setup to make sure data was also suitable for the SMB model. Note that the proposed model domains shown in this section are not the final model domains but an approximation.

2.2 Bathymetric Data

2.2.1 Coastline Data

Two coastline data sets have been obtained for use in this study the Global Self-consistent, Hierarchical, High-resolution Shoreline ( GSHHS) distributed by National Geophysical Data Centre ( NGDC) in the US, and Ordnance Survey Mapping.

The GSHHS coastline comes in different resolutions. For the UK, the best resolution available is the World Vector Shoreline ( WVS) designed to be used at a resolution of 1:250,000. The GSHHS coastlines have been data processed to ensure they are free of internal inconsistencies such as erratic points and crossing segments.

The Ordnance Survey ( OS) Vector Map District contains tidal boundary polylines, which are at Mean High Water Spring level ( MHWS) in Scotland and MHW in England and Wales. These are at higher spatial resolution than the GSHHS shoreline dataset. The GSHHS data is considered appropriate for use in areas where the model resolution is coarse, the OS vector map district MHWS line should be used in areas of higher resolution, such as for St Magnus Bay.

2.2.2 Global/Regional Gridded Data Sets

Three existing coarse resolution bathymetry data sets have been identified which cover the study area the GEBCO_08, the ETOPO-1 grid and the EMODnet grid. These are described briefly below. Details regarding these datasets are provided in Halcrow (2012).

2.2.2.1 General Bathymetric Chart of the Oceans ( GEBCO)

The GEBCO_08 data set is a global DTM at 0.5 minute resolution generated from a database of bathymetric soundings with interpolation between soundings guided by satellite-derived gravity data. The dataset is produced by GEBCO ( http://www.gebco.net).

Known errors or discontinuities in the data set occur between regions where data is derived from satellite data and detailed bathymetric survey - this is evident in a grid pattern in the Southern North Sea Region, and a discontinuity at 0°E. Marine Scotland has highlighted errors where false banks occur on the shelf around the Shetland Island (Hughes, 2014).

Figure 2-1 shows the GEBCO_08 bathymetry for the British Shelf and the source of the data. The discontinuity at 0°E and the grid pattern in the North Sea are clearly visible. There does not appear to be any discontinuities in the immediate area of St Magnus Bay.

2.2.2.2 ETOPO-1

ETOPO-1 is a global DTM at 1 minute resolution produced by NOAA National Geophysical Data Center. The documentation states that this uses the GEBCO_08 data set for the British Shelf. Due to the lower resolution this dataset has not been considered further.

2.2.2.3 European Marine Observation and Data Network ( EMODnet)

The European Marine Observation and Data Network ( EMODnet) have produced DTMs for the Greater North Sea and Celtic Seas at 0.25 minute resolution (about 250m east-west direction and 450m north-south directions). The grids are based on bathymetric surveys and terrain models developed by external data providers including the UK Hydrographic Office ( UKHO), and the GEBCO_08 Grid 0.5 minute resolution dataset where no other data is available. Data sets are made available through the EMODnet website http://www.emodnet-hydrography.eu/

Further details of EMODnet are provided in Halcrow (2012).

Figure 2-2 shows where UK Hydrographic office data has been incorporated into the EMODnet dataset and the differences between the EMODnet and GEBCO_08 bathymetry. Comparison of the EMODnet and GEBCO_08 data sets shows significant differences where the data from the UKHO and other hydrographic offices has been included. Differences are generally greater in areas where the GEBCO_08 has been interpolated, and the UKHO data has been used in the EMODnet bathymetry, for example around 1.5°W 56.3°N, due east of the Firth of Tay. The large differences west of Norway are due to incorporation of Norwegian hydrographic office data. There are also differences north west of the British Shelf around Iceland, where the EMODnet data is sourced from the GEBCO_08 grid. However these have not been investigated as they are not considered important for the study area. It can be seen in Figure 2-2 that there is more detailed bathymetry in St Magnus Bay from UKHO data.

Due to the inclusion of the majority of the UKHO data, the EMODnet bathymetry is considered appropriate for use as the base bathymetry for model construction in areas where the resolution was in the order of one kilometre. Higher resolution bathymetry data is however required in areas where the model mesh is finer to represent bed or flow features. Therefore other datasets are required as described below.

2.2.3 Hydrographic Data

Three sources of hydrographic survey data have been identified; the United Kingdom Hydrographic Office ( UKHO), the International Council for Exploration of the Sea ( ICES) and Marine Scotland's data sets.

The UKHO have a memorandum of understanding with Marine Scotland making their high resolution bathymetric survey available. Most of these data have already been incorporated into the EMODnet bathymetry, however further data has since become available. The location of the UKHO data is shown in Figure 2-3.

The ICES surface dataset holds over 100 years of ship based observations, including soundings. There are over 2 million data points in the ICES data set within the study area, providing a good coverage over most areas. The ICES website ( http://ocean.ices.dk/) states that data are quality controlled by contributing organisation and visually inspected by experienced staff to further improve the quality of these data. However it is expected that due to the age of some of the sounding data and the differences in measurement methods, data logging and processing that there may be significant differences or scatter between the soundings. Marine Scotland used the ICES dataset to identify and correct anomalies in the GEBCO_08 data set off the coast of Shetland. See Halcrow, (2012), for more detail regarding hydrographic data and the differences observed between datasets.

2.2.4 Other data sources

Other identified data sources include digital Admiralty charts ( C-MAP) and SeaZone. However, these datasets were not used for this study due to licensing restrictions as discussed fully in Halcrow (2012). A licence enabling Halcrow to digitise the required Admiralty Charts was obtained from the Hydrographic Office and the digitising undertaken. This allows the data to be used into the future for this project without paying a licence fee every year. The digitised Admiralty Charts are used to fill the gaps in the digital bathymetry data available for the SMB model.

2.2.5 Summary of bathymetry data availability for the St Magnus Bay Area

A map of data availability for the Shetland Islands, including the proposed model domain in and around St Magnus Bay, is shown in Figure 2-4. For the Shetland Islands there is no high resolution data east of the Mainland and through the Yell Sound, however this area is not within the SMB model domain. UKHO bathymetry data does exist within St Magnus Bay and was used in preference to other datasets, Admiralty Chart data being the second preference followed by the coarser EMODnet data. Admiralty Chart data is required in the margins of the Bay and in the smaller channels.

To summarise, there appears generally to be sufficient bathymetry data in the open water areas, however there is limited data in the smaller channels. These gaps have been filled with data obtained by digitising the appropriate Admiralty Charts (after first obtaining a licence to do so from the Hydrographic Office).

2.3 Forcing Data

2.3.1 Introduction

Forcing data is required for a six month climatological model run of the SMB flow model and for calibration using observed data for approximate 1 month period. The following forcing data is required;

• meteorological - including wind speed/stress, atmospheric pressure, surface heat flux, precipitation and evaporation
• hydrological - river flux
• oceanic open boundaries - including temperature, salinity and velocity
• tides

As the SMB model lies wholly within the PFOW model domain, the boundary conditions came directly from that model for the calibration runs and from the shelf model for the climatology.

2.3.2 Meteorological forcing

2.3.2.1 UK Met Office Model Data

Two data streams from the Met Office forecast models have been archived at NOC (Liverpool) for operational modelling:

• for operational tide-surge modelling on the continental shelf, using the 2d tide-surge model ( CS3 and CS3X).
  • These data comprise of surface wind and atmospheric pressure only, at 1-h intervals, from May 1991 to present. From 1991 to 1995 the data is at 50 km resolution, post 1995 the data is at 12 km resolution.
• for Irish Sea Observatory operational modelling system, running the 3d baroclinic hydrodynamic model, POLCOMS, on (i) the Atlantic Margin Model ( AMM, ~12km) and (ii) the nested Irish Sea model ( IRS, ~2km). The data comprise the following, from 2004 to 2007 with some gaps, and continuously from 2007 to 2011, all at 12 km resolution:
  • Global model output for the Atlantic at 6-hour intervals - 10m wind (E and N components); sea level pressure; low, medium and high level cloud coverage; specific humidity at 1.5m, air temperature at 1.5m; total accumulated precipitation; sensible heat flux.
  • Mesoscale model output at 3-hour intervals - same variables.

2.3.2.2 Climatological Forcing

Climatological forcing could be derived from the ERA40 and ERA-Interim datasets, which were used to force the POLCOMS AMM (~12km) model for the 45 year hindcast (1960-2004). See Wakelin et al. (2012) and Holt et al., (2012). A licence to use these data has been provided by the European Centre for Medium range Weather Forecasting ( ECMWF) for this study. A one-year climatological forcing for the temperature and salinity ( i.e. heat flux and precipitation) has been derived.

2.3.3 Meteorological observations

The Marine Scotland Science survey vessel MRV Scotia undertook two surveys for this project, including one in St Magnus Bay, Shetland (October 2012). During these surveys wind measurements were made from the vessel.

2.3.4 Hydrological Data (Fresh Water Inflows)

In order to simulate the effect that river flow has upon salinity in coastal waters, river flux data are required. The Centre for Ecology and Hydrology ( CEH) Grid-to-Grid ( G2G) model was used to supply freshwater inflows to the various coastal models for this study. For the SMB model the G2G model is being extended to provide conditions for the Shetland Isles which were not available in the existing dataset.

The output that CEH provided from the G2G model were

1. Provision of river discharge data (time series data) at all coastal locations in Scottish waters with the G2G model. The data was supplied for a period covering 1 March 2007 to 30 September 2010 at 15 minute intervals.

2. Provision of river discharge data (time series data) at all coastal locations around Shetland and Northern Ireland with the G2G model. The data was supplied for a period covering 1 March 2007 to 30 September 2010.

3. Provision of river discharge climatological data (long term daily/seasonal discharge data) at all coastal locations for Scotland (including Shetland) and Northern Ireland with the G2G model. Daily averaged data was provided, the averaging period covered 1962-2011.

2.3.5 Tide

For the SMB Model, the boundary data was derived from the PFOW model which in turn gets its boundary data from NOC-L's Atlantic Margin Model ( AMM) with a 12km resolution. Water levels along with temperature and salinity timeseries was applied at the model boundaries for specific periods coincident with times that calibration data is available. Climatological runs were forced using shelf model climatology results whose boundary conditions were taken from the results of the POLCOMS model hindcast from 1960-2004, which was run on the AMM 12km grid. This is available for monthly means but also held in-house at NOC-L as daily mean 3D temperature and salinity and current residual fields, together with hourly barotropic currents and elevations.

2.4 Calibration Data

2.4.1 Introduction

Model calibration was undertaken against observation datasets for periods of up to 1 month. Calibration is required for water level, currents, temperature, salinity and surface waves. In addition validation is required for the 1 year climatological runs against accepted general flow characteristics including residual current speed and direction (seasonal variability) and seasonal temperature and salinity cycles.

2.4.2 Water Level

Figure 2-5 shows all the locations of water level observations that are available in the SMB region. These come from three main sources: tide gauge data from the BODC National Oceanographic Database ( NODB) and bottom pressure data from the NODB, analysed tidal data from NOC. Those data which are available post year 2000 are shown in Figure 2-6.

In addition, we have access to tidal data from TotalTide - a digital version of the UK Admiralty tide tables, from the UK Hydrographic Office. The locations of these datasets are shown in Figure 2-7. Because these data are based on harmonic analyses, water level estimates for any past or future date are obtainable, or via the use of constituents from the Admiralty tide tables. All available water level data available post year 2000 are shown in Figure 2-8.

2.4.3 Currents

Datasets on currents have been found from a number of sources; locations of many of these are shown in Figure 2-9. These come from the BODC National Oceanographic Database ( NODB) and the TotalTide software, from UK Hydrographic Office. As Figure 2-10 shows, there are only a few datasets from the BODC National Oceanographic Database since year 2000. In addition, some of these datasets (shown in red) may not be freely available. In some cases, vertical current profiles are available; these are shown in Figure 2-11.

Fish Farm data was also obtained from Alan Hills of SEPA. This data consisted of 30 days of current measurements at three depths (surface, mid and bottom). This data has proved to be useful for comparison with the model within the SMB area.

The methodology used by TotalTide for calculating currents is not known exactly but is likely to be an interpolation of tidal diamond chart data to cover different range tides. In addition, these data have been estimated for the use of shipping; therefore, a greater weighting may be placed on surface currents than currents near the sea bed.

Additionally the MRV Scotia collected current and CTD measurements in and around St Magnus Bay in Shetland in October 2012 (shown as ADCP Data from MRV Scotia on Figure 2-9, 2-10 and 2-11 with more detail shown on Figure 2-12). These data are considered essential for the calibration of the SMB model.

The Atlas of UK Marine Renewable Energy Resources ( www.renewables-atlas.info) contains information on peak tidal current speeds over a mean spring and a mean neap tide. The dataset was derived from the POL HRCS Model, with peak spring and neap current speeds calculated from the major 2 or 4 tidal harmonics. Although this dataset is limited, it is freely available on a 0.0167° x 0.025° (latitude x longitude) grid throughout the region shown in Figure 2-13.

2.4.4 Temperature and Salinity

Temperature and salinity validation was carried out using selected hydrographic stations which were identified from the British Oceanographic Data Centre data holdings for UK. There are a very large number of datasets from CTD and bottle casts, both from the BODC National Oceanographic Database and the ICES database. Additionally, some of the CEFAS WaveNet buoys record sea surface temperature. Additionally the Marine Scotland survey in SMB also provides temperature and salinity data, which together with the ADCP data was the data used for model calibration.

Figure 2-14 shows the locations of the temperature observations and Figure 2-15 shows the locations of the salinity observations. As Figure 2-16 shows, the temperature and salinity observations have occurred throughout the last two decades, with many observations throughout all model domains having occurred over the last few years. Figure 2-17 shows which of these observations include profiles over the entire water depth. Most temperature and salinity observations occurred at the same location and time.

In addition, the Ocean Data analYsis System for SEA ( ODYSSEA) dataset is a re-analysis of satellite observations of sea surface temperature. Daily mean average sea surface temperatures since 01/10/2007 have been obtained, on a 0.1° x 0.1° grid.

The results from the climatic run were compared with climatological atlas information for temperature and salinity, from the World Ocean Atlas ( WOA) and International Council for Exploration of the Seas ( ICES) climatological datasets.

2.4.5 Summary of data availability for the SMB model

This section summarises the availability of calibration and validation data for the SMB model area and identifies any gaps in the available data. Furthermore, recommendations are made on how to fill the gaps.

Table 2‑1 summarises the available current, temperature/salinity and Meteorological/river flow data available for calibration of the SMB model. It shows that for all three years sufficient data exists for tidal hydrodynamic calibration, however 2009 is the only year suitable for the baroclinic calibration. Calibration of the model is carried out using 2012 while 2009 and 2001 are used for temperature/salinity and tidal currents validation respectively.

Table 2‑1 Case Study models and available data

Sub model	Year	Water level	Currents	Temperature /salinity	Meteorological	Wind	River
St Magnus Bay	2001	√	√	√	X	X	X
	2009	√	√	√	√	√	√
	2012	√	√	√	X	√	X

2.5 Conclusions and Recommendations

A review has been undertaken to identify and in many cases request / obtain data that are relevant to the setting up, forcing and calibration of the SMB model. It has been found that there are many datasets available providing coverage over a wide spatial and temporal field.

2.5.1 Bathymetry

The EMODnet data is considered appropriate for use as the base bathymetry for model construction. This data forms our base coarser resolution data but is supplemented with higher resolution data.

Further UKHO data have been used to replace the coarser resolution data in areas that they overlap, with appropriate checks for consistency. However even with these data there are areas which have been identified in the data review report (Halcrow, 2012) as not having sufficient bathymetry data at a fine enough resolution. In this case data from digitised Admiralty Charts have been used.

2.5.2 Forcing data

For this case study tidal forcing, temperature and salinity data have been obtained from the PFOW model which in turn obtained its boundary conditions from the NOC-L AMM model.

Meteorological forcing for the SMB model was derived from the Met Office model data that NOC-L holds. The Met Office data provides wind data from 1991 to present day, however other parameters such as sea level pressure, low, medium and high level cloud coverage, specific humidity at 1.5m, air temperature at 1.5m, total accumulated precipitation and sensible heat flux are only available from 2007 to 2011. This therefore limits the periods where calibration data are available coincident with full meteorological forcing. Therefore for the model calibration only wind forcing was available (coincident with the MRV Scotia data in St Magnus Bay).

Fluvial inputs were derived from G2G river flow data obtained from CEH for the SMB area for the 2009 validation. Additional G2G runs were undertaken to provide missing data in Shetland although this is not coincident with the 2012 survey but was useful for climatological simulations.

2.5.3 Calibration Data

Section 2.4.5 presents information about which data are available for the SMB model. In general there is sufficient data with which to undertake calibration for, water level, currents, temperature and salinity, with the main period for comparison being in October 2012 when the MRV Scotia made her measurements. In summary we believe that there are sufficient data for the calibration of the SMB model.