Scottish Shelf Model. Part 4: East Coast of Lewis and Harris Sub-Domain

2 Available data for model development

2.1 Introduction

In order to carry out the numerical modelling works for the East Coast of Lewis and Harris ( ECLH), the following data have been collated:

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

This section of the report describes the data collated for the East Coast of Lewis and Harris ( ECLH) model area. Where appropriate, reference is made to the overall project data review report (Halcrow, 2012). 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. 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.

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 Isles (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 although this does not affect this model.

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.

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 in the vicinity of the ECLH model domain is shown in Figure 2‑3 where it has been indicated on top of the EMODnet data.

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 NOOS 1.0

NOOS 1.0: A gridded dataset for the UK continental shelf at 1 arc-minute resolution was produced under the aegis of NOOS (an operational oceanography organisation for the NW European Shelf (see Halcrow, 2012 for more information). The NOOS bathymetry incorporates local datasets made available by oceanographic institutions in countries around the North Sea, however no detailed source attribution information is available for the bathymetry, and it was last revised in 2004. Bathymetric surveys collected by the UKHO post 2004 are therefore not incorporated in to the bathymetry, and it is uncertain to what extent earlier UKHO and other national hydrographic office datasets were incorporated.

After consideration of this data and comparison against other datasets (Halcrow, 2012) it was concluded that the NOOS bathymetry should not be used west of 0°E and has therefore not been used for the ECLH model.

2.2.5 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 ECLH model.

2.2.6 Summary of bathymetry data availability for the East Coast of Lewis and Harris Area

Figure 2‑3 shows data availability for the east coast of Lewis and Harris model. High resolution bathymetric survey is available for the east coast of the islands with the exception of the section between the Point of Ness and Stornoway. There are numerous soundings in the ICES database in this region, and the area is covered by Admiralty Charts. The GEBCO_08 bathymetry in this region is mainly derived from soundings, and is more likely to be accurate based on comparisons with other areas where UKHO data is available. ICES sounding data and Admiralty Charts should be used to increase the resolution between Point of Ness, Stornoway and the mainland above that in the EMODnet bathymetry.

For the wider model the UKHO data has not yet been incorporated in the EMODnet bathmetry for the area between Lewis and the mainland and the area between Lewis and the St Kilda and the bathymetry is largely GEBCO_08. West of Benbecula, South Uist, Barra and Mingulay few observations are incorporated into the GEBCO_08 bathymetry. Where ship soundings have been used ship tracks are clearly visible in the GEBCO_08/ EMODnet bathymetry. As an example, the bathymetry is approximately 30m (50%) lower west of South Uist where the ship tracks are present than where the bathymetry is based on interpolation (compare with Figure 2‑2). The planned Civil Hydrography programme survey of Barra should significantly improve accuracy of the bathymetry in this area, however these data were not available within our programme.

Use of the EMODnet bathymetry where it is derived from the GEBCO_08 grid is not appropriate for the area west of Noth Uist to Mingulay. Therefore digitised Admiralty Chart data were used and ICES depth soundings were used to adjust the GEBCO_08 data in this region.

To summarise, there appears to be reasonable coverage to the east of Lewis and Harris however this does not necessarily completely cover the shallower nearshore areas ( UKHO and ship tracks). For example, ship tracks cover the lochs of interest (Lochs Erisort, Ouirn, Shell, Seaforth, Scalpay and East Loch Talbert) where Farm Management Areas ( FMA) exist, but do not cover them in their entirety in shallower areas. There are little survey data to the west and southwest of Uist. Additional bathymetry data was digitised from Admiralty Charts in areas of interest and close to the shore where higher resolution was required and not available in the existing data.

2.3 Forcing Data

2.3.1 Introduction

Forcing data is required for a six month climatological model run of the ECLH flow model and for calibration using observed data for approximate 1 month periods. 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

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 was 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. A detailed description of the methodology used to derive the climatology forcing is provided in the Scottish Shelf Waters Model report (Wolf et al. 2015).

2.3.3 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.

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.

2.3.4 Tide

For the ECLH Model, the boundary data was derived from NOC-L's Atlantic Margin Model ( AMM) with a 12km resolution. Water levels along with temperature and salinity timeseries were 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 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 and salinity. In addition validation is required for the 6 month climatological runs against accepted general flow characteristics including current speed and direction (seasonal variability) and seasonal temperature and salinity cycles.

2.4.2 Water Level

Figure 2‑4 shows all the locations of water level observations that are available in the ECLH region. These come from three main sources: tide gauge data from the BODC National Oceanographic Database ( NODB); bottom pressure data from the NODB, and analysed tidal data from NOC.

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 2‑5. 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 2‑6.

2.4.3 Currents

Datasets on currents have been found from a number of sources; all locations are shown in Figure 2‑7. These come from the BODC National Oceanographic Database ( NODB) and the TotalTide software, from UK Hydrographic Office. As Figure 2‑8 shows, there are only a few datasets from the BODC National Oceanographic Database since year 2000. In some cases, vertical current profiles are available; these are shown in Figure 2‑9.

The methodology used by TotalTide for calculating currents is not known. 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.

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 2‑10.

Direct measurements of current speed and direction at a number of fish farms are available within the area of interest, i.e. the east coast of Lewis and Harris (Figure 2‑11). Not all of these data set were suitable to use for calibration purposes, being in shallow water or outside of the calibration period. The data set selected for calibration purposes at 13 sites are shown in Figure 2‑12 along with information on the sampling period and location. Two of the sites selected are not associated with fish farms and the measurements were made by Marine Scotland at the request of Halcrow, these are LMSL (Little Minch Seaforth Loch) and LMO (Little Minch Offshore), taken at the mouth of Seaforth Loch and Offshore of Seaforth Loch respectively.

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.

Figure 2‑13 shows the locations of the temperature observations and Figure 2‑14 shows the locations of the salinity observations. As Figure 2‑15 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 two years. Figure 2‑16 shows which of these observations include profiles over the entire water depth. Most temperature and salinity observations occurred at the same location and time. Figure 2‑17 and Figure 2‑18 show there are some temperature and salinity profiles within the model domain, both during the 1997 and 2001 periods when some current observations are also available.

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 ECLH model

This section summarises the availability of calibration and validation data for the ECLH 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 ECLH model.

Table 2‑1 Case Study models and available data

Sub model	Year	Water level	Currents	Temperature /salinity	Meteorological	Wind	River
East coast of Lewis and Harris	1997	√	√	√	X	√	X
	2001	√	√	√	X	√	X
	2009 + other years	√	X	√	√	√	√

Tide gauge data have been obtained from Stornoway, Ullapool and Kinlochbervie. Figure 2‑6 and Figure 2‑8 show that many recent water level and current datasets exist near the west coast of the Scottish mainland. However most of the post-2000 current datasets are located close to the shore of the Scottish Mainland rather than the Outer Hebrides. No current measurements have been identified along the east coast of Lewis and Harris, or through the Sound of Harris. However there are three measurements across the northern entrance to the Minch, one between Skye and Benbecula, and another east of Skye. Tidal diamonds are effectively the only source of information for calibrating near shore currents in this region.

Figure 2-11 shows available fish farm current data and the Marine Scotland survey data locations. Figure 2‑17 and Figure 2‑18 show the available current observations, which occurred between April and July 1997, and between April and July 2001, with temperature and salinity observations occurring during the same time period. There is poor overlap between current and temperature and salinity observations; there are many CTD casts are available for 1992, 1998, 2002, 2004, 2005, 2006, 2007, 2008 and 2009 for the sea lochs on the east coast of Lewis and Harris, but CTD casts for 1997 and 2001 are some distance off shore.

The quantity of available datasets suggests they should be sufficient to calibrate for currents in the Minch, and for temperature and salinity inshore. However, multiple calibration periods may be required as current and inshore temperature and salinity data are not available for concurrent periods. Furthermore meteorological forcing data for temperature and salinity has only been archived at NOC-L post 2007 (going back to 2004 with some gaps).

2.5 Conclusions and Recommendations

A review has been undertaken to identify and obtain data that are relevant to the setting up, forcing and calibration of the ECLH model. It has been found that there are 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 and other higher resolution datasets from ICES and Marine Scotland 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 NOC-L AMM mode to provide boundary conditions to the ECLH model.

Meteorological forcing for the ECLH 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. Due to the lack of full meteorological forcing during many of the potential calibration periods, all calibration and validation runs will be during 2009, although no current measurements are available for this period harmonic analysis of the results can be carried out for comparison with observed data.

Fluvial inputs were derived from G2G river flow data obtained from CEH for the ECLH area.

2.5.3 Calibration and validation data

Section 2.4 presents information about which data were available for the ECLH model. In general there was sufficient data with which to undertake calibration for water level, currents, temperature and salinity. A summary of the dates where suitable calibration and validation data is available is provided in Table 2‑1.