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Cowes Harbour Commission English Channel Regional Hydrodynamic Model Calibration Report R.2492 March 2016

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Cowes Harbour Commission English Channel Regional Hydrodynamic Model Calibration Date: March 2016 Project Ref: R/4327/1 Report No: R.2492 © ABP Marine Environmental Research Ltd

Version Details of Change Date 1 Issue for Client Comment 15.09.2015 2 Final version reflecting review comments and inclusion of abbreviations list 30.03.2016

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30.03.2016

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30.03.2016

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30.03.2016

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English Channel Regional Hydrodynamic Model Calibration

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English Channel Regional Hydrodynamic Model Calibration Contents

Page

1. Introduction ................................................................................................................................. 7 2. Model Configuration ................................................................................................................... 9

2.1 Software ........................................................................................................................ 9 2.2 Model Mesh – Extent ..................................................................................................... 9 2.3 Model Mesh – Location of Open Boundaries ............................................................... 10 2.4 Model Mesh – Resolution ............................................................................................ 10 2.5 Model Bathymetry ........................................................................................................ 14 2.6 Open Boundary Conditions .......................................................................................... 18 2.7 Bed Roughness ........................................................................................................... 19

3. Model Calibration and Validation .............................................................................................. 22 3.1 Guidelines ................................................................................................................... 22 3.2 Calibration and Validation Data Sources ..................................................................... 24

3.2.1 Water Level Data Sources ......................................................................................... 24 3.2.2 Current Data Sources ................................................................................................ 27

3.3 Calibration of Water Levels .......................................................................................... 28 3.4 Validation of Water Levels ........................................................................................... 39 3.5 Validation of Currents .................................................................................................. 46

4. Summary .................................................................................................................................. 52 5. References ............................................................................................................................... 53 6. Abbreviations ............................................................................................................................ 54 Appendix A. Bathymetry Review Tables 1. Values of C100 ........................................................................................................................... 20 2. Summary of water level data sources ....................................................................................... 25 3. Summary of current data .......................................................................................................... 28 4. Water level calibration statistics for a spring neap period ......................................................... 32 5. Water level calibration statistics for a spring period .................................................................. 32 6. Water level calibration statistics for a neap period .................................................................... 32 7. Water level validation statistics for a spring neap period .......................................................... 40 8. Water level validation statistics for a spring period ................................................................... 40 9. Water level validation statistics for a neap period ..................................................................... 40 10. Current speed and direction validation statistics for a spring neap period ................................ 49


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Figures 1. Model Extent ............................................................................................................................. 10 2. Model Mesh Resolution – Whole Model Extent: North Sea and English Channel .................... 11 3. Model Mesh Resolution – Detail: Approaches to the Solent ..................................................... 12 4. Model Mesh Resolution – Detail: East and West Solent and Southampton Water ................... 13 5. Model Mesh Resolution – Detail: Cowes Harbour and Approaches ......................................... 14 6. Model Bathymetry – Whole Model Extent: North Sea and English Channel ............................. 15 7. Model Bathymetry – Detail: Approaches to the Solent .............................................................. 16 8. Model Bathymetry – Detail: East and West Solent and Southampton Water ............................ 17 9. Model Bathymetry – Detail: Cowes Harbour and Approaches .................................................. 18 10. Model Bed Roughness (Mannings M) ....................................................................................... 21 11. Locations of Model Calibration and Validation Data Sites ........................................................ 26 12. Modelled Tidal Water Level Co-phase Contours from an Approximately Mean Spring

Range Period ............................................................................................................................ 29 13. Modelled Tidal Water Level Co-range Contours from an Approximately Mean Spring

Range Period ............................................................................................................................ 30 14. Water Level Calibration at Whitby ............................................................................................ 33 15. Water Level Calibration at Devonport ....................................................................................... 34 16. Water Level Calibration at Lowestoft ........................................................................................ 34 17. Water Level Calibration at Dover .............................................................................................. 35 18. Water Level Calibration at Newhaven....................................................................................... 35 19. Water Level Calibration at Weymouth ...................................................................................... 36 20. Water Level Calibration at Bournemouth .................................................................................. 36 21. Water Level Calibration at Portsmouth ..................................................................................... 37 22. Water Level Calibration at Calshot ........................................................................................... 37 23. Water Level Calibration at Dock Head ...................................................................................... 38 24. Water Level Calibration at Lymington ....................................................................................... 38 25. Water Level Validation at Whitby .............................................................................................. 41 26. Water Level Validation at Devonport ........................................................................................ 41 27. Water Level Validation at Lowestoft ......................................................................................... 42 28. Water Level Validation at Dover ............................................................................................... 42 29. Water Level Validation at Newhaven ........................................................................................ 43 30. Water Level Validation at Weymouth ........................................................................................ 43 31. Water Level Validation at Bournemouth ................................................................................... 44 32. Water Level Validation at Portsmouth....................................................................................... 44 33. Water Level Validation at Calshot ............................................................................................. 45 34. Water Level Validation at Dock Head ....................................................................................... 45 35. Water Level Validation at Lymington ........................................................................................ 46 36. Tidal Stream Atlas of the English Channel high water-5 to High Water as Calculated by

the Model. Phase Relative to high water Dover ........................................................................ 47 37. Tidal Stream Atlas of the English Channel high water +1 to High Water +6 as Calculated

by the Model. Phase Relative to high water Dover ................................................................... 48 38. Current Speed and Direction Validation at the ‘Dover’ Site ...................................................... 50 39. Current Speed and Direction Validation at the ‘West IOW’ site ................................................ 50 40. Current Speed and Direction Validation at the ‘Channel’ Site ................................................... 51 41. Current Speed and Direction Validation at the ‘East IOW’ site ................................................. 51


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1. Introduction

Following the partial construction of the Cowes Outer Breakwater, ABP Marine Environmental Research Ltd (ABPmer) has been commissioned by Cowes Harbour Commission (CHC) to develop an enhanced and robust numerical simulation tool (tidal model) for Cowes. Once completed, the model will be used to simulate changes to patterns of currents and sedimentation for various design options for development of the harbour layout and infrastructure. The model will also be used to investigate apparent changes in the hydrodynamic regime of the Solent (potentially affecting flows at Cowes) over the period 2005 to 2014. The construction and performance details of the regional and local aspects of the new suite of models (hereafter ‘the new model’ or ‘the model’) are reported in: ▪ English Channel Regional Hydrodynamic Model Calibration (this report); ▪ Cowes Local Hydrodynamic Model Calibration (ABPmer, 2016a), and; ▪ Cowes Local Model Sediment Transport Model Calibration and Validation (ABPmer,

2016b). This report outlines the development and calibration of the model at a regional scale (within the whole model extent but mainly outside of the Solent). Prior to further use on behalf of CHC, this model will be further calibrated and validated within the Solent and at Cowes, which will be separately reported. To facilitate model calibration and validation, modelled water levels and currents are compared to suitable observed and other predicted data at multiple locations within the model extent. This report demonstrates the suitability of the regional model performance as a basis for further local model refinement and development within the Solent and at Cowes. The model calibrates well at most sites throughout the English Channel and captures the key hydrodynamic processes which are important to the area of interest for further local model development. This model has been developed in conjunction with recommendations and comments provided by Atkins. The updated format of the model help to overcome potential limitations of previously developed local scale models of Cowes Advantages and improvements include: i. Using the most up to date version of the numerical software. Previously, other models

were also developed using the current software version at the time, however, these have since been superseded;

ii. The new model makes use of the ‘flexible’ mesh approach (a net of triangular elements of varying size) which has become available in the newer versions of the software. This approach allows better control of local model grid alignment and spatial resolution than the previously used ‘rectilinear’ mesh approach;


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iii. The model has a relatively large extent (including parts of the North Sea and all of the

English Channel), which is required to more accurately simulate a wider range of tidal conditions within the area of interest.

iv. The description of bathymetry (water depth) throughout the new model includes many more recently collected and updated data sets. The data used are thereby more representative of the present day conditions and are typically of better spatial resolution and accuracy than was previously available;

v. Once fully developed for use in the Solent and Cowes, the new model will therefore contain an updated description of the coastline position and seabed bathymetry to include various recent development works in the area (e.g. new marina infrastructure, dredging works and (optionally) the various construction stages of the Cowes Outer Harbour Breakwater); and

vi. The new model utilises tidal harmonic constituents to define its boundaries and so can be run for any period in time. The accuracy and resolution of boundary condition data at regional scales (based on analysis of satellite altimetry data) has improved in the last decade. Previous models were somewhat restricted to simulating particular periods of time for which locally measured water levels and flow data were available. The new model will therefore be able to provide comparative simulations of past, present and future tidal conditions (in conjunction with the present day bathymetry and coastline position).

Further improvements of the model are being made with respect to the simulation of water levels, currents and sedimentation within the Solent and at Cowes. The present report demonstrates the ability of the regional model in its present state to deliver suitable boundary conditions to the Solent, as a basis for this further development. Details of the further development, including more details of local calibration and validation, will be separately reported.


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2. Model Configuration 2.1 Software

The model is built using the hydrodynamic module of DHI MIKE 21. This state of the art software package allows for the simulation of flows and water levels in complex coastal and estuarine environments. The software is under continuous development with updates routinely being released. This model has been developed using the most up to date version of the software at the time of writing (Release 2014, Service Pack 3). There have been continual improvements in the software and hardware available for and in support of tidal modelling, in the ten years since the previously used models of Cowes Harbour were originally built. These improvements, in conjunction with a greater availability of more accurate input data, improve the potential flexibility and accuracy of the model. The following sections describe the configuration of, and data inputs to the model.

2.2 Model Mesh – Extent The model domain extent is shown in Figure 1. The model domain includes the English Channel and the southern extent of the North Sea. The model is bounded by land on its western extent by the UK and on its eastern extent by France, Belgium, Netherlands, Germany and Denmark. The model extent includes a relatively large area with respect to tidal processes and so is termed a ‘regional scale’ model. Regional scale tidal models simulate the larger, regional scale tidal processes that causes and control more complex local tidal behaviour. The accuracy and resolution of boundary condition data at regional scales (based on analysis of satellite altimetry data) has improved in the last decade. In comparison, models previously used at Cowes were ‘local scale’ models, and were limited to a much smaller extent, namely, the approaches to Cowes Harbour. Local models are smaller and so require less input data during construction in terms of coastlines and bathymetry. However, local model performance is typically restricted by the duration and quality of the data that are available to inform the model boundaries and calibration. Such data either need to be informed by locally observed data, or by output from other regional scale models. Tidal behaviour (the relative timing and magnitude of tidal variance in water levels and currents) in the central English Channel and Solent region is primarily controlled by the shape of the English Channel basin, interacting with the tidal wave coming from the Atlantic. There is also significant interaction at the Dover Straits, between the tidal waves in the English Channel and in the southern North Sea. The southern North Sea tidal wave is, in turn, affected by other tidal waves/processes in the central and northern North Sea. The relatively large model extent used is required to provide the details of tidal behaviour in the area of interest, whilst enabling the simulation of any required time window and so a wider range of possible tidal behaviours.


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Figure 1. Model Extent

2.3 Model Mesh – Location of Open Boundaries The model has two open boundaries where water levels are forced to rise and fall in a pre-determined manner (representing the tidal wave along that line), so that the tidal wave then propagates through the rest of the model domain. The open boundary in the north is a line between Peterhead, Scotland and Hanstholm, Denmark. The open boundary in the south-west is a line between The Lizard, Cornwall and Porspoder, France. The open boundary (water level) conditions applied at the open boundary locations are described in Section 2.6. The two open model boundaries are deliberately located to approximately follow tidal co-phase lines (along which the phasing of the tidal signal is very similar), and at the edge of tidal amphidromes (an area at the centre of which there is a theoretical zero tidal range and around which the tidal wave rotates). This approach to the location of the open boundaries reduces the complexity of the tidal signal entering the model, improving overall model performance.

2.4 Model Mesh – Resolution The spatial resolution of the mesh is deliberately varied within the model domain, as shown in Figure 2.


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Resolution is lower (coarser) in areas far from the main locations of interest, where the detail of local tidal processes does not significantly affect tidal processes in other (more important) parts of the model. Lower resolution in these areas improves the efficiency of the model in terms of run time and results data volume, and does not adversely affect the performance of the model at the sites of interest. Conversely, resolution is higher (finer) closer to the sites of interest in order to better describe physical features in the bathymetry or coastline that might locally affect tidal behaviour. Higher resolution also enables the model to resolve the development and propagation of local flow features (e.g. eddies, jets and other strong local gradients in current speed or direction). Resolution is otherwise varied smoothly over the mesh. As such, the particular choice of resolution applied locally is a balance between computational efficiency and accuracy. The spatial variation in the resolution of the mesh is illustrated in Figure 2 to Figure 5.

Figure 2. Model Mesh Resolution – Whole Model Extent: North Sea and English

Channel


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As shown in the coarsest resolution, ~8,250 m, is used in the North Sea, increasing to 5,000 m in the English Channel. Within the central English Channel, the model grid resolution becomes progressively finer towards the Solent. With reference also to Figure 1, Figure 2 and Figure 3, the coastline of the model away from the Solent has been smoothened to the local resolution of the mesh, excluding some finer details of coastline shape, estuaries and small islands, etc. The excluded details and features are not likely to significantly affect model performance in the Central English Channel and Solent.

Figure 3. Model Mesh Resolution – Detail: Approaches to the Solent As shown in Figure 3, the mesh and coastlines are more detailed in the region of the Solent to account for features making significant contributions to local tidal processes. To this end, the model includes: Langstone, Portsmouth and Chichester Harbours; Fishbourne and Lymington Harbours; and the Medina, Hamble, Test, Itchen and Beaulieu (River) Estuaries. In many


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cases, features are only partially represented in terms of extent or complexity, due to a limited extent, resolution or accuracy of suitable bathymetry data. Poole, Christchurch, Yarmouth, Newtown and Pagham Harbours are not included in the model mesh but are not considered likely to significantly affect Solent wide tidal processes. These are complex features requiring detailed resolution to perform in a suitably accurate manner, but are subject to limitations in local data availability.

Figure 4. Model Mesh Resolution – Detail: East and West Solent and Southampton

Water As shown in Figure 4 and Figure 5, the model grid resolution becomes progressively finer inside the Solent, typically between ~150 and ~50 m, to a maximum resolution of 9 m in Cowes Harbour. This resolution is used to describe more complex local and Solent wide flow patterns and hydrodynamic features that are potentially important to processes in Cowes Harbour. Additional resolution has been specifically added within Cowes Outer Harbour to enable a detailed description of the new breakwater to be included as a bathymetric feature.


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Figure 5. Model Mesh Resolution – Detail: Cowes Harbour and Approaches As shown in Figure 5, additional resolution has been specifically added within Cowes Outer Harbour to enable a detailed description of the new breakwater to be included as a bathymetric feature.

2.5 Model Bathymetry An extensive review of available bathymetry data sources was undertaken to develop the most suitable and accurate data set to inform bathymetry in the model grid. A summary of the bathymetry data review process and the datasets contributing to the final model setup are provided in Appendix A. The combined bathymetry data set was interpolated onto the model mesh using a natural neighbour method. In most locations, the resolution of the available bathymetry data is higher than the local model mesh resolution and so the process of interpolation has minimal potential to affect the accuracy of the source data.


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The interpolated model bathymetry is shown in Figure 6 to Figure 9. The vertical datum of the processed bathymetry data, and so the model mesh, is mean sea level. This is the most appropriate and practicable choice for regional scale models with long open boundaries where the water level boundaries are also relative to mean sea level (see Section 2.6).

Figure 6. Model Bathymetry – Whole Model Extent: North Sea and English Channel The regional model bathymetry (as shown in Figure 6) includes representations of key bathymetric features at the local resolution of the mesh (e.g. Hurd Deep, Thames Estuary, Dogger Bank, etc.).


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Figure 7. Model Bathymetry – Detail: Approaches to the Solent The model bathymetry in the vicinity of the Isle of Wight (as shown in Figure 7) includes representations of key bathymetric features at the local resolution of the mesh (e.g. St Catherines Deep, Shingles Bank etc.).


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Figure 8. Model Bathymetry – Detail: East and West Solent and Southampton

Water The model bathymetry in the Solent and Southampton Water (as shown in Figure 8) includes representations of key bathymetric features at the local resolution of the mesh (e.g. Bramble Bank, deep water and dredged navigation channels, etc.).


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Figure 9. Model Bathymetry – Detail: Cowes Harbour and Approaches The model bathymetry in the vicinity of Cowes Harbour (as shown in Figure 9) includes representations of key coastal and bathymetric features at the local resolution of the mesh (e.g. local marina infrastructure, Prince Consort Shoal, navigation channels, the Shrape Mud, Cowes Outer Harbour Breakwater etc.).

2.6 Open Boundary Conditions At the two open boundaries (see Section 2.3), the model is driven by a spatially varying time-series of water levels. The boundary data are obtained from the global tidal constituent model DTU10 (Andersen and Knudsen, 2009). Data were extracted at 51 equally spaced positions along the North Sea Boundary and at 21 equally spaced locations along the English Channel boundary, at a frequency of 15 minutes. The model software interpolates these as required by linear interpolation to provide a continuous description of tidally varying water levels along the boundaries. Consistent with the model mesh, the vertical datum of the boundary information is also mean sea level.

The river Medina is included to the tidal limit at Newport.


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No additional sources of water flow, e.g. rivers, are included in the regional model set up. The calibration and validation periods tested in this report are in November and December 2014 (see Sections 3.3 and 3.4 for more details).

2.7 Bed Roughness Bed roughness in the model describes the friction ‘felt’ by moving water. Changing the magnitude of bed roughness locally affects the rate at which water moves in that area and so can affect tidal water level range and phasing, and (mainly the speed of) tidal currents. Applying patterns of spatially varying bed roughness within the model domain can produce a more complex effect. As such, bed roughness is a key variable in the model that can be varied to optimise the model calibration in comparison to coincident measured data. In the marine environment, bed roughness naturally varies as a result of differences in seabed type. For example, rocky, gravelly, sandy or muddy seabed types can be expected to present varying amounts of friction; the additional presence of seabed features will further increase friction at the seabed. In the MIKE model, the parameter ‘Manning’s M’ is used to quantify the effect of bed roughness. To more accurately reproduce the complexity of flow patterns in the English Channel, it is necessary to apply spatially varying bed roughness in the form of a ‘roughness map’ as a means of calibrating the model. The Manning ‘M’ values were initially calculated as a function of seabed or sediment type, and water depth:

M =

1�Cd/g∙h1/6

( 1)

Where g is acceleration due to gravity, h is water depth and Cd is the drag coefficient calculated as:

Cd = �1

√0.32 h�𝟏𝟏/𝟕𝟕

∙ 𝑪𝑪𝟏𝟏𝟏𝟏𝟏𝟏

( 2)

Here C100 is the drag coefficient referenced to 1 m above the bed. Values of C100 for a range of generic seabed types were obtained from Soulsby (1997). The spatial distribution of sediment types was obtained from the European Marine Observation and Data Network (EMODnet) substrate dataset. A range of C100 values were subsequently tested to optimise the calibration of water levels so far as possible. The values used to create the initial bed roughness map are summarised in Table 1.


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Table 1. Values of C100

Substrate C100

Rock or other hard strata 0.0010 Coarse sediment 0.0017 Seabed 0.0015 Sand to muddy sand 0.0017 Mud to sandy mud 0.0015

The resulting roughness map provided values that were both water depth and seabed type dependent and should, theoretically, present a realistic amount of friction to the local flow. An initial roughness map was developed using the theoretical approach described above, which provided improved model results during the calibration process. Further local modifications to the bed roughness magnitude and distribution were also subsequently made to enhance local calibration. The calibration efforts were aimed at improving the skill of the model in reproducing three key regional (Solent) scale hydrodynamic processes which are known to be important to processes occurring in Cowes Harbour: ▪ Overall tidal ranges and phasing in the central English Channel and Solent; ▪ The Young Flood Stand in the Solent, and ▪ The double high water in certain parts of the Solent. As observed in the model, the double high water observed during larger tides in the Solent is caused by the return of the tidal wave from the eastern end of the English Channel. The rate of propagation and the nominal centre of rotation of the tidal wave within the English Channel were adjusted by varying roughness in the central part of the Channel. Between Lyme Bay and Cherbourg, the northern side of the English Channel was made rougher and the southern side smoother. This had the effect of re-orientating the tidal wave as it passed through this area of the model, resulting in a retained higher water level around the Isle of Wight as the main tidal wave continues east. The Young Flood Stand in the Solent is caused by the difference in phase between the high water entering through the western Solent and the high water entering through the eastern Solent. Additional roughness was added to the south west of the Isle of Wight to delay the incoming tidal wave from entering the eastern Solent. The extra time taken for the rotation of the wave around the Isle of Wight before it enters the eastern Solent allows for a longer Young Flood Stand before the flood restarts. The double high water at Calshot and Dock Head is a result of a standing wave effect that develops within Southampton Water and the Test Estuary. The main channel and the banks of the estuary were roughened to more accurately reproduce this process. Close to the south western boundary, roughness has been adjusted with the effect of increasing tidal range at all sites. The final roughness map is presented in Figure 10.


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Figure 10. Model Bed Roughness (Mannings M)

Whole model extent

Detail: Solent and approaches


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3. Model Calibration and Validation

Following initial model build, ‘calibration’ is the process by which the performance of the model is improved and optimised by adjustment of the model set up, prior to specific use. Model performance is measured during the calibration (and validation) process by comparison of the model results with other data sources, usually using a range of quantitative statistical metrics appropriate for the data type. Model calibration is reported as a summary of the performance of the model following calibration, against the selected data used for this purpose. Following calibration, ‘validation’ is the process by which the performance of the model, without further adjustment, is again tested and quantified but using data other than that previously used for calibration, i.e. the model performance is tested for data time periods, data locations and/or data types for which the model has not previously been specifically optimised. The validation statistics thereby provide a quantitative measure of the expected accuracy of the model when applied in future studies. Calibration and validation of this regional model has been undertaken at a similarly regional scale. The objective of this stage of the model development is to calibrate and validate the regional model to deliver suitable tidal conditions to the wider Solent region. As such, the performance of the model is tested within the Solent, but not specifically at Cowes at this stage of model development.

3.1 Guidelines The hydrodynamic model performance was assessed against a selected set of performance metrics defined in an internal guidance note (ABPmer, 2011). The guidance recommends that tests of tidal model performance examine amplitude, phase, direction (if relevant) and asymmetry of water level and current parameters. Data sources used for comparison can be measured or anecdotal in nature. Appropriate consideration should be given to the nature and potential accuracy and limitations of any data sources used. To quantify the temporal aspect of the model calibration, statistical approaches are applied to coincident measured or independently predicted, and modelled time-series data, to demonstrate the level of confidence that can be placed in the model performance in a clear and understandable way. The particular quantitative performance metrics used to assess the tidal model performance in this report are set out below. These metrics provide a quantitative measure of the performance of the model for phase and amplitude in relation to the equivalent observational record. Water Level Calibration and Validation Metrics: ▪ Mean difference in absolute water level (of high and low waters). The mean

difference in peak high and low water levels (model minus corresponding observed value, independent of phase differences) in the period being considered. Calculated based on absolute differences in water level for each tide;


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▪ Mean difference in relative water level (of high and low waters). The mean

difference in peak high and low water levels (model minus corresponding observed value, independent of phase differences) in the period being considered. Calculated based on differences in water level expressed as a proportion of the corresponding tidal range for each tide;

▪ Mean difference in absolute phase (of high and low waters). Calculated as the

mean time difference between the model and observed data for high or low waters, in the period being considered.

Water Level Calibration and Validation Quality Thresholds: ▪ For coastal sites mean water level differences should be within ±0.1 m, while the

percentage differences in peak water levels should be within 10% of spring tidal ranges and 15% of neap tidal ranges. Water level phasing at high and low water should be to within ±15 minutes.

▪ For estuarine sites mean level differences should be within ±0.1 m at the mouth and

±0.3 m at the head, while the percentage differences in peak levels should be within 10% of spring tidal ranges and 15% of neap tidal ranges. Water level phasing at high and low water should be to within ±15 minutes at the mouth and to within ±25 minutes at the head.

Current Speed Calibration and Validation Metrics: ▪ Mean difference in absolute peak current speed (of ebb and flood tides). The

mean difference in peak current speed (model minus corresponding observed value, independent of phase differences) in the period being considered. Calculated based on absolute differences in peak current speed for each tide.

▪ Mean difference in relative peak current speed (of ebb and flood tides). The mean difference in peak current speed (model minus corresponding observed value, independent of phase differences) in the period being considered. Calculated based on differences in peak current speed expressed as a proportion of the corresponding observed peak current speed for each tide.

▪ Mean difference in absolute peak current direction (of ebb and flood tides).

Calculated as the mean difference in current direction associated with the peak current speed between the model and observed data for ebb and flood tides, in the period being considered.

▪ Mean difference in absolute peak current speed phase (of ebb and flood tides).

Calculated as the mean difference in the time of peak current speed between the model and observed data for ebb and flood tides, in the period being considered.


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Current Speed Calibration and Validation Quality Thresholds: ▪ Modelled speeds should be within ±0.2 m/s or ±10 to 20% of peak observed speeds,

while modelled directions should be within ±10° and 15° of observed directions for coastal and estuarine sites respectively, and phasing within ±20 minutes.

Some differences between the observations and model are expected, due to variation in how the data was captured (a discrete point in space and time, collected using particular instrumentation and possibly also analysed with particular methodologies or assumptions) and due to the potential effect of non-tidal influences (meteorological forcing), compared to the model result (tide only, depth and time averaged, potentially also spatially averaged in a coarse region of the model mesh). More noticeable differences can be tolerated where the accuracy of observational data is questionable. Further discussion is provided within this report where such concerns arise. The above metrics measure the gross skill of the model in reproducing overall tidal range and the correct phase of high and low water / flood and ebb processes. They do not specifically measure the skill of the model in reproducing the Young Flood Stand or the double high water feature observed in the Solent, or similar tidal features at other locations. Time-series plots of coincident modelled and measured data are provided to enable semi-quantitative visual assessment of the skill of the model in this regard. Semi-quantitative visual comparisons are also made between independently mapped data sources (e.g. tidal stream atlas, and maps of tidal co-range and co-phase contours) and equivalent data from the model. It is usually not possible to provide meaningful quantitative statistics from such comparisons due to the limited format and resolution of the external data source.

3.2 Calibration and Validation Data Sources Generally, any data source can be applied equally for the purposes of calibration or validation; however, any given individual data set (data from a certain location and timeframe) should not be used for both. A range of different data sources and data types might be available for comparison with the model results, which may include location specific qualities, statistics or time-series data, or mapped information. Data might be measured in situ or might have been created synthetically (e.g. from other models or from analysis of other data sources). Most data sources are likely to have been subject to data processing or analysis of some kind, and so will be subject to certain assumptions or modifications from the underlying original observations. It is important to understand the provenance and nature of the data source (type, quality, quantity, accuracy, etc.) so that appropriate consideration can be given for potential uncertainties in the interpretation of comparisons made with corresponding model results.

3.2.1 Water Level Data Sources The National Tidal and Sea Level Facility holds archived primary tide gauge records from 45 standard ports around the UK. The data are collected, processed and archived centrally to


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provide long time-series of reliable and accurate sea levels. The level data from the National Tidal and Sea Level Facility (NTSLF) are provided as observed total water levels, with a separate non-tidal residual component, estimated in conjunction with the expected tidal signal (based on harmonic analysis of longer periods of the observed total water levels). A simple subtraction of the residual from the measured total level provides the tidal water level at the gauge, which can be compared directly with the tidal model outputs. After performing quality checks on the available water level records, 11 sites were selected to provide data for comparison with the model output. The locations of the sites were chosen to ensure the processes in the model were representative on a regional scale. Additional tide gauge data collected at Calshot and Dock Head in Southampton Water were obtained from the local port operator ABP. Tide gauge data collected at Lymington in the western Solent were also obtained from the Channel Coastal Observatory (http://www.channelcoast.org/). These additional data sources are commercial grade tide gauges, but are not subject to the same (very) level of quality assurance, (long) data record length, and data processing as a primary tide gauge. As a result, they provide only the total measured water level (including non-tidal influences), which is not always directly comparable with the tidal model simulation and results. The nature of these data sources does not presently allow tidal and non-tidal influences to be robustly separated (e.g. using harmonic analysis) at these stations. The locations of the measurement sites used are shown in Figure 11 and summarised in Table 2. All of the measured tidal water level data are provided relative to the local Chart Datum (CD) (approximately Lowest Astronomical Tide, LAT). To enable model comparison the data were converted to a mean sea level (MSL) datum, using Admiralty conversion factors. The locations of the 11 data sites, and associated metadata, are summarised in Table 2. Table 2. Summary of water level data sources

Location CD to MSL Conversion (m)

Latitude (° N)

Longitude (° E)

Calshot 2.74 50° 49.241’ -1° 18.511’ Portsmouth 2.87 50° 48.131' -1° 06.671' Dock Head 2.74 50° 52.980’ -1° 23.666’ Lymington 1.98 50° 44.419' -1° 30.426' Bournemouth 1.60 50° 42.816' -1° 52.482' Weymouth 1.04 50° 36.510' -2° 26.876' Newhaven 3.65 50° 46.907' 0° 03.422' Devonport 3.34 50° 22.104' -4° 11.112' Dover 3.77 51° 06.864' 1° 19.356' Lowestoft 1.66 52° 28.374' 1° 44.994' Whitby 3.36 54° 29.400' -0° 36.862' * Data were obtained for most sites for the whole of year 2014; only a shorter period of data was available for Dock Head and Lymington

(November and December 2014). The data encompasses the calibration and validation periods tested (see Section 3.3 and 3.4).

http://www.channelcoast.org/


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Figure 11. Locations of Model Calibration and Validation Data Sites The increased complexity of hydrodynamics in estuaries has been considered in the calibration of this model. For the purpose of this study, Calshot, Portsmouth and Dock Head are classified as estuarine sites. The other eight water level calibration sites are classified as coastal sites. Tide gauge data have also been collected in Cowes Harbour to inform local operations, and as part of site surveys. As previously noted, the objective of the present study is to calibrate and validate the regional model. Data from Cowes will be used to inform further development of the model within the Solent and Cowes Harbour, which will be separately reported.


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3.2.2 Current Data Sources Regional scale tidal atlas information is available from a number of publications, e.g. Admiralty (1992) and Reeds Nautical Almanac (2015). These data sources provide hard copy or digital maps of current speed and direction shown as vector arrows over large areas (e.g. the English Channel), with some quantification of current speed for representative tidal range conditions (mean spring and mean neap information is often provided). The images are typically provided at a temporal resolution of one hour, defined relative to the time of high water at a standard reference port. The underlying data sources used to create the maps are not known but are likely similar to that for tidal diamonds (discussed further below). Whilst there is uncertainty associated with the instantaneous or local accuracy of such data, they are an established and commonly used product for navigation and so are expected to provide reasonable information at the regional scale. Site specific estimated current speed and direction time-series information was also obtained from the TotalTide (TT) software package, produced by the UK Hydrographic Office (UKHO). TT provides scaled tidal diamond type information, based on a usually limited amount of measured data. As such, the accuracy of the predicted currents is not expected to be high, and in any case is not as reliable as measured data. The value of TT data in this case comes in its ability to provide indicative conditions for any time period for a wide distribution of locations within the model domain. It should be noted here that synthetic TT data are subject to a range of uncertainties, which can impact upon data accuracy, particularly in areas of complex or changing seabed bathymetry in morphologically active areas. The following limitations apply to these TT data sources: ▪ Any original measurements used to inform the predicted values may have only been

collected over a short period and are likely to be simply scaled for other time periods assuming a linear relationship between spring and neap, or sometime between peak current speed and associated tidal range;

▪ At source, predicted TT data may be of limited temporal resolution, typically only half

hourly or hourly, and ▪ Synthesised water levels are given to the nearest 0.1 m, and speeds are given to the

nearest 0.1 m/s. Location specific time-series of current speeds were obtained at four locations (shown in Figure 11 and summarised in Table 3). Three sites are relatively close to the Solent, namely, west of the Isle of Wight (West IOW), east of the Isle of Wight (East IOW) and directly to the South of the Isle of Wight (Channel). These locations were chosen to ensure that patterns of currents (i.e. the mass movement of water) are correctly reproduced in the region of the study area. A fourth site was also identified at Dover to represent currents in the Dover Straits. The records extracted from TT cover a two month period from the 1 November to 23 December 2014 including a spring neap cycle. A quantitative comparison of currents was performed over this full period and a qualitative comparison performed two days either side of Spring high water (HW).


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Table 3. Summary of current data

Location Latitude (° N) Longitude (° E) Start of Record End of Record Dover 51° 06.64' 1° 20.30' 03/11/2014 00:00 24/24/2014 23:50 West IOW 50° 33.73' -2° 00.08' 01/11/2014 00:00 24/24/2014 23:50 Channel 50° 11.20’ -1° 03.69' 01/11/2014 00:00 24/24/2014 23:50 East IOW 50° 37.95' -0° 25.50' 01/11/2014 00:00 24/24/2014 23:50

Various other current speed and direction data have also been collected at a limited number of locations in the Solent and in Cowes Harbour as part of various surveys. As previously noted, the objective of the present study is to calibrate and validate the regional model. More data from Cowes and elsewhere in the Solent will be used to inform further development of the model within this area, which will be separately reported.

3.3 Calibration of Water Levels Various options were initially tested regarding model extent and mesh design, but thereafter the model was calibrated primarily by selective adjustment of the bed roughness map, as described in Section 2.7. Water depth, eddy viscosity and other parameters were also investigated as calibration variables, but were not found to consistently improve the model performance. The following section describes the performance of the model following calibration, within the calibration data period. Modelled and observed data are compared over one spring-neap cycle from 10 to 25 December 2014 (approximately fifteen days), including one mean spring range tide and one mean neap range tide (as defined by the Admiralty tidal statistics for these sites). Additional statistics are also provided separately for spring tides and neap tides in the same spring-neap period (including four high water and four low water peaks). Calibration was performed by making iterative adjustments to optimise the model setup with the aim of reproducing the coincident measured data (primarily tidal water level) as closely as possible, particularly in the region of the Solent. In the English Channel, the model correctly reproduces the flood tide propagating in from the Atlantic, from west to east on the southern side. As high water reaches the Dover Straits it combines with the high water from the tidal wave in the North Sea, returning westward along the coast of the UK, producing the characteristic double high water in the central English Channel. The model correctly reproduces the position of the degenerate amphidrome over southern England and the distribution of tidal co-phase and co-range contours (i.e. the large scale patterns of spatial variation in the characteristic range and timing of the tide). Tidal co-phase and co-range contours derived from a mean spring tidal range period in the model results are presented in Figure 12 and Figure 13. Comparative distributions are readily available from third party publications, but cannot be replicated here due to copyright restrictions.


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In the North Sea, the distribution of tidal co-phase and co-range contours and the position of amphidromic points in the two amphidromic systems present are also correctly reproduced.

Figure 12. Modelled Tidal Water Level Co-phase Contours from an Approximately

Mean Spring Range Period


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Figure 13. Modelled Tidal Water Level Co-range Contours from an Approximately

Mean Spring Range Period


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To provide a quantitative assessment of the model performance, a statistical analysis based on observed and modelled high water and low water levels was undertaken using the metrics outlined in Section 3.1. The results are presented in Table 4, Table 5 and Table 6. Positive values indicate that water levels in the model are higher, or times are later, than those observed and vice versa. The calibration statistics are based on analysis of the following time periods: ▪ Spring-neap cycle - 10/12/2014 13:00 to 25/12/2014 17:45 (15 days, typically providing

up to approximately 29 high and low water peaks for comparison, subject to data gaps);

▪ Spring tides - 22/12/14 05:00 to 24/12/14 17:15 (using only the first four high and low water peaks for comparison); and

▪ Neap tides - 14/12/14 22:45 to 17/12/14 11:15 (using only the first four high and low water peaks for comparison).

Most of the individual site metrics meet the calibration guidelines as presented in Section 3.1. Varying the calibration parameters in the North Sea was found to have only a small effect on water levels and processes in the English Channel. For this reason, less calibration effort was afforded to Whitby, Lowestoft and Dover. Performance within the Solent (at Calshot, Portsmouth, Dock Head and Lymington) is measured against total observed water levels (i.e. including non-tidal influences) that will appear to reduce model accuracy. Model calibration within the Solent is not yet completed and so performance at these sites is not yet fully optimised. In terms of the calibration metrics and visual comparison of levels throughout the tidal cycle, the best level of calibration in the English Channel was obtained at Devonport, Bournemouth and Newhaven. These sites perform relatively better on neap tides than on spring tides. Newhaven and Devonport do not meet calibration guideline targets during high water at spring tides and Devonport and Bournemouth do not meet calibration guideline targets at low water spring tides. Calshot, Portsmouth and Dock Head do not meet calibration guideline targets at all stages of the tide. These sites do, however, show relatively better predictions of high water over spring tides and of low water over neap tides. Given that tidal process elsewhere in the English Channel, and in the approaches to the Solent are well represented, it is expected that model performance within the Solent will be improved further in the next round of model development, focussed on this area. Much of the model calibration was focused on replicating processes such as the Young Flood Stand and double high water on the south coast of the UK, processes which are not easily quantified, and are therefore not included specifically as metrics in the calibration standards. To make this assessment, the modelled and measured time-series are compared qualitatively.


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Table 4. Water level calibration statistics for a spring neap period

Location Mean High Water Level Difference Mean Low Water Level Difference Mean Phase Difference

(m) (% of Range) (m) (% of Range) High Water (Minutes)

Low Water (Minutes)

Calshot -0.16 -5.8 -0.04 -1.5 -16 4 Portsmouth -0.12 -4.0 0.01 0.3 -23 0 Dock Head -0.11 -3.7 -0.04 -1.4 -15 6 Lymington -0.01 -0.7 -0.01 -0.5 -13 -74 Bournemouth 0.01 0.7 0.02 1.7 19 19 Weymouth -0.04 -3.1 -0.22 -17.1 14 5 Newhaven -0.06 -1.3 0.01 0.3 8 8 Devonport -0.03 -1.0 -0.01 -0.2 1 -13 Dover 0.14 3.0 -0.14 -3.1 12 -3 Lowestoft -0.01 -0.8 -0.12 -8.5 16 34 Whitby -0.05 -1.3 -0.02 -0.6 -5 16 * Entries that exceed the guideline values are highlighted. Positive values in red (water levels in the model are higher, or times are later,

than those observed) and negative values in blue (vice versa). Table 5. Water level calibration statistics for a spring period



Low Water (Minutes)

Calshot -0.14 -3.6 -0.06 -1.5 -38 5 Portsmouth -0.14 -3.5 0.03 0.7 -33 1 Dock Head -0.08 -2.0 -0.05 -1.3 -25 5 Lymington 0.11 4.5 0.07 2.7 -13 -3 Bournemouth -0.07 -4.1 0.06 3.7 10 16 Weymouth 0.11 5.7 -0.26 -13.6 19 -6 Newhaven -0.02 -0.4 0.02 0.4 4 2 Devonport 0.09 1.8 0.01 0.2 -9 -27 Dover 0.23 3.9 -0.16 -2.8 11 -14 Lowestoft -0.02 -1.2 -0.08 -4.0 21 27 Whitby -0.01 -0.2 0.03 0.6 -6 22 * Entries that exceed the guideline values are highlighted. Positive values in red (water levels in the model are higher, or times are later,

than those observed) and negative values in blue (vice versa).

Table 6. Water level calibration statistics for a neap period



Low Water (Minutes)

Calshot -0.24 -13.5 -0.07 -4.3 -16 13 Portsmouth -0.09 -4.9 -0.03 -1.8 -23 -13 Dock Head -0.22 -12.1 -0.07 -4.0 -16 15 Lymington -0.07 -6.6 -0.04 -3.8 -9 -22 Bournemouth 0.02 5.0 -0.04 -10.3 19 36 Weymouth -0.12 -22.6 -0.22 -40.1 24 27 Newhaven -0.01 -0.4 -0.04 -1.5 -7 8 Devonport -0.09 -4.2 -0.08 -3.8 -6 -6 Dover 0.09 3.0 -0.15 -4.7 7 8 Lowestoft 0 0.5 -0.18 -17.8 31 54 Whitby -0.12 -5.8 -0.08 -3.9 4 16 * Entries that exceed the guideline values are highlighted. Positive values in red (water levels in the model are higher, or times are later,



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Modelled water levels are presented against processed measured water levels for a mean spring tide, a mean neap tide and for the complete model run (Figure 14 to Figure 24). The spring period presented is also coincident with the period over which survey data was previously collected for the calibration of flow speeds in Cowes Harbour. The model replicates the oscillation of high water in the English Channel well with accurate predictions of the level and timing of the second high water at Portsmouth and Bournemouth. As the second high water approaches Weymouth in the model it is smaller and later than in observations. The Young Flood Stand is replicated at Calshot and Dock Head. The initial increase in water level at Portsmouth (from the South East) is captured well. The second increase occurs at slower rate than observed, particularly over spring tides. The double high water which occurs inside Southampton Water is generally lower and more exaggerated in the model. This process is local to Southampton Water and should not affect the hydrodynamics outside Southampton Water.

Figure 14. Water Level Calibration at Whitby


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Figure 15. Water Level Calibration at Devonport

Figure 16. Water Level Calibration at Lowestoft


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Figure 17. Water Level Calibration at Dover

Figure 18. Water Level Calibration at Newhaven


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Figure 19. Water Level Calibration at Weymouth

Figure 20. Water Level Calibration at Bournemouth


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Figure 21. Water Level Calibration at Portsmouth

Figure 22. Water Level Calibration at Calshot


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Figure 23. Water Level Calibration at Dock Head

Figure 24. Water Level Calibration at Lymington


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3.4 Validation of Water Levels

The following section validates the performance of the model following calibration, without further adjustment, for a period of time for which the model has not been explicitly calibrated. Modelled and observed data are compared over one spring-neap cycle from 25/11/2014 12:30 to 10/12/2014 13:15 (approximately 15 days). This period includes several mean spring and larger tidal range events; the smallest neap tidal range in this period is larger than a mean neap range (as defined by the Admiralty tidal statistics for these sites). Additional statistics are also provided separately for spring tides and neap tides in the same spring-neap period (including four high water and four low water peaks). Qualitatively, the model again replicates regional scale tidal processes in the English Channel and North Sea, to a similar standard as previously described in the model calibration period. To provide a quantitative validation assessment of the model performance for times other than the calibration period, a statistical analysis based on observed and modelled high water and low water levels was undertaken using the metrics outlined in Section 3.1. The results are presented in Table 7, Table 8 and Table 9. Positive values indicate that water levels in the model are higher, or times are later, than those observed and vice versa. The validation statistics are based on analysis of the following time periods: ▪ Spring-neap cycle - 25/11/2014 12:30 to 10/12/2014 13:15 (15 days, typically providing

up to approximately 29 high and low water peaks for comparison, subject to data gaps);

▪ Spring tides - 06/12/14 04:30 to 08/12/14 17:00 (using only the first four high and low water peaks for comparison); and

▪ Neap tides - 29/11/14 22:45 to 02/12/14 11:15 (using only the first four high and low water peaks for comparison).

Most of the individual site metrics meet the validation guidelines as presented in Section 3.1. Modelled water levels are visually presented against processed measured water levels for a mean spring tide, a mean neap tide and for the complete validation period in Figure 25 to Figure 35.


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Table 7. Water level validation statistics for a spring neap period



Low Water (Minutes)

Calshot -0.04 -1.3 -0.05 -1.6 -23 -4 Portsmouth -0.09 -2.6 -0.11 -3.4 -22 -10 Dock Head 0.03 1.0 -0.05 -1.6 -32 -3 Lymington 0.03 1.3 -0.03 -1.3 -38 -8 Bournemouth 0.03 2.9 -0.05 -4.4 21 8 Weymouth 0.06 4.4 -0.31 -21.9 15 1 Newhaven 0.07 1.5 -0.07 -1.3 -1 -5 Devonport 0.06 1.6 -0.11 -2.7 -1 -14 Dover 0.30 5.9 -0.26 -5.2 1 3 Lowestoft 0.00 0.1 -0.18 -10.8 17 27 Whitby 0.04 1.0 -0.13 -3.5 -8 18 * Entries that exceed the guideline values are highlighted. Positive values in red (water levels in the model are higher, or times are later,


Table 8. Water level validation statistics for a spring period



Low Water (Minutes)

Calshot -0.04 -1.2 0.09 2.6 -39 -18 Portsmouth -0.15 -3.9 0 0.1 -33 -23 Dock Head 0.03 0.8 0.11 2.9 -6 -18 Lymington -0.03 -1.3 -0.02 -0.7 -103 -2 Bournemouth -0.02 -1.2 0.06 3.9 23 14 Weymouth 0.10 5.5 -0.31 -17.8 12 -2 Newhaven -0.01 -0.1 0.01 0.1 -1 2 Devonport 0.06 1.4 -0.02 -0.4 15 -4 Dover 0.27 4.8 -0.25 -4.5 4 -11 Lowestoft -0.02 -1.2 -0.13 -7.1 17 22 Whitby 0.04 0.8 -0.07 -1.7 -7 19 * Entries that exceed the guideline values are highlighted. Positive values in red (water levels in the model are higher, or times are later,

than those observed) and negative values in blue (vice versa). ** The apparently large difference in the time of high water is due to the sensitivity of the identifiable high water position, to the particular

form of the double high water peak at this location. Table 9. Water level validation statistics for a neap period



Low Water (Minutes)

Calshot -0.05 -1.9 -0.19 -7.4 -10 -2 Portsmouth 0.00 -0.1 -0.20 -7.4 -39 -16 Dock Head 0.00 0.1 -0.20 -7.7 -15 0 Lymington 0.07 4.0 -0.08 -4.7 -34 -3 Bournemouth 0.11 12.1 -0.09 -10.4 16 18 Weymouth 0.01 1.7 -0.32 -36.8 -7 8 Newhaven 0.17 4.0 -0.16 -3.9 -1 -8 Devonport 0.05 1.7 -0.17 -5.7 -11 -12 Dover 0.34 8.0 -0.33 -7.8 -4 8 Lowestoft 0.03 2.5 -0.23 -16.4 34 22 Whitby 0.04 1.4 -0.21 -6.8 0 14 * Entries that exceed the guideline values are highlighted. Positive values in red (water levels in the model are higher, or times are later,



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Figure 25. Water Level Validation at Whitby

Figure 26. Water Level Validation at Devonport


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Figure 27. Water Level Validation at Lowestoft

Figure 28. Water Level Validation at Dover


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Figure 29. Water Level Validation at Newhaven

Figure 30. Water Level Validation at Weymouth


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Figure 31. Water Level Validation at Bournemouth

Figure 32. Water Level Validation at Portsmouth


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Figure 33. Water Level Validation at Calshot

Figure 34. Water Level Validation at Dock Head


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Figure 35. Water Level Validation at Lymington

3.5 Validation of Currents The regional model was calibrated primarily with respect to tidal water levels. No specific calibration of the regional model has been undertaken with respect to tidal currents. The following validation information is provided to demonstrate the general performance of the regional model in replicating depth averaged tidal current speed and direction, i.e. the rate and direction of the mass movement of water in key offshore areas of the model domain. In the next stage of model development, further local calibration and validation will be undertaken for the more detailed patterns of tidal currents in the Solent and, in particular, in and around Cowes Harbour. Qualitatively, spatial patterns of current speed and direction, and the timing of variance in such patterns of currents predicted by the model in the English Channel, are closely comparable with those shown in other third-party tidal stream atlas’, such as those published by the Admiralty (1992) and Reeds Nautical Almanac (2015). These third party publications are readily available but cannot be replicated here due to copyright restrictions. The results of the model are provided in Figure 36 and Figure 37 to enable comparisons to be made with such third party publications.


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Figure 36. Tidal Stream Atlas of the English Channel high water-5 to High Water as Calculated by the Model. Phase Relative to high water Dover


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Figure 37. Tidal Stream Atlas of the English Channel High Water +1 to high water +6 as Calculated by the Model. Phase Relative to high water Dover


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Current speeds from the model were compared with TT data at the four sites shown in Table 3. Positive values indicate that current speeds in the model are higher, or directions are relatively veered (clockwise), in comparison to those observed, and vice versa. The model performs well on ebb tides, meeting the validation guideline targets at all but the Channel location. The model also performs well on flood tides, meeting the validation guideline targets at all but the Dover location. Table 10. Current speed and direction validation statistics for a spring neap period

Location Mean Difference in Peak Speed of Ebb

Mean Difference in Peak Speed of Flood

(m/s) (% of Peak Observed Speed) (m/s) (% of Peak

Observed Speed)

Dover -0.60 -29.6 -0.04 -3.2 West IOW 0.00 0.0 -0.01 -0.7 Channel -0.36 -19.7 0.04 2.5 East IOW -0.02 -2.1 -0.12 -10.0

Location Mean Difference in Direction

at Peak Speed Mean Phase Difference

Ebb (Degrees)

Flood (Degrees)

Ebb (Minutes)

Flood (Minutes)

Dover -10 7 -15 38 West IOW -7 -3 -30 19 Channel 9 3 -94 -41 East IOW -6 -2 -41 -41 * Entries that exceed the guideline values are highlighted. Positive values in red (current speeds in the model are higher, or directions

are veered (clockwise), relative to those observed) and negative values in blue (vice versa).

With allowance for the potential accuracy of the TT data, Figure 38 to Figure 41 visually illustrates a generally good level of calibration. In the middle of the Channel, the model appears to predict patterns of currents occurring earlier than the TT prediction and peak ebb current speeds are relatively under predicted. However, currents at nearby locations to the east and west of the Isle of Wight appear to describe the timing and magnitude of currents much more closely, including relative ebb/flood dominance. The majority of apparent discrepancies are most likely attributable to limitations in the accuracy of the TT data.


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Figure 38. Current Speed and Direction Validation at the ‘Dover’ Site

Figure 39. Current Speed and Direction Validation at the ‘West IOW’ site


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Figure 40. Current Speed and Direction Validation at the ‘Channel’ Site

Figure 41. Current Speed and Direction Validation at the ‘East IOW’ site


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4. Summary A regional scale tidal model has been created, including a large part of the English Channel and North Sea. The model has so far been developed and calibrated with the aim of delivering enhanced and robust tidal water level and current boundary conditions to the area of the central English Channel and Solent. Planned further development of this model (to be separately reported) aims to further improve predictions of tidal processes in and around Cowes Harbour and the Solent. The model uses the most up to date version of the MIKE by DHI modelling software and is built using a flexible mesh which allows better control of local model grid alignment and spatial resolution. The model has a relatively large extent (including parts of the North Sea and all of the English Channel), which allows more accurate simulation of a wider range of tidal conditions within the area of interest. The model mesh is informed by a range of relatively recently collected or updated bathymetry data sets, which improves model performance and is more representative of present day conditions. During calibration, the model was found to be primarily sensitive to the bed roughness parameter. A roughness map was initially created that was a function of seabed type and water depth but further adjustments were ultimately also required to calibrate the model adequately. Some of the changes that have been made to the roughness map cannot be directly attributed to known or actual variations in seabed type; however, the performance of the model has been measurably improved as a result. Tidal water level behaviour is reproduced well throughout the extent of the regional model, providing a generally accurate description of tidal range, phase and asymmetry at the regional scale in the North Sea and English Channel. At some locations subject to particularly strong tidal features or asymmetry, and particularly at sites within the Solent, the accuracy of the model to reproduce such features is expected to improve further during the next stage of model development. The quantitative statistics that can be produced for these locations are also sensitive to the relative asymmetry of double high or low waters, and the skill of the algorithms used to interpret them. Spatial patterns of depth averaged current speed and direction in the English Channel, and the timing of variations in that pattern throughout a tidal cycle, are shown to be well reproduced at the regional scale. Tested locally at a small number of selected locations, the timing and magnitude of current speeds and directions are well reproduced around the Isle of Wight. The model is potentially less effective at reproducing currents in the central English Channel (south of the Isle of Wight) and in the Dover Straits, however, these local comparisons are subject to uncertainty in the accuracy of the available data. The calibration and validation information presented in this report demonstrates that the newly developed hydrodynamic model is presently suitable to inform further local scale modelling and model development activities within the Solent . This model is shown to provide a suitable regional basis for further model refinement and calibration within the Solent, with the ultimate aim of enhancing the accuracy, scope and flexibility of future investigations of hydrodynamic processes in and around Cowes Harbour.


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5. References

ABPmer (2011). Numerical Model Calibration and Validation Guidance. ABP Marine Environmental Research Ltd, File Note R/1400/112 ABPmer, 2016a. Cowes Local Hydrodynamic Model Calibration. ABP Marine Environmental Research Ltd, Report R.2517 ABPmer, 2016b. Cowes Local Model Sediment Transport Model Calibration and Validation. ABP Marine Environmental Research Ltd, Report R.2591 Admiralty (1992). Admiralty Tidal Stream Atlas: English Channel. NP250. Admiralty Publications. Andersen O. B, & Knudsen P, (2009). The DNSC08 mean sea surface and mean dynamic topography. J. Geophys. Res., 114, C11, doi:10.1029/2008JC005179, 2009. Channel Coastal Observatory (2015). Website: http://www.channelcoast.org/. EMODnet (2015). Website: http://portal.emodnet-bathymetry.eu/ GEBCO (2015). Website: http://www.gebco.net/data_and_products/gridded_bathymetry_data/ gebco_30_second_grid/ Reeds (2015). Reeds Nautical Almanac. Soulsby, R., (1997). Dynamics of marine sands: a manual for practical applications. Thomas Telford. University College London. (2012). Vertical Offshore Reference Frames. Available [Online] https://www.ucl.ac.uk/vorf. Accessed 23/08/2015.

http://www.channelcoast.org/

http://portal.emodnet-bathymetry.eu/

http://www.gebco.net/data_and_products/gridded_bathymetry_data/%20gebco_30_second_grid/

http://www.gebco.net/data_and_products/gridded_bathymetry_data/%20gebco_30_second_grid/

https://www.ucl.ac.uk/vorf


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6. Abbreviations

ABPmer ABP Marine Environmental Research Ltd ASCII American Standard Code for Information Interchange CCO Channel Coastal Observatory CD Chart Datum CHC Cowes Harbour Commission DEM Digital Elevation Model DHI Danish Hydraulic Institute DTU10 Danish Technical University (2010) Global Ocean Tide Model EMODnet European Marine Observation and Data Network FM Flexible Mesh GEBCO General Bathymetric Chart of the Oceans HW High Water IOW Isle of Wight LAT Lowest Astronomical Tide LiDAR Light Detection And Ranging LW Low Water M Mannings Coefficient of Roughness MSL Mean Sea Level NTSLF National Tidal and Sea Level Facility ODN Ordnance Datum Newlyn OSGB Ordnance Survey of Great Britain TT TotalTide UK United Kingdom UKHO United Kingdom Hydrographic Office UTM Universal Transverse Mercator VORF Vertical Offshore Reference Frame WGS 1984 World Geodetic System 1984 WL Water Level Cardinal points/directions are used unless otherwise stated. SI units are used unless otherwise stated. Conversion of speeds: ▪ 1 m/s equals 1.94 knots ▪ 1 knot equals 0.54 m/s

Appendix A Bathymetry Review


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A. Bathymetry Review The model study area extends from the south west of the English Channel to the north of the North Sea. A bathymetry layer was generated for the model using a number of open source data sets along with recent ABP and Cowes Harbour Commission (CHC) surveys in Southampton water and Cowes Harbour, both of which were used with permission granted by the data provider. The final bathymetry layer is presented in WGS 1984 UTM 30N projection with a vertical datum of Mean Sea Level (MSL). To facilitate the incorporation of multiple resolutions of bathymetry data, the study area is segmented into 5 separate sub areas, detailed below and shown in Figure A1: ▪ SA1: Cowes; ▪ SA2: Isle of Wight; ▪ SA3: Portland to Selsey; ▪ SA4: English Channel; and ▪ SA5: North Sea.

Figure A1. Map Showing Locations of the Study Area Sub Areas


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During the identification and collation of bathymetry data sets it was decided that SA3 (Portland to Selsey) would be incorporated into SA4 (English Channel). This was due to the availability and resolution of open source data sets within this sub area, which were not seen to be significantly finer resolution than EMODnet DEM which was used in SA4. The main sources of higher resolution survey data used were the United Kingdom Hydrographic Office (UKHO) Inspire Portal and the Channel Coastal Observatory (CCO) Data Catalogue. The UKHO Inspire Portal holds survey data used in the production of Admiralty Charts with survey dates ranging from 1956 to the present day. The portal is a very useful source of data covering large survey areas and provides nearly complete coverage around the Isle of Wight (SA2). The CCO Data Catalogue provides intertidal and sub tidal data over the past 20 years. This information is provided in LiDAR, single-beam and multi-beam formats depending on the location of the data and was used to make sure that the intertidal and sub tidal areas around the coast in both SA1 and SA2 had complete and accurate coverage. A.1 SA1 Cowes The Cowes sub area encompasses the area around Cowes and the River Medina, a number of data layers were used to create the most up to date and accurate bathymetry layer for this region of the model. This sub area has the highest resolution final bathymetry layer with 5 m spacing. Table A1 lists the datasets used, where data could be sourced that was surveyed in the last couple of years these were used, however for the River medina and for the intertidal areas near Cowes the earliest data sourced was from 2007 and 2008. The majority of the data layers were sourced in CD, therefore the final combined data layer was created in CD before being converted to MSL using the Vertical Offshore Reference Frame (VORF) data layers (Section A.5). Any data layers not in CD were converted to CD before combining into the final data layer, VORF data layers were used to do this conversion. Table A1. Data sets used within SA1 Cowes sub area

Data Set Source Projection Vertical Datum

Depth Values Resolution Year

Atkins Breakwater Survey 2015 Atkins OSGB CD plus 0.5 m 2015

Atkins Siltation Survey 2015 Atkins OSGB CD plus 3 m 2015

CHC Cowes Survey October 2014

Cowes Harbour Commission OSGB CD minus 5 m 2014

CHC Cowes Survey Cowes Harbour Commission OSGB CD plus 5 m 2013

CCO Intertidal LiDAR CCO OSGB ODN minus 1 to 2 m 2008 ABP Survey – North of Cowes

ABP Southampton OSGB CD minus 1 m 2014

2011 Hydrographic Multi-Beam CCO OSGB ODN minus 1 m 2011

2007 2007-029367 Isle of Wight - River Medina

UKHO Inspire Portal WGS 1984 CD plus 5 m 2007

All of the datasets listed in Table A1 were mosaicked into a single data layer, it is important that the correct order is used to mosaic the data so that the newest and best resolution data layers are used in preference over any older data layers. Considerations such as the extent of the data layers and there


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locations to the coast were also taken into account when deciding the order for mosaic. Where necessary a mask was created for each individual data layer, this allows for the bathymetry to be clipped to the correct extent for the data to be mosaicked, this was especially important for the data layers at the top of the mosaic. The dataset mosaic order is listed below with 1 being the top layer and 8 being the bottom layer: 1. Atkins Breakwater Survey 2015 2. Atkins Siltation Survey 2015 3. CHC Cowes Survey October 2014 4. CHC Cowes Survey 2013 5. ABP Survey – North of Cowes 6. 2011 Hydrographic MultiBeam 7. CCO Intertidal LiDAR 2008 8. 2007 2007-029367 Isle of Wight - River Medina The resulting combined dataset for SA1 is shown in Figure A2.

Figure A2. Cowes SA1 Bathymetry


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A.2 SA2 Isle of Wight The area around the Isle of Wight (SA2) required finer resolution data than which was available within the EMODnet and GEBCO composite bathymetry layers. To generate a finer resolution bathymetry layer multiple higher resolution survey data layers were also identified and compiled. Table A2 lists the datasets used. The addition of ABP and CHC survey data was incorporated where available. The survey data sets span multiple years and were available to a variety of resolutions, therefore when the data was compiled into a composite surface, it was important to order the data layers so the finest resolution and most recent data layers were mosaicked on top of the coarser and older data layers. The majority of the data layers were sourced in Chart Datum (CD); therefore the final composite bathymetry layer was created in CD then converted to MSL using VORF data (Section A.5). Table A2. Data sets used within SA2 Isle of Wight sub area

Data Set Source Projection Vertical

Datum Depth Values Resolution Year

CCO Intertidal LiDAR

CCO data catalogue http://www.channelcoast.org/data_management/online_data_catalogue/metadata/search/index2.php

OSGB 1936 ODN minus 1 to 2 m Multiple 2004 - 2008

CCO Singlebeam Portsmouth, Langstone, Chichester Harbours


OSGB 1936 ODN minus 15 to 25 m 2004 & 2005

CCO Singlebeam – Southampton Water, Hurst Bank & North of Cowes


OSGB 1936 ODN minus 15 to 25 m Multiple

ABP Survey – North of Cowes ABP Southampton OSGB 1936 CD minus 1 m 2014

ABP Post deepening channel survey ABP Southampton OSGB 1936 CD minus 1 m 2014

2013 HI1437 Selsey Bill to Lee-on-Solent 1 m FMCUBE

UKHO Inspire Portal https://www.gov.uk/inspire-portal-and-medin-bathymetry-data-archive-centre

WGS 1984 CD minus 1 m 2013

2012 HI1366 Poole Bay 2 m SB



2006 HI1156 Hampstead Ledge to Stansore Point 2 m



2010 HI1317 Hurst Spit 2 m SB



http://www.channelcoast.org/data_management/online_data_catalogue/metadata/search/index2.php












https://www.gov.uk/inspire-portal-and-medin-bathymetry-data-archive-centre













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2008 HI1279 Eastern Approaches to Nab Channel 2m SB



2010 HI1323 South Wight 2 m SB



2011 HI1315 WP09 E-G Isle of Wight 2 m SB



2003_HI1005_Southern_Approaches_to_Nab_Tower_30N


WGS 1984 CD plus 50 m 2003

2003_HI1004_Medmery_Bank_30N


WGS 1984 CD plus 100 m 2003

2002_HI962_Anvil_Point_to_Nab_Tower_Blk9_30N


WGS 1984 CD plus 50 m 2012

2002_HI1003_Selsey_Bill_to_Hooe_Bank_BlkB_30N


WGS 1984 CD plus 50 m 2002

1989_HI366_Nab_Tower_to_Old_Castle_Point_Blk3_30N


WGS 1984 CD plus 50 m 1989

1987_HI365_The_Needles_to_Egypt_Point_Blk2_30N


WGS 1984 CD plus 50 m 1987

Hamble Bathymetry 2005 Cowes model OSGB 1936 CD plus 10 m Unknown

Beaulieu Channel Bathymetry 2005 Cowes model OSGB 1936 CD minus 20 m Unknown

ABP Survey Data for Charting ABP Southampton OSGB 1936 CD plus

Varies around 20

m 2002-2007





























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The data sources listed in Table A2 were mosaicked into a single data layer using the same method as described in Section A.1, taking account of data layer age, location and resolution when determining the order of mosaic. The dataset mosaic order is listed below with 1 being the top layer and 24 being the bottom layer: 1. Langstone Harbour CCO Single Beam 2005 2. Portsmouth Harbour CCO Single Beam 2005 3. Chichester Harbour CCO Single Beam 2004 4. All CCO Intertidal LiDAR Data 5. ABP North of Cowes Survey 2014 6. ABP Post Channel Deepening Survey 2014 7. Selsey Bill to Lee-on-Solent 2013 8. Poole Bay 2012 9. Hurst Spit 2010 10. Isle of Wight 2011 11. South Wight 2010 12. Eastern Approaches to Nab Channel 2008 13. Hampstead Ledge to Stansore Point 2006 14. Southern Approaches to Nab Tower 2003 15. Medmerry Bank 2003 16. Anvil Point to Nab Tower 2002 17. Selsey Bill to Hooe Bank 2002 18. Nab Tower to Old Castle Point 1988 19. Needles to Egypt Point 1987 20. CCO Singlebeam – Southampton Water, Hurst Bank & North of Cowes 21. Royal Pier Model Bathymetry 22. ABP Survey Data for Charting 23. Hamble Bathymetry 24. Beaulieu Channel Bathymetry The resulting combined dataset for SA2 is shown in Figure A3.


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Figure A3. Isle of Wight SA2 Bathymetry


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A.3 SA3 Portland To Selsey and SA4 English Channel Two main data sources were identified to inform the SA3 Portland To Selsey and SA4 English Channel sub areas: GEBCO 2014 grid; and EMODnet 2014 DEM. Table A3 lists the datasets used. The EMODnet data layer has a finer resolution (200 m) than the GEBCO layer (800 m) and was therefore used to cover north side of the English Channel, with GEBCO used on the south side. The coarser resolution data were used where the model is further away from Cowes as this will reduce the overall size of the bathymetry data and make the data easier to incorporate into the model. It should be noted that both GEBCO and EMODnet state that their data is not consistently accurate to a specific vertical datum however they are closest to Lowest Astronomical Tide (LAT). The sourced bathymetry layers were processed to convert them to UTM 30N projection with vertical datum in MSL. The VORF extended data layer (Section A.5) was used to complete the vertical datum conversion for both data layers.

Table A3. Data sets used within SA3 Portland To Selsey and SA4 English Channel sub

areas



EMODnet Composite DEM

EMODnet http://portal.emodnet-bathymetry.eu/

WGS 1984 LAT minus Approximately 200 m

2015 (however the data is based on multiple datasets which will be older than 2015).

GEBCO 2014 grid

GEBCO http://www.gebco.net/data_and_products/gridded_bathymetry_data/gebco_30_second_grid/



The resulting combined dataset for SA3 and SA4 is shown in Figure A4.



http://www.gebco.net/data_and_products/gridded_bathymetry_data/gebco_30_second_grid/





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Figure A4. SA4 English Channel Bathymetry (Incorporating SA3 Portland To Selsey) The relative accuracy of EMODnet bathymetry data was tested within the extent of SA2 Isle of Wight. Additional data were obtained from EMODnet within the extent of SA2. Without any adjustment, these data were compared to the collated (higher resolution) survey bathymetry data (see Section A.2). The difference in local bed level between the two data sets is shown in Figure A5. The figure shows that the difference between the two data sources is small (typically within ±1 to 2 m) in most locations. Examination of the frequency distribution of all difference values (not shown) indicates an approximately even distribution (positive and negative) about a modal difference of zero. Locally greater differences are found at the edges of topographic features where water depth is changing rapidly over short distances. Local differences between the data sets occur where changes in depth are better resolved by the higher resolution survey data, but are spatially averaged in the more coarsely resolved EMODnet data.


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Figure A5. Local Difference Within SA2, Between EMODnet and the Collated SA2 Survey

Bathymetry The magnitude of local differences between EMODnet and survey bathymetry data is therefore shown to be relatively small in magnitude (e.g. less than the typical spring tidal range in this area, 2 to 4 m) and without particular bias. This validation of the accuracy of the EMODnet data in this area is assumed to be indicative of the accuracy that can be expected in other areas of SA3, SA4 and SA5, where EMODnet data have been used.


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A.4 SA5 North Sea The bathymetry for the North Sea was obtained from GEBCO. Table A4 lists the datasets used. The data were downloaded as ASCII files which were merged to create one raster grid. The grid was then processed to convert it to UTM 30N projection with the vertical datum in MSL. Vertical Offshore Reference Frame (VORF) was used to apply the vertical datum change (University College London, 2012). VORF is currently limited to UK waters and does not cover the whole of the North Sea, to allow for a complete vertical datum conversion the VORF data was extended to have full coverage of the study area inclusive of all the sub areas, the extension of VORF methodology is explained within Section A.5. The resulting dataset for SA5 is shown in Figure A6. Table A4. Data sets used within SA5 North Sea sub area

Data Set Source Projection Vertical

Datum Depth Values Resolution Year

GEBCO 2014 grid

GEBCO http://www.gebco.net/data_and_products/gridded_bathymetry_data/gebco_30_second_grid/








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Figure A6. North Sea SA5 Bathymetry


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A.5 Vertical Offshore Reference Frames Vertical Offshore Reference Frames (VORF) is a data product that provides maps of the vertical offset between various vertical datums in UK waters (http://www.ucl.ac.uk/vorf). The project was run as collaboration between University College London and the UKHO and was completed in 2008. Vertical offsets are provided as spatially varying maps at any point within UK waters and allow for conversions between a number of datums, including; CD, MSL, LAT, Mean Low Water Springs and Ordnance Datum Newlyn (ODN) (University College London, 2012). These surfaces were used to make accurate vertical datum shifts for the bathymetry surfaces created in each sub area to make sure the final sub area bathymetries were consistently relative to MSL. However, the model domain reaches through the English Channel and the North Sea, and for SA4 and SA5 the domain reaches beyond UK waters. In these sub areas a mixture of EMODnet and GEBCO bathymetry data are used to cover the extents. The data from both EMODnet and GEBCO bathymetry layers is most in line with LAT, although it should be noted that the vertical datum for these Digital Surface Models (DSM) is not stated to be accurate to a vertical datum as smoothing algorithms are used to make sure the final DSM is has no steps in the data. To be able to do the conversion from LAT to MSL in SA4 and SA5 the VORF LAT to MSL spatial surface was extended across the North Sea and the English Channel. To do this a number of points were located on the European coasts that provide a value for the vertical datum shift between LAT and MSL shown in Figure A7. The VORF surface was converted to points and thinned to allow for quicker processing time. The VORF and European points were re-projected into UTM 30N then merged together and a surface created from the points. A polyline data layer showing the co-tidal ranges for UK waters was used to verify the pattern of the output surface matched the co-tidal lines. Figure A8 shows the final extended VORF LAT to MSL conversion surface.

http://www.ucl.ac.uk/vorf


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Figure A7. Extent of the VORF Data and the Location of the European Point.


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Figure A8. VORF Extended LAT to MSL Difference Surface A.6 Reference University College London. (2012). Vertical Offshore Reference Frames. Available [Online] https://www.ucl.ac.uk/vorf. Accessed 23/08/2015.

https://www.ucl.ac.uk/vorf

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