P090-1 CELL ELEVEN WAVE, TIDE AND SEDIMENT STUDY. Darren Price 1, Mikkel Andersen 2, Brian Joyner 3, Nigel Pontee 4, Andy Parsons 5

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1 CELL ELEVE WAVE, TIDE AD SEDIMET STUDY Darren Price 1, Mikkel Andersen 2, Brian Joyner 3, igel Pontee 4, Andy Parsons 5 1. Halcrow Group Ltd, Manchester, UK 2. Halcrow Group, ew York, USA 3. Halcrow Group, Tampa, USA 4. Halcrow Group Ltd, Burderop Park, Swindon UK 5. Halcrow Group Ltd, York, UK Keywords Sediment transport modelling, hydrodynamic modelling, wave modelling, Cell 11 SMP, calibration, regional wave model, Regional flow model, DHI MIKE HDFM, DHI MIKE ST, DHI MIKE SW, mathematical modelling. 1. ITRODUCTIO The Cell 11 region of the UK extends from Great Orme s Head in orth Wales to the Scottish Border in the Solway Firth (Figure 1, but excludes the Isle of Man). The first round of Shoreline Management Plans (SMP s), which set policies for managing coastal flood and erosion risks, identified a range of uncertainties related to coastal processes. These uncertainties related to tidal currents, water levels, waves and sediment transport at a regional scale. The present paper describes some of the modelling work which has been undertaken to help reduce these uncertainties. The paper describes two main studies: a Joint Probability Study (JPS) and; a Tide and Sediment Study (CETaSS). The Joint Probability Study (JPS) was undertaken to provide joint probability curves for large waves and high water levels at a range of nearshore locations throughout the region. This paper describes the setting up, approach and calibration of the wave models used to derive the wave climate. The CETaSS study has involved a considerable amount of coastal numerical modelling. This paper covers the setting up and calibration of regional hydrodynamic models and their application, with wave model results, to derive sediment transport pathways across the entire Cell 11 region. Additional littoral transport calculation on a regional scale were also undertaken but not presented here. P090-1

2 The provision of necessary data for this study was made through the Cell 11 Regional Monitoring Study (CERMS) which is collating data for the CELL 11 region. SCOTLAD Solway Firth Cumbria EGLAD Isle of Man Duddon Estuary Morecambe Bay Blackpool Ribble Estuary Great Orme s Head Mersey Estuary ORTH WALES Dee Estuary Figure 1: Cell 11 region encompassing much of the shoreline of the north east Irish Sea between Great Orme s Head and Scotland (excluding the Isle of Man). 2. JOIT PROBABILITY STUDY 2.1 Model choice To calculate the joint probability of large waves and high water level, the JOISEA software (Hawkes 2007) is to be used. Some 51 nearshore points were identified for analysis throughout the Cell 11 region. The analysis requires coincident timeseries of waves and water level over a sufficient time period (>7 years) as well as marginal extremes of these parameters. A UK wide national study will be reporting extreme water levels later this year and long water level timeseries will be used from a hydrodynamic surge model of the region used in the national study. What was missing was wave timeseries at the 51 nearshore locations and extreme wave analysis of this data. umerical wave modelling was therefore required and the MIKE21SW spectral wave model has been used for this purpose. P090-2

3 In addition, CETaSS included a study to investigate and calculate littoral transport rates around the shoreline of Cell 11 and modelled wave timeseries were also required for this purpose. Regional (broad-scale) wave transformation model Review of data from UK Met Office European Waters and UK Waters hindcast archives indicated that this wave data could not be used directly to drive a nearshore wave transformation model. The European Waters model has a coarse grid and it does not represent the presence of the Isle of Man, thus over predicting wave energy from dominant wind directions at the eastern (lee side) of the Isle; Figure 2 shows the difference between the UK waters model (point GL1800) and the EU waters model (MOGF) with more wave energy from the west in the EU model due to the lack of representation of the Isle of Man. The European Waters hindcast also does not represent the island of Anglesey in sufficient detail for the present study s purposes. The UK Waters hindcast model does represent the Isle of Man and Anglesey in more detail, but the available data time period ( ) is too short for the marginal extreme value analyses and does not overlap sufficiently with the available tide and surge model output for the subsequent joint-probability analyses. GL1800 March ov 2008 (with Isle of Man) Calm 0.00 % 5 % Hs (m) Above Hs (m) Above Below Below % MOGF June Sept 2007 (no Isle of Man) Calm 0.00 % 5 % Hs (m) Above % Hs (m) Above Below Below Figure 2: Wave height roses for UK Met Office grid cells GL1800 (UK Waters) and MOGF (European Waters). P090-3

4 A broad-scale, regional wave model was therefore constructed to transform European Waters hindcast data from a boundary to the west of the Isle of Man and Anglesey, together with wind wave growth using gridded wind data to the boundary of a nearshore model. The UK Waters hindcast data were used to crosscheck the results of the broad-scale model. The primary calibration data for the broad-scale and nearshore models were actual measured wave data at a series of Cefas instrument locations. earshore wave transformation model The calibrated broad-scale model was used to provide boundary conditions to a higher resolution nearshore wave transformation model which more precisely resolves the bathymetric features near the Cell 11 coastline. The resulting wave conditions from the nearshore model will be used in combination with concurrent water level records to conduct the joint probability analysis at a later stage of the Cell 11 Joint Probability Study. 2.2 Model set up and calibration Regional (broad scale) Wave Model Setup The bathymetry and wave model mesh for the broad-scale wave model can be seen in Figure 3. It is a relatively coarse mesh but finer than either of the UK Met Office models available. Bathymetry data for the Irish Sea and orth-east Irish sea area, i.e. East of Isle of Man (SMP Cell11) was obtained from digitised Admiralty Charts from SeaZone as well as LiDAR data and local bathymetry survey data :00:00 12/30/1899 Time Step 0 of 0. Bathymetry [m] Above Below -100 Undefined Value Figure 3: Broad-scale wave model domain, resolution, and bathymetry (mod). P090-4

5 The MIKE21SW model was run in fully spectral mode for the broad scale model using the EU waters Met office data from locations shown in Figure 2 to provide wave boundary conditions to the west of the Isle of Man and the other locations used to provide wind timeseries as a spatial and time-varying parameter. Comparisons between the broadscale wave model, the met office datasets as well as wave buoy data were undertaken. Figure 4 presents the comparison between the broad scale model predictions and observed wave conditions at a fixed location outside of Liverpool Bay (obtained from Cefas wavenet archive). It can be seen that the reproduction of wave conditions at this location was good Cefas Liverpool Bay: Hm0 [m] Broad-scale model: Hm0 [m] Wave height January February March Cefas Liverpool Bay: Tpeak [sec] Broad-scale model: Tp [sec] Wave period Cefas Liverpool Bay: Tz [sec] Broad-scale model: Wave Period, T01 [sec] January February March -2 Cefas Liverpool Bay: Peak Wave Direction [deg] Broad-scale model: MWD [deg] Wave direction January February March 0 Figure 4: Calibrated Broad-scale model (red) vs. Cefas WAVEET data (black) at point Liverpool Bay The final parameters used and other information regarding the model calibration can be seen in Table 1. During the calibration process, the model was tested for sensitivity to the following items or parameters: Wave period. Wind speed (sensitivity to wind speed from met office EU model was undertaken and it was found that an increase of 15% was required to get adequate calibration). Frequency discretisation. P090-5

6 Table 1: Calibrated broad-scale model formulations and parameter values Mesh / Bathymetry Simulation Description Directional / Frequency Discretisation ominal resolution = 5000m, except banks east of Isle of Man at nominal resolution of 2500m; Isle of Man and Anglesey not triangulated (e.g. as land). Fully Spectral formulation (FS), instationary, low-order dir=10deg, number of frequencies=27, C=1.13, minimum frequency = (maximum resolved wave period = 22seconds) Time Period 1 Jan - 1 Jan 2007 Water Level Spectral Wave Boundary Conditions Winds White-capping Variable, Irish Sea regional HD model hindcast WL analyzed for constituents. Resulting constituents re-predicted into the 20-year time series and interpolated onto a 10km x 10km grid UK Met Office European Waters hindcast series at MOGA - MOGW, total resultant wave conditions Variable, European Water model hindcast winds interpolated onto 10km x 10km grid files, corrected from 20m to 10m altitude, then increased to 115% C dis =4.5, DELTA=0.5 Bottom Friction ikuradse roughness, constant value of k n = Wave Breaking Gamma 1 =1.5 (steepness limiting), Gamma 2 =0.8 (depth limiting), alpha=1 Table 2: Statistical comparison of calibrated broad-scale model with Cefas measurements at Liverpool Bay, 1-hour intervals. H m0 T z RMSE = bias = dif bias BI = me 1 1 = i= 1 i= 1 dif i dif i RMSE SI = me The statistics included in Table 2 are RMS error (RMSE), Bias, Bias Index, and Scatter Index. The statistics are computed as: RMSE = 1 i= 1 dif i 2 bias = dif = 1 i= 1 dif i P090-6

7 bias BI = me SI = RMSE me where: me i = measured; mo i = model; dif i = mo i - me i and me = 1 i= 1 me i In addition to the RMSE statistic typically employed in model calibration, both the bias and BI statistics were particularly useful in identifying whether the model was generally under predicting or over predicting the measured data values. A negative bias indicates under prediction, and the BI indicates the relative magnitude of the bias relative to the mean measured value (the BI approximates percent error). Once the broad scale model had been calibrated, it was run for a continuous 20 year period. earshore Wave Model The nearshore model resolves a nearshore subset of the broad-scale model in significantly greater spatial detail. The purpose of this nearshore wave transformation model is to provide detailed wave information at many locations along the Cell 11 coastline in support of a joint-probability analysis and alongshore littoral drift sediment transport calculations in the linked CETaSS studies. Like the broad-scale model, the nearshore model was set up in the MIKE 21 SW software. Figure 5 shows the model domain extent and bathymetry. The nearshore model mesh is significantly finer than the Broadscale model, see Figure 6 for part of the model mesh. The nearshore wave model was run in directionally decoupled and quasistationary mode due to the relatively fine mesh resolution and large number of elements. To run a 20 year fully spectral model with this mesh would have been prohibitive. Figure 7 presents the wave model calibration at a buoy located in Liverpool Bay. A good level of reproduction of the wave height, period and direction can be seen. Table 3 lists model formulation selections and parameter values for the calibrated nearshore model. The statisitical fit at the location shown in Figure 5 can be seen in Table 4. Other locations were also examined in the model calibration including data from an HF Radar. The model was deemed to be calibrated and was subsequently used for the littoral transport study and subsequent joint probability analysis. The model was run for 20 years in blocks of 1 year providing the possibility to extract 20 years of wave data throughout the model domain. P090-7

8 gl1740 mogc mogf mogg gl1800 gl mogj mogk mogl mogo mogp mogq gl1919 Barrow OWF calibration point mogt mogu mogv Liverpool Bay calibration point Bathymetry [m] Above Below -40 Undefined Value Figure 5: earshore wave model domain and bathymetry (mod). Also shown are Met Office data points and some calibration data set locations :00:00 12/30/1899 Time Step 0 of 0. Bathymetry [m] Above Below -30 Undefined Value Figure 6: Model Mesh along orth Wales and Merseyside coastlines P090-8

9 Cefas Liverpool Bay: Hm0 [m] earshore model: Hm0 [m] January February March Cefas Liverpool Bay: Tpeak [sec] earshore model: Tp [sec] Cefas Liverpool Bay: Tz [sec] earshore model: Wave Period, T01 [sec] January February March Cefas Liverpool Bay: Peak Wave Direction [deg] earshore model: MWD [deg] January February March 0 Figure 7: Calibrated earshore model vs. Cefas WAVEET measured data at point Liverpool Bay Table 3: Calibrated nearshore model formulations and parameter values Mesh / Bathymetry Simulation Description Directional Discretisation Outer band nominal resolution = 2500m; Inner bank nominal resolution = 1500m. Mesh was refined with respect to bed elevation with the endpoints (- 20m OD; 1000m resolution) and (+2.0m OD; 500m resolution) Directional Decoupled (DD), quasi-stationary, low-order dir=10deg Time Period 1 Jan - 8 May 2007 Water Level Spectral Wave Boundary Conditions Winds Spatially and temporally varying grid developed from a combination of longterm tide+surge hindcast data supplemented with Class A tide gauge data Wave conditions read from spatially-varying files saved directly from the broad-scale model 20-year continuous simulations Variable, EU Waters model hindcast winds interpolated onto 10km x 10km grid files (corrected for the winds 20m altitude and then increased by 15% during broad-scale model calibration); SPM84 wind generation method Bottom Friction ikuradse roughness, constant value of k n = Wave Breaking Gamma 1 =1.5 (steepness limiting), Gamma 2 =0.8 (depth limiting) P090-9

10 Table 4: Statistical comparison of calibrated nearshore model with Cefas measurements at Liverpool Bay (January May 2007), 1-hour intervals. H m0 (m) T z (s) RMSE Bias Bias Index Scatter Index CETaSS (Cell 11 Tide and Sediment Study) 3.1 Model choice For the CETaSS study both the tidal and sediment transport regime in the north east Irish Sea were to be examined. A depth-averaged hydrodynamic model was developed of the whole Irish Sea for this purpose. This was calibrated and provided boundary conditions to a finer resolution north east Irish Sea (EIS) hydrodynamic model. The EIS hydrodynamic model setup using MIKE21HD FM, was used as the driving force for the sand transport model (MIKE21ST) along with a wave model (MIKE21SW) covering the EIS area which would be used to provide additional bed shear stress due to waves. Once calibrated, the EIS model was run for a 30 day spring-neap tidal cycle and the current only sediment transport was calibrated. From this a representative tidal cycle (actually two tides) was chosen such that the general sediment transport patterns and rates, once factored up, matched reasonably well the results from the 30 day simulation. Subsequent simulations and analysis, for example to consider changes due to future increases in mean sea level, were analysed over the representative tidal period to cut down simulation time. 3.2 Model setup and callibration Regional Irish Sea model The regional Irish Sea model had been established prior to the studies under an internal R&D study, so is not be presented in great detail here with more emphasis placed on the new EIS model. The regional Irish Sea model is forced with tidal boundaries extracted from the DHI global tidal model shipped with MIKE21. The model domain, bathymetry and model mesh can be seen in Figure 8. P090-10

11 Figure 8: Regional Irish Sea tide model computational mesh Guidelines for the calibration of a hydrodynamic model are set out in the Environment Agency report Quality Control Manual for Computational Estuarine Modelling (Environment Agency, 1998). The guidelines for hydrodynamic models are as follows: For the open coast: water levels to within +/- 0.1m; speeds to within +/-0.1m/s; directions to within+/-10 degrees; and, timing of high water to within 15 minutes. Expressed as percentages: o o speeds to within +/-10-20% of observed speed; and, water levels to within 10% of spring tidal ranges or 15% of neap tidal ranges. In addition, the Environment Agency guidelines indicate that where these conditions are too testing, then a less stringent expectation might be that the conditions are satisfied for 90% of position/time. These guidelines were adopted for the calibration of the hydrodynamic models developed in CETaSS. To evaluate the accuracy of the calibration, the root mean square (RMS) error for water levels is calculated for a 3 month simulation for P090-11

12 selected stations. The RMS error is compared as percentages towards the spring and neap tidal ranges. The spring and neap tidal ranges are provided from Admiralty tide tables. The estimation of the phase error is calculated based on the measured water level data. The water level RMS error is determined as the difference between the measured and simulated water level. To estimate the phase error the simulated water level timeseries is shifted in time. For each shift the RMS is calculated. The phase error is defined as the phase shift for which the RMS error is minimal. A positive phase error is defined as the situation when the simulation lags behind the observed data. There are 9 class 'A' tide gauges available within the domain of the regional Irish Sea model which were subsequently used for calibration. This data can be obtained from The level of calibration fitted in well with the EA guidelines and therefore the model was deemed to be calibrated for water levels and taken forward to provide boundary conditions to a higher resolution model. EIS model The regional E Irish Sea hydrodynamic model was used as a basis for much of the subsequent sediment transport modelling. This tidal model was used to describe the tidal flows within the E Irish Sea. The model resolution was chosen so that increased resolution is applied nearshore as well as at important features such as offshore sandbanks and estuary mouths. In general resolution in the nearshore from depths less than -20m has been increased significantly with typical mesh size of 500m, decreasing to 150m in the most refined areas. The regional E Irish Sea model bathymetry and model mesh are shown in Figure 9. Additional calibration runs were carried out with the E model, however the best overall model performance was achieved with basically the same model calibration coefficients as the regional Irish Sea model. Bed roughness around the boundaries was increased slightly to reduce potential for instability. Model calibration simulations were carried out for a 3 month period in 2007 (1st January-1st April 2007). This period is coincident with most of the available water level and current data that had been obtained for calibration. A model verification simulation was carried out for the period April 1st to July 1st. The results of the EIS model water level calibration against Class A tide gauge data are summarised in Table 5. Root mean square errors (RMS) are in the range 0.16 m to 0.30 m. The Environment Agency guidelines for water levels are achieved for all locations. The phase error is also listed in Table 5. The phase error is within the range -4 min to +6 min and also in line with Environment Agency guidelines. Compared to the regional Irish Sea model calibration the result of the E regional model is moderately better for all stations except for Port Erin (which is close to the model boundary of the E regional model). The phase error has improved relative to the regional Irish Sea model and for 3 of the 5 stations there is no phase error at all. P090-12

13 Table 5: EIS model water level calibration results Class A gauges, RMS and phase error Class A water level station Root Mean Square error (m) Spring range error(%) eap range error Spring Range (m) eap Range (m) Phase error (min) 1 Workington % 5.7% Heysham % 7.3% Liverpool (Gladstone dock) % 6.0% Llandudno % 7.0% Port Erin % 7.1% Figure 9: EIS model mesh Current speed and direction calibration was undertaken against a number of sources, fixed locations (at the same locations offshore from Liverpool that the wave data was available for) and HF radar data (downloaded for free from The locations of the fixed stations are shown in Figure 10 and referred to as COA1 and COB1. Additionally on this figure, HF (high frequency) radar data is shown by red dots. At each of these locations wave height and near surface current speed and direction was available at 20 minute intervals. P090-13

14 Comparison between the model predictions and measured current speed data at COA1 and COB1 shows differences in current speeds of less than 0.1m/s giving a reasonable level of calibration and within the EA guidelines. Comparison of model flow vectors (depth-averaged) against the HF radar spatial data (near surface flow vectors) can be seen in Figure 11 with a reasonable level of agreement. The model was deemed to be calibrated for both water levels and currents following the above and additional analysis. Figure 10: Current meter measurements. Fixed stations (COA1 and COB1) and outline of radar grid P090-14

15 Figure 11: EIS model calibration. Vector current speed and direction comparison at flood tide. Blue vectors are measured currents from surface HF radar grid. Red vectors are simulated depth mean current speeds and directions. 4. RESULTS 4.1 Hydrodynamic Regime Once calibrated the hydrodynamic model has been run for a 30 day period. The main characteristics of tidal current patterns are: There is a general flow toward the east in the open sea and into the estuaries during the flood tide. Flood currents are generally greater than those on the ebb; The highest current speeds are seen to the north and south of the Isle of Man with velocities approaching 1.5 to 2m/s. P090-15

16 Peak ebb tide shows a strong flow out of the orth East Irish sea and also out of the Estuaries. The residual current speeds and vectors were calculated throughout the simulation and are presented in Figure 12 for the case without wind. In general throughout much of the southeast part of the region the residual tidal flow is small with areas around the estuaries where this becomes slightly larger; likely due to the freshwater inflow influence. However in the northern part of the study area there appears to be a large clockwise residual circulation centred less than 10km offshore of the north eastern tip of the Isle of Man. There appears to be a general residual flow from orth to South. The model was also run with a wind of 15m/s blowing from the dominant southwest direction. This strong wind has the effect of creating an eastward nearshore residual flow along the orth Wales and Wirral coastlines in the South and a northerly residual flow along the eastern coastal margins. This northerly residual partially interferes with the southerly directed flow in the northern part of the region but still leaves the clockwise residual circulation to the east of the Isle of Man. The hydrodynamic models were also run with an increase of mean sea level of 0.5m applied at the boundary of the regional Irish Sea model. Boundary conditions were then extracted and applied to the EIS model. Figure 13 shows the increases in maximum water levels. An assumption was made that the bed levels throughout the model remain the same; it would be expected however that there would be some accretion within the estuaries given an adequate supply which would reduce this increase in water depth. However the results do give an indication that an increase in mean sea level of 0.5m will cause an increase in high water levels of about 0.1m (on top of the 0.5m rise) within most estuaries with a larger increase of 0.3m (on top of the 0.5m rise) in the upper reaches of the Solway Firth (the northern most and largest estuary). Differences in low water levels were not so apparent although in the Solway Firth low water was found to be up to about 0.3m lower. 4.2 Sediment transport regime The sediment transport modelling was needed to help understand the existing sediment transport regime in Cell 11 and assess if pathways exist to supply sediment to the shore. The model was also used to assess the potential impact from an increase in mean sea level and storm surge. As well as calculating net yearly sediment transport pathways, the potential annual flux of sediment across a number of cross-sections, including the mouths of the main estuaries, was calculated for the existing and sea level rise scenarios. This provided a means of assessing the impact that sea level rise has upon the sediment transport rates. However in this paper only the general overall sediment transport patterns are presented. The sediment transport modelling included the effect of waves as an added enhancement to the bed shear stress. It is impractical to run the orth East Irish sea regional hydrodynamic and sediment transport models for all wave conditions throughout a year due to long simulation times. Therefore an approach was required where the yearly wave climate is reduced to a small number of representative wave conditions which when simulated with the hydrodynamic and sediment transport models and combined together form a representation of the yearly sediment transport. The characteristic offshore wave conditions were determined based on the contributions of different wave events in the 17-year P090-16

17 Met. Office wave time series at the model boundaries to the annual energy flux. This assumes that the sand transport rate from a given direction is related to the energy flux from that direction. This approach is described in Johnson, et al., Figure 12: Residual current speed (spring-neap tide) (m/s). P090-17

18 Figure 13: Changes in maximum water level for mean sea level rise of 0.5m. The wave model used the same mesh as that of the EIS hydrodynamic model, along with the derived representative waves, an associated wind field was also prescribed. Sediment type data from BGS was used to define spatially varying representative grain sizes. Rather than run the sediment transport model for a 30 day period, a representative tide was chosen. The sediment transport rates at a range of locations were examined in order that a two tide period of the 30 day simulation, when factored up to a year, produced reasonably similar transport rates and patterns to the 30 day simulation results. This proved to be approximately a mean spring tide. In the simulations where wave forcing is included the annual sand transport pattern is estimated as the weighted combination of the tide only case (calm case) and the six combined wave and tide cases. The calculated maps of sand transport rates for the six characteristic wave conditions and the pure tide condition were multiplied by the corresponding frequency of occurrence in a year and added together to create an annual sand transport pattern for the study area. The net yearly potential sediment transport rates and vectors are presented in Figure 14. It can be seen that in the southern part of the region the net transport is from west to east. This direction is mainly due to the higher flood tide current speeds in this region. Along the orth Wales coastline there is an indication of alongshore and onshore transport of sediment. Along the eastern margin in the south of the region, transport is predominantly directed towards the shore and P090-18

19 the estuaries. In the orthern region, the potential transport rates appear higher, especially between the Isle of Man and Scotland. Transport is directed towards and into the Solway Firth, but also southwards between the Isle of Man and the Cumbrian coast. The study therefore confirmed pathways for potential supply of sediment towards the shoreline in most of Cell 11, but along the SW facing Cumbrian coast there is no obvious pathway for the supply of offshore sediment towards the shore. Examination of Figure 1 shows that along the Cumbrian coast the nearshore contours are closer together whereas towards the south they are wider indicating that there has been more sediment accumulation where this region has acted as a sink for sediment. Figure 14: Yearly estimated sand transport from representative with tide and wave forcing. In Liverpool Bay at the south of the Cell 11 area some convergence is predicted at the mouths of the two estuaries (the Dee and the Mersey) where extensive sandbanks exist. Offshore sediment is directed towards these estuaries and there is a potential for sand sized sediment within the main channels to be transported out of them. These estuaries have accreted over the years mainly due to finer sediment accreting onto the intertidal areas. Analysis with a more detailed Ribble Estuary model showed that although there appeared to be a net potential export P090-19

20 of sediment in the main channel, there was a net import onto the intertidal regions. Offshore from the Ribble there is an onshore directed transport supplying sediment towards the estuary. Within the main channel of the Ribble Estuary the potential transport is out of the estuary. This convergence has helped form a series of sand banks at the mouth of the estuary. Within the intertidal margins of the estuary the net transport is directed into the estuary confirming the depositional nature of this estuary. It can be seen north of the Ribble Estuary that the net transport direction turns towards the north, coinciding and likely to partly be the cause of increased erosion risk along this shoreline. The sediment transport model was also run with increased mean sea level and for a surge event. For the surge event the model showed a general increase in transport towards the shoreline. For the increased mean sea level there was also a general increase in sediment transport towards the coast. 5. DISCUSSIO Two regional wave models have been setup and succesfully calibrated against the available data. It was found that wind speeds derived from the UK Met Office European Waters model needed to be increased by 15% inorder that appropriate nearshore wave conditions were obtained from the model. The results from the nearshore wave model will subsequently be used to undertake a joint probability analysis at over 50 locations around the Cell 11 coastline. A regional hydrodynamic model has been setup and calibrated against both water levels and current speeds with the specific aim of addressing uncertainties for coastal management decisions. This model has been used to help simulate and understand the tidal flow regime in the Cell 11 region in terms of flood, ebb and residual flow patterns. Using the hydrodynamic flow results as well as representative wave conditions, a sediment transport model has been used to simulate and examine both tide only and tide plus wave effects upon sediment mobility, to help identify and quantify the main sediment transport pathways within the Cell 11 region. The sediment transport modelling shows there to be a potential mechanism for the supply of sediment to the coastline and the main estuaries in Cell 11. The region can be divided into two main areas in terms of the general direction of offshore sediment transport. In the southern part of the Cell 11 region, Morecambe Bay to orth Wales, the general sub-tidal sediment transport direction is from west to east. This has the effect of being able supply sediment so the coast and shoreline in the south-eastern half of the region. In the northern area, there is a mechanism to transport sediment towards the Solway Firth, but further south the dominant transport direction is in a southerly direction parallel to the Cumbrian shoreline towards Morecambe Bay. The project has gone on to quantify the potential transport rates and the effect that sea level rise upon the rates of supply but this has not been reported on in this paper. In addition to the sub-tidal sediment transport modelling presented in this paper, results from the wave modelling have been used to calculate bulk littoral transport rates at 141 locations around the Cell 11 coastline. This has shown that in general the net littoral transport is in an easterly direction along the orth Wales coast and in a northerly direction along the eastern coastline. At the mouths of the main estuaries on the eastern coastline there is a net littoral transport towards the estuary mouths. P090-20

21 The modelling of waves, tides and sediment transport, together with interpretation and review of previous studies has helped to resolve a number of uncertainties identified in SMPs for the region. Key finding from the studies have included: - The sub-tidal sediment modelling confirmed and provided improved spatial understanding of shorewards sediment transport pathways previously proposed from coarser modelling (Pingree and Griffiths, (1979) and sand wave observations studies; - With raised mean sea levels the sediment pathways remain very similar, but the potential for shorewards transport is increased. - As the sub-tidal sediment transport is dominated by tidal flows, surge tides can have a significant effect, with potential for stronger shorewards transport during storm surges. - The finding that raising regional mean sea levels leads to amplification of high tides in the estuaries has implications for the allowances used for future sea level change in design of flood defences. The results from the modelling will help focus future coastal monitoring effort in CERMS (Cell 11 regional Monitoring Study). 6. COCLUSIOS The regional wave model of the north east Irish Sea provides coastal scientists and coastal defence managers with a significant improvement over previous regional or sub-regional models, which may not have incorporated the influence of the Isle of Man if the European Met Office wave model had been used for boundary conditions. This model, in conjunction with a new regional hydrodynamic model, has allowed the consistent assessment of sediment transport across the whole region, both in terms of the subtidal and littoral regions. The regional hydrodynamic and subtidal sediment transport modelling clearly shows sub-tidal sediment transport pathways, which are key to understanding the large scale sediment budgets for the coastal region. In the southern half of Cell 11, the potential sediment transport is directed from west to east, with an onshore component along the orth Wales coast. Similarly, in the north, the transport is directed towards, and into, the Solway Firth. However, along the Cumbrian coast, from St Bees Head to Morecambe Bay, the potential sediment transport pathway is parallel to the shoreline in a south easterly direction. The effect of this is evident when looking at the bathymetry of Cell 11 with narrow steeper shorelines in the north and much wider shallower margins in the south. This general onshore transport of sand sized sediment also provides a supply to the estuaries in Cell 11, especially the larger ones in the south of Cell 11, which have extensive sand banks around their mouths. This improved understanding will be used to inform future shoreline management decisions in the Cell 11 region, including the second round SMP which was being developed in parallel with these studies. The Joint probability Study is due to complete by the end of this year. This will provide definitive joint probability curves for large waves and high water levels for a range of return periods. These data will be available for subsequent more localised studies which require joint probability data for example to assess P090-21

22 performance of flood defences, coastal flood propagation models and preliminary design studies for coastal defences. ACKOWLEDGEMETS The authors would like to thank the Cell 11 SMP2 client group representative Fiona Crayston at Blackpool Council for overseeing this project. Also the help in providing data is acknowledged from Paul Wisse at Sefton Metropolitan Borough Council and Lee Swift at the Environment Agency. REFERECES Environment Agency (1998) Quality Control Manual for Computational Estuarine Modelling. Appendix A4. R&D Technical report W16 HAWKES, P.J., (2007) Joint probability analysis for estimation of extremes. Special Issue of Journal of Hydraulic Research. Paper 6. Johnson, H.K., Appendini, C.M., Soldati, M., Elfrink, B., Sørensen, P., (2001) umerical modelling of morphological changes due to shoreface nourishment. Proceedings of Coastal Dynamics 01, Lund, Sweden. Pingree and Griffiths (1979) Sand transport paths around the British Isles resulting from M2 and M4 tidal interactions. Journal of the Marine Biological Association of the UK, 59, P090-22