Appendix 3. Hydrodynamic and sedimentation studies

Transcription

1 Appendix 3

2

3 Report EX 4945 Rev 3.0 November 2004

4 Document Information Project Report title Client Borough of Poole Client Representative Mr Stuart Terry Project No. DDR3553 Report No. EX 4945 Doc. ref. EX4945-Poole Harbour studies rev3-0.doc Project Manager T J Chesher Project Sponsor Dr M P Dearnaley Document History Date Revision Prepared Approved Authorised Notes 03/03/ jse tjc Interim calibration report 03//06/ jse tjc Full report (Draft for circulation) 31/10/ jvb tjc ES amendments 05//11/ jvb tjc mpd Final Prepared Approved Authorised HR Wallingford Limited HR Wallingford accepts no liability for the use by third parties of results or methods presented in this report. The Company also stresses that various sections of this report rely on data supplied by or drawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data. EX 4945 ii Rev 3.0

5 Summary Report EX 4945 November 2004 In order to improve access for vessels, Poole Harbour Commissioners (PHC) are proposing to undertake capital dredging of the approach channel to Poole, as follows: The Swash Channel is proposed to be deepened to 8.5mCD, maintaining the existing 130m base width with side slopes of 1:10; The Middle Ship Channel is proposed to be deepened to -8mCD and widened to 100m with side slopes of 1:5; The turning basin at the ferry port is proposed to be deepened to 8mCD with no change to the area footprint and side slopes of 1:5; Little Channel (to the north of the turning basin) is proposed to be dredged to 6.5mCD with no change to the area footprint and side slopes of 1:5. In November 2003, HR Wallingford was commissioned by Borough of Poole to undertake numerical modelling studies of the area in support of the Environmental Impact Assessment being undertaken by Posford Haskoning. After an extensive exercise to collate the available data and robust calibration and validation of the numerical models against the observations the models were used to assess the short term impacts of the proposed deepening on tidal currents and wave conditions and on patterns of erosion and deposition. The dispersion of fine material released during dredging and disposal activities was also modelled. Numerical models were also employed to assess the impact of a notional larger vessel on shipwash and drawdown. The results from the numerical models were further used to assess future maintenance dredging requirements for Poole as well as long-term impacts of the channel deepening on the coastal processes and morphology of Poole Harbour and Poole Bay. For further details please contact either Dr M P Dearnaley or Mr T J Chesher in the Estuaries and Dredging Group at HR Wallingford. EX 4945 iii Rev 3.0

6 EX 4945 iv Rev 3.0

7 Contents Title page Document Information Summary Contents i ii iii v 1. Introduction Background Scope of work Report structure Data collation Introduction Bathymetry Tidal levels and Tidal Currents Collated data Data collected specifically for present study Waves Sediment and Sedimentation Non-cohesive sediment data sources Cohesive sediment data sources Channel Design specification Hydrodynamics Methodology D Flow model Calibration Regional model Calibration Local model Calibration Baseline conditions Conditions with proposed channel Detailed 3D flow modelling at Poole Harbour entrance Introduction Model set-up Model calibration (spring tide) Model validation (neap tide) Changes to the flow field in Poole Harbour entrance as a consequence of deepening approach channel Changes to the flow field in Poole Harbour entrance as a consequence of deepening approach channel and dredging Chapman s Peak Sediment transport and channel infill Introduction Non cohesive sediment transport studies Modelling approach Model establishment and verification (baseline conditions) Post deepening conditions Cohesive sediment transport studies Introduction Model establishment and calibration Baseline conditions...37 EX 4945 v Rev 3.0

8 Contents continued Post deepening conditions Context simulation 1990 conditions before and after Middle Ship Channel deepening Summary of channel sedimentation predictions Assessment of the effects of vessel activity Introduction Vessel Shipwash wave activity Vessel generated waves Comparison of vessel wave energy with locally generated wave energy Summary Vessel-generated draw-down Coastal impact assessment Aim of this chapter Review of concerns and the present state of the coastline Mechanisms for effects Changes in hydrodynamics Changes in sediment transport and coastal morphology Summary of potential changes in coastal processes and morphology Swanage Bay Studland Bay Poole Harbour entrance Sandbanks coastline (including Hook Sands) to central Poole Bay Whitley Lake Brownsea Island Stone Island Lake Parkstone Bay Town Quay Western and Southern parts of Poole Harbour Long-term morphological changes within Poole Harbour Introduction Data Analysis Analysis of Numerical model predictions Impact of capital dredging Dispersion of dredged sediment Introduction Potential impacts in Poole Harbour arising from capital dredging operations Methodology Results of short term plume simulation Results of long term plume simulation Dredging at other locations Background concentrations within Poole Harbour Discussion Potential impacts in Poole Bay arising from capital disposal operations Methodology EX 4945 vi Rev 3.0

9 Contents continued Results of simulation Effect of placement at different position within the disposal area Background concentrations Discussion Considerations arising from changes in maintenance Conclusions Introduction Summary of studies undertaken Flow studies Wave studies Sediment transport studies Studies of long-term morphological changes within Poole Harbour Studies of dispersion from dredging and disposal activities Studies of Ship effects Assessment of impact on coastal processes References...80 Tables Table 2.1 Tidal levels at Poole Harbour Entrance (mcd)...4 Table 2.4 Volumes of material dredged from Poole Harbour Navigation Channel ( ) (NB each year is defined as from April to March)...6 Table 2.2a Offshore wave climate showing significant wave height against wave direction (Jan 1974 Feb 1992). Data in parts per 100, Table 2.2b Offshore wave climate for significant wave height against mean wave period (Jan 1974 Feb 1992) Data in parts per 100, Table 2.3a Offshore wave climate for significant wave height against wave direction Oct 1986-Mar Data in parts per hundred thousand...11 Table 2.3b Offshore wave climate for significant wave height against mean wave period Oct 1986-Mar Data in parts per hundred thousand...12 Table 3.1 Existing and predicted water levels at locations in Poole Harbour...17 Table 3.2 Minimum and maximum changes to water levels in intertidal areas...18 Table 4.1 Representative wave conditions for sediment transport enhanced by wave stirring...27 Table 4.2 Water levels at various tidal states...28 Table 4.3 Predicted sand infill in the navigation channel - baseline conditions...29 Table 4.4 Predicted sand infill in the navigation channel - after deepening...32 Table 4.5 Predicted wind-generated wave conditions in Poole Harbour...34 Table 5.1 Characteristics of modelled ships...41 Table 5.2 Predicted ferry-generated wave heights at selected points in Poole Harbour...41 Table 5.3 Draw down effect of the Barfleur and proposed vessel...43 Table 6.1 Coastal wave analysis locations...47 Table 6.2 Wave conditions simulated to assess the changes in wave energy at the coastline due to the channel deepening...48 Table 7.1 Area above Chart Datum for Admiralty Chart 2611 dated between 1984 and Table 7.2 Average change in intertidal area Table 7.3 Impact of deepened channel on intertidal area...58 EX 4945 vii Rev 3.0

10 Contents continued Table 7.4 Impact of deposition in basin on intertidal area in western harbour Table 7.5 Impact of release of fines from capital dredging Table 8.1 Summary of HR Wallingford background concentration measurements Table 8.2 Summary of EA sampled background concentration measurements Table 8.3 Proportion of placed fine sediment settling at temporary/permanent sinks in Poole Bay Figures Figure 1.1 Location map Figure 2.1 Tidal diamond and current meter positions from previous field measurement campaigns Figure 2.2 Tidal diamond and current meter positions from previous field measurement campaigns in the harbour Figure 2.3 Observed tidal variation at Poole Harbour Entrance Figure 2.4 Spatial distribution of sand and gravel (based on median grain size) in Swash Channel (also showing contour of peak speed of 1m/s from modelling study) Figure 2.5 Poole Entrance seabed sampling analysis sediment distribution Figure 2.6 Poole Entrance seabed sampling analysis median grain diameter Figure 2.7 Measured bathymetry change between 1999 and 2001 Swash Channel Figure 2.8 Measured bathymetry change between 2001 and 2003 Swash Channel Figure 2.9 Summary of present channel infill areas and volumes. Source: Poole Harbour Commissioners Figure 2.10 Proposed deepening in the Swash Channel Figure 2.11 Proposed deepening in the Middle Ship Channel and Port Area Figure 3.1 Numerical model areas Figure 3.2 Regional model mesh Figure 3.3 Comparison of measured and predicted (local model) water levels Figure 3.4 Water level data locations Figure 3.5 Comparison of measured and predicted (regional model) tidal currents Figure 3.6 Local model extent and mesh size Figure 3.7 Example detail of local model mesh. Top: Poole Harbour entrance; Bottom: Port area Figure 3.8 Comparison of measured and predicted (local model) water levels. Mean spring tide Figure 3.9 Comparison of measured and predicted (local model) water levels Jan Figure 3.10 Comparison of measured and predicted (local model) tidal currents April 1990 Figure 3.11 Comparison of measured and predicted (local model) tidal currents May 1990 Figure 3.12 Comparison of measured and predicted (local model) tidal currents 11 March 1990 Figure 3.13 Comparison of measured and predicted (local model) tidal currents 12 March 1990 Figure 3.14 Comparison of measured and predicted (local model) tidal currents 29 January 1990 Figure 3.15 Comparison of measured and predicted (local model) tidal currents 30 January 1990 Figure 3.16 Comparison of measured and predicted (local model) tidal currents 11 May 1990 Figure 3.17 Comparison of measured and predicted (local model) tidal currents Admiralty Diamonds Figure 3.18 Spring tide ebb current vectors. Baseline conditions. Figure 3.19 Spring tide peak ebb speed. Baseline conditions EX 4945 viii Rev 3.0

11 Contents continued Figure 3.20 Spring tide flood current vectors. Baseline conditions Figure 3.21 Spring tide peak flood speed. Baseline conditions Figure 3.22 Change in spring tide peak ebb speeds following channel deepening Figure 3.23 Change in spring tide peak flood speeds following channel deepening Figure 3.24 Water level analysis point locations Figure 3.25 Spring tide water levels: baseline and following channel deepening Figure 3.26 Model domain and mesh Figure 3.27 Model bathymetry Figure 3.28 Observed and modelled spring tidal levels Figure 3.29 Current meter locations Figure 3.30 Observed and modelled spring tide current speeds and directions (to) at 1.6m below the surface Figure 3.31 Observed and modelled spring tide current speeds and directions (to) at mid-depth in the water column Figure 3.32 Observed and modelled spring tide current speeds and directions (to) at 1.2m above the bed Figure 3.33 Vectors of near-surface, mid-depth and near-bed observed currents, at spring tide peak flood and ebb Figure 3.34 Observed and modelled spring tide current vectors at 1.6m below the surface Figure 3.35 Observed and modelled spring tide current vectors at mid-depth in the water column Figure 3.36 Observed and modelled spring tide current vectors at 1.2m above the bed Figure 3.37 Observed and modelled neap tidal levels Figure 3.38 Observed and modelled neap tide current speeds and directions (to) at 1.6m below the surface Figure 3.39 Observed and modelled neap tide current speeds and directions (to) at mid-depth in the water column Figure 3.40 Observed and modelled neap tide current speeds and directions (to) at 1.2m above the bed Figure 3.41 Vectors of near-surface, mid-depth and near-bed observed currents, at neap tide peak flood and ebb Figure 3.42 Observed and modelled neap tide current vectors at 1.6m below the surface Figure 3.43 Observed and modelled neap tide current vectors at mid-depth in the water column Figure 3.44 Observed and modelled neap tide current vectors at 1.2m above the bed Figure 3.45 Observed and modelled neap tide depth-mean current vectors Figure 3.46 Model bathymetry before and after dredging of approach channel; local change in bathymetry Figure 3.47 Modelled surface currents at peak flood and ebb, before and after dredging of approach channel Figure 3.48 Modelled mid-depth currents at peak flood and ebb, before and after dredging of approach channel Figure 3.49 Modelled currents at bed at peak flood and ebb, before and after dredging of approach channel Figure 3.50 Impact on modelled surface speeds of dredging of approach channel, at peak flood and ebb Figure 3.51 Impact on modelled mid-depth speeds of dredging of approach channel, at peak flood and ebb Figure 3.52 Impact on modelled speeds at bed of dredging of approach channel, at peak flood and ebb EX 4945 ix Rev 3.0

12 Contents continued Figure 3.53 Modelled tidal residual velocity at surface, before and after dredging of approach channel Figure 3.54 Modelled tidal residual velocity at mid-depth, before and after dredging of approach channel Figure 3.55 Modelled tidal residual velocity at bed, before and after dredging of approach channel Figure 3.56 Modelled flow through Section 1 at peak flood and ebb, before and after dredging of approach channel Figure 3.57 Model bathymetry before and after removal of Chapman s Peak; local change in bathymetry Figure 3.58 Modelled surface currents at peak flood and ebb, before and after removal of Chapman s Peak Figure 3.59 Modelled mid-depth currents at peak flood and ebb before and after removal of Chapman s Peak Figure 3.60 Modelled currents at bed at peak flood and ebb, before and after removal of Chapman s Peak Figure 3.61 Impact on modelled surface speeds of removal of Chapman s Peak, at peak flood and ebb Figure 3.62 Impact on modelled mid-depth speeds of removal of Chapman s Peak, at peak flood and ebb Figure 3.63 Impact on modelled speeds at bed of removal of Chapman s Peak, at peak flood and ebb Figure 3.64 Modelled tidal residual velocity at surface, before and after removal of Chapman s Peak Figure 3.65 Modelled tidal residual velocity at mid-depth, before and after removal of Chapman s Peak Figure 3.66 Modelled tidal residual velocity at bed, before and after removal of Chapman s Peak Figure 3.67 Modelled flow through Section 2 at peak flood and ebb, before and removal of Chapman s Peak Figure 4.1 PISCES structure Figure 4.2 Example wave height field for 1.7m, 4.5s waves propagating from 190 o N Figure 4.3 Example wave orbital velocity field for 1.7m, 4.5s waves propagating from 190 o N Figure 4.4 Initial distribution of bed deposits imposed in the SANDFLOW model Figure 4.5 Predicted annual erosion and deposition patterns in the approach channel. 0.25mm sand. Baseline conditions Figure 4.6 Net tidal patterns of erosion and deposition. Spring tide, 0.25mm sand. Baseline conditions Figure 4.7 Net tidal patterns of erosion and deposition. Neap tide, 0.25mm sand. Baseline conditions Figure 4.8 Net tidal patterns of erosion and deposition. Highest astronomical tide, 0.25mm sand. Baseline conditions Figure 4.9 Net tidal patterns of erosion and deposition due to a spring tide with wave stirring. 0.25mm sand. Baseline conditions Figure 4.10 Net tidal patterns of erosion and deposition due to spring tides. 0.18mm sand. Baseline conditions Figure 4.11 Net tidal patterns of erosion and deposition due to spring tides. 3.00mm sand. Baseline conditions Figure 4.12 Net tidal patterns of erosion and deposition due to spring tides with local wave stirring. 0.25mm sand. Baseline conditions EX 4945 x Rev 3.0

13 Contents continued Figure 4.13 Integrated patterns of annual erosion and deposition due to tides with wave stirring. 0.25mm sand. Post deepening conditions Figure 4.14 Predicted annual erosion and deposition patterns in the approach channel.0.25mm sand. Proposed conditions Figure 4.15 Increase in annual deposition following deepening mm sand Figure 4.16 Distribution of bed sediment in NE Poole Harbour Figure 4.17 Comparison of observed and simulated suspended concentration March 1990 Figure 4.18 Comparison of observed and simulated suspended concentration January 1990 Figure 4.19 Net tidal deposition and erosion during a spring tide. Baseline conditions Figure 4.20 Maximum suspended sediment concentrations. Baseline conditions Figure 4.21 Predicted annual accretion in NE Poole Harbour - baseline conditions Figure 4.22 Predicted annual accretion in NE Poole Harbour Post channel deepening Figure 4.23 Change in predicted annual accretion. NE Poole Harbour Figure 4.24 Predicted annual accretion in NE Poole Harbour conditions Figure 4.25 Predicted annual accretion in NE Poole Harbour conditions with Middle Ship Channel dredged to present day configuration Figure 4.26 Change in predicted annual accretion from Middle Ship Channel deepening Fig Figure 5.1 Ship-generated wave pattern Figure 5.2 Vessel generated wave heights (assuming constant water depth) Figure 5.3 Variation of wave height with water depth (12 knots) Figure 5.4 Calculated vessel draw-down Barfleur sailing at 6m/s Figure 5.5 Calculated vessel draw-down Proposed design ferry sailing at 6m/s Figure 6.1 Coastal wave analysis locations Figure 6.2 Example wave heights In Poole Harbour entrance: baseline and following deepening. 4.4m 12s waves from 220 o N Figure 6.3 Example wave heights In Poole Harbour entrance: baseline and following deepening. 3.0m 10s waves from 140 o N Figure 6.4 Example wave heights In Poole Harbour entrance: baseline and following deepening. 1.2m 4s waves from 120 o N Figure 7.1 Observed bed change in South Deep ( ) Figure 7.2 0mCD contour from Admiralty Chart 2611: Figure 7.3 Drogue tracks before and after channel deepening Figure 8.1 Location of simulated dredging Figure 8.2 Predicted (a) peak depth-averaged sediment concentration increases above background occurring over the dredging simulation and (b) deposition of fine sediment arising from dredging at Middle Ship Channel Figure 8.3 Time series of predicted depth-averaged suspended sediment concentration increases above background arising from dredging Figure 8.4 Predicted mean depth-averaged suspended sediment concentration increases above background occurring over the simulation Figure 8.5 Time series of suspended sediment concentrations at six locations within the Harbour Figure 8.6 Time series of deposition at six locations within the Harbour Figure 8.7 Envelopes of predicted peak deposition and peak concentration Figure 8.8 Predicted peak depth-averaged suspended sediment concentration increases above background arising from disposal at NW of site Figure 8.9 Time series of predicted depth-averaged suspended sediment concentration increases above background arising from disposal at NW of site Figure 8.10 Predicted mean depth-averaged suspended sediment concentration increases above background arising from disposal at NW of site EX 4945 xi Rev 3.0

14 Contents continued Figure 8.11 Predicted deposition of fine sediment occurring at the end of the simulation of dispersion arising from disposal at NW of site Figure 8.12 Predicted maximum deposition of fine sediment arising from disposal at NW of site Figure 8.13 Time series of predicted deposition arising from disposal at NW of site Figure 8.14 Predicted peak depth-averaged suspended sediment concentration increases above background from disposal at SE of site Figure 8.15 Predicted maximum deposition of fine sediment arising from disposal at SE of site Figure 8.16 Time series of predicted depth-averaged suspended sediment concentration increases above background arising from disposal at SE of site Figure 8.17 Time series of predicted deposition arising from disposal at SE of site Appendices Appendix A Appendix B Appendix C Appendix D Particle size analysis Details of TELEMAC, COWADIS, SUBIEF-2D and SANDFLOW models Representative wave calculations Wave changes at Poole Harbour Entrance due to deepening EX 4945 xii Rev 3.0

15 1. Introduction 1.1 BACKGROUND In order to improve access for vessels, Poole Harbour Commissioners (PHC) are proposing to undertake capital dredging of the approach channel to Poole (see Figure 1.1). The capital dredging is required for several reasons. Firstly, a deeper approach channel is required in order to meet the needs of modern ferries, the majority of which now have a draft of 6.5 metres or more. In addition, the dredging would also meet the needs of the short-sea vessels, which are progressively increasing in draft. In summary, therefore, the approach channel dredging is proposed to accommodate larger vessels in order to secure the long-term viability of the Port. In November 2003, HR Wallingford was commissioned to undertake numerical modelling studies of the area in support of the Environmental Impact Assessment being undertaken by Posford Haskoning. The hydrodynamic models were used to drive sediment transport models that simulate the transport and deposition of both sand and mud. The results from these models were used to assess future maintenance dredging requirements and the changes to existing patterns of erosion and deposition. In conjunction with analysis of existing data on morphological changes in the area, the result of this model was also used to assess long-term impacts of the channel deepening. The hydrodynamic models have also been used to provide information regarding the changes associated with the channel deepening to an assessment of impact on coastal processes. The results of the tidal flow modelling have been used to predict the likely dispersion of fine material released into the water column during the dredging activity inside the Harbour and the dispersion and fate of dredged material placed at the offshore licensed disposal site to the east of Swanage Bay. Suitable material derived from the maintenance dredging of the channel during the operational phase could be used for beach renourishment works within Poole Bay. The detail of the location or locations of any future beach renourishment works is currently unknown, as this will depend on the need for such works and the nature (i.e. particle size distribution) of the material arising from the maintenance dredging. Given that no specific schemes currently exist, the potential hydrodynamic and sedimentological impacts associated with renourishment have not been assessed in this technical report. 1.2 SCOPE OF WORK The scope of work for this study comprised the following: Establishment of new hydrodynamic and sediment transport models that were used to determine the effect of the channel deepening on the hydrodynamic and sediment regime of Poole Harbour and its approaches; Determination of the consequences of the channel deepening on the maintenance dredging requirement for the channel; Determination of the impact of dispersion of sediment from the dredging within the Harbour and from disposal at the licensed disposal site; Consideration of the potential impact of the development in respect of beach response; EX Rev 3.0

16 Consideration of the longer term morphological changes taking place within the Harbour and approaches; Consideration of ship effects in the context of the physical processes within the harbour and assessment of any changes to these linked to the proposed larger vessel; Provision of information to Posford Haskoning for input to the Environmental Statement. 1.3 REPORT STRUCTURE The remainder of this report is structured as follows: Chapter 2 summarises data sources used in this study. Chapter 3 describes the hydrodynamics of the area and how the various tide and wave fields area modified by the channel deepening. Chapter 4 summarises the sediment transport conditions within the study area and the impact of the proposed deepening on these fields. The assessment of vessel effects is presented in Chapter 5, and Chapter 6 describes the issues associated with consideration of the coastal impacts in the study area. Chapter 7 summarises an assessment of the longerterm morphological changes within Poole Harbour, and how the proposed deepening may affect these changes. Chapter 8 describes an assessment of the dispersion of fine material from dredging within the Harbour and from disposal of dredged material at the licensed disposal site. Conclusions arising from the study are presented in Chapter 9. EX Rev 3.0

17 2. Data collation 2.1 INTRODUCTION Data for the study was derived from a number of sources. Many of these sources were related to previous studies undertaken by HR Wallingford, and our experience of working at this location proved valuable in retrieving useful information both for model calibration and to understand the various physical processes operating at the site. A wealth of field data was collected in the Poole bay and Harbour area over the period , in relation to various studies that took place during this time including a previous approach channel deepening assessment and studies associated with the creation of a proposed offshore island. Data sets arising from these previous studies have been used in this study to calibrate the flow models using bathymetric information relating to the same period. The aim of the calibration procedure is to evaluate model parameters (e.g. bed friction), as a means of accurately reproducing the tidal current fields, and that these parameters are then considered to be appropriate in simulations using more recent bathymetric data (where calibration data has not been collected). In the case of the present studies particular current data was collected in the Harbour Entrance as a means of calibrating the vertical variation in the tidal currents simulated by a local area 3D numerical model. The key data sets used in this study are discussed in the following sections. 2.2 BATHYMETRY For this project, three bathymetric data sets were compiled: Representing conditions in 1990 (calibration condition); Representing present day bathymetric conditions (baseline conditions); Representing conditions after the proposed deepening (proposed conditions). For the model of conditions in 1990, the bathymetry was derived from a previously established bathymetry data set comprising information from: Depth soundings from Admiralty Charts 2611, 2175 and 2172; Additional soundings from Van Oord Dredging UK (surveyed 1990); Poole Harbour Commissioners (PHC) data, and Data collected by HR Wallingford and BP/Geoteam (Geoteam (1990)). For the model of the existing conditions the bathymetry was obtained by combining three data sources: Depth soundings provided by PHC covering most of Poole Harbour and the Swash Channel; LIDAR data provided by Borough of Poole (BoP) for the northern Harbour intertidal areas; The most up to date Admiralty Charts 2611, 2175 and 2172 for Poole Bay. For the model representing post deepening conditions: Schematic plans for the planned dredging were provided by PHC. EX Rev 3.0

18 Additional bathymetric data from surveys of the Middle Ship Channel and Swash Channel, pre and post dredge were also provided by PHC for 1998/99 (October 1998 and March 1999), 2001 (January and March) and 2003 (January and April). Channel infill values were derived from these to gain an indication of the rate and distribution of deposition for the calibration of the infill model. 2.3 TIDAL LEVELS AND TIDAL CURRENTS Collated data The locations of the tidal elevation and current observations used for the present studies are shown in Figures 2.1 and 2.2. The table below summarises the tidal levels at Poole Harbour entrance, taken from Admiralty Tide Tables. Table 2.1 Tidal levels at Poole Harbour Entrance (mcd) HAT MHWS MHWN MWL MLWN MLWS LAT Tidal water levels measured at Poole Harbour entrance and at the Continental Freight Ferry Terminal at Hamworthy were provided to HR by PHC as part of the 2003 strategy studies. Figure 2.3 shows the tidal variation over a spring neap cycle at Poole Harbour Entrance. The generally consistent shape of the spring tides can be seen with a double high water, although the relative height of the two high waters is shown to vary considerably. During the neap tide period the tide shape is shown to be extremely variable as during neap tide periods the tide curve is dominated by the higher order constituents (quarter-diurnal, sixth-diurnal). Velocity observations (speed and direction) from the Middle Ship Channel Study (HR Wallingford, 1990c) taken on 29 th and 30 th January 1990, 11 th and 12 th March 1990 and 11 th May 1990 were also recorded at positions inside the harbour. Speed and direction observations in Poole Bay were also recorded for the BP Island Study (HR Wallingford, 1990a). Tidal current data both inside and outside Poole Harbour was also obtained from tidal diamonds on the various Admiralty Charts covering the area. However, it should be noted that these diamonds results from data collected in the Harbour prior to the deepening of Middle Ship Channel undertaken in Data collected specifically for present study As part of the present studies tidal currents were measured using a vessel mounted Acoustic Doppler Current Profiler (ADCP) across three transects at the harbour entrance during two days: a large spring tide (8 th April 2004) and a neap tide (13 th April 2004). These observations were collected to aid understanding of the complex tidal currents in the harbour entrance where the combination of sharp bends in the flow and variable bathymetry creates complex, secondary, currents. The data was also collected to provide calibration data for a detailed 3D numerical flow model of the harbour entrance (See Section 3.5). This model was necessary because the 2D depth integrated model used for the general impact studies would not be able to represent the vertical variation of current direction caused by secondary currents. This 3D model was used to study the detailed 3D currents in the harbour entrance and the impacts of a number of EX Rev 3.0

19 minor adjustments to the proposed dredged channel in the vicinity of the harbour entrance. 2.4 WAVES In a previous study (HR Wallingford, 1995) the HR Wallingford HINDWAVE model was calibrated to provide an offshore wave climate using wind data from the anemometer at Portland Coastguard Station. This wind record covered 18 years from January 1974 to February HINDWAVE uses the JONSWAP wave generation formulation which hindcasts wave conditions at a given location from wind conditions and fetch lengths measured at constant angular increments from the wave prediction point. HINDWAVE produces a set of site-specific offshore wave forecasting tables, giving wave height, period and direction for a wide range of wind speeds, directions and durations. HINDWAVE then uses these tables with the wind data to produce a synthetic wave climate at the offshore wave prediction point. In a subsequent study (HR Wallingford 2003a), the above data was supplemented with additional wave climate data derived using HINDWAVE with UK Met. Office European Model wind data for a location offshore of Poole Harbour. The offshore point from which the fetches were measured was located at W N which lies approximately 6km east of Durlston Head, which is close to the local model offshore boundary. This data set covers the period October 1986 to March 2001, and information from these two sources was used in this study. These two wave climates are presented in Tables 2.2 and 2.3 (pp 8-9). Predictions for locally generated waves as stated in Appendix B of HR Wallingford (1990b) and in the Poole Borough Coastal Strategy Study (HR Wallingford, 1995) were used to select wave values for the mud modelling with SUBIEF-2D. These values were revised in line with recent developments in the understanding of wave propagation in shallow waters as described in Section SEDIMENT AND SEDIMENTATION Non-cohesive sediment data sources Information on the composition of the seabed is derived from the following sources: Hydrographic surveys by BP/Geoteam (Martin, 1988 and Geoteam, 1990). PHC/HR Wallingford/BP/University of Southampton survey which consisted of core samples and grab samples (HR Wallingford, 1991). Grab samples collected and analysed by HR Wallingford (HR Wallingford, 1990c). Most recent studies have included; seabed samples from the Middle Ship Channel (BoP, tested by ACS Testing (provided by PHC) and 46 grab samples collected on behalf of Bournemouth Borough Council in the Swash Channel (Halcrow, 2003). The spatial distribution of the particle sizes in Swash Channel is shown in Figure 2.4. Seabed sampling at the Harbour entrance undertaken as part of this study. Analysis of this data to provide the particle size distribution and median grain diameter is presented in Figures 2.5 and 2.6. The full particle size analysis is presented in Appendix A. This information highlights both the variation in the median particle size distribution within the study area, ranging from medium sand in Poole Bay to fine silts in Poole EX Rev 3.0

20 Harbour, and also the distribution of the surface sediments. The BP/Geoteam surveys found that at the entrance to Poole Harbour the sand deposits are relatively thin overlying rocky substrata. Finer material in the entrance to Poole Harbour would be removed by the strong tidal currents experienced there, leaving relatively coarse surface deposits. Information on the magnitude and distribution of channel infill was derived from two sources. Firstly PHC bathymetric data sets for the period 1999, 2001, and These surveys were pre- and post- dredging surveys for maintenance dredging campaigns. Changes in the bed level In the Swash Channel over the periods between 1999 and 2001, and 2001 and 2003 are presented in Figures 2.7 and 2.8. Secondly dredging records were supplied by PHC covering maintenance and capital dredging for the entire navigation channel from Swash Channel to the ferry turning basin. This information is summarised in Table 2.4 below (NB Middle Ship Channel and Ro-Ro Terminal volumes include both sand and silt, whereas Swash Channel is predominantly sand). The table also shows volumes dredged from the numerous sites elsewhere within Poole Harbour. Table 2.4 Volumes of material dredged from Poole Harbour Navigation Channel ( ) (NB each year is defined as from April to March) Year Middle Ship Channel Volume dredged (m 3 ) Swash Channel Turning Basin Elsewhere within the Harbour 1990/ / / / / / / / / / / / / / A summary assessment of the typical volume of infill and distribution, determined by PHC is shown in Figure 2.9. From the data sources on channel sedimentation above it is concluded that the average annual volumes of infill in the navigation areas are as follows: Swash Channel (sand) order 21,000m 3 /year Middle Ship Channel (predominantly sand) order 20,500m 3 /year Turning basin (predominantly silt) order 13,000m 3 /year. Elsewhere, the volume removed as part of maintenance activities is approximately 34,000m 3 /year, comprising predominantly silt. This volume is mostly associated with the maintenance of the various marinas and slipways within the harbour EX Rev 3.0

21 Information on suspended sand concentrations is relatively sparse. In 1990 sand flux measurements were made in Poole Bay on a bed frame at three locations (SF1, SF2, SF3 see HR Wallingford, 1990a). However, this information highlighted the relatively low levels of suspended sediment concentration in that area, which is related to the low tidal currents. Whilst the model can be shown to agree in the area of the observations (by also reproducing low levels of suspended sediment concentration) this is not a robust procedure for calibration of the model in areas of high sediment transport (most notably in the entrance to Poole Harbour and the northern end of Swash Channel and East Looe Channel). Consequently calibration of the sand transport model was undertaken by comparison with the patterns of channel infill from bathymetric analysis Cohesive sediment data sources The available information on fine sediment concentrations in the area came from the previous Middle Ship Channel Studies (HR Wallingford, 1990) and from sampling undertaken by the Environment Agency (EA). The EA data indicated that within the main channels in Poole Harbour general background concentrations are of the order of 10mg/l or less. However, background concentrations on intertidal areas or in creeks within intertidal areas were higher and appeared to be of the order of 50mg/l. The HR Wallingford data (derived from spring tide conditions) which indicated that outside the main channels (depth-averaged) mean concentrations in the Harbour were 20mg/l or more (30mg/l or more near the bed), with peak concentrations of over 50mg/l regularly experienced on calm spring tides. During times of increased wave activity (depth-averaged) mean concentrations in the Harbour were mg/l with peak (depth-averaged) concentrations mainly of the order of mg/l. Although the HR Wallingford observations appear generally higher than those from the EA the combination of these observations would suggest low suspended concentrations during calm periods with a fair degree of spatial variation. During storm events concentrations rise markedly, again with a level of spatial variation due to the spatial variation of the locally generated wave conditions. As described above, PHC supplied HR with volumes of material removed from the turning basin off the Continental Freight Ferry Terminal as part of the maintenance dredging of the area. These results gave a typical infill rate of approximately 13,000 cubic metres per year predominantly of silts (Reference PHC Pers. Comm.). This infill rate in combination with the 1990 observations of suspended solids concentrations provided the target data for the numerical cohesive sediment model calibration. Sediment settling velocity observations were undertaken in Holes Bay in June 1988 (HR Wallingford, 1988). The observations showed a range of median settling velocities in the range mm/s for observed suspended solids concentrations in the range mg/l. The bed exchange parameters (critical stress for erosion, erosion rate coefficient) vary greatly over an area such as Poole Harbour and are defined as part of a model calibration procedure. For the present studies the critical stress for erosion was defined by using the same value (0.2 N/m 2 ) as that used in the previous Middle Ship Channel Studies (HR Wallingford, 1990b) and an erosion constant of was used. Information about the distribution of cohesive sediments on the bed came from the 1990 survey (HR Wallingford, 1990b) and a general view of the distribution of bed sediments is shown in Figure EX Rev 3.0

22 2.6 CHANNEL DESIGN SPECIFICATION In addition to measured bathymetric and dredging data, PHC provided HR Wallingford with the proposed channel deepening information (PHC, 2003) in which the changes to the navigation channel are as follows: The Swash Channel is proposed to be deepened from 6.0mCD (published depth) to 7.5mCD (published depth) with dredging down to 8.5mCD, maintaining the existing 130m base width. The proposed side slope will be dredged at 1:10 (steeper than the present 1:30). The Middle Ship Channel is proposed to be deepened from 6mCD (published depth) to 7.5mCD (published depth) with dredging down to -8mCD and also widened on its northern side over most of its length to increase the total width from 80m to 100m. The proposed side slopes will be dredged to 1:5. The turning basin at the ferry port is proposed to be deepened from 6mCD (published depth) to 7.5mCD (published depth) with dredging down to 8mCD with no change to the area footprint. The proposed side slopes will be dredged to 1:5. Little Channel (to the north of the turning basin) is proposed to be deepened from 5.0mCD to 6.0mCD (published depth) with dredging down to 6.5mCD with no change to the area footprint. The proposed side slopes will be dredged to 1:5. Based on the most recent bathymetric survey of the study area, in the Swash Channel the main areas of cut are in the far north west of the channel (Northern Shell Bay area) where up to 3 metres of sediment would be removed and on the south eastern bank at the seaward end of the channel (removal of up to 2.5 metres). Generally, up to 1.5 to 2 metres of sediment shall be removed from the Swash Channel to achieve the required depth. Figure 2.10 shows the change to the bathymetry from the proposed dredging in the area. The left-hand figure shows bottom of the channel side slopes and a hypothetical line where the side slope would intersect a level of 0mCD. The right-hand plot shows the actual depth change that would be introduced by the scheme. In the Middle Ship Channel, also based on the most recent bathymetric information, in the vicinity of the Aunt Betty buoy the proposed depth would require removal of up to 2.5m of material, mostly on the northern/eastern slope. Further along the channel towards Poole, close to the Diver Buoy, where the channel is to be widened, over 3m of material would require removal to achieve the proposed depth. Figure 2.11 shows the change to the bathymetry from the proposed dredging in the Harbour. As above the left hand plot indicates the change in the proposed side slopes whereas the right hand plot shown the actual bed change required for the proposed deepening scheme. EX Rev 3.0

23 Table 2.2a Offshore wave climate showing significant wave height against wave direction (Jan 1974 Feb 1992). Data in parts per 100,000 Wave direction in degrees North H1 To H2 P(H>H1) Parts per thousand for each direction EX Rev 3.0

24 Table 2.2b Offshore wave climate for significant wave height against mean wave period (Jan 1974 Feb 1992) Data in parts per 100,000 H1 To H2 P(H>H1) Mean wave period in seconds (Tm) Parts per thousand for each period EX Rev 3.0

25 Table 2.3a Offshore wave climate for significant wave height against wave direction Oct 1986-Mar Data in parts per hundred thousand Wave direction in degrees North H1 To H2 P(H>H1) Parts per thousand for each 34 direction EX Rev 3.0