ARDMORE SEWAGE SCHEME MARINE OUTFALL SURVEY REPORT

Transcription

1 Document Ref.: 845/1/2001 ARDMORE SEWAGE SCHEME MARINE OUTFALL SURVEY REPORT Submitted to:- E. G. Pettit & Co. Limited, Consulting Engineers, Blackrock Road, Cork. Prepared by:- Irish Hydrodata Limited, Rathmacullig West, Ballygarvan, Co. Cork. Tel (021) Fax (021) th July 2001 Rev 10 th November 2003 EPA Export :02:19:39

2 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) CONTENTS Section Content Page 1. Introduction 2. Oceanographic Characteristics of Area 3. Field Survey Methodologies 4. Survey Results 5. Modelling Methods 6. Simulation of Discharges 7. Conclusions Appendix A - Model Output Plots EPA Export :02:19:39

3 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 1. INTRODUCTION 1.1 BACKGROUND TO STUDY This report presents the findings of marine survey studies conducted to assess the effects of a treated effluent discharge from the town of Ardmore to the sea. At present, partially treated municipal wastes discharge to the sea near the pier on the southern shoreline of the bay. Marine studies were conducted over a wide area as no preferred outfall route was identified prior to surveys commencing. The surveys locations and works were chosen to maximise information that could subsequently be used in assessing potential outfall locations. 1.2 STUDY BRIEF The objectives of the study as defined by E.G.Pettit & Co. was: - to make an assessment of the bacteriological effects of treated municipal waste water discharges to the sea for a number of outfall locations; - to select the most appropriate outfall location; The brief called for various scenarios to be focused on, particularly those relating to the summer season with onshore sea breezes. The study area was to include both Ardmore and Curragh bathing beaches to the south and north of the Black Rock. The legislatory requirements of the Urban Waste Water Treatment Directive and the Bathing Water Directive and the voluntary requirements of the Blue Flag Scheme were to be considered in making the assessment. 1.3 SUMMARY OF STUDY WORKS Following a full review of available data and bearing in mind the objectives of the brief the following study works were undertaken: - various field measurements in Ardmore Bay; - a two dimensional flow model was set-up for the sea area stretching from Ram 1 EPA Export :02:19:39

4 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) Head to Mine Head, a distance of approximately 12km. The model extended 5km offshore and was used to simulate water circulation; - a two dimensional dispersion model of the sea area outlined above was used to compare the impacts of discharges from various outfall points; The results of these works are described in the following sections of the report. 2 EPA Export :02:19:39

5 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 2. OCEANOGRAPHIC CHARACTERISTICS OF AREA 2.1 BATHYMETRY AND TOPOGRAPHY The town of Ardmore is situated some kilometres west of Dungarvan and rates as a significant urban centre on the southern seaboard of Ireland. The coast is exposed to the Celtic Sea and the shoreline is fringed by lowland, lying mostly within 15 metres of sea level and carrying a mantle of drift material. Sandy bathing beaches are present adjacent to the town. For this study information relating to water depths and general bathymetric features of the area were obtained from Admiralty Chart No These were supplemented by bathymetric surveys conducted during the course of the study. 2.2 TIDES AND WATER LEVEL CHANGES Astronomical Tides The tidal regime off the south coast is almost purely semidiurnal with two high waters (HW) and two low waters (LW) each day (24 hours). Limited data is available in the Admiralty Tide Tables (ATT, ref:2) while accurate predictions are available for the nearest standard port of Cobh. Data recorded during the 2001 study provides additional information on the tidal patterns in the area. The main features are summarised in Table 2.1. (Note: Chart Datum -2.5m Malin Datum) LEVEL Malin Head Chart Datum Mean High Water Spring (MHWS) Mean High Water Neap (MHWN) Mean Low Water Neap (MLWN) Mean Low Water Spring (MLWS) Table Tidal statistics (m) relative to Malin Head and Chart Datum. Storm Surges Storm surge refers to the rise in water levels above expected tidally induced values due to the action of wind stress and atmospheric pressure on the water surface. In the absence of any site specific data recourse was made to results obtained from the Institute of Oceanographic Sciences (UK) Continental Shelf storm surge model. This predicts the 50-year storm surge elevation for the sea area off Ardmore to be about +0.75m (ref: 6). 3 EPA Export :02:19:39

6 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 2.3 COASTAL TIDAL STREAMS AND CURRENTS The general circulation patterns of the Celtic Sea have been well documented (ref:1,5) through the years. Admiralty charts and tidal atlases provide a more specific indication of the current patterns. The tidal currents generally run parallel to the coast. Speeds are relatively low reaching maximum values of about 1.0 knots (.5m/s) during spring tides. 2.4 COASTAL WIND CLIMATE Met Eireann s network of coastal stations comprise a total of 8 sites at which reliable measurements of wind speed and direction have been collected over varying periods between 1940-to date. The coastal station closest to Ardmore is the one at Roches Point (40km). Information from this site (ref: 11) has been used to establish the likely wind patterns at Ardmore relevant to the study. During the bathing season (June-August) the onshore wind distribution will generally be as shown in Table 2.2a. The frequency of occurrence of moderate breezes (Force 4, 11 knots) from these directions (indicated as T, bearing from true North) are given in Table 2.2b while the occurrence of moderate to strong breezes (Force 5+, 17 knots) are given in Table 2.2c. SECTORS ( o T) June July August % 4.7% 6.0% % 7.1% 7.7% % 11.8% 11.6% Totals % 23.60% 25.30% Table 2.2a - Percentage occurrence of onshore winds (all speeds) for the months June-August. (Data: Roches Point Airport, , ref:11). SECTORS ( o T) June July August % 1.7% 2.1% % 1.3% 1.8% % 3.6% 4.4% Totals % 6.60% 8.30% Table 2.2b - Percentage occurrence of onshore winds greater than 11knots (5.5m/s) for the months June-August. (Data: Roches Point, , ref:11). SECTORS ( o T) June July August % 0.2% 0.3% % 0.1% 0.4% % 1.2% 1.4% Totals % 1.50% 2.10% Table 2.2c - Percentage occurrence of onshore winds greater than 17knots (8.5m/s) for the months June-August. (Data: Roches Point, , ref:11). 4 EPA Export :02:19:39

7 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 3. FIELD STUDIES METHODOLOGY Various survey measurements were made in accordance with the specification prepared by the consulting engineers. The various operations are summarised briefly below. 3.1 Drogue Tracking Cruciform shaped drogues were released from various locations adjacent to the shoreline on two dates to establish the general water movements during spring and neap tides. These drogues were set to track the water mass at depths of 0.5m, 3.5m and 6m below the water surface. 3.2 Dye Releases The effects of the proposed discharges were simulated by discharging Rhodamine WT tracer dye at different locations on five occasions. The dye was diluted with water to provide a patch that would mix readily with the surrounding waters in the manner of municipal effluent. The volume of dye released varied from 200ml to 2000ml depending on the location. For one of the releases dye was discharged continuously for a 12h period at a rate of 5 ml/minute. 3.3 Current Metering Doppler current meter profiling was conducted at three transects over a 12h period on a spring tide to establish circulation patterns in the bay. Along each transect speed and direction data was recorded at approx. 2 second intervals. In the vertical data was recorded in 0.5m depth bins. 3.4 Anemometer. Wind speed and direction data was recorded at hourly intervals during each days field measurement using a hand held Deuta anemometer. 3.5 Recording Current Meter & Tide Gauge Recording tide and current instruments were deployed at offshore locations adjacent to the Black Rock for a period of 17 days. 3.6 Echo Sounding Detailed echo sounding surveys were conducted off Ardmore beach and Curragh beach. Depth data were reduced to Chart Datum ( -2.5m Malin Head). 5 EPA Export :02:19:39

8 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 3.7 Horizontal Control The survey vessel was positioned by differential GPS with corrections taken from the nearest IALA transmitter. All positions are in Irish National Grid to an accuracy of +/- 1m. 3.8 Vertical Control All tidal heights and water depths are referenced to a TBM established on the northern end of the old pier. This was levelled in from Ordnance Survey benchmarks in the town. 3.9 Survey Activities The field works were conducted at various times during the months of May, June and July EPA Export :02:19:39

9 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 4. SURVEY RESULTS 4.1 DROGUE TRACKING Drogue Tracking No.1-24/5/2001 This high water drogue release took place along a line extending out from the rocky headland to the north of Ardmore Beach. The line is approx 400m due south of the Black Rock which separates Ardmore Beach from Curragh Beach. Six drogues were released. Their sails were set to track at depths of 0.5, 3 and 6m below the surface. Weather conditions on the day were good with light winds from the southeast direction. Tidal conditions corresponded to a spring tide. As the tide ebbed the drogues followed a south-south westerly trajectory. The surface drogues were deflected to the west by the southeasterly winds. Over the duration of the ebb typical drogue excursions were approx. 1km. At low water a number of drogues were recovered and re-released at differing locations. All were tracked for the flooding tide and were recovered at high water having travelled distances of up to 1km. (Figure 4.1). Drogue Tracking No.2-19/6/2001 This drogue release took place along the same line as adopted for release number 1. Weather conditions on the day were initially good with light winds from the southeast direction early in the morning. However, as the day progressed the wind speeds increased to a moderate southerly Force 4. Tidal conditions corresponded to an average tide. The release commenced approx. 3 hours after high water on an ebbing tide. The surface drogues tracked to the northeast against the expected tidal flow direction, probably the result of wind induced surface water circulations or drogue drag, while the deeper drogues travelled to the south for a short period. (Figure 4.2). All drogues were recovered at local low water and re-released at the original drop points. The drogues moved rapidly to the northeast and were recovered as they approached the northern shoreline of the bay, 3 hours later, having travelled distances of approx. 1.6km. 7 EPA Export :02:19:39

10 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) The drogues were re-released two more times over the duration of the tide and followed similar trajectories. 4.2 DYE TRACKING Dye Release No. 1-25/5/2001 This dye release took place at a distance of 300m from the shoreline, adjacent to Black Rock, during spring tides. Weather conditions on the day were good with moderate westerly winds that calmed later in the day. Two litres of Rhodamine dye were released on the sea surface. After release the patch moved steadily to the south (Figure 4.3). Five hours later, when the tide changed, the patch had expanded to a length of approx. 800m and was lying to the west of Ram Head. Dye Release No. 2 - Route 1-25/5/2001 This dye release took place at a distance of 300m to the east of Ardmore Point. Two litres of Rhodamine dye were again released on the sea surface. Winds were a moderate Force 4 from the south-south west. After release the patch moved steadily to the north-east (Figure 4.4) and away from Ardmore Bay. Tracking was discontinued after 4 hours when the winds increased to Force 5. Dye Release No. 3-13/7/2001 This dye release took place at a point adjacent to the southern shoreline approximately 200m from the high water mark. Weather conditions on the day were good with a light southerly wind (Force 1). One litre of Rhodamine dye was released on the sea surface. After release the patch remained relatively stationary for a long period of time, elongating in an east-west direction parallel to the shoreline. The patch moved eastwards around Ardmore Head as the tide changed. (Figure 4.5). Dye Release No. 4-13/7/2001 This dye release took place from a point in the centre of the bay at approx local low water. Winds were initially calm but later increased to a light Force 1-2 from the northwest. 8 EPA Export :02:19:40

11 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) After release the patch moved towards the north and adjacent to a tide line. The patch then began to elongate very rapidly in an north-easterly direction as it was sheared by the current. At approx HW-3 hours the patch, which at this time had extended over a wide area, began to move in a southerly direction (Figure 4.6). Dye Release No. 5 20/6/2001 This continuous dye release took place 300m to the southeast of the shoreline adjacent to the Black Rock during average tides. Weather conditions on the day were good with light winds in the forenoon giving way to moderate to fresh westerly winds by evening. Initially the plume travelled in a southeasterly direction on the ebbing tide (Figure 4.7). Later in the day as slack water approached and the ebb commenced the plume rotated in a clockwise direction, sweeping around to the west. As the flood strengthened, the plume adopted a northeasterly track away from the shoreline. 4.3 TIDE & CURRENT MEASUREMENTS A bottom mounted tide gauge was deployed approx 100m off the Black Rock for 17 days. It recorded water levels at 20-minute intervals. These were corrected for atmospheric pressure fluctuations and reduced to Chart datum. A time series plot of the data is presented in Figure 4.8. A recording current meter was deployed approx 200m of the Black Rock in 6m of water. It operated for 1 day after which impact with an unknown body damaged the unit and resulted in invalid readings for the remainder of the deployment. Acoustic doppler current profiling was conducted along the transects shown in Figure 4.9 during spring tides of 23/5/2001. Current speed and direction data were recorded at approx. 5 m intervals in the horizontal and 0.5m intervals in the vertical every hour. Depth averaged information has been extracted from this data for various locations and time series produced. 4.4 ECHO SOUNDING Echo sounding was conducted over a wide area off Ardmore as indicated in Figure 4.10, to establish the bathymetry and identify any unusual seabed features. The seabed depths are generally in keeping with those indicated on the admiralty chart for the area. 9 EPA Export :02:19:40

12 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 5 PREDICTIVE MODEL OF WATER QUALITY 5.1 REQUIREMENTS The main requirements of the models were that they should be site-specific and capable of incorporating real bathymetry, topography and tidal variations. The modelling approaches adopted were: (i) a two-dimensional flow field model (ii) a two-dimensional effluent dispersion model. 5.2 OVERVIEW OF MODELLING APPROACH AND METHODS A two dimensional depth averaged flow model (M2D, ref:10) was used to simulate the tidal circulation in the study area and provide an half hourly flow pattern for both the spring and neap tidal cycles. The effects of wind were included in terms of enhanced horizontal mixing along the axis of the wind vector. The model used for predicting the effluent dispersion patterns (TRACK, ref: 9) is based on the concept of particle tracking. With this technique, the continuum of dispersed contaminant is simulated by a cloud of discrete particles. The model operates on the same grid as that employed in the flow model D - FLOW MODEL Model Framework In the 2D coastal circulation model the bathymetry was defined on a rectangular grid with cells of horizontal dimension 25m x 50m. The model extended for 242 grid cells east-west (12km) and 200 grid cells north south (5km) with Ardmore Bay towards the west of the model area. (Figure 5.1). The large model area was necessary to ensure that numerical boundary effects did not adversely impact the predicted flows in the Ardmore Bay region. Current Analysis Method The computer model (M2D) was used to determine the current flow patterns based on the bathymetric grid described above. The model is based on a finite difference solution of the equations of motion of fluid flow and incorporates non-linear effects, wind induced currents and drying bank features. Essentially the area to be modelled is divided into a large number of discrete cells that represent the shape and bathymetry of the site. Boundary conditions are then applied in the form of tidal 10 EPA Export :02:19:40

13 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) oscillations and the equations of motion solved for each cell at successive time steps. Boundary conditions were initially taken from co-tidal charts (ref:3) and other references. These indicate typical spring tidal amplitudes of approximately 3.6m and 3.7m on the western and eastern boundaries with a phase difference of about 1 o. Several model runs were then conducted for a range of boundary parameters to obtain simulations equivalent to mean neap and mean spring tides. 2 D - Flow Model Calibration The mechanics of water flow are particularly amenable to mathematical analysis by finite difference solutions as employed in M2D. Calibration of the model is achieved by comparing predicted current speeds with field measurements and adjusting model coefficients as required. The method is robust in that once bathymetry and tidal elevations are specified to a reasonable accuracy and typical model coefficients employed, good predictions will be obtained without the need for specific adjustments. A typical model output, for maximum flood and ebb on a spring tide, is presented in Figure 5.2. Highest speeds occur to the south of the area, near Ram Head while the slackest ones occur off Curragh Beach, north of the Black Rock. Figure 5.3 compares measured currents at profiling locations C and D (refer to Figure 4.9) with those predicted by the model. The main features of the flow are reproduced to an acceptable accuracy. Simulated mid-depth drogue trajectories corresponding to the spring tide release of 25/5/2001 (figure 4.1) are shown in Figures 5.4a and 5.4b. Excursions and trajectories are simulated to a good accuracy D - DISPERSION MODEL (TRACK) Model Approach Dispersion was simulated using the particle-tracking model TRACK. In the model, the discharge of effluent material is represented by a number of discrete particles. As the simulation progresses through time a series of particles are released at the outfall location. During each time step the particles are moved horizontally by the current flows. In addition to these advective steps, each particle is moved by random steps along the coordinate axes in order to simulate the effects of horizontal diffusion. The particle step length, which simulated diffusion in the models, was 11 EPA Export :02:19:40

14 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) selected randomly in the range +/- infinity according to an appropriate Gaussian probability density function. Dispersion Coefficients In shallow coastal waters dispersion results from a combination of physical mechanisms. These principally relate to the current and the manner in which it varies both vertically and laterally. The greater the 'velocity shear' the more rapid will be the dilution of the effluent. The horizontal diffusion coefficient was determined from the effective value arising from the shear dispersion. The effect of wind is to promote more rapid mixing. This was simulated by an increased diffusion coefficient. Decay The process of bacterial decay was included in the model by evaluation of the probability of decay for each particle during each time step. This was expressed as a function of T 90 where T 90 is the time for 90 percent decay. An actual decay event occurred when a random number was less than this probability. In the simulations produced for this study, the decay time was defined to be 12 hours and so the results achieved equilibrium within 10% after one tide and within 1% after two tides. Model Simulations The simulations made by the model were of 24.8 hours duration (two tides) using a time step of 10 minutes. Each time step 600 particles were released giving particles overall. However, due to the effects of decay (T 90 = 12 hours) there were particles remaining in the model at the end of each simulation. Wind Wind statistics from Roches Point were used to determine the wind conditions during the 'bathing' season. This data provides simultaneous occurrences of wind speed and direction in 10-degree angular sectors and 13 speed bands. Onshore winds were considered those in the sector 070 degrees to 180 degrees inclusive. The onshore wind speed which was exceeded for 8% of the time (during the bathing season) in this sector was found to be approx. 5.5m/s. (speed band knots). The higher speeds of 8.5m/s (speed band knots) is exceeded for only 2% of the time. Based on this data the wind speed corresponding to a 5% occurrence is estimated to be 7m/s. 12 EPA Export :02:19:40

15 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) Calculation of Bacterial Concentrations Concentrations of effluent were estimated by counting the number of particles in each model grid cell (25m x 50m). This produced the number of model particles in a volume of water which was determined by the horizontal cell dimensions and the water depth at that point. If the water depth exceeded 5m then the vertical dimension was set to 5m. 5.5 DISPERSION MODEL CALIBRATION Verification of the combined flow/particle track dispersion model was achieved by comparing simulated drogue tracks with field data and similarly patch releases with dye track data. Model drogues and dye patches were released and tracked in the current field. Examples of these simulations are shown in Figures 5.5 and 5.6. The agreement is good in that the excursions and patch dimensions are very similar to those recorded. 13 EPA Export :02:19:40

16 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 6. SIMULATION OF EFFLUENT DISCHARGES Following evaluation of the field work data, modelling was undertaken to simulate effects of treated wastewater discharges to the bay. Six effluent release points (Figure 6.1) were chosen as being representative of wider conditions in the bay. The associated outfall parameters are listed in Table 6.1. Model simulations for each outfall location were made for spring and neap tides and various onshore wind conditions. In addition simulations were conducted for stream water discharges at points G, N and K. (Figure 6.1). Discharge Location Outfall Length from HW (m) A F J K L M Water Depth at Outer End (m Chart Datum) Table Outfall Dimensions. (Note: Chart Datum approximates to lowest tide level) 6.1 EFFLUENT CHARACTERISTICS Effluent parameters used in the simulations were: Flow Rate: Bacterial Concentration: Decay Time: litres per second; fc/100ml; T 90 = 12 hours The freshwater stream entering Ardmore Bay, adjacent to beach contains bacterial contamination. The level of contamination is variable but for the purposes of this study typical high and low values, as listed below, were adopted. Stream Flow Rate: Stream Faecal Bacterial Concentration: Decay Time: 120 litres per second; 5000 and fc/100ml; T 90 = 12 hours The decay time of 12 hours represents a conservative value, typically adopted for coastal waters. 6.2 MODEL RESULTS 14 EPA Export :02:19:40

17 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) The model simulations were run for two tidal cycles and outputs generated at 0.5-hour intervals. Outputs are presented in two formats: (a) (b) contour plots of bacterial concentration; time series plots of bacterial concentration on selected shoreline sites. The contour plot output (a) shows the movement of the effluent plume as it is advected and dispersed over the tidal cycle. Only high and low waters are presented in this report (Appendix A) corresponding approximately to times of maximum flood and ebb excursion. During calm or light wind conditions the plots show that plumes remain relatively compact with clearly defined trajectories. Increasing wind speed leads to greater mixing and greater dispersion of the plume. Simulations of dispersion corresponding to onshore wind conditions of 7m/s and 10m/s are presented. While the contour plot output (a) provides a good indication of the plume s impact on the shoreline it is possible to miss worst-case situations unless each output time (24 per simulation) is examined. Therefore, the time series plot output (b) is provided as a more rigorous method of comparing the impacts of differing discharge locations. The output represents the average plume bacterial concentration where it enters a shoreline strip and is derived from all model output times. The chosen shoreline strip locations are indicated in Figure 6.2 and are representative of Ardmore Beach, the Black Rock and Curragh Beach. Time series output plots for each strip are included in Appendix A with a summary of predicted bacterial concentrations extracted from these in Tables 6.2 to 6.4. During calm conditions all outfall locations are predicted to ensure compliance with Blue Flag requirements (<100fc/100ml) on the bathing beaches at Ardmore and Curragh. Onshore wind (7m/s) simulations indicate that the discharge locations are also satisfactory under these conditions. (Location J was not modelled for a 7m/s wind as this is a longer outfall than those at locations A and M and assumed to produce better results). A F J K L M Ardmore Beach 1 4 < Black Rock Curragh Beach < Table Predicted Faecal Coliform Concentration fc/100ml for Calm Conditions. A F J K L M Ardmore Beach Black Rock EPA Export :02:19:40

18 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03). Curragh Beach Table Predicted Faecal Coliform Concentration fc/100ml. for Onshore Wind - 7m/s (5% ile). A F J K L M Ardmore Beach Black Rock Curragh Beach Table Predicted Faecal Coliform Concentration fc/100ml. for Onshore Wind - 10m/s ( 2% ile). The impact of the freshwater stream can be significant as it discharges in the immediate proximity of bathing waters. In a worst case scenario bacterial levels in the stream plume may approach source levels (5000 to 50000fc/100ml). Results from model simulations of the discharges are presented in Table 6.5. These indicate peak calm weather values of 250fc/100ml for a source concentration of 5000 fc./100ml. (Note that the model averages over a 25m x 50m cell). Onshore wind promotes increased mixing reducing the bacterial levels accordingly. Location Length from HWM Calm Wind 7m/s Wind 10m/s Stream Stream Stream <5 <5 Table Predicted Faecal Coliform Concentration (fc/100ml.) on Ardmore Bathing Beach resulting from the freshwater stream with a bacterial concentration of 5000 fc/100ml. 16 EPA Export :02:19:40

19 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 7. CLOSURE 7.1 This report presents the findings of a marine study of the proposed treated wastewater discharges from the town of Ardmore. This study assesses the dispersive characteristics of the bay and makes recommendations for siting the outfall so as to ensure that local bathing beaches will maintain Blue Flag status. 7.2 The general oceanography of the region is typical of open coastal sites. The tidal currents flow parallel to the shoreline, sweeping in to the bay, with peak speeds in excess of 0.25m/s. Dispersion characteristics are good as shown by dye and drogue data. Tidal ranges in the area are approx. 3.7m on springs and 1.8m on neaps. 7.3 A two-dimensional flow model together with a particle track dispersion model was used to simulate the discharges. Recorded data from current meter, drogue and dye releases were used for calibration and validation purposes. Results in the form of graphical plots were prepared for 24 separate simulations. These are presented in Appendix A. 7.4 The impacts of proposed discharges on bathing water quality at three shoreline strip locations are summarised in Tables 7.1 and 7.2. These data are maximum values and have been extracted from the detailed data in Appendix A. Discharge Location Ardmore Beach Black Rock Headland A 1 11 <1 F J <1 0 0 Curragh Beach K L M Table Predicted Maximum Average Coliform levels (fc/100ml) at Selected Shoreline Locations for calm conditions. Discharge Ardmore Beach Black Rock Curragh Beach Location Headland A F J K L M Table Predicted Maximum Average Coliform levels (fc/100ml) at Selected Shoreline Locations for onshore wind conditions (5% ile). 7.5 The model results show that for the level of treatment provided any of the locations modelled will provide sufficient dilution to ensure bacterial levels on the bathing beaches remain within Blue Flag limits. 17 EPA Export :02:19:40

20 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) 7.6 The freshwater stream flowing onto the south western corner of Ardmore Beach (model location G) represents a significant source of bacterial contamination on the beach. The impact of this will depend on the bacterial concentration in the stream waters and flow rate at the particular time. For the characteristics adopted (120 litres/sec and 5000 fc/100ml) the model predicts calm weather bacterial levels on Ardmore Beach to be 250 fc/100ml. 18 EPA Export :02:19:40

21 Ardmore Sewerage Scheme - Marine Survey Report, July 2001(Rev Nov 03) REFERENCES & BIBLIOGRAPHY 1. Admiralty, 1968, Irish Coasts Pilot. The Hydrographer of the Navy, London. 2. Admiralty, 2001, Admiralty Tide Tables. The Hydrographer of the Navy, Taunton. 3. Admiralty, 1980, Chart 5058, Co-tidal and Co-range Lines for the British Isles and adjacent waters. 4. Admiralty, 1979, Chart 12049, Old Head of Kinsale to Tuskar Rock. 5. Admiralty, 1974 Tidal Stream Atlas for the Celtic Sea, NP Department of Energy, 1990, Offshore Installations: Guidance on Design, Construction and Certification. HMSO. 7. EC, 1976, (76/160/EEC), Council Directive of 8 December 1975 on the quality of bathing water. 8. EC, 1991, (91/271/EEC), Council Directive of 21st May 1991 concerning municipal wastewater treatment. 9. Hunter, J.R., 1986, The Application of Lagrangian Particle-Tracking Techniques to Modelling of Dispersion in the Sea, In: Numerical Modelling - Applications to Marine Systems (Ed. J. Noye), North- Holland. 10. Hunter, J.R., 1987, User manual for numerical hydrodynamic models of marine systems and associated plotting package, Centre for Marine Science and Technology, Curtin University of Technology, Report C Meteorological Service, Meteorological data for coastal station at Roches Point 19 EPA Export :02:19:40

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