MAY0 COUNTY COUNCIL W.ESTPORT MARINE SURVEY

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1 MAY0 COUNTY COUNCIL W.ESTPORT MARINE SURVEY VOLUME 1 HYDRODYNAMIC & WATER QUALITY MODELLINQ Tolaphano Fwlmlk a=z Morton

2 PREFACE,* I Kirk McClure Morton, in conjunction with MarEnCo, were commissioned to undertake a survey of the water quality and tidal movement in Westport Bay in order to make provision for the proper treatment and disposal of waste water and contaminated storm water from Westport and its environs. The purpose of the survey was to establish both the present status of the marine ecosystem in the vicinity of the existing untreated waste outfalls and to establish the magnitude and influence of factors such as tidal currents, wind etc on the movement of treated effluent discharged in the future at a number of potential discharge points. This report presents the results of the study. The report is divided into two volumes: (1) Hydrodynamic and Water Quality Modelling. (2) Baseline Survey. Volume 1 comprises the following sections: (i) (ii) Synopsis. Beneficial uses of Westport Bay. (iii) (iv) Morphology of Westport Bay. Part 1 - Hydrodynamic Modelling. (v) Part 2 - Water Quality Modelling. Appendix 1 Tidal Data. Appendix 2 Appendix 3 Wind Data. Confirmatory soundings within the Study Area.

3 SYNOPSIS Part 1 - Hydrodynamic Modelling The tidal flow characteristics of Clew Bay were calculated with the aid of a computerized mathematical model developed at Hydraulics Research Ltd. The tidal computations were carried out using an implicit finite difference method over the complete tidal cycle of 12 hours and 20 minutes. The tidal flow regime depicted by the model was successfully calibrated and validated for both spring and neap tides by giving close agreement with field survey observations. The hydrodynamic model was therefore considered to be capable of accurately assessing the tidal flow regime within the study area of Westport Bay during any given set of tidal conditions. Part 2 - Water Quality Modelling Computational models for the management of water quality in Westport Bay comprised a suite of post processing packages to the TIDEWAY-2D hydrodynamic model. The water quality models were verified by simulating the dispersal of two discrete conservative dye releases and one continuous conservative dye release within the study area. The advection of the dye releases were successfully validated for both Spring and Neap tides by giving close agreement with field survey observations. The water quality models were therefore considered to be capable of accurately assessing the dispersion of effluent from proposed discharge points during any given set of tidal conditions. A series of 20 model runs were designed to provide an assessment of the effects on the receiving waters of a release of effluent from a proposed sewage treatment works and long sea outfall within Westport Bay. It was recognised that the Blue Flag bathing beaches at Bertra were not at risk The main emphasis of the water quality simulations was therefore to aid identitication of the optimum discharge point with regard to the Shellfish Directives. The plume simulation runs considered six potential outfall locations for a range of Dry Weather Flows treatment options and meteorological conditions. Background bacterial counts from river inputs and septic tanks were also considered.

4 BENEFICIAL USES OF WESTPORT BAY Westport has a population of some 4,000 persons within a catchment area likely to be serviced by a proposed Waste Disposal Works. At present the main drainage system in the Urban Area discharges via two trunk sewers to twin outfalls which terminate at or near the high water mark in Cleavlagh Bay to the north of Roman Island and Westport Quay. The eastern half of Clew Bay, east of a line joining Old Head near Louisburgh on the southern shore to Gobbaun Point at Dooghbeg on the northern shore, is designated under the Fisheries Acts 1959 to 1980 as an area where Aquaculture by the Clew Bay Oyster Co-operative under licence is permitted up to the high water mark A substantial industry has developed pursuant to this designation. The coastline from Westport to Murrisk, Lecanvey and Louisburgh is a particularly scenic part of County Mayo and the bathing area at Bertra is a Blue Flag Beach in accordance with the Bathing Water Directive. Other beneficial uses in the Inner Westport Bay Area include sailing, with the Sailing Club based at Rossmoney and the Glenans Sailing School located on the southern extremity of Collan More Island. The many islands of the inner part of Clew Bay are a collective amenity for wildlife, boating and picnicking. The Westport quadrant of Clew Bay is one of the principal Inshore Boat Angling centres in Ireland and hosts an annual International Angling Festival. A monkfish "hot spot" exists near the Perch which is located due west of Roman Island and due south of Rosslee Point. Westport Quay and Harbour, together with Westport House, impart a distinctive character to the area from Rossymailley inland to the Urban Area itself. There is every prospect that with an otherwise clean water environment, further beneficial uses in addition to those outlined above will be developed as the Tourism Industry in this part of County Mayo develops. Mayo County Council propose to make provision for the proper disposal of the existing effluent load, both domestic and industrial from Westport in accordance with its obligations under the Bathing Water and Shelliish Directives, and other EC Directives which may apply (including EC Commission Proposals for Directives), so as to protect the environment of Westport Bay. It is intended that these provisions will include for the future expansion of Westport and its industrial base.

5 Bertra Strand Bertra is a gravel spit attached to the mainland coast west of Murrisk as shown in Figure(ii). It runs out some 2 h from this attachment point in a north-north-easterly direction terminating in Bertra Island, which itself is constructed of a series of gravel recurves. Both the island and the spit are topped by sand dunes. Half-way along Bertra spit, there is an offshore transverse gravel ridge that would appear to be acting as a sediment source for the spit. The dune system on the spit has a relatively recent morphology, originating around 1900, based on a comparison of OS maps. Also, when the present position of Bertra spit is compared to its position in 1830, we observe the that whole struc+e has Y moved some 20 m to the east. The whole system is therefore dynamic and further changes are likely in the future. The importance of Bertra Strand as a tourist site has recently been acknowledged by acquiring a blue flag from the European Community. Its popularity may ultimately lead to the destabilization of the dune system if vegetation is destroyed by human impact. At the southern end, the underlying gravel ridge is at its lowest and if exposed it could easily be overtopped at high water creating a new outlet for Westport Bay. The Dorinish Complex The Dorinish Complex consists of Dorinish More, Dorinish Beag, and their associated barriers and spits as shown in Figure (iii). The Dorinish Beag north spit and the Dorinish More south spit have joined to form the Dorinish Barrier. The Barrier and Spits are on average some 7 m to 8 m above low water and are rarely overtopped except under extreme storm conditions. It is estimated that around 75% of the Dorinish Islands have been eroded and that the complex has passed its most active phase. Though material is no longer passing over the barrier or spits, material is still being transported along the barrier.

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7 - Boulder/cobbles Gravel Matrix Sand Figure (ii) Bertra Strand

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10 1.0 MODELUNG THE TIDAL FLOW General The bathymetry of Westport Bay comprises a complex pattern of islands, shoals and drying areas. There are, nevertheless, computational packages available which are capable of accurately simulating the tidal flow processes within Westport Bay provided that the estuary's somewhat unique features are clearly identified and understood. Clew Bay, and in particular Westport Bay, are well known to the Marine Survey Team. The Queen's University of Belfast have studied both seasonal and long-term sediment transport mechanisms within the area for several years. The tidal flow regime in Westport Bay is dependent on its interaction with the main body of water within Clew Bay and is also influenced by the movement of water between the archipelago of islands to the north of the study area. Extensive fieldwork would be required to establish the flow conditions close to the boundaries of Westport Bay so it was considered necessary to locate the boundaries to the hydrodynamic model at some significant distance from the study area. A hydrodynamic model covering much of Clew Bay was therefore developed. The tidal flow regime within Clew Bay was established by a fully computerised suite of tidal processes, TIDEWAY-2D, developed by Hydraulics Research Ltd. The tidal currents were calculated by depth integrated flow equations which are solved using implicit finite difference techniques based on the Crank-Nicholson method. Modelling the Seabed Characteristics The analysis required the bathymetry of Clew Bay to be modelled accurately. This required a grid system to be set up over the entire region and the depth value at each nodal point input to a computer. A grid spacing of 90 m was used throughout the whole of Clew Bay in order to model the changes in depth accurately. A dynamically linked "patched model of the entire survey area was then incorporated into the main model with a fine grid spacing of 30 m. This required over 72,000 points to adequately model the seabed.

11 The data were obtained from Admiralty charts, the Survey Source Data Sheets for the area and additional soundings recorded during the survey period as described in Appendix 3. All points were converted to the same datum, chosen at LAT (Lowest Astronomic Tide), before being entered to the computer. It is important to note that the Consulting Engineer's Report relates all levels to Ordnance Datum at Malin Head. The level at LAT is 2.59 m below OD Malin. A contour map of the seabed generated by the computer model is shown in Figure 1. The bathymetry includes all of the information dray from Admiralty Charts, Source Data Sheets and confirmatory soundings. The contours have been drawn relative to LAT at 5 m intervals up to a depth of 30 m. A more detailed map of the study area is also presented in Figure 2, again created by the digital ground modelling process. The contours in this figure have been drawn at 2 m intervals ftom LAT to 6 m below LAT. 1.3 Implementation of the Tidal Flow Model The tidal computations were performed on a SUN 4 Workstation using an implicit finite difference method. The flow results, ie water levels, velocities and discharges between each model element were stored in the computer at 10 minute intervals throughout the tidal cycle for subsequent processing. An implicit finite difference method was used because the solution to the dynamic regime should always be stable. However, to maintain accuracy, the time step was limited to five times the explicit stability limit Ate where: and s dmax Vmax = minimum grid spacing = maximum water depth = maximum velocity The time step used for the Westport Bay model was 3.3 seconds.

12 .~. To prove stability, the complete model was re-run doubling the time step to 6.6 seconds for three tides. Tidal heights and velocities predicted after the second and third tides were compared and found to be identical showing that the model had again given a stable solution. Calibration and Validation of the Tidal Flow Model The tidal flow model was calibrated for spring tides. Adjustments were made to both the model bed roughness and turbulent diffusivity until the model results were representative of field survey obsemations. The hydrodynamic model was then validated by running the model for neap tides, without any further adjustment to the hydrodynamic parameters, and comparing the model results with additional field survey obsemations. A field survey was undertaken to provide general information about the tidal regime in Clew Bay and data for calibrating and validating the model. This included the measurement of simultaneous tidal levels at seven locations within Clew Bay as shown in Figure 3. The data was recorded on 10 August 1990 during spring tides and on 14 August 1990 during neap tides. Tidal levels were recorded every 15 minutes over the complete tidal cycle of 12 hours 20 minutes. In all cases, tidal levels were reduced to ~.. ~.~ ~.-.~. - Ordnance Datum at Poolbeg, ~ ubk. /- -~. ~/- One of the recording stations comprised a continuous trace from an automatic tide level recording system, supplied by W S Oceans Ltd which was positioned close to the entrance to Clew Bay as described in Appendix 1. The data from this station was used for the boundary conditions to the hydrodynamic model. Velocity measurements were also taken at two locations, Stations A and B, within the estuary as shown in Figure 3. The current measurement stations were established within the channels to the north and south of Inishraher where the main tidal flow was expected. These stations were set up specifically to calibrate the hydrodynamic model. Measurements were recorded during spring tides, corresponding to the times of the tidal height measurements.

13 Fig 1. Inner Clew Bay Bathyrnetry 90m Grid Model

14 Fig 2. Inner Westport Bay Bathymetry 90m Grid Model

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16 A comparison between the model tidal height results and the field results used to calibrate the hydrodynamic model are presented in Figures 4 to 9. The close agreement between the tidal height range and the slope of the curves, indicating current velocity, show that the tidal heights have been modelled correctly. At Mallaranny Pier and Old Head Pier, Figures 4 and 9, it can be observed that the model has correctly depicted a smooth tidal curve, similar to that recorded in the open waters of Clew Bay by the automatic recording tide gauge. The results show, however, that at Mallaranny Pier during low tide conditions on 10 August 1990, the water level was higher than that predicted by the hydrodynamic model. This was due to strong south-westerly winds on the day of the fieldwork causing a rise in sea level along the northern shoreline due to wave set up. At Mayo Sailing Club and Leckanvy Pier, Figures 6 and 8, it can be observed that the model has depicted the correct tidal curve close to high water. The sites at which the field measurements were recorded, however, dried out close to low tide. The model results represent the closest model point in the model bathymetry matrix immediately offshore to the actual position of the field station. Within Westport Bay itself, it can be observed that the shape of the tidal curves become more irregular due to the influence of the islands, shoals and drying areas. Again, this phenomenon was observed by the automatic recording tide gauge positioned off Inishraher. At Westport Quay, Figure 7, it can be observed that both the model and fieldwork results exhibit a period of approximately 1.5 hours at high water at which the water level remains constant. A comparison between the model velocity results and the field results used to calibrate the hydrodynamic model are presented in Figures 10 to 11.

17 The predicted and observed tidal current velocities at the Fixed Current Stations are also presented in Figures 18 to 20. Again the model was able to predict the correct current velocity trends throughout Westport Bay. The hydrodynamic model was therefore considered to be successfully validated and was capable of accurately assessing the tidal flow regime within Clew Bay during any given set of tidal conditions.