Oweninny Wind Farm. Oweninny Environmental Impact Statement Appendix 16. Erosion and Sediment Control Plan

Transcription

1 Oweninny Wind Farm Oweninny Environmental Impact Statement Appendix 16 Erosion and Sediment Control Plan Copyright ESB International Limited, all rights reserved.

2 Farm

3 Executive Summary The approach to surface water management for Oweninny Wind Farm Project integrates with the methods already agreed with the environmental authorities for bog rehabilitation at Oweninny. These methods are tailored to the characteristics of the site and to the specific properties of the peat, and they form part of a suite of techniques used in the Oweninny Bog Rehabilitation Plan and in Sustainable Drainage Systems (SuDS). The bog rehabilitation programme at Oweninny is based on three measures aimed at encouraging re-vegetation of the site and stabilising it to minimise suspended solids loading to receiving rivers: undisturbed buffer areas alongside rivers; rewetting of areas by blocking drains; and ploughing of a small number of areas with little or no peat to promote revegetation. The objective of the wind farm hydrology and sediment control design is to replicate these natural drainage patterns within the project, by: Limiting the impermeable fraction of the development, with particular attention to sensitive locations. Re-directing upslope clean surface water around structures and providing first stage treatment to construction/operation water locally at structures to remove and isolate contamination at source. Thereafter, spreading surface runoff across the surface to maximise the benefits of the existing site characteristics through use of buffer zones and rehabilitation areas. The percentage impermeable area added as a result of the development will be less than 1% which is negligible, particularly since the existing percentage runoff is already high. Hence the development will not adversely affect runoff from the site in terms of flowrate. Surface water generated on the development area will predominantly continue to drain as it would under pre-development conditions. Access tracks and structures are generally located on relatively high areas and on local watershed boundaries, away from rivers. The buffer zone distance from each structure to a river is more than 100m, except for turbine T110 which has an additional settlement pond. The system is a diffuse system. First stage local treatment is provided throughout the site in settlement ponds and lagoons, with the type of pond and lagoon designed on the basis of the bog rehabilitation and SuDS features available and on the risk to the nearest river. In very sensitive areas, the number of turbines is minimised and additional settlement SuDS measures are proposed. During very extreme rainstorms, continued operation of the wind farm substations is provided for in the diffuse SuDS approach. Implementation of this plan will ensure control of hydrology and sediment discharges from the proposed development during all stages, within a certified ISO Environmental Management System. ESBI i 13/06/2013

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5 Contents Executive Summary i 1 INTRODUCTION 1 2 SITE CHARACTERISATION Catchment Areas and Landuse Water Balance Sensitive Receptors 6 3 PROPOSED MEASURES WITHIN BOG REHABILITATION PLAN Rehabilitation Measures Implemented Existing Surface Water Features Guidelines for Developers 14 4 PROPOSED ADDITIONAL SuDS SYSTEMS General Flow and Erosion Control Measures Settlement Ponds and Lagoons Other Construction Settlement Control Measures 23 5 SPECIFIC DESIGNS Layout Turbine Hardstanding Areas Access Tracks Borrow Pit Peat Repository Batching Plant Forest Clearfelling 29 6 FLOODING 31 7 MONITORING AND MAINTENANCE Construction Phase Operational Phase Decommissioning Phase Emergency Response Equipment, Training and Corrective Action 34 8 CONCLUSION 35 9 REFERENCES 36 ESBI iii 13/06/2013

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7 1 INTRODUCTION Oweninny Power Ltd is planning to develop the Oweninny Wind Energy Project at the former Bord Na Móna peat harvesting site adjacent to the decommissioned ESB Bellacorick Power Station site in Oweninny, Bellacorick, Co. Mayo. The company is a joint venture between ESB Wind Development Limited, which is a wholly-owned independent subsidiary company of Electricity Supply Board (ESB), and Bord Na Móna Energy Limited, a wholly-owned subsidiary of Bord Na Móna. The wind farm site is located in North Mayo, west of Crossmolina and east of Bangor, just north of the N59 road. It extends to approximately 5,000 hectares. Peat harvesting has ceased and the lands comprise cutover and cutaway bog land. The proposed development comprises 112 turbines, associated site works and ancillary electrical infrastructure. The total installed capacity is expected to be approximately 370 MW. This report is a preliminary drainage and sediment control plan for the proposed wind farm. It will be updated in stages from planning approval, tendering documentation and contractor method statements through to Construction Environmental Management Plan and finally it will form part of the Wind Farm Operational Environmental Management System. The plan is based primarily on: o Contoured Lidar Mapping and OSi Aerial Photography, Ref. 1 o Bord na Móna Oweninny Bog Rehabilitation Plan, Ref. 2 o CIRIA, The SuDS Manual C697, Planning for SuDS C687, SuDS Best Practice Manual C523, Site handbook for the construction of SuDS C698, Control of water pollution from linear construction projects C648 and Designing for exceedance in urban drainage C635, Ref. 3 o EPA river maps and catchments (EPA ENVision environmental mapping system [4] and EPA Hydrotool, Ref. 4 and Ref. 5 o OPW and Mayo Co. Co. Flood hazard mapping and preliminary flood risk assessment, Ref. 6, Ref. 7 and Ref. 8, and o Surveys and site investigation listed in the set of planning drawings. These and other references are provided at the end of this plan. ESBI 1 13/06/2013

8 2 SITE CHARACTERISATION 2.1 Catchment Areas and Landuse The site is located on a catchment boundary between three catchments, the Oweninny/Owenmore, the Cloonaghmore/Owenmore and the Moy, see Figure 2-1. Figure 2-1 Oweninny wind farm and the extent of the Owenmore/Oweninny Catchment (green shading), Cloonaghmore/Owenmore (Orange shading) and Deel/Shanvolahan (purple shading) The Owenmore River which drains the western and central part of the site rises to the north in the Sheskin area and flows south and then west. The Oweninny River is a tributary of the Owenmore. The Oweninny River is also joined by the Muing River which drains Lough Dahybaun within the site. The Owenmore drains a catchment of approximately 332 km 2 before entering the sea at Tullaghan Bay. The north-eastern part of the site is drained by small tributary rivers which rise in Shanvodinnaun and flow eastwards to the main easterly flowing river, also named the Owenmore. Becoming the Cloonaghmore River, it enters the sea at Rathfran Bay which is within Killala Bay. The Cloonaghmore River drains a catchment of approximately 132 km 2. The south-eastern part of the site drains to tributaries of the Shanvolahan River before entering the Deel River which drains to Lough Conn and ESBI 2 13/06/2013

9 eventually joins the River Moy at Ballina before entering the sea at Killala Bay. The River Moy drains a catchment area of approximately 1,966 km 2 before entering the sea at Killala Bay. The area of the Shanvolahan catchment before it enters the Deel River is approximately 23.7 km 2. Error! Reference source not found. summarises the three main river catchments and the extent of proposed turbines and access tracks in each. Catchment Table 2-1: Catchments and Turbines Draining to Area sq km No of Turbines Owenmore (Oweninny) Tullaghan Bay Cloonaghmore (Owenmore) Rathfran Bay Deel (Shanvolahan) River Moy 1, The area is an Atlantic blanket peatland, comprising largely drained cutaway bog but with significant areas of rehabilitated cutaway and also remnant bog. The peat deposits generally vary in depth from 0.5m to 3.5m but 80% of the construction area has a depth less than one metre. Glacial till occurs in places and this generally underlies the peat. Small areas of glacial sand and gravel deposits are also present, particularly in the south and east of the site. Finally, alluvial deposits are shown to be present along the course of the Oweninny/Owenmore River. Site Figure 2-2 Turbine Locations and Rivers ESBI 3 13/06/2013

10 There are natural rivers within the site as shown in Figure 2-2 and the area is drained by a network of manmade drainage ditches. 2.2 Water Balance Long term rainfall and evaporation data was sourced from Met Éireann. The long-term (30-year) annual average rainfall (AAR) for the Oweninny/Owenmore Catchment of 187 sq km, to immediately downstream of the site is 1554mm. The closest synoptic station where the average potential evapotranspiration (PE) is recorded is at Belmullet, Co. Mayo, located approximately 32km west of the site. The long term average annual PE for this station is 518mm/year. To determine Actual Evapotranspiration (AE), a standard crop factor of 1.3 has been regularly applied to blanket bog settings in Ireland where the surface of the bog is dominated by sphagnum. However, at the Bellacorick site there is little or no sphagnum and a value of 1.3 would overestimate AE here. A conservative estimate of 1.1 is used for the Bellacorick site which equates to an AE of 570mm/year. A factor of 0.95 is generally used for grassland sites. The effective annual average rainfall (ER) represents the water available for runoff or groundwater recharge and is the rainfall less the actual evapotranspiration. The ER for the site is 984mm. During the months April to October, there is very little runoff or recharge. Most of the effective rainfall at field scale occurs in the Winter months. There are no longterm recording surface water flow gauging stations in or near the site. In the 1950s and 1960s however, three water level gauging stations associated with the original Power Station recorded levels in the rivers for a short period and flows were measured at one of these, in the Owenmore River downstream of the site. These provide an indication of the surface water response at the time. Anecdotal observations at Oweninny during harvesting agree with field measurements of blanket peatland drainage made in a neighbouring bog at Glenamoy, Ref. 10, which concluded that flood runoff was reduced in frequency and amount, and summer flow of streams was increased. The bog rehabilitation programme would have reversed this process to some extent. A water level recession curve measured in 1962, prior to peat harvesting, is shown in Figure 2-3 for the channel downstream of the site. This plot indicates some storage in and around the peat and possibly in the underlying groundwater. More recently, an investigation of groundwater at the Bellacorick Iron Flush SAC area within the site recorded water tables, phreatic pressures and flows where links between surface and groundwater were identified, Ref. 9. ESBI 4 13/06/2013

11 Water Level mod Sep Oct Oct Oct Oct-62 Figure 2-3 Sample water level recession curve measured in 1962 The estimated flow duration curve for the Oweninny/Owenmore River immediately downstream of the site is shown in Figure 2-4. This shows estimated flow that is exceeded for any percentage of time, using the EPA Hydrometric System (Hydrotool), and also suggests a relatively fast runoff, which is a characteristic of blanket peatlands. This estimation method does not account for the cutaway nature of the site, with rewetting and revegetation, explicitly. This rehabilitation has stabilised the sediment. Figure 2-4 Estimated flow in River Owenmore/Oweninny exceeded for various percentages of time ESBI 5 13/06/2013

12 2.3 Sensitive Receptors A full discussion of all environmentally sensitive receptors in relation to ecology, hydrogeology, etc are stated in the project EIS. Sediment loss to any of the receiving rivers would pose a significant risk to salmonid spawning areas and juvenile fish and hence the control of flow and sediment throughout the site is critical. All Special Areas of Conservation (SAC) and Natural Heritage Areas (NHA) in the area have been considered in the design. Five areas are mentioned here for special attention. Shanvolahan-Deel River An area in the southeast of the site is particularly important as it drains to the Deel (Shanvolahan) River, which supports freshwater pearl mussel populations in downstream reaches. Lough Dahybaun The catchment area of Lough Dahybaun, partly located within the site, has been delineated from detailed LIDAR data. Geoheritage of Oweninny River The only geoheritage feature within the environs of the site is the geomorphology of the banks of the Oweninny River itself, as it traverses through the central part of the site. Bellacorick Iron Flush The Surface Water Groundwater interaction at the Bellacorick Iron Flush SAC within the site was investigated in 2012 and is reported in Ref. 9. Bog Remnants There are small areas of relatively undisturbed bog within the site. ESBI 6 13/06/2013

13 3 PROPOSED MEASURES WITHIN BOG REHABILITATION PLAN The hydrology and sediment plan is based on integration of the proposed wind farm with the existing bog rehabilitation plan for the site, using Sustainable Drainage Systems. The Cutaway Bog Rehabilitation Plan was prepared in 2003 and has been implemented on the site. This involved detailed consultations with relevant Agencies, Authorities and affected parties to arrive at an implementation procedure to ensure minimum impact to the environment. The plan is based on three measures aimed at encouraging re-vegetation of the site and stabilising it to minimise suspended solids loading to receiving rivers: o undisturbed buffer areas alongside rivers; o rewetting of areas by blocking drains; and o ploughing of a small number of areas with little or no peat to promote revegetation. Plate 3-1 and Plate 3-2 illustrate the impact of the rehabilitation. This process of rehabilitation is heavily dependent on the presence and detention of surface water through natural and artificial ponding and raised water table in target areas. 3.1 Rehabilitation Measures Implemented The bog rehabilitation programme is discussed here in relation to the mutually beneficial relationship with the proposed development of surface water drainage proposals. A comprehensive assessment of the impact on fisheries arising from peat silt discharges from Bord na Móna bogs in North Mayo was prepared by Inland Fisheries Ireland. It indicated that significant peat loss to waters occurred from bare peat areas within the site in the past but these have reduced significantly due to the bog rehabilitation works undertaken by Bord na Móna since The integration of the wind farm with the programme provides an opportunity to reduce the rate of runoff from the development and to develop sediment control measures tailored to the characteristics of the peat present on the site. This will provide second-stage treatment across the surface of the site, following the first-stage treatment at each structure. ESBI 7 13/06/2013

14 Plate 3-1. Area prior to rewetting, Ref. 18 Plate 3-2. The same general area as Plate 3-1 in 2010, Ref. 18 ESBI 8 13/06/2013

15 Drain Blocking The process of drain blocking involves blocking of existing drainage channels at regular intervals. The drainage channels had been created to drain the bog during the former peat production process on the site. The purpose is to create wetted areas with conditions required for native peatland vegetation to establish. Early Stage Rehabilitation Areas Early stage rehabilitation areas are characterised by many small deep ponds, many larger shallow ponds and exposed peat surface areas. Varying levels of vegetation have established in these areas. The small deep ponds are located along the line of drainage channels created during the former peat production cycle of the bog. These drainage channels have been filled in at regular intervals creating a series of small ponds on the line of the former drainage channels. The larger shallower ponds are simply formed by natural depressions in the peat surface now that former drainage channels have been blocked and water table level has risen. Access track drainage will be directed to these pond areas where feasible, providing for treatment and attenuation of surface water generated on access tracks. The ponds will also provide secondary treatment and attenuation of surface water generated on non linear construction areas such as turbine hardstandings and electrical substations. Early stage bog rehabilitation or regeneration areas are at different stages of development throughout the former peat production site. Vegetation first begins to establish in the ponds and spreads out to what is still an exposed peat surface between the former drainage channels. As vegetation establishes, further filtration of surface water runoff from the development is achieved. Established Bog Rehabilitation Areas Established bog rehabilitation areas are locations on the site where former peat production areas have been substantially re-vegetated with native peatland vegetation. This is the situation throughout most of the site. These areas are mature wetland areas offering substantial benefits to surface water drainage. Where development runoff can be directed to these areas, significant detention and attenuation of runoff can be achieved. The wetland vegetation provides significant treatment through filtration, detention and evapotranspiration. These mature wetland areas are also interspersed with ponds providing additional detention possibilities for the settlement of suspended solids. Bog rehabilitation areas offer an ideal vegetated wetland buffer between development areas and local rivers. ESBI 9 13/06/2013

16 Plate 3-3 Existing bog rehabilitation area close to turbine 91 Bog Remnants Sections of original bog were either deliberately left undisturbed during the former peat production use of the site or were drained but never harvested. These bog remnants are strategically located around the site. There are significant remnant strips providing a buffer between the local rivers and the former peat production site. Plate 3-4. Surface water drainage channel in existing bog remnant ESBI 10 13/06/2013

17 3.2 Existing Surface Water Features Vegetated Filter Strips Despite the former peat harvesting activities, large areas of the site have remained vegetated or have re-vegetated since the cessation of peat harvesting. These vegetated areas provide an opportunity for filtration of surface water runoff from the development areas. Wetlands There are natural wetlands and constructed wetlands located throughout the site. The constructed wetlands are part of the ongoing bog rehabilitation programme. These bog rehabilitation areas are located on the former peat harvested areas and in effect occupy the majority of the site area. Consequently, the majority of the development area is in close proximity to a wetland area. Insofar as is possible the development areas will not be located within the most established and advanced wetlands for practical and ecological reasons. Access tracks, turbine hardstandings and electrical substations have been located on local watersheds and higher ground relative to surrounding areas where practical in the design. Plate 3-5. Existing wetland formed as part of bog rehabilitation interspersed with existing ponds between Turbines 23 and 29 ESBI 11 13/06/2013

18 It is proposed to discharge runoff from access trackways directly to adjacent wetland areas where possible. Runoff from larger development areas will also discharge to wetland where possible following first-stage treatment. Existing Ponds There are existing ponds of various sizes scattered around the site. These ponds have in many cases formed as a result of the drain blocking process to create wetlands on the site. Many of these are on the natural flow path from the development areas. Existing ponds offer increased opportunity for surface water detention and treatment of runoff from development areas. Plate 3-6. Existing pond close to turbine 100 Vegetated Drainage Channels A number of minor drainage channels remain on the site and many of these are naturally formed drainage routes due to the topography of the area. These well established natural drainage routes are generally well vegetated. ESBI 12 13/06/2013

19 Plate 3-7. Well vegetated drainage channel near Turbine 41 There are areas throughout the site where discharge from proposed development areas is located adjacent to small ditches. Where these ditches are heavily vegetated, surface water will be discharged directly to these flow corridors as they offer significant water treatment value through filtration, settlement and evapotranspiration. Existing Settlement Ponds There is an extensive series of existing settlement ponds on the site. The majority of these are located at natural discharge points from the site to the local rivers. These settlement ponds were constructed for the much more intensive peat harvesting activities which once occupied a large proportion of the site. Where elements of the proposed wind farm development are located within 100m of rivers, discharge of surface water generated on these areas after first-stage treatment will be to surface flow paths which will convey the runoff through existing settlement ponds or additional settlement ponds or lagoons. The existing settlement ponds vary in size and amount of vegetation. They will provide an additional level of treatment prior to discharge to rivers. ESBI 13 13/06/2013

20 Plate 3-8. Well vegetated existing settlement pond near Turbine Guidelines for Developers To assist potential developers in designing a development for the area a number of criteria have been outlined within the Bog Rehabilitation Plan: 1. Within the development design, avoid bog remnants and areas of intact bog. 2. Minimise disturbance of bog remnant and intact bog edges. Areas proximal, particularly to bog remnants, may be required to maintain and/or enhance the ecological integrity of the habitats. Therefore, there should be no new excavations adjacent to these areas Any development should focus on the higher points and/or areas of shallower peat in order to minimise peat excavation and therefore potential environmental damage. 4. Lowest points in production areas, generally the location of main outfalls and terminal silt ponds should be avoided. These areas are focal points for the rehabilitation plan in developing replacement siltation areas and providing sustainable filtration mechanisms. 5. Minimise disturbance of established rehabilitated areas identified as of ongoing scientific interest, namely Area 3. This particular area has been 1 Restrict new developments within 15m (one peat production field width). ESBI 14 13/06/2013

21 established to provide an insight to the development of the future landscape. 6. Avoid disturbance of peat, or any activity that could destabilise peat banks, particularly on peripheral areas or steep slopes, and/or adjacent to streams and rivers. 7. A minimum distance from streams and rivers 2 should be considered where there is no margin of intact bog to isolate and/or mitigate against the potential run-off of peat into streams and rivers. 8. Where a development includes new development bog (i.e. areas not developed completely for peat production purposes), a policy of minimum impact on these areas should be adopted Where possible, use the existing road infrastructure as entry points to the sites. 10. The construction methods employed in any development should operate in a fashion to mitigate against potential peat run-off and siltation of streams and rivers. 2 A minimum distance of 50m should be considered. 3 For example, if there is a road network passing through these areas, the road construction method should be adapted to minimise the footprint in the area. Concurrently, in advance of peat excavation for roads, the vegetative layer should be laid to one side and replaced on the excavated peat, which in turn should be deposited within 10m either side of the excavated roadway. When the drainage for the roadway is established, the peatland area between roadways should be rehabilitated. ESBI 15 13/06/2013

22 4 PROPOSED ADDITIONAL SuDS SYSTEMS This section provides a list of measures that will be applied throughout the wind farm site, in addition to the SuDS measures in the bog rehabilitation plan. The SuDS approach is outlined below in Error! Reference source not found General The objective of the wind farm hydrology and sediment design is to replicate the natural drainage patterns within the project. The following design philosophy has therefore been adopted: Limit the impermeable fraction of the development, with particular attention to sensitive locations. Re-direct upslope clean surface water around structures and providing first-stage treatment to construction/operation water locally at structures to remove and isolate contamination at source. Thereafter, spread surface runoff across the surface to maximise the benefits of the existing site characteristics through use of buffer zones and rehabilitation areas. The level of first-stage treatment was assessed for each structure separately, depending on the available second-stage treatment from bog rehabilitation and depending on the local risk to rivers. The purpose of local first-stage treatment at structures is initial attenuation and sediment control, based on previous experience on the site licensed by EPA in accordance with best practice. Sediment generation and the potential for pollution of drainage channels and rivers are greatest during the construction stage of the project. ESBI 16 13/06/2013

23 Rainfall Roads Turbine Hardstanding Areas Substaions, Interpretive Centre, Batching Plant, Borrow Pit, etc 4th Level of Treatment 3rd Level of Treatment 2nd Level of Treatment 1st Level of Treatment Source Control Prevention (permeable surfacing, swales, v-notch drainage ditch with check dams, oil separators) (Rainwater Re-use) Road Runoff Control Development Area Control Filter strips Surface Water Detention - Settlement Ponds with check dams On Site Controls (vegetated filter strips, existing ponds, existing wetland/ marsh, newly formed wetland and vegetated channels) Conveyance System (vegetated channels & overland flow paths) Regional Site Control (existing settlement ponds on discharge points from site) Discharge to Local Watercourse Figure 4-1. Outline of SuDS Approach The SuDS design is site-specific, suited to the well-humified peat. The experience gained by Bord na Móna and by Teagasc in the area over many years will be employed throughout the project, extending from the work undertaken in the neighbouring bog at Glenamoy in the 1960s, Ref. 10, to the present rehabilitation programme. ESBI 17 13/06/2013

24 Initially, water tables were lowered in the bog by constructing shallow drainage ditches of 0.75m depth there was no benefit at greater depths. The ditches need to be at a very close interval of 4.5m as the well-humified peat is impermeable except in the pores near the surface. In the cutaway bog, peat banks remain vertical and the water table can be about 100mm below the surface. Drainage work in a wind farm would normally pro-actively drain working areas and direct all flow and sediment within a designed drainage network. There is generally sufficient depth of outfall nearby all construction areas. At the undulating cutaway peat area of Oweninny, slopes are generally very low, some areas are flat and the likely outfall is a considerable distance from many construction areas. Although an extensive drainage system was installed during peat extraction in most areas, this drainage system is being revoked by the bog rehabilitation programme. 4.2 Flow and Erosion Control Measures Erosion control by reducing or preventing runoff is much more effective than subsequent sediment control. Runoff from surrounding areas will be prevented from flowing across exposed or excavated ground. This is achieved primarily by the provision of diversion drains, or clean water cut off drains, to channel runoff from upslope portions of a catchment around any construction areas. They will discharge within the same catchment areas as in pre-development conditions. Additional construction erosion control measures will include the following: Minimise the area of exposed ground. Backfilling and construction of access trackway will occur in conjunction with excavation and excavation will not proceed faster than rate of construction of the linear project. Construction access tracks will have falls not exceeding 15%. Monitoring of the weather forecast prior to planning excavation works. Wheel wash facilities will be provided and utilised for heavy goods vehicles leaving the site during construction. Runoff from this area will enter a dedicated lagoon where the resultant sludge will be removed from site by a fully licensed contractor with the relevant waste collection and disposal permits. Sediment control structures and associated local drainage will be constructed prior to the main construction at each site. The layout design for the development has minimised the area to be disturbed by construction through rationalising the access track network serving turbine locations and locating turbine hardstandings and access tracks in areas of shallow peat depths where feasible. ESBI 18 13/06/2013

25 4.3 Settlement Ponds and Lagoons Settlement ponds and/or lagoons are the main features proposed for firststage treatment at each structure, followed by subsequent bog rehabilitation SuDS measures between the structure and the nearest river. The subsequent measures will also control sediment and they include spreading flow across the peat surface and/or into large artificial ponds and wetlands. First-stage treatment settlement ponds are proposed at all non-linear development areas i.e. all areas of development other than the access track network: Turbine and associated hardstanding areas Electrical substation sites; Peat repository and borrow pit locations; Batching plant; The visitor/ interpretive centre and car park. Operation and Maintenance building; Contractors laydown area. First-stage settlement measures have been designed to provide initial detention for the settlement of solids activated during local earthworks, based on their contributing area and rainfall depths for the particular risk assessed for the specific structure location and for the particle settlement size required. Although they are not comparable to the existing cutaway settlement ponds which treat large areas and lead directly to rivers, they provide similar protection in most instances. Rainfall return periods of 10, 30 and 100 years have been applied to different areas of the site in accordance with identified local risk to rivers, that is, depending on: the number and quality of second-stage SuDS measures of surface water treatment available following discharge from the pond, and on the proximity of the pond to a local river. The rainfall for these return periods were obtained from Met Eireann, Ref. 11, and runoff flowrates for these return periods were derived using the Rational Formula with a runoff coefficient (C v ) of 0.70, which is consistent with the upper limit of the range recommended for stripped ground. A tenminute duration is appropriate for all structure areas apart from the larger area of the peat repository. A conservative instantaneous time of entry is assumed. Ponds comply with CIRIA Guidance C648. Six types of ponds have been designed and, depending on their location within the site, they have been sized for different return periods, in accordance with Table 4-1. ESBI 19 13/06/2013

26 Table 4-1. River Risk and Design Rainfall Return Periods Location relative to River and Number of Subsequent Treatment Measures Rainfall Return Period years Peak Rainfall Intensity mm/10min River in close proximity and 1 subsequent SuDS treatment measure River nearby and 1 to 2 subsequent SuDS treatment measures Large distance to nearest river and 2 or more subsequent SuDS treatment measures About 50% of eroded peat particles are finer than 0.2mm in terms of particle size diameter, with 10% finer than 0.035mm, Ref. 26. The pond hydraulic and sediment design criteria are: o Pond throughflow velocity less than 0.1m/s to reduce re-agitation of settled sediment. o Settling velocities for eroded hill peat and water-transported milled peat are similar. The mean settling velocity of eroded hill peat is between 2.2mm/s and 2.5mm/s for sediment concentrations less than 1,000mg/l,Ref. 26. o Standard dimensions of base width 5m, top width 8m, depth 2m (water depth 1.5m) and side slopes 1:0.75 were chosen to suit a standard 12 tonne excavator with a 1.5m jib extension. Pond lengths are then varied to suit local risk and load. This excavator can clean the pond from both sides with minimum disturbance. o Each structure has its own development area generating runoff to its pond. The pond discharge location has its own second-stage treatment in the subsequent bog rehabilitation or additional SuDS measures. The overall local risk was assessed, requiring either a 10, 30 or 100 year rainstorm. Turbine hardstandings all have the same development area and they were allocated Type 1, 2 or 3 pond lengths based on their location risk. Structures with larger areas require Type 4, 5 or 6 pond lengths. o Discharge during foundation construction in peat will be via a 450mm diameter twin-wall pipe with a flap valve installed, to provide isolation during cleaning. Thereafter discharge will be across a surface weir. The valve arrangement will remain in place at the peat repository ponds and at sensitive locations. o In the five very sensitive areas listed earlier in Chapter 2, near the Shanvolahan River, Lough Dahybaun, Oweninny channel, Iron Flush and Bog Remnants, a piped inlet with a manually operated flap valve will be installed on the settlement pond. This valve can be closed ESBI 20 13/06/2013

27 preventing incoming flow in the event of an environmental incident, thus protecting the pond. o At structures that are a long way from rivers and with extensive second-stage treatment, the first-stage treatment settlement pond may be replaced altogether by an artificial SuDS lagoon. Such a lagoon would be larger than a designed settlement pond. It would be fitted to the local topography allowing for continuous wetting to suit the bog rehabilitation plan. It will allow inlet and outlet flows across a wider area. Table 4-2: Design Criteria Design Criteria Runoff Coefficient 0.7 Pond Water Depth 1.5 m Side Slopes 1: 0.75 Base Width 5 m Cross section Area 9.2 m 2 Peat Settlement Velocity Vs m/s Particle Settling Time 600 s The basis for the settlement pond calculation of pond length for a chosen return period flow is to compare: Mean hydraulic detention time in minutes, calculated as Pond Volume (m 3 ) divided by Flow (m 3 /s), with The required particle settling time in minutes, which is calculated as the Depth (m) divided by the Particle Settling Velocity (m/s). If the detention time is 50% of the required settling time, then 50% of the particles are removed and the efficiency is 50%. If the detention time is 200% of the required settling time, the efficiency is 200%, then all of the particles will be removed, but this occurs within the first half of the pond length. Pond lengths were derived in accordance with the criteria above, and are shown in Table 4-3 and Table 4-4. ESBI 21 13/06/2013

28 Table 4-3: Preliminary Settlement Pond Retention Time Risk Type and Return Period (years) Rainfall Duration (mins) Rainfall Intensity (mm/hr) Area (m 2 ) Flow Q (m 3 /s) Mean Through Flow Velocity (m/s) Pond Length (m) Retention Time (min) Type 1: , Type 2: , Type 3: , Type 4: , Type 5: , Type 6: , The required settling time is the same for all ponds since they have the same depth and cross-section area. Table 4-4: Preliminary Settlement Pond Required Settling Time Risk Type and Return Period (years) Rainfall Duration (mins) Depth (m) Settling Velocity (m/s) Required Settling Time (min) Pond Trap Efficiency Type 1: Type 2: Type 3: Type 4: Type 5: Type 6: The calculated efficiencies are for one pond at each structure; these results will be considered in a final design for each structure to arrive at an optimum local arrangement of ponds and lagoons that will meet this criterion and also suit the local topography and contractor s methods. The area of the peat repository is large. The preliminary design is for two ponds, each serving half of the total area, hence a rainstorm duration of 30 minutes was checked as shown, in comparison with 10 minutes at structures. Other durations were also checked. Adjustments may be made to existing local SuDS features that will achieve the same criteria. In this case, there are existing tracks on three sides of the repository and a new track is proposed on the fourth, with a large area inside the enclosure separate from the repository. Restricting culvert outlets through the track and excavating a lagoon upstream in the enclosure will provide local settlement. In addition, there is a large artificial lake located relatively closeby along the downslope peat surface. Details of the calculations ESBI 22 13/06/2013

29 above will be adjusted locally at each structure to account for lagoons, artificial lakes, and additional ponds where necessary. All settlement pond outlets will be lined with stone. This design is resilient in the context of climate change predictions. 4.4 Other Construction Settlement Control Measures Additional temporary lagoons and if necessary settlement ponds will be constructed around the site should the need for these be identified. Temporary wheel wash facilities will be provided and utilised for heavy goods vehicles leaving the site. Runoff from this area will enter a dedicated lagoon where the resultant sludge will be removed from site by a fully licensed contractor with the relevant waste collection and disposal permits. In areas of significant crossfall, clean water runoff drains up to 0.5m deep will direct flow to peat surface areas away from the works. This will provide a significant reduction of the volumes of potentially discoloured run-off that would otherwise require treatment. Swales A swale is an open gently sloping vegetated drainage channel. A swale will be used to collect and convey drainage water to the lagoons and special ponds, trapping sediment and enhancing filtration. Check Dams Check dams are small temporary barriers that will be constructed across larger areas of concentrated flow at structures. Their purpose is to reduce the velocity and to slow the rate of runoff. In steeper parts of site, check dams will be placed in the drainage channels, effectively creating ponding which will assist in sediment removal, see Plate 4-1. Plate 4-1: Typical stone check dam in a drainage ditch ESBI 23 13/06/2013

30 Oweninny Hydrology and Sediment Plan 5 SPECIFIC DESIGNS Design decisions relating to specific structures are presented below. 5.1 Layout The first SUDS measure is the location of developments away from rivers, leaving buffer zones from rivers with no direct hydraulic connection. Then, where possible, developments are located on high ground or along local watershed boundaries, so that there is little interaction with drains and ditches within the site. This is shown on a map of the site in Figure 5-1, with topography of the area and with turbine and access track locations. Figure 5-1. Topography with high point at Furnought, and Turbine and Access Tracks As stated earlier, the pre-development surfacing on the site is cutaway bog, which is very mixed, ranging from wetland, original bog, exposed peat harvested areas, forestry and exposed scree in places. This has a relatively high percentage runoff, in comparison to mineral soils, and therefore it will ESBI 24 13/06/2013

31 be largely unaffected by a high percentage runoff from the area of development. The predominant surface treatment of the proposed wind farm development is stone. The access tracks and turbine hardstanding areas are all proposed to be constructed with stone and capped with Clause 804 or equivalent. This surfacing will have a greater permeability than surfaces usually considered as impermeable. The Clause 804 stone surfacing of the development area will most likely have a greater permeability than much of the existing site area of exposed peat and wetland, leading to similar flow rates from the stone and surrounding peat. Only the length of track from the site entrance to the visitors centre will be sealed or tarmaced. The surface area of the development has been minimised to the extent possible: The footprint of the development (areas of tracks, cranepads, borrow pits, substations, visitor interpretive centre and batching plant) will occupy approximately 0.45% of the upper Oweninny/Owenmore catchment after it passes through the site. The footprint of the development will occupy approximately 0.67% of the upper Cloonaghmore/Owenmore catchment after it passes through the site. The footprint of the development will occupy approximately 0.35% of the upper Deel/Shanvolahan catchment after it passes through the site. These percentages reduce significantly as the rivers extend further downstream. The localised footprint areas associated with the tributaries subcatchment areas vary from 0.0% to 1.9%, and the highest percentages occur at the Tawnaghmore and Muing tributaries. Excluding the stone access tracks, which are not impervious, the percentages range from 0.0% to 1.3%. Surface water generated on the development area is designed to continue to drain locally as it would under pre-development conditions. A continuous drainage network leading to rivers or large bog drains is not included so that the SuDS benefits of the bog are realised. There will be no point discharges. The method of local collection and conveyance of surface water generated on development areas will be with open channels. The layout design at very sensitive areas is discussed below. Fiddaunatooghaun Stream (Shanvolahan/Deel Catchment) Seven proposed wind turbine construction sites and access trackways are located within the upper watershed area of the Fiddaunatooghaun stream catchment. An existing railway embankment, which was part of the Bord na Móna peat harvesting operations, forms a physical barrier between six of the turbines and the small first and second order streams in the upper catchment. One turbine site, T110, is within 100m of the first order stream ESBI 25 13/06/2013

32 feeding into the Fiddaunatooghaun. This is located on bare earth by design, where all peat has been removed. The drainage from this turbine passes through a settlement pond / lagoon and subsequently to an existing Bord na Móna silt pond within the stream before entering the Fiddaunatooghaun. The construction site is approximately 8km from the nearest freshwater pearl mussel recorded population which is located at the confluence of the Deel and Shanvolahan Rivers. Lough Dahybaun The design of the wind farm layout has examined the Dahybaun catchment and there is a suitable location for a turbine and crane hardstand, with 1.2 km of access track. The access track will be constructed along the existing railway line which minimises sediment disturbance. Additionally this access track forms a physical barrier separating the wind turbine construction area from the lake. The construction area is located approximately 950m from the lake and no direct discharge from the construction site to drainage flowing to the lake will occur. The construction site drainage will be directed to a settlement pond/lagoon and subsequently to overland flow as indicated previously. In addition a large artificial lagoon, previously installed by Bord na Móna as part of the Oweninny peat harvesting operations, is located between the access track and Lough Dahybaun and will provide additional settlement for solids. All existing land drains in this area will be audited and check dams installed as required. Geoheritage of Oweninny RIver No works are proposed on the river banks, with the exception of the replacement or upgrading of the existing river crossing within the site. It is not anticipated that this upgrade will affect the river bank substantially beyond the extent of the existing crossing and is unlikely to present a significant risk. Bellacorick Iron Flush The layout surrounding the Bellacorick Iron Flush is discussed in Ref. 9, where two turbine locations are considered in detail. There will be no significant impact. Bog Remnants Particular attention was given to keeping turbines away from bog remnants, to the extent feasible, and away from replacement siltation areas and riparian zones. Nearly all bog remnants have been avoided except in a small number of cases where minor incursions occur due to the necessity of the wind resource design and where existing Bord na Móna roads and paths were already established. ESBI 26 13/06/2013

33 5.2 Turbine Hardstanding Areas An open drainage ditch will be located on the downslope side(s) of each of the turbine hardstanding development areas. Where possible this drainage ditch will be profiled as a swale and will be easily maintained from the edges of the hardstanding. It will discharge to first-stage treatment in a dedicated settlement pond / lagoon. The type of settlement pond / lagoon at each location has been selected on the basis of the sensitivity of the area. Finally, discharge from the first-stage treatment system will be overland for secondstage treatment on the surface. In some locations where artificial ponds are located nearby, the discharge will be directed into the ponds. Peat excavated for the construction of the hardstanding will be placed in an adjoining area to the hardstanding where this is geotechnically feasible so that runoff from this area will also be routed through the specific first-stage treatment measures at the hardstanding. Peat excavated for the construction of the hardstanding in deep peat areas arising in Phase 3 of the project will be placed in the peat repository area, so that runoff from this area will also be routed through specific first-stage treatment measures. 5.3 Access Tracks The process of rewetting through drain blocking on the site has detained surface water on site thereby creating wetted areas and the conditions required for native peatland and other vegetation to establish. This is also very effective at trapping peat silt particles. Early stage rehabilitation areas are characterised by many small deep ponds, many larger shallow ponds and exposed peat surface areas. Access track drainage will be directed onto the peat surface and to these pond areas where feasible, providing for treatment and attenuation of surface water generated on access tracks. Some mature wetland areas offer substantial benefits to surface water drainage where significant local detention and attenuation of runoff can be achieved. Peat excavated for access track construction will be spread upslope from the track. A clean water cutoff drain will be provided upslope of the peat deposition area to collect and divert clean water under the road to the downslope side. Construction/operation water interceptor drains will be constructed downslope of the access track to collect surface runoff from the construction area. These will be diverted at intervals to lateral drains. In the deep peat areas settlement ponds / lagoons will be provided (a Type 1 pond or lagoon will be constructed for every 300m section of road). These will discharge to overland flow. Access tracks will be above the existing ground level where peat is shallow, that is, across 80% of the site. Where peat excavated for the access track construction is sidecast there will be gaps left in this side cast peat to allow ESBI 27 13/06/2013

34 surface water flow paths through to the surrounding area. Where possible, the access tracks have been strategically positioned on watersheds so as to allow runoff in both directions. However, in the limited areas where the natural overland flow paths are interrupted by the line of the access track, regular culverts/ drainage paths will be provided so that the access tracks do not interfere with the natural hydrology of the site. These drainage paths will have check dams at regular intervals on any access trackside drains where the longitudinal access track gradient exceeds 10%. This will further promote treatment and detention of surface water runoff from the access track. Surface cross drains will be installed on tracks that are particularly steep and have long gradients. Access tracks will be below existing ground level, where peat depths in excess of one metre cannot be avoided, that is, across 20% of the site to the east. Temporary local drainage will be installed to allow construction of the track sub-base. The final track will camber to both sides and be provided with a v-notch drain at both sides. This drain will follow the longitudinal fall in the track until the access track level is again above the surrounding ground level. At this point, finger drains perpendicular to the access track will discharge runoff from the access track over surrounding lower lying ground. Should a low point of the access track and consequently the drains coincide with the access track level below existing ground, the access trackside drain will diverge from the access track in the direction of nearest lower ground. Check dams will be provided at regular intervals on any access trackside drains where the longitudinal access track gradient exceeds 10%. This will further promote treatment and detention of surface water runoff from the access track. 5.4 Borrow Pit Investigation of the site has indicated that materials suitable to support construction are available on site. These materials will be won through the excavation of a shallow borrow pit which may be inundated by groundwater. It is intended that the working of the pit will take place through the water table in a water filled excavation, i.e. no pumping of groundwater will take place to facilitate excavation. Excavated material will be stored in a gravel storage area adjacent to the site, and then loaded onto trucks for construction purposes. The gravel storage area will be drained to a settlement pond / lagoon and then to overland flow for approximately 450m to a small tributary of the Sruffaunnamuingabatia River. 5.5 Peat Repository In Phase 3 of the project, peat will be excavated from areas, such as deep peat areas, that are not suitable for sidecasting of peat. This may arise due ESBI 28 13/06/2013