Glyn Rhonwy Pumped Storage Development Consent Order
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Glyn Rhonwy Pumped Storage Development Consent Order Appendix 9.1 Water Framework Directive Assessment Table of Contents 1 Introduction ....................................................................................................................................................................... 1 2 Approach and Methodology ............................................................................................................................................. 6 3 Environmental Baseline .................................................................................................................................................. 13 4 WFD Appraisal ................................................................................................................................................................. 23 5 Summary and Conclusion .............................................................................................................................................. 35 References .................................................................................................................................................................................... 37 Glossary ........................................................................................................................................................................................ 38 Appendix A Study Area Water Bodies ........................................................................................................................................ 39 Appendix B Water Balance Technical Note ................................................................................................................................ 40 Appendix C Water Quality Data and Analysis ............................................................................................................................ 51 AECOM Pumped Storage Scheme WFD Compliance Appraisal 1 1 Introduction This Water Framework Directive (WFD) Appraisal has been prepared to support the preparation of an Environmental Statement and an application for a Development Consent Order (DCO) for a proposed pumped storage development near Llanberis. 1.1 Description of the Proposed Scheme Snowdonia Pumped Hydro Ltd (SPH) proposes to construct and operate a pumped storage hydropower plant using existing former quarries to the west of Llyn Padarn and to the northwest of Llanberis. The extent of the Development and the location of any nearby Water Framework Directive (WFD) designated water bodies are shown by the figure in Appendix A. Please refer to Chapter 4 Project Description of the Environmental Statement for further details. 1.1.1 Construction Chapter 4 Project Description of the Environmental Statement presents an overview of the construction method with details of mitigation measures described in the Code of Construction Practice (Appendix 16.1 of the Environmental Statement). The following are specific details relevant to this assessment. It is estimated that the headpond (henceforth referred to as Q1 – see Photo 1) currently holds approximately 500 m3 of water, and the tailpond (henceforth referred to as Q6 – see Photo 2) approximately 100,000 m3 of water (this is a worst case assumption based on an estimate of 50,000 m3 of water in the standing water column and a further 50,000 m3 in the voids of the slate waste believed to reside in the bottom), respectively (Please refer to Appendix A). This water will need to be removed prior to construction works, together with any ongoing dewatering of natural runoff and groundwater seepage. Photo 1 Q1 Headpond Photo 2 Q6 Tailpond It is preferred that this water will be discharged to Llyn Padarn via a temporary above ground pumping main until either the existing scour pipe is completed or the existing system that drains the quarry is fully understood. Water quality in Llyn Padarn will be monitored during the discharge operation, the scope of which will be agreed when an application for a discharge consent is made under the Environmental Permitting (England and Wales) Regulations 2010 (as amended). 1 AECOM Pumped Storage Scheme WFD Compliance Appraisal 2 1.1.2 Commissioning It is proposed to abstract water from Llyn Padarn to commission the pumped storage scheme proposed at Glyn Rhonwy by Snowdonia Pumped Hydro. In July 2015 Snowdonia Pumped Hydro obtained an abstraction license from NRW under the Water Resources Act 1991 (as amended) for the following (yet to be utilised): - Permission to abstract up to 2,000 m3/day from Llyn Padarn; and - Permission to abstract up to 550,000 m3 in a 12 month period (equivalent to 1,507m3/day for the full year). An additional condition was that no abstraction from Llyn Padarn should occur when river levels in the downstream Afon Seiont are less than 0.343 m at the Peblig Mill gauging station (henceforth referred to as the ‘hands off flow’). It had previously been estimated that 1.1 million m3 (Mm3) of water will be required to fill the quarries during the commissioning period. Subsequent to gaining the current abstraction licence Snowdonia Pumped Hydro identified from recent bathymetric surveys of the quarry system that a greater volume of 1.3 Mm3 can now be stored It is now proposed to abstract water at a rate of up to 3,300 m3 per day within a total period of approximately 18 months, depending on licence conditions linked to the lake water level, flows along the Afon Seiont and environmental considerations yet to be agreed with NRW. The abstraction of water from Llyn Padarn will require an amendment to the existing abstraction licence. Until this is determined it has been assumed by this assessment that abstraction will be made at the maximum proposed rate providing there is sufficient water in Llyn Padarn to maintain the hands off water level on the Afon Seiont at the Peblig Mill gauging station, as required by the current approved abstraction licence. A new intake and pumping station would be located at approximately SH 57150 61200 in the centre of the southern bank of the lake. The intake will extend approximately 30 m into the lake and approximately 5 m wide at its largest extents. It will be positioned so that it is at least 5 m below the average minimum water level of the lake and a similar depth above the typical shallowest depth of the thermocline in order to avoid potentially adverse impacts on ecological and recreational receptors. The intake (and adjacent outfall) pipe will be buried, being constructed using an open cut method but within a dry working area supported by a coffer dam and an outer silt curtain. 1.1.3 Operation Throughout the following section please refer to Chapter 4, Project Description, and the scheme drawings presented in Appendix 4.2-4.15 of the Environmental Statement. Once in operation, water will generally be allowed to drain from Q1 through the penstock, turbines and the tailrace to Q6 during peak daytime demand. Water will then be pumped back to Q1 during the night exploiting cheaper electricity rates. However, it is highly likely that both reservoirs will be part filled for the majority of the time. The scheme is a ‘closed system’ with water being constantly recycled between Q1 and Q6 and vice versa. However, it will be necessary to ‘bleed off’ excess water that enters the quarry from direct rainfall and runoff from the quarry walls. It is also necessary to provide a way to rapidly drain down each reservoir in an emergency (i.e. as a result of concerns over dam safety), other than by transferring to the other reservoir. To do this two overflows (i.e. pipes to outfalls) will be constructed, one from Q6 to Llyn Padarn and from Q1 to the Nant-y-Betws stream. From Q6, the overflow will be an approximately 800 mm diameter pipe that will be buried beneath the littoral margins of Llyn Padarn before emerging from the bed at a water depth of at least 5 m below the minimum average lake water level (and thus into water above that typically used by Arctic charr for spawning) to where a concrete headwall, diffuser head with suitably sized fish screen will be fitted. From Q1 the overflow will consist of approximately 800 mm (overflow) and 450 mm (combined scour and Rising Main) diameter pipes that will discharge to the Nant-y-Betws stream via a stilling basin and an outfall fitted with flow disruptors (to reduce flow velocity and dissipate energy) and a trash screen. It has been agreed with NRW that the detail design of this outfall can be subject to a Requirement of the DCO. This will ensure that adequate measures are fitted to the outfall to minimise any risk of erosion occurring in the Nant-y-Betws. 2 AECOM Pumped Storage Scheme WFD Compliance Appraisal 3 Under normal operation, excess water that builds up in the system from natural runoff will be syphoned off from Q6 using an overflow positioned just above the maximum operational water level (i.e. 154 mAOD) and discharged to Llyn Padarn via an overflow. Should this not be possible due to a failure of the pumping system, restriction on the discharge from Q6 to Llyn Padarn, or a large sudden storm event, excess water in Q1 could be discharged via an overflow (set just above the maximum operational water level of 392 mAOD) to the Nant-y-Betws stream. However, this scenario is extremely unlikely as the optimal position would be to discharge from Q1 to Q6 in these events by opening the penstock between the two reservoirs. Only if the penstock was unavailable and the headroom in Q1 utilised, would the relief valve then release to the