PROPOSED HOUSING DEVELOPMENT, TREFNANT INN, TREFNANT DRAINAGE STRATEGY 1.0 INTRODUCTION 1.1 Background 1.1.1 Social Housing Prov

PROPOSED HOUSING DEVELOPMENT, TREFNANT INN, TREFNANT DRAINAGE STRATEGY 1.0 INTRODUCTION 1.1 Background 1.1.1 Social housing provider Cartrefi Cymunedol Gwynedd (CCG) is seeking planning permission to develop a parcel of land in Trefnant (adjacent to Trefnant Inn), grid reference 305185E 370597N, for residential use. The site is ‘greenfield’; it has no previous history of development and is situated on the edge of an urban environment. Its’ location is shown in figure 1 below. Figure 1: Site Location Plan 1.2 Existing site features 1.2.1 The land falls from southwest to northeast, towards the A525, which runs along the eastern boundary of the site. 1.2.2 There is a small watercourse running adjacent to the southern site boundary, within the grounds of ‘The Chase’. This stream passes (in a 225mm diameter pipe) through the eastern boundary wall of ‘The Chase’ before entering a culvert (via a gully cover), as illustrated in Figure 2. Figure 2: Water course on southern site boundary 1.2.3 A CCTV survey was carried out by Dwr Cymru Welsh Water (DCWW) on 6th June 2018 to establish if the watercourse ultimately connected into the DCWW adopted Surface Water (SW) sewer to the rear of the properties in Maes Teg. A drawing summarising the findings of this investigation is enclosed in Appendix 2. The conclusion reached was that the water course connects into the SW sewer at DCWW chamber reference SJ05702602 475A, located in the back garden of 5 Maes Teg and ultimately this SW sewer discharges into an unnamed small watercourse at grid reference 305587E 370766N. 1.2.4 Flows which do not reach the boundary watercourse would, in extreme rainfall flow onto the A525, where it is noted there has been instances of standing water on the highway. 1.3 Proposed Development 1.3.1 The site has an area of approximately 0.37Ha. The proposed development consists of 12 semi- detached houses and one bungalow. The table below summarises the various areas associated with each proposed use: Table 1: Proposed Areas Envisaged Area Proposed Use (m2) Building 668 Hardstanding 962 Roads 457 Footways 341 Gardens 1,039 TOTAL 3,467 2.0 FOUL DRAINAGE 2.1 Location of Public Sewer Network 2.1.1 Information provided by Dwr Cymru Welsh Water (DCWW) (Appendix 3) confirms that Public Sewer Network (PSN) exists in both the B5428 to the north of the site and also in the village hall car park to the east of the site. Due to the site topography, forming a gravity sewer connection into the PSN within the B5428 is impractical. DCWW have confirmed (Appendix 3) that connection to the PSN within the village hall car park (at chamber SJ05702632) would be acceptable to them. 2.1.2 An indicative layout for the foul drainage of the site is shown on drawing 3413-CAU-XX-XX-DR- D-1600 included in Appendix 1. All of this network (other than spurs to individual properties) will be offered to DCWW for adoption. 3.0 SUSTAINABLE DRAINAGE 3.1 Project Approach to Sustainable Drainage 3.1.1 The objective of this section of the report is to summarise the outline design for the disposal of surface water from the site. The approach outlined in the CIRIA SuDS manual seeks to manage the quality and quantity of rainwater runoff close to where it falls and to allow its use in a manner which provides amenity benefits to site users and also encourages biodiversity. 3.1.2 SuDS design should, wherever possible be based around the following principles: • Use surface water as a resource • Manage rainwater close to where it falls • Manage runoff on the surface • Allow rainwater to soak into the ground • Promote evapotranspiration • Slow and store runoff to mimic natural runoff characteristics • Reduce contamination of runoff through pollution prevention and controlling the runoff at source • Treat runoff to reduce the risk of urban contaminants causing environmental pollution 3.1.3 The subsequent paragraphs of this section of the report outline how these objectives can be achieved within the outline drainage design for the Trefnant Inn site. 3.2 Surface Water as a Resource 3.2.1 A drainage design which follows SUDS principles will initially seek to manage surface water at source, with the objective of delaying both the timing and magnitude of peak surface water run-off from urban areas entering any watercourse. 3.2.2 To this end the following source control measures are proposed for the proposed development for the Trefnant Inn site: • Use of permeable paving (either block paving and/or porous asphalt) for driveways and hardstandings. • Rainwater harvesting. As a minimum water butts to be provided. 3.3 Managing Rainwater at Source 3.3.1 For small residential developments the use source control methods such as green roofs are rarely economically viable. Another option for the management of rainwater at source may be to discharge some of the water from the roofs into an area of the garden containing water- tolerant plants. The area to the rear of the proposed clawdd (adjacent to the A525) may be suited to this purpose and the practicalities of discharging rainwater from the rear roofs of plots 8 to 13 to this area for subsequent disposal by envirotranspiration or infiltration will be considered. 3.4 Allow Rainwater to Soak into Ground 3.4.1 Three trial pits were excavated for percolation tests on 25th January 2018. The location of the trial pits and the test results are included in Appendix 4. The ground exposed consisted of a shallow layer of made ground or topsoil (typically 0.3m deep) overlying stiff to very stiff clay. All three pits being excavated to a depth of 2.5m. The percolation tests were conducted on a single day, with negligible infiltration being noted during the 4 hours of testing. The results are included in Appendix 4, with conclusion reached is that the ground tested is effectively impermeable and the disposal of surface water using conventional infiltration devices is impractical. 3.4.2 Due to the impermeable nature of the ground the disposal of surface water from areas such as building roofs using infiltration devices is impractical. 3.4.3 The proposed Trefnant site is located over bedrock described as ‘a principal aquifier’. The installation of a deep soakaway within the bed rock below the impermeable clay has been considered in principle but is not economically viable and its use is not compatible with a surface water drainage scheme which can be offered for adoption. 3.4.4 The Welsh Government produced document ‘Sustainable Drainage Systems Standards in Wales’ document notes (1.15) that it is still practical to dispose of less intense or less concentrated volumes of rainwater into the ground, even when the ground has a low infiltration rate by using permeable paving. The use of permeable pavings are proposed for all surfaces which will not be offered to the Highway Authority for adoption including driveways and hardstandings. 3.4.5 Surface water which has not been re-used, managed at source or cannot soak into the ground needs to be disposed of by evapotranspiration or to a watercourse. The nearest watercourse is the small stream described in 1.2.2 which passes in a culvert below the A525. 3.5 Envirotranspiration 3.5.1 Ultimately it is envisaged that there will be some need to dispose of surface water into a watercourse. This is described in 3.6 below. While there is an opportunity to lose some water by envirotranspiration if it can be conveyed to the watercourse by open drainage features, the potential to use this mechanism on the Trefnant site is limited by the small area of the development. 3.6 Disposal to Water Course 3.6.1 As noted in 1.2.2 and 1.2.3 above, there is a small watercourse which runs adjacent to the southern site boundary. Options to connect into this watercourse are limited and constrained by the Root Protection Areas (RPAs) of significant trees on both the southern and eastern boundaries of the site. The proposed route for the surface water drain (which avoids the RPAs) is shown on drawing 3413-CAU-XX-XX-DR-D-1600 in Appendix 1. It is envisaged that the drain run in the A525 will be offered to DCWW for adoption as part of a surface water (SW) sewer serving the proposed development. 3.6.2 The site has an area of 0.37 Ha, with a 1 in 1 year greenfield run off rate of 1.9 l/s (see calculation in Appendix 5). Given that this flow rate is lower than that the practical minimum size for a flow control device it is proposed to limit discharges from the site to 5.0 l/s, with attenuation being provided to store any surplus surface water flows generated by the development. 3.6.3 The design criteria for surface water sewers in the UK is to have no flooding for events with a return period up to and including 1 in 30 years, with surface water for events up to the 1 in 100 year (+ climate change allowance) being retained within the development boundaries. 3.6.4 Further calculations included in Appendix 6 establish the indicative volume required for attenuation, both below ground for the 1 in 30 year event and also above ground for the 1 in 30 year to 1 in 100+CC event. Both of these values are based on the areas for buildings, roads and footways in Table 1 of 1,466m2.

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