NEW HOUSE, CABRICH KIRKHILL Drainage Statement October 2020 First Floor, 36B Longman Drive, Inverness, IV1 1SU New House at Cabrich, Kirkhill Drainage Statement REPORT CONTROL SHEET: CLIENT: Mr J. Dalgetty PROJECT TITLE: New House at Cabrich, Kirkhill REPORT TITLE: Drainage Statement PROJECT REFERENCE: J3683 VERSION & APPROVAL SCHEDULE: Date Version Author Approved By 27/10/2020 1 C. MacLennan K. Tough 27/11/2020 2 C. MacLennan K. Tough J3683 October 2020 New House at Cabrich, Kirkhill Drainage Statement TABLE OF CONTENTS: 1.0 Introduction 2.0 Site Investigation 3.0 Foul Water Disposal 4.0 Surface Water Drainage/SUDS Strategy 5.0 Flooding 6.0 Maintenance APPENDICES: 1 – Trial Pit Logs 2 – J3683-001 – Drainage Layout Plan 3 – SEPA Flood Map J3683 October 2020 New House at Cabrich, Kirkhill Drainage Statement 1.0 Introduction The development site is located approximately 3 miles South West of Kirkhill village centre – Inverness-shire. (254050E 842966N). The proposal is to construct a new 3-bedroom dwelling with associated access road and parking. The total overall site area equals approximately 0.2 hectares. 2.0 Site Investigation A site investigation was carried out by Caintech in October 2020 to determine the suitability of the subsoil for foul & surface water drainage disposal by infiltration. Five trial holes were excavated within the development site. The results of this investigation revealed the ground conditions to generally consist of 0.4m topsoil overlaying a medium density clayey sand, with pockets of boulder clay also discovered. Water ingress was recorded via the side wall of trial hole number two. This was considered surface water run-off from the slope above the trial hole. Percolation tests were attempted in trial pit numbers one, two and three in accordance with BS 6297:2007. However, due to the silty, clayey nature of the subsoil and poor rates recorded, it was determined that the subsoil was unsuitable for the disposal of treated foul and surface water effluent. Infiltration by traditional soakaways would not be feasible. Three number percolation tests were then undertaken within the topsoil to determine the suitability of an above ground foul water treatment system. The results were not suitable to support an above ground system, with limited data recorded in the time available. It should be noted that the week prior to the day of the investigation had seen a high level of rainfall which likely contributed to the poor percolation recorded. The area available for drainage infrastructure is also at the base of a steep slope. Ordnance Survey mapping has been consulted, it was noted that the site is bound by several watercourses and so a treated discharge to one of these was considered the most appropriate and feasible discharge option. 3.0 Foul Water Disposal Due to the ground conditions being unsuitable for below ground soakaways, the options available for the disposal of foul water (in accordance with SEPA Regulatory Method WAT-RM- 03 & SEPA Regulatory Method WAT-RM-04) from the proposed dwelling are: • Connection to the public sewer. • Discharge to a suitable watercourse following treatment. • Mound filter system or Puraflo wastewater treatment system. No public sewers are recorded near to the development and so a sewer connection will not be feasible. To the South of the site an un-named watercourse flows through the adjacent woodland and down into an excavated well with piped outlet. This buried pipe flows downhill through croft land under the ownership of the client and down past the East boundary of the proposed development site. From here it skirts to the East of the existing site access track, before discharging into a roadside gully. The client confirmed that this gully feeds into a piped roadside drain constructed in the verge by Highland Council. The drain flows in an Easterly direction towards the property named “Birchfield” where it discharges into an existing concrete manhole, located just behind the roadside verge. The pipe is culverted below the road at this point before discharging to an un-named watercourse on the North side of the J3683 Octber 2020 New House at Cabrich, Kirkhill Drainage Statement single-track road. Here it converges with another un-named watercourse which flows between the boundaries of “Echo Wood” and “The Lair”. Both are charted on the Ordnance Survey Map at 1:25,000 scale. On the day of the site investigation both watercourses were observed to be fast flowing. It is believed that these watercourses are tributaries of the Conan Water. Therefore, it is considered that discharge to the piped drain that runs through the client’s land is the most feasible option for disposal of foul and surface water from the dwelling. Foul water will be collected from the dwelling and discharged into a suitably sized wastewater treatment plant (Klargester BioDisc or equal and approved). The outflow will then gravitate into a below ground partial soakaway of a minimum 25 square meters area. The remaining flows that do not percolate through the soakaway will be conveyed via new gravity fed pipework with a discharge into a sampling chamber where they will be combined with the surface water flows. Pipework will then convey the combined flow to the East corner of the site where a branch connection will be made to the existing piped field drain. It is proposed that the existing drain will be left to discharge as per the existing arrangement, with the treated flows discharging to the un-named watercourse at approximate NGR NH 54175 42981. This arrangement will require authorisation from SEPA in accordance with the Water Environment (Controlled Activities) (Scotland) Regulations 2011 (CAR). Approval should also be sought from Highland Council TECS department to discharge treated effluent to the roadside drainage pipe. Refer to drawing J3683-001 for details of this arrangement. 4.0 Surface Water Drainage/SUDS Strategy Access Road Water Run-off In the clients planning application, it is proposed that the new dwelling will utilise the existing farm access track. We would recommend that the access formed onto the public is improved so that it is constructed in accordance with section 4.0 of Highland Council “Access to Single Houses and Small Housing Developments”. Surface water runoff from the new access will be conveyed away from the public road and discharged into the existing roadside gully to the East side of the existing access. The new access road & parking area will be formed with a recycled granular construction and so no formal drainage is required. Excess surface water run-off (during long periods of rainfall) from the new access road will be conveyed away from new dwelling & existing farm barn building. Roof Water Run-off Treatment & Disposal Roof water run-off from the dwelling will be collected and discharged into a stone filled filter trench, which will be wrapped with an impermeable membrane (Visqueen urban drainage geomembrane, grade 460g/m² or equal & approved). The outlet from the filter trench will discharge into a new sampling chamber in the East corner of the site, where it will combine with the treated foul flows. The combined flow will from here connect to the existing piped field drain on the East boundary via a branch connection. Refer to drawing J3683-001 for details of this arrangement. 5.0 Flooding Potential Sources of Flood Risk At this location there are several potential sources of flooding that may require to be considered. These are outlined below along with a qualitative assessment of the risk they pose to the development. J3683 Octber 2020 New House at Cabrich, Kirkhill Drainage Statement • Pluvial flooding: The SEPA Indicative Flood Map shows the location of the site is out with any area that is at risk from pluvial flooding. • Fluvial flooding: The SEPA Indicative Flood Map shows the location of the site is out with any area that is at risk from fluvial flooding. Conclusions & Recommendations As highlighted above, the site is not at risk of flooding from any source nor will the development have any impact on any neighbouring properties. 6.0 Maintenance All components of the proposed foul water and surface water systems will be privately owned and maintained. J3683 Octber 2020 New House at Cabrich, Kirkhill Drainage Statement APPENDICES: 1 – Trial Pit Logs 2 – J3683-001 – Drainage Layout Plan 3 – SEPA Flood Map J3683 Octber 2020 First Floor 36B Longman Drive Inverness IV1 1SU Tel: 01463 545000 Website: www.caintechltd.com Project: Logged By: Date: Job No: New House at Cabrich, Kirkhill CM 26/10/2020 J3683 Trial Pit Log: TP1 Depth (mm) Description Water Depth Notes (mm) 0-400 Loose, Light brown, TOPSOIL with roots. Heavy rain in the week prior and during site investigation is likely to have negatively affected 400-800 Medium – dense, orange/brown, slightly gravelly, percolation rate. slightly silty, clayey SAND with angular cobbles. Sand is medium to coarse. 800-1000 Medium – dense (becoming increasingly dense with depth), yellow/grey, sandy BOULDER CLAY with angular cobbles. Comments: Soil density descriptions by visual assessment only. Trial pit side walls stable during excavation. Percolation test attempted at base of pit. Test failed due to impermeable nature of subsoil. First Floor 36B Longman Drive Inverness IV1 1SU Tel: 01463 545000 Website: www.caintechltd.com Project: Logged By: Date: Job No: New House at Cabrich, Kirkhill CM 26/10/2020 J3683 Trial Pit Log: TP2 Depth (mm) Description Water Depth Notes (mm) 0-300 Loose, Light brown, TOPSOIL with roots, some Heavy rain in the week prior and during site shallow angular cobbles. investigation is likely to have negatively affected percolation rate. 300-550 Medium – dense, orange/brown, slightly gravelly, slightly silty, clayey SAND with angular cobbles.