2.3 Proposed Temporary Surface Water Drainage Strategy
2.3.1 Overview
Temporary construction drainage has been proposed for all elements of the Proposed Development and associated ancillary infrastructure.
In the vicinity of the onshore substation, it is proposed that an arrangement similar to the permanent drainage solution is implemented at as early a stage of the construction programme as possible, to enable what will eventually be the permanent SuDS pond to be used for temporary drainage and to control silt-laden runoff. Silt fencing and cut-off ditches (fitted with check dams) are proposed at the toe of the platform fill embankments, with flows collected and routed to appropriately sized settlement ponds.
A similar approach is proposed at the various construction and HDD compounds, with runoff intercepted using subsurface herringbone systems discharging to construction drainage ditches (fitted with check dams) and routed to settlement ponds. Should additional areas be used for laydown or material storage / stockpiles, further silt fences downstream of these areas are proposed, with cut-off ditches to direct flows to settlement ponds.
The outflows of settlement lagoons will be discharged to watercourses where available in the vicinity or to level spreaders to disperse the flow overland as per the pre-development hydrological regime where a watercourse is not located nearby.
Typical temporary drainage details have also been provided for managing runoff from stockpiles, haul roads, access and the general cable corridor. Construction drainage for the areas will include silt fencing, cut-off ditches (fitted with checks dams) and settlement ponds.
Full details of the proposed temporary drainage strategy are shown in drawings TDW-000 to TDW-009.
The temporary drainage mitigation measures proposed are not final nor exhaustive. Other alternative measures may be implemented to achieve similar objectives and this would be at the discretion of the appointed Principal Designer / Contractor at the post planning stages.
2.3.2 Design Criteria
The temporary drainage design outlined in the subsequent section and relevant drawings have been prepared to an outline detail for the full Proposed Development. Detailed design will be undertaken at the post planning stage and should be designed in accordance with the following key guidance documents:
SEPA (2021) Water run-off from construction sites (WAT-SG-75)
CIRIA (2001) Control of water pollution from construction sites (C532)
CIRIA (2006) Control of water pollution from linear construction projects (C648/C649)
The outline designs provided at this stage have been undertaken in reference to the above documents.
3. Foul Water Drainage Scheme
With reference SEPA’s Regulatory Method for Indirect Sewage Discharges to Groundwater[5], SEPA’s order of preference for means of discharge are:
Connect to public sewer
Discharge to land
Discharge to watercourse
Connection to a public sewer is not viable due to the distance and elevation difference between the wastewater generation facilities within the site (i.e. the substation) and the closest sewer. Discharge to land generally requires a percolation value (Vp) between 15 and 100 secs/mm. The results of the ground investigation indicated poor infiltration capacity of the underlying material, suggesting that VP values are likely to be >140 secs/mm; therefore, the following SEPA guidance applies:
“If the Vp value is: >140 secs/mm Consider other disposal options including appropriately treated effluent to surface waters (normally via a partial soakaway) or for a discharge to land, a filtration system or construction of a mound soakaway in accordance with the aforementioned Technical Handbook.” |
Discharge to land is therefore viable ensuring the above guidance is adhered to and the proposed secondary treatment to meet the above guidance is discussed below.
The Proposed Development will likely accommodate 1-2 persons visiting per day. However, in the event of maintenance being undertaken there may be up to 20 visitors in total. In order to provide a conservative design, the foul drainage design has been sized to accommodate a P.E. of 15.
Primary treatment of the foul flows is to be provided by a private packaged sewage treatment plant (Klargester Biodisc or similar approved).
In order to provide secondary treatment of the foul flows, a filter mound is proposed. This has been designed in accordance with BRE 478[6] and a summary of the filter mound parameters is included in Table 7 below.
Table 7 Summary of Filter Mound Parameters
Parameter | Units | Value | Notes |
|---|---|---|---|
Population Equivalent (p.e.) | No. persons | 15 | Calculated based on data provided by SSE and reference to SEPA’s guidelines |
Filter Material Percolation Rate (Vf) | s/mm | 45 | Taken from BRE 478: Filter Mound Design |
Native Soil Percolation Rate (Vp) | s/mm | >140 | Approximated, given material encountered during GI and negligible infiltration during soakaway testing |
Minimum Distribution Area | m2 | 168 | = p.e. x Vf x 0.25 |
No. Infiltration Pipes | - | 5 | - |
Infiltration Pipe Diameter | mm | 100 | 80mm is the minimum diameter for gravity fed system |
Infiltration Pipe Spacing | m | 1.50 |
|
Distribution Layer Width [A] | m | 7 | Includes 100mm cover at each end pipe |
Distribution Layer Length [B] | m | 24 | - |
Gravel Depth [F] | mm | 300 | Depth of gravel in the distribution layer |
Depth of Filter Material [D] | mm | 700 | Minimum depth for level base application |
Cap Depth at Edge of Distribution Layer [G] | mm | 300 | Minimum allowable depth |
Cap Depth at Centre of Distribution Layer [H] | mm | 450 | Minimum allowable depth for raised top |
Upslope Taper [J] | m | 3.90 | = (D+F+G) x 3:1 slope |
Downslope Taper [C] | m | 3.90 | Same calculation as [J] for level application |
Sideslope Taper [K] | m | 4.35 | = (D+F+H) x 3:1 slope |
Mound Length [L] | m | 32.7 | = B + 2K |
Mound Width [W] | m | 14.8 | = J + A + C |
Mound Base Area | m2 | 484.0 | = L x W |
It is noted that BR478 design methodology is for domestic applications. As such the population equivalent used within the design calculations and to inform filter mound sizing assumes a larger daily loading per person than would be expected for maintenance workers on a substation site. Therefore the proposed filter mound dimensions calculated provides a conservative design.
Details of the proposed foul drainage strategy and filter mound details are presented as Drawing DRA-002 and Drawing DRA-003 respectively.
4. Conclusions
ITPEnergised (ITP) has been appointed by SSE Renewables (The Client) to provide support and input to the onshore component of the Environmental Impact Assessment Report (EIAR) submission to support a planning application for Berwick Bank Offshore Windfarm.
This report provides the relevant design information relating to the permanent surface water and foul water drainage of the proposed substation including details of the proposed Sustainable Drainage Systems (SuDS) and also includes details of temporary construction drainage for the full Proposed Development.
The proposed permanent surface water drainage strategy presented within this report demonstrates that adequate SuDS space provision is afforded within the design and the scheme is feasible and compliant to appropriate best practice and regulatory requirements.
Outline temporary drainage mitigation measures for the construction phase have been presented within the report and associated supporting drawings.
The proposed foul water drainage strategy presented indicates that foul water arisings from the Proposed Development can be discharged safely to ground via a mound soakaway and secondary treatment provision.
Taking all of the above into account it is considered there is no impediment to the development being granted planning permission on the grounds of surface water and foul water drainage provision.
[1] CIRIA (2015) The SuDS Manual, Report C753
[2] East Lothian Council (2018) LDP 2018, Sustainable Drainage Systems, Supplementary Planning Guidance
[3] Institute of Hydrology Report No.124 (1994) (IH R124), Flood estimation for small catchments, June 1994
[4] MicroDrainage (2020), Innovyze Drainage Design and Modelling Software (Version 2020.1)
[5] SEPA, Regulatory Method (WAT-RM-04): Indirect Sewage Discharges to Groundwater (2017)
[6] BRE 478: Mound Filter Systems for the Treatment of Domestic Wastewater (2008)