3. Atkins Report - Proposed Works
3.1. Scope of Works
FCC proposes to relocate the existing pumping station from its current location to an open space located to the North of Holywell Way and include an underground 24 hour emergency storage tank. The proposed works will comprise of the construction of the foul pumping station including underground Wet Well, Valve Chamber, Emergency Overflow Chamber, 24 Hour Storage Tank, Inlet manhole, meter chamber, over ground Wet Kiosk, Control Kiosk, Vent Stack and associated works to allow for the diversion, construction and decommissioning of existing services, utilities and pumping station. The proposed pumping station and associated tank are self- contained infrastructural items with no connection to surface water infrastructure know to be in the vicinity.
3.2. Pumping Station
The proposed pumping station including Wet Well, Valve Chamber, Emergency Overflow Chamber, 24 Hour Storage Tank as indicated on drawing C004 will be located underground with surface access covers to allow for maintenance and access to each of the chambers. 2No. Kiosks and a vent stack will be located at surface level as indicated on Section A on drawing C005. The proposed pumping station will be designed and constructed in accordance with Irish Water Code of Practice for Wastewater Infrastructure ‘A design and construction guide for developers’ and ‘Wastewater Infrastructure Standard Details’.
The proposed access covers to the pumping station will be located greater than 15m from the nearest dwelling in accordance with the current Irish Water Code of Practice for Wastewater Infrastructure ‘A design and construction guide for developers’
It is noted that the proposed pumping station will not increase the waste water loading from the site to the receiving Swords Waste Water Treatment Plant located at Broadmeadow Estuary, Swords, Co. Dublin.
The proposed Kiosks will be similar to the Kiosk as indicated in figure 3-1. The Kiosk will be 1.2m high x 0.6m wide. It is proposed to reuse the existing control building and also allow for re-use / upgrade of the telemetry and monitor equipment within the control building.
Figure 3-1 Wet Kiosk and Control Kiosk
The proposed surface access covers that will be used for access to the Wet Well, Valve Chamber and 24 Hour Emergency Storage Tank. Each cover will be lockable and flush with the existing ground level. Refer to figure 3-2 below for image of access covers when opened for maintenance
Figure 3-2 Surface Access covers
To limit the amount of required hard standing area within the open space a trafficable reinforced grass road will be installed around the access covers as indicated on drawing C003. The reinforced grass road will allow for access of services vehicles, reduce the hard standing impermeable area required and maintain a visual appearance of grass within the open space. Refer to figure 3-3 below for image of reinforced grass road.
Figure 3-3 Reinforced Grass Road
3.3. 24 Hour Emergency Storage Tank
The proposed 639m3 24 Hour Emergency Storage Tank will be located underground to the west of the proposed pumping station. The tank will not be visible from ground level and will be grassed over to provide continued use of the existing open space. In accordance with the Irish Water Code of Practice for Wastewater Infrastructure ‘A design and construction guide for developers’ a pumping station servicing a development similar in size to Holywell is considered to be a class 3 pumping station. Irish Water requires that class 3 pumping stations have a 24 hour storage facility to allow for emergency in the event of pump failure. The volume of the tank is based upon the calculations below.
The estimated foul loading calculations are as follows; Dwellings
Average day Demand per unit = 600 litres / unit / day*
Dry weather flow (DWF) = 600 x 1188 (units) / 1000 = 713 m3 / day 713 / 24 = 29.7 m3 / hour
* 600 litres / unit / day based on the average figure 3 person per unit (as per Irish Water Water & Sewerage Codes of Practice)
Peak flow rate
6DWF = 29.7 m3 x 6 = 178.2 m3 / hour Infiltration allowance (10%)
178.2 m3 / hour + 10% = 196.02 m3 / hour
School
School loading = 300 PE (1 PE = 180 litres / day)
Dry weather flow (DWF) = 300 x 180 litres / 1000 = 54 m3 / day 54 / 24 = 2.25 m3 / hour
300 PE as indicated by Fingal County Council Drainage Department in school planning permission ref; F11A/0246
Peak flow rate
6DWF = 2.25 m3 x 6 = 13.5 m3 / hour Infiltration allowance (10%)
13.5 m3 / hour + 10% = 14.85 m3 / hour
Wet Well & Emergency Storage Design
Storage Capacity = Total inflow / hour by No. of pump starts / hour Total Inflow = 196.02 m3 + 14.85 m3 = 211 m3
10 No. pump starts / hour = 211 m3 / 10 = 21.1 m3 Wet Well storage capacity = 21.1m3
4 Hour Wet Well Storage Capacity = 4 x DWF DWF Inflow / hour = 29.7 m3 + 2.25 m3 = 31.95 m3 Wet Well Storage = 31.95m3 x 4 = 127.8m3
4 Hour Wet Well Storage required by Fingal County Council
20 Hour Emergency Storage requirement = 20 x Total DWF Storage = 31.95 m3 x 20 hours = 639 m3
20 Hour Emergency Storage required by Fingal County Council
3.4. Services Diversions
The proposed works will also include the diversion of the existing foul and storm drainage locally as indicated on drawing C007. A new rising main will also be constructed along the length of the proposed 24 Hour Storage Tank where it will connect back into the existing 150mm dia. rising main.
To the North of the proposed pumping station is an existing storm drainage culvert. The culvert is 3m x 1.5m precast concrete. The exact location of the culvert will be determined through detailed investigations on site prior to detailed design to determine the proximity and level of the culvert.
3.5. Utility Diversions
The proposed works are located close to existing underground ESB, Eircom and Watermain Utilities. A review of existing utilities has been carried out prior to the part 8 planning submission. It is proposed to carry out site investigation works in including GPR survey at detailed design stage to permit for the detailed mapping of all underground services.
3.6. Site Investigations
Site Investigations in the form of a detailed desktop review of relevant historical site investigations in the area has been undertaken. These are included in appendix A of this report. The Investigations were carried out during the construction of the M1 – Airport to Balbriggan bypass located directly to the East of the proposed site. The investigations indicate that the ground conditions are soft to firm between a depths of 1.8m to 4.2m. This would indicate suitable site conditions for the proposed works. It is proposed to carry full site investigation works prior to detailed design stage to allow for assessment of the ground conditions in the specific area of the proposed works.
3.7. Decommissioning of Existing Pumping Station
The Decommission of the existing pumping station will be carried out by suitably qualified contractors who are licensed with the EPA, local Authority and who have a current waste collection permit.
All sludge, pumps and fittings will be removed from the existing pumping station. The existing pumping station will be cleaned and backfilled with a suitable fill material to the underside of the tank cover slab.
The existing access covers will be removed and the area will be landscaped to a similar standard to the surrounding area.