A network of silt ponds exists across all BnM bogs. They are typically approximately 50 – 300m in length and 8m or 12m wide.
The silt ponds are all located hydraulically upgradient or before the outfall points. Silt ponds are generally excavated to a depth of 1.5 metres below the pipe invert level and they are cleaned twice a year.
Field drains have been installed at approximately 15 metre intervals across the bogs, the drains generally fall towards the headland which is located at both ends of each production field. This headland allows for the plant such as harrowers, millers or ridgers to turn from one field into the next field. The open drains are generally piped across the end of each production field to facilitate production plant and machinery to travel from field to field. The drainage network continues by either open channel or pipe to a silt pond or ponds prior to discharging to a local watercourse.
Drainage is by gravity flow where possible, however there are a number of pumping stations in some bogs, where required (due to low or absent flow gradients, i.e. water levels in surrounding watercourses are higher than those on the bogs), to drain the bogs.
In accordance with the existing Integrated Pollution Control licences, all drainage water from bog units in the licensed area is discharged via an appropriately designed silt pond treatment arrangement. The silt ponds serving operation bogs have been sized in accordance with a condition in the existing Integrated Pollution Control Licences which states:
Within three years of date of grant of this licence all existing silt ponds serving operational bogs
shall achieve the following minimum performance criteria (flood periods excepted):
Maximum flow velocity < 10 cms-1
Silt design capacity of lagoons, minimum 50m3 per nett ha of bog serviced
Silt ponds were generally designed and constructed with a width of 8 metres, however in some cases silt ponds are up to 12 metres in width. Silt ponds of 12m width are only provided in areas where access is available to both sides of the silt ponds for cleaning.
The length of the silt pond will vary depending on the capacity required (i.e. proportional to the area of catchment being drained). In some locations, baffles have been provided within the ponds to reduce the energy in the flow and elongate the pond thereby increasing residence time and aiding settlement.
Silt ponds are generally excavated to a depth of 1.5 metres below the pipe invert level, however in some locations, due to restricted space the silt pond depth is greater than this.
Flow velocity through the silt pond is generally controlled by inlet and outlet pipes at the silt ponds or up-stream of the silt pond. These pipes control the velocity of the flow into and out of the silt ponds so that the velocity within the silt pond itself is less than 0.1 m/sec.
Pipelines and pump capacities are assessed based on a run-off rate of 1.7 litres per second per hectare. This run-off rate is based on a report prepared by Bord na Móna in the 1980’s and the run-off equates to approximately 60% of 25mm of rainfall falling in 24 hours (these details are taken directly from Bord na Móna SWMPs for each Bog Group).
As bogs have been developed and the draining of areas of bog by gravity is no longer feasible, pumps have been installed to drain part or all of the catchment. Bord na Móna have used Archimedean screw pumps for many years throughout their bogs and in recent years, submersible pumps have become more common. Pumps are usually installed on a duty/ standby basis with level sensors.