SRA End of Year Report 2015-16: Maintaining newly dredged profiles (year 1 of 4-year cycle)
Background: As discussed in Pioneer dredging – high level carriers – River Parrett, 8km of the Rivers Parrett and Tone was dredged in 2014. Once costly pioneer dredging has been completed, it makes sense to monitor and to maintain. This preserves the benefits of the initial work. It also avoids the substantial capital costs that could be incurred by not doing enough maintenance and then having to repeat the initial work.
Problem: The benefits of restored flow capacity, and of the reduced risk of serious flooding, can be quickly lost as silt is re-deposited through natural processes in rivers.
Achieved: The Parrett Internal Drainage Board, on behalf of the SRA, used interim funding to specify and supervise maintenance dredging upstream of Northmoor Pumping Station, along part of the Parrett and Tone that was pioneer-dredged in 2014. A 2.2km reach was identified, where the most significant loss of channel cross-section had occurred and where the greatest benefits from maintenance dredging would therefore be accrued.
The main technical aim was to ensure that the design profiles achieved by the 2014 pioneer dredge were retained. Work largely followed the dredging methods used then. Material removed was placed on the back of the river banks under a ‘D1 Waste Exemption’ or incorporated into neighbouring fields using a ‘Standard Rules SR2010No4 Mobile plant for land spreading permit’, so reducing the need to haul material along the road and thereby avoiding disruption.
The dredge was the biggest ever done by the Parrett IDB. Work began in late November 2015 and finished just before Easter 2016.
Four weeks of work was done from floating platforms near Westonzoyland Pumping Station, as access from the banks is severely restricted there. A 300-tonne crane was used to lift a tug boat, hopper barges, floating pontoons and a 20t excavator onto the river, whilst a further six excavators including two 45-tonne units, each with a reach of 22 metres, carried out works from the banks.
In total, 21,234m3 of silt was removed.