Maintenance dredging forms an important part of the Commissioners’ channel maintenance works. It is critical to the efficient operation of the Commissioners’ artificially pumped and gravity systems to dredge silt deposits from channel beds as, over time, watercourses’ and drainage channels’ silt deposits naturally build up as they are carried into them by water flows.
The gradual build-up of silt and decaying plant material along the channel bed prevents efficient water conveyancing during heavy rainfall and pumping events, and can also make statutory navigation of the Commissioners’ system difficult.
The Commissioners periodically undertake channel bed surveys of their watercourses. The survey information is plotted and used to help prioritise and programme where future maintenance dredging works are required.
Most maintenance dredging work is pre-planned to be undertaken during the colder winter months. This helps to reduce the impact on local wildlife habitat, ecology and navigation traffic.
Contractors are used to undertake maintenance dredging works, using Long Reach Excavators. They are able to reach out into the water channels and efficiently remove silt to a uniformed pre-determined bed level.
The silt dredgings at this point have a high-water content, and are placed along the back of the grass embankments to dry out sufficiently until they can be re-handled later in the summer months.
Silt drying can take a few months, and sometimes requires a digger to return to site to ‘open up’ the silt, inverting the wet material at the bottom of the piles, to allows the entire depth of material to be dried out uniformly.
Once the silt material has dried out, it is either levelled on adjacent arable fields and returned back into the cropping rotation in the autumn months, or if the bank crest height adjacent to the dredging site is deemed to be low, and the dredged material is of the correct quality, it is placed onto the bank top and levelled out to raise the bank height to meet the Commissioners’ bank level requirements. This provides a great saving to the Commissioners’ expenditure and prevents the need of importing bank raising materials from elsewhere to complete the task.
When the silt is levelled out onto the newly raised bank crest, it is sown with a hard-wearing grass seed mixture and left for a few months to establish.
From silt survey to grass seed establishment, the Maintenance Dredging and Bank Raising process takes approximately a year to complete.