Maintaining Banks & Channels.
Aquatic Weed Control
The Middle Level Commissioners undertake aquatic weed clearance along watercourses under their control during the warmer summer and early autumn months when weed growth is most prolific, using their fleet of small work boats.
The weed clearance work is essential to provide statutory navigation of approximately 100 miles of watercourses within the Middle Level system.
Weed cutting also facilitates the efficient and effective conveyance of water flows along channels within the Commissioners’ system during rainfall events.
The Commissioners’ weed cutting boats are fitted with a hydraulic arm to which hydraulically powered reciprocating knife blade sections are attached. The cutting arm can be operated either above or below the waterline, depending of the type of weed requiring to be cut.
The weed ‘lifter boats’ work in attendance to the cutting boat. The ‘lifters’ gather up the floating cut weed and aquatic vegetation and place it on adjacent bank sides, where it is left to decompose.
Below is a short video demonstrating the Commissioners’ new weed cutting boat, ‘Celeste’, operating with rear mounted, trailing knives, cutting along the river bed. Note the two collecting boats working in tandem behind Celeste to clear the cut weeds and vegetation. This is deposited along the bank sides to decompose.
Programmed bush cutting is undertaken during the dormant winter months.
Reactive tree cutting work is sometimes required at other times of the growing season, when bushes or trees in full canopy become storm damaged and fall across rivers and water channels impeding or blocking statutory navigation and water flows.
Pollarding of trees is often undertaken. This reduces the tree canopy which in turn initiates new growth in mature trees during the following season. This can also extend a mature tree’s life span significantly.
Bank Raising and Desilting
Maintenance desilting 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 bathymetric surveys of their watercourses, using a remote control boat fitted with a sonar. The survey information is plotted and used to help prioritise and programme where future maintenance dredging works are required.
Most maintenance desilting 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 desilting, 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 desilting 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 Desilting and Bank Raising process takes approximately a year to complete.
Modern day flail mowing machinery has evolved over the last half century or so from the gangs of labourers that were employed to work their way along river banks wielding hand scythes, to the sophisticated flail mowing machines that are used for mowing river banks today.
The Commissioners’ flail mowing rounds are undertaken in accordance with our Operations Manual, and timed to minimise any impact on grass embankments, wildlife or habitat.
The ‘Main cut’ commences in mid-July and is generally completed by the end of November in most seasons, before the earth embankments become too wet for the mowing machines to operate.
Regular mowing of the banks creates a tight grass sward, which helps to stabilise the embankments. Mulched plant material is left behind the mowing machines to decompose over the autumn and winter months, and provides a beneficial ‘thatch’ for many species of wildlife.