Dredging has been underway on the Canal in Hampshire for the last few weeks and is due to continue almost until the end of March. But what is dredging and why does the Canal need it?
“dredge verb – to remove mud, stones, etc. from the bottom of a river, canal, etc.”[Oxford English Dictionary]
Why do we dredge the Canal?
The Basingstoke Canal, as designed, was really in two halves – west of Ash Lock and east of Ash Lock. The section from Ash Lock to Basingstoke was intended to be 4ft 6in deep and act as a reservoir for the part in Surrey which was 3ft 6in deep. When the canal was used as a commercial waterway, few trees were allowed to grow alongside it, but this has not been the case for many years.
Today the canal is one of the most heavily wooded in the country. Although very attractive, the many trees drop their leaves into the canal which then decay and sink to the bottom, building up a layer of silt. The Canal Society used the steam-powered dredger, Perseverance, from 1975 to 1992 to clear the channel from King John’s castle in North Warnborough to Pondtail in Fleet and in the process removed an estimated quarter of a million cubic metres of silt from the canal.
Since the Canal re-opened in 1991, some local dredging has been carried out, mainly by contractors, but silt has continued to build up, in some places more than others due to different patterns of water flow, leaf fall etc. This causes difficulties for boats, which are exacerbated when water levels fall during hot summers. The John Pinkerton II is often forced to suspend trips westward from Colt Hill because of the difficulty in passing through Swan Cutting on its way to King John’s castle. Even with the most recent dredging efforts the crew on the John Pinkerton II still managed to find a couple of ‘bumps’ in the cut when returning from dry dock.
In addition, dredging allows more water to be stored in the Canal itself; generates material for reuse (such as bank repairs, soil improvement) and allows harmful substances and invasive species to be removed from the water.
Nevertheless, there are certain disadvantages to this process. Dredging can be seen as destructive in the short to medium term, especially regarding plants and invertebrates. It creates excessive turbidity and releases stored carbon. It is expensive to perform. Finally, any contaminated material cannot be reused and as a result, needs to be disposed of in landfill.
There are two methods of dredging, which are mechanical where the use of an excavator or dredger digs out the silt and suction (or hydraulic) where the liquified slurry and water are pumped into a lagoon. Typically, it is the mechanical method which is employed on the Basingstoke Canal.
In general, companies tendered by the BCA typically carry out spot dredging. This means targeting spots of shallower canal which have been previously identified by boats and depth surveys and is typically on a smaller scale to other types. Spot dredging occurs relatively frequently on the Canal, having been performed initially by the dredger Unity from 1989-2013 and subsequently by contractors from 2011-present.
Main channel or systematic dredging is larger scale maintenance which typically performed once in a generation and is considerably more expensive as a result. This was last performed on the Canal between 1997 and 2001. The third type is restoration dredging which is where a heavily silted waterbody requires recreation and as previously mentioned, this was last performed on the Canal between 1974-1993 led by the BCS steam dredger Perseverance (an appropriate name, given the huge amount of work needed to be undertaken during this time).
So, whilst dredging is an expensive and labour-intensive process, it is a necessary process to allow boats to navigate smoothly along the Canal.