Canal Dredging Explained

Working in partnership with our framework contractor, MacKenzie Construction, our Maintenance Team has just finished dredging a bustling stretch of the Union Canal on the outskirts of Edinburgh (from Gogar Bridge towards the city). There are many considerations to weigh before initiating dredging works, but we’ll start with the why and how. 

Why we decide to dredge 

Scottish Canals is responsible for managing 141-miles of waterways in Scotland. With limited budgets, we prioritise which sections of the network – the hotspots for sediment – that we will dredge in any given year.  Sometimes, we have to get permissions from SEPA (or Marine Scotland if it is a coastal location), and we have to test the material in the canal before attempting to dredge.

Public safety and navigability are paramount: we focus investment on dredging areas where sediment build-up could risk damage to vessels. That means ensuring the canals retain sufficient water depth and space for boats to pass through. On the Union Canal, these are primarily barges and small boats, but up on the Caledonian Canal we also have heavy commercial vessels, too. 

The other compelling consideration which dictates where we dredge is flood risk. Our network comprises not just canals but 19 reservoirs and four lochs (including Loch Ness!), which means that we have to have robust control over the flow of immense volumes of water.  

How we dredge 

During the recent dredging on the Union Canal, our operators set up floating platforms for their excavators. From there, they dug silt from the bottom of the canal and loaded it into a nearby hopper, which was then transported by tug-boat to an access point for onward transportation.  

Other dredging methods include suction dredging (as used on the Caledonian Canal) and plough dredging (as used in Ardrishaig Harbour). Going back to the beginning of the canals, steam-powered dredging was used to dredge through some of the shallower lochs along the Caledonian Canal to achieve sufficient draft for the sea-going vessels (circa 1815). 

Heritage is also a consideration when planning dredging works as the design and shape of the canals prevent dredging near to the edges, and especially near to the towpaths. If dredging operations cause damage to the puddle clay, breaking the seal, there could be significant water loss from the canal. Digging deeper near to the wharfs or the towpath could lead to wash wall collapse. 

When possible, we try to carry out dredging works over the winter and early spring before the canals re-open for navigation. We often leave towpaths open to the public while dredging, so if you are interested in what is lying at the bottom of your canal, you can get a good view (but please stay behind any safety fencing). 

Finding ways to use dredged ‘waste’ 

We try to follow a circular approach to disposal wherever possible, and what we dredge from the canals is often reusable. The dredged material – which is typically composed of decayed organic material and sediments – can often be processed, and reused, for example, as infill for construction projects, for fertiliser in agriculture, or for building flood and erosion protection systems. 

For example, material from our current dredging works in Edinburgh was transported to Anglo Scottish Biosolids, an Edinburgh-based company that specialises in the transport, treatment and recycling of bio-materials. 

Within the first three weeks of dredging at Gogar this year, the team progressed more than 400 metres along the canal and dredged 969 tonnes of materials for reuse (that’s equivalent to the weight of 77 double-decker buses!).  

Managing disruption to wildlife 

Our team is well-experienced in managing dredging works and undertaking environmental monitoring so that they have minimal impact on habitats along the canal channel. They avoid the sensitive sites and breeding seasons for the unique mix of wildlife living on each canal.  

A key consideration is avoiding disruption to the breeding cycle of fish. There are a number of triggers for fish to start spawning, including water temperature. Our staff monitor the temperature throughout dredging works to ensure that it has not reached point that would trigger fish to spawn. For example, at our recent dredging project on the Union Canal, the water temperature remained below threshold for spawning. 

We also need to control the frequency of dredging. Dredging a waterway too often can damage its delicate ecosystem, which means we have to balance the need to keep the channels clear for vessels with the ecological health of each canal.  

Dredging with the times – what’s next?  

For the next chapter in our history, we are looking to leverage new technologies and ecological research to ensure we can meet the needs of the ever-increasing numbers of vessels (barges, pleasure boats and freight).