De-Silting

"Always fit the tool to the job and not the job to the tool"

Conventional approaches to de-silting/dredging of inland waterways has been fundamentally based taking standard construction plant and occasionally turning it to the task of de-silting/dredging as and when the need arises. A messy job for anyone, rather than a task left to those with specialist training and technologies.

Today, the power of regulations aimed at safe-guarding the environment and embracing sustainable development bring us up to date with a system that rewards specialist solutions that respect the intrinsic complexity and sensitivity of a de-silting/dredging operation.

De-silting and dredging often takes place from the banks of rivers, canals and drainage channels, etc. Where the reach required is in the range of 2-4m, contractors can use machines with a range of weights between 3-13tn. This is at the middle-lower end of the range of excavator sizes. However, such a short reach requirement is very unlikely in practice and even at this lighter end of the spectrum it is difficult to avoid immediate physical damage.

A typical 13tn excavator reach is 6m, however a safe distance from the bank crest must be maintained, reducing this to 2-4m.​ Frequently, the reach required is greater than 4m and this necessitates a much heavier machine in the range of 21-42tn, which ​ presents a high risk of physical damage. Machines of this size deform relatively stable ground just by tracking along with high ground pressures and vibration. They are also at very high risk of causing bank slumping and sliding/toppling into watercourses with obvious health and safety implications. When excavators work from the bank, the interaction between the bucket and the bed can cause significant damage to the geomorphology and habitats where not just silt is removed. ​Where machines enter the water, this damage is clearly compounded by the tracks deforming the bed.  

One way of avoiding the physical damage highlighted above is to deploy in-water de-silting plant. Here the governing dimension is generally just the minimum channel width of 2.5-3m. ​There is no damage to banks and the potential damage to the bed and watercourse habitats is also reduced. Purpose designed silt pumps operate by agitating a small plume of silt at the pump head, which is sucked immediately into the pump housing. The agitator is a relatively slow turning, blunt screw or impeller and only silt is removed, leaving stones and gravels in situ. Increasing the volume of silt removed to match that rate at which it can be removed by a conventional excavator bucket is a matter of engineering and selecting the right balance of pump power, weight and positioning plant. Ebsford have deployed in-channel amphibious plant in a vast range of environments, from Environment Agency sites, SSSIs and World Heritage Sites, to wastewater treatment works and Local Authority lakes/ponds and canals.

Ebsford are at the leading edge of applying technology developed in the Oil & Gas and Minerals sectors, where high-tech dredgers have been developed with performance and reliability in mind in the most highly aggressive environments. Ebsford are passionate about the safety of operatives and it’s our objective to be the leading practitioners of unmanned de-silting plant applied to the UK’s inland waterways.

Sometimes there is no way to reach target silt with an excavator or in-channel amphibious machine. It may also not be viable to jet the material to a location outside the hard to reach work area.​ High performance vacuum suction tankers are a highly sophisticated and industrial version of a domestic vacuum cleaner. They can be parked safely up to 150m away from the work area and easily suck up the silt and take it via flexible hose to its integrated storage vessel. It can also reverse the process to blow the silt out of the vessel, along the hose to a receptor that may otherwise be impossible to reach. Ebsford have exploited the benefits of this technology on behalf of the Environment Agency to remove silt from an urban brook that had not been de-silted for decades due in part to the high density of domestic dwellings that have been built immediately adjacent to most of the channel, making it impossible to access without making temporary accesses through the gardens and property of residents. We were able to remove 800m3 of silt from a 1km stretch of otherwise inaccessible brook, without going through or impacting on any private land or property. ​

Silt as an asset and perhaps not automatically waste for disposal. Silt can be an incredible resource due to its organic and nutrient content and we are passionate about releasing this potential, rather than wasting it. There is a raft of regulations, exemptions and permits associated with the extraction, handling and disposal of silts. Ebsford believe that finding the right and proper route through the regulatory system is the ethical and smart way to approach silt management. There are many ways of keeping silt on-site, for-example by integrating it into combined native planting and erosion control schemes.

Contaminated silts present greater challenges where even straightforward de-watering can be extremely difficult due to the required handling of the potentially contaminated water produced, as well as the risk of mobilisation of in-situ contaminants during extraction. Ebsford work with independent geo-environmental consultants to develop viable methodologies to manage contaminated silts.

Where off-site disposal is required, we select the most appropriate storage and dewatering method bearing in mind factors like the footprint and timescales available. We have deployed a range of solutions including low-footprint geo-synthetic de-watering bags/tubes, and passive straw bale structures designed to handle up to 10,000m3 of silt with minimal cost. Traffic management is a key consideration in any off-site disposal operation and Ebsford work pro-actively with clients, CDM coordinators and local authorities to ensure the system is safe, clean and of minimal disturbance to road users and the public.