“Dredging: What Is It and Is There a Need to Do It?”
What is sediment and how is it generated?
Fluvial and coastal sediment can be described as solid fragments of inorganic or organic material, which can range in grain size from boulders >256mm to colloids <1um. Coastal and Fluvial processes such as erosion, transportation and deposition can be dramatically altered over time as a result of the movement and settlement of various types of sediment. The origin of sediments lies in the geomorphology (soil types) of the river basin and estuarial zones from which the sediments originates.
The flow of water over a waterway bed, may cause erosion and movement of the particles. The effective movement of the particles is governed by two key factors;
1).The behaviours of the water body (channel shape, profile and velocity)
2).The particle type (rock, clay, sand etc.) grain size, volume, shape and density can also affect the rate at which this may occur.
The greater the quantities of sediment particles already in motion the greater the levels of vertical and horizontal erosion, a process known as abrasion, which is particularly extensive during times of flood. The distance sediment is transported is influenced by the flow velocity of the fluid in which it is entrained and/or the gradient of the surface over which it flows. Transportation can be in the form of bedload siltation which may be more dense/course/gravelly components bouncing along the bed or suspended loads consisting of fine/lighter material within the water column itself. A river meander for example is formed and enhanced as the outer bank is subject to erosion as a result of accelerated flow velocity whilst the inner bank is subject to deposition due to reduced flow velocity. As water flows over shallower areas further deposition occurs as flow energy is reduced.
What is Dredging and is there a need to do it?
There are two main types of dredging: Capital dredging and Maintenance Dredging. Capital dredging is generally undertaken to remove a required depth of sediment or virgin bed materials to enable civil engineering works such as: harbour basins, docks, canals, marinas to be undertaken. This can also include mining or excavation of earth resources for use as: aggregate, metal resources, precious elements. This is generally undertaken on virgin material in un-polluted regions. Maintenance dredging is the process name for maintaining existing waterways and channels for nautical navigation, to reduce flooding or preserve hydrological depth for other reasons. It in involves the moving or removal of relatively recent sediment deposits. Arguably the most common form of maintenance dredging is to reduce flooding and maintain nautical navigation.
Dredging can also be key in removing sediment contamination and improving biodiversity, where the chemical and ecological quality status of a watercourse and the surrounding area may have been affected. Waterbodies experiencing eutrophication* (see below) can often benefit from a dredging campaign combined with activities to reduce the further introduction of nutrients into the system. Elevated Phosphates for example can be notoriously difficult to treat in-situ and therefore physical removal of the Phosphate bound particles can be prudent.
*Eutrophication, also referred to as ‘hypertrophication’ is when the environment becomes enriched with nutrients. In marine habitats such as lakes this can be a problem as it can cause algal blooms. It is generally the ecosystem response to the addition of artificial or natural substances, mainly phosphates to an aquatic system. This can be from detergents, fertilisers or sewage. For instance, fertilisers are often used in farming, sometimes these fertilisers run-off the land into nearby water causing an increase in nutrient levels, which can be dispersed throughout a river basin as “diffuse polluction”.
Historical contamination from industrial regions, which were not subject to the regulations of today, can profit from dredging with the contaminated sediment being removed and treated via land based remediation operations. Areas subject to extensive oil spills can benefit from emergency dredging to physically remove the oil contaminated sediment before the contaminants can disperse further and negatively affect flora and fauna.
Dredging to improve Navigation and Flood Risk
Allied to population growth, there is an increasing pressure for development on or near to river systems, floodplains and in coastal regions. This has led to extensive artificial and modified river and coastal engineering schemes in an attempt to provide safety measures and minimize the damage caused by flooding.
The natural water carrying capacity of a river channel, for example, can be stretched when development and intensive land uses can amplify the effect of heavy rainfall, as the rate of run-off from paved and hard surfaces accelerates (spikes) the volumes of water entering the system. Particularly after heavy precipitation events extensive sediment deposition can occur at certain locations within a river system, notably where manmade structures and interventions have interrupted the natural behaviors of the watercourse. Dredging is often deemed essential under certain circumstances to maintain the through-flow of water. Maintaining systems by the removal of plant matter, debris, urban litter and sediment transported from upstream riverine areas is often key to maintaining the performance of the wider catchment.
(Image courtesy of Western Morning News 20/1/2014)
Somerset flooding in 2014 as a result of reduced channel carrying capacity combined with extensive and prolonged rainfall, and sustained high tidal influences.
The Navigation of waterways as a result of commercial, operational and recreational activities has led to the need for dredging to maintain suitably safe channel networks with a sufficient water depth to accommodate craft.
It is also essential to maintain the storage capacity of reservoirs and lakes which have a propensity to become silted up over a period of time, as the accretion of sediments occurs when feeder streams and run off waters reach the relatively low velocity environments of these waterbodies.
When dredging a particular region to maintain navigation it is often valuable to use the dredged material for land reclamation or replenishment purposes. This may be to extend or enhance an area of land for commercial purposes i.e.: a neighbouring area of land in a harbour. Artificial beach replenishment may be undertaken to maintain prominent areas of tourism and reduce flood risk in an area as the water flow energy is dissipated as it moves over the sand particles. Replenishing mud flats and other biodiverse areas can also be valuable in preserving such areas by increasing ecological value and providing protection from extensive erosional processes. The re-use of sediments in these scenarios is subject to considerable testing, analysis and background studies to determine the suitability or otherwise of the materials. There is a complex balance of appropriate use/ecological impact/commercial viability and sustainability to be assessed in determining an end use for sediments. Unsuitable materials are disposed of to landfill or at an appropriate off-shore, licenced disposal site in a low impact environment.
Types of dredging methods
Water Injection Dredging
Water injection dredging (WID), known as hydrodynamic dredging, is a process where basal sediment is agitated and diluted by the injection of pressurised water jets, this causes the material to rise and flow in suspension. A strong river flow, significant downward gradient and/or a high energy tidal regime are required to transfer the suspended material downstream. The injection/water pressure breaks down the cohesion between the sediment particles. WID can only operate on certain fine-grained bed materials, which are capable of reducing their density to the point where they act as a fluid and flow hydraulically.
Generally the aim of WID is for the material to be carried out to sea directly as a density current, which rolls along the basal layers of the watercourse and does not discolour or make turbid the whole water column. The methodology is largely mimicking the behaviour of the watercourse during periods of adverse weather but with less turbidity expected due to the basal density current created. The sediment column can move rapidly and ‘naturally’ within the river flow dynamics. WID is considered good environmental practice, under the correct circumstances, due to the sustainable release of sediment within this system, reducing the reliance on landfill disposal options and transportation.
Potential water bodies where WID is considered should be firstly be subject to an environmental assessment to ascertain the likely outcome of the movement of the sediments and potential areas of re-deposition. Environmental monitoring is usually required by the authorities to assess and monitor the effects of WID on the local environment. Often the types of water bodies, where WID is suitable, experience naturally elevated background levels of turbidity, due to the high energy river flow or tidal regime hydraulically agitating the present sediments. Often however the main concern with WID is a lasting detrimental effect on dissolved oxygen concentrations within the water. It is therefore prudent to assess the background water quality particularly dissolved oxygen % concentrations and turbidity to ensure that the water quality is not already sub-standard. Monitoring the effects on the water quality pre- and post- dredging as well as during the operation allows observations to be made to determine whether the dredging operation has the potential to negatively impact the water quality.
(Image courtesy of Deme Group) (Image courtesy of Land and Water)
Water injection dredgers are often used in small, shallow ports and marinas because they have good maneuverability and can dredge very close to embankments, quay walls and also under static structures such as pontoons. This method, if suited to the circumstances, can be considerable cheaper than other dredging processes, as there is no by-product waste\ sediment to be physically re-used/disposed of.
Cutter Suction Dredgers
A cutter suction dredger is a stationary or self-propelled vessel that uses a rotating cutter head to loosen the material in the bed (‘cutting’). A suction inlet located beneath the cutter head (known as the suction mouth) is connected by a suction tube directly to one or more centrifugal pumps. The vacuum force at the suction inlet sucks up the loosened material and water mix. The suction tube and cutter head are attached to a ladder. The ladder with cutter head is positioned at the fore of the vessel. The sizes of these dredgers vary for marine dredging and smaller vessels tailored to inland water bodies.
(Image from Maritime Journal 2006)
This method is highly effective under certain circumstances, for example in estuarine marinas which can be dredged to the required depth suitable for navigable craft and the sediment pumped from the marina into the main estuarine flow or to a receiving lagoons or reclamation sites. Typical pump-mix concentrations for a cutter suction dredger vary from 8-20% sediment/water mix depending on the pump distances and material types.
Trailing Suction Hopper Dredger
A trailing suction hopper dredger has large, powerful pumps and engines that enable it to suck up sand, clay, sludge and even gravel from ocean or river beds. One or two suction pipes run from the vessel to the bed at up to 80m deep. A draghead is attached to the end of the pipe and lowered to just above the bed, making it possible to regulate the mixture of sediment and water that it takes in. A trailing suction hopper dredger generally stores the dredged material in its own hopper and discharges the left-over water overboard.
(Trailer dredger discharges its cargo of sand on to a beach, Image courtesy of Boskalis Westminster Ltd)
Back Hoe Dredgers
Backhoe or dipper dredgers are usually a pontoon mounted excavator. Pontoons generally have ballast tanks or weighting added to provide stability and spud legs for anchoring. The excavator arm digs below the vessel and empties material into waiting hopper barges for transportation to its final resting place at sea or on land. Buckets or grabs can be used to retrieve material.
(Image courtesy of Jandenul)
The sizes of this type of dredger are vast from enormous marine vessels capable of digging to 32m depth to specially designed narrow gauge dredgers to navigate pinch points and locks in canals and inland waterways. Long reach excavators are used to extend dredging depths.
(Image courtesy of Land and Water)
Long Reach Excavators
These can be used to dredge from stable banks along rivers and waterways. Satellite guided equipment can be used to aid accuracy when working in difficult or unsighted areas.
(Image courtesy of Land and Water)
An amphibious excavator is a type of construction plant that can perform activities such as dredging while afloat in shallow water. This unique feature makes amphibious excavators more suited and better adapted for removing silty clay, clearing silted trenches, river maintenance and general canal bank management in and around shallow water areas. The amphibious excavator can walk or work in water, because the chassis crawler floats on sealed pontoons. It moves on water using a dual-body boat form buoyancy tank.
(Image courtesy of Land and Water)
Licensing, Consents and Disposal
Where dredging activities involve the displacement of sediment which will be retained within the coastal or riverine system (generally contained below water level) a Marine Licence for disposal would generally be required from the Marine Management Organisation (MMO) or consent from the Environment Agency (EA) for inland waterways.
If the dredged material is to be removed and discarded via a land-based activity then it technically becomes a waste and is therefore subject to Environment Agency waste management regulations as either non-hazardous dredgings EWC code 17 05 06 (Mirror entry Non-hazardous) or hazardous dredgings 17 05 05 (Mirror entry Hazardous).
It is a legal requirement to apply the waste hierarchy (in order of preference: reduce, re-use, recycle/ compost, energy recovery, disposal) when considering waste management.
Therefore; often, site-specific assessments are undertaken to determine the most appropriate management techniques. Generally this process is initiated by obtaining a relevant number of sediment samples which are analysed and assessed to determine the characteristics of the waste depending on the localised options available.
This decision making process may be dependent on many factors such as:
· Sediment classification- Is the sediment hazardous/ non-hazardous/ suitable for various re-use options;
· If hazardous can the waste be treated in-situ or ex-situ;
· Spatial logistics;
· Particle size/ other characteristics of the sediment (or dredged leaf litter which can be composted), i.e.: use of gravel/ sand as aggregate or fine nutrient-rich silt as agri-improver;
· Proximity of sensitive receptors;
· Potential opportunity for improvement in an area, i.e.: beach replenishment, habitat improvements, bank stabilisation.
The finer fractions of sediment can be very useful for agricultural improvement as they often contain high levels of organic matter, Nitrogen, Sulphur with appropriate levels of Phosphorus, Potassium and Magnesium. Dredged sediment can be used to replace the need for artificial fertilisers. In 2014 after the flooding issues in Somerset an emergency dredging campaign was initiated. Under 33No Standard Rule EA permit deployments approx. 78,202 tonnes of sediment was spread over various agricultural field locations. Combined with the bank raising and stabilisation design schemes 0% of the dredged silt from the River Parrett & Tone in the 2014 campaign was disposed to landfill.
There are various others additional permits available whereby dredged sediment could be successfully recovered and re-used under specific circumstances. Using waste exemptions (from The Waste Management Licensing Regulations) can also be extremely beneficial in maintaining channel capacity and providing additional enhancement techniques which can prevent flooding and increase biodiversity.
To conclude, dredging activities can often be an essential component in maintaining coastal, riverine and man-made systems to meet the increased demands brought by intensive land uses, particularly when controlling flooding and maintaining navigational activities. With a strong consideration for the environment dredging can also be undertaken with minimal environmental harm, which can enhance and provide benefit to various ecosystems.