Dredging

Types of dredging, good practice and what you need to do before you start.

What is it?

The removal of sediment from the bed of a river, lake, canal, harbour, estuary or coastal water.

Why is it done?

Sediment is removed for a number of reasons:

to provide increased water depth for safe passage and mooring for vessels
to build or expand a port or waterway
to protect the integrity of flood and coastal defences, and promote flow for flood risk management purposes
to gather minerals e.g. sand and gravel for construction or beach nourishment
to collect shellfish from the seabed
to remove contaminated sediment for environmental benefit

How is it done?

Dredging normally involves cutting into the sediment and raising the material out of the water with buckets or pipelines. The material is then transported ashore by boat or pipeline, or disposed of elsewhere on the sea bed. Sometimes water jets are used to dislodge sediment and transport the material away from the site under the influence of natural flow or tidal currents.

Good Practice

Dredging and dredged material placement involves the disturbance of existing sediments this can result in impacts on the environment. Different substrates will lead to different habitats which support aquatic life. Sediments are often lost into the water during the process of dredging this can effect the clarity and quality of the water. If the sediment has been storing toxic chemicals, then these may also be released into the water and become available to fish, plants and invertebrates. It is therefore good practice to limit sediment loss where there is a risk to the environment.

Potential impacts can also be minimised by careful planning even before the project starts. This would include considering the timing, location and minimise the amount of the dredge. Periods of fish spawning and migration should be avoided, and known fish nursery areas and shellfish beds should be avoided. During the dredge the most appropriate dredging plant should be used with careful consideration of dredging production rates.

In some estuaries, removing sediment may affect the overall budget of sediment in the ecological system. This could change the effectiveness of nearby mud or sand flats to provide habitat for wildlife. Dredging and disposal methods or timings can be designed to keep sediments within the system.

What do I need to do if I want to dredge?

The first step is to find out what type of dredge you think you might need and what type of licence it requires.

By undertaking a strategy for all dredging within a particular system it is far easier to take account of the different social, economic and environmental interests. A project should consider a number of management practices which might have been identified as suitable within the strategy.