Beneath the Surface: The Art of Dredging

Dr. Ben Carroll, In-house expert and Assistant Project Manager at MDL Marinas, shares his insight into the art of dredging and why this is an essential practice for all marinas.

Maintaining an exemplary marina which offers an efficient service is the utmost priority for all marina managers. However much of the work at a marina takes place under the surface.

Marinas across the world are, for the most part, subject to constant siltation and many require regular dredging to maintain water depths, which ensures boats do not run aground and have safe access when mooring.

The pattern and scale of siltation at any given marina will depend on a unique range of physical factors, which will inherently differ between sites. Therefore, the dredging requirement of any two marinas will never be the same.

Each year MDL invests up to £1million into its dredging strategy, dependent upon the requirements across the marina group. Executing a comprehensive dredging programme in seamless conjunction with additional services is a key factor to ensuring marinas stay competitive and somewhere for boaters to enjoy.

MINIMAL DISRUPTION

In many respects, dredging is seen as a necessary evil, with a degree of disruption inevitable when operating large mechanical equipment within a confined marina basin. Due care must be taken to ensure no damage occurs to customer boats or the marina infrastructure, whilst also minimising any potential disruption to marina activities.

In order to minimise disruption, we always schedule the MDL dredging campaign for the winter period, between November and March, when the marina is usually quieter. However, one potential challenge of a winter dredge campaign is the risk of delay due to bad weather, whereby disposals to sea are hindered. This should always be factored in when undertaking a winter dredging programme and contingency plans should be considered in the event of unpredictable weather.

PLANNING FOR SUCCESS                 

The path to undertaking dredging requires meticulous planning as there are strict legislative requirements to adhere to. The first step of planning is the granting of a marine licence from the Marine Management Organisation, which permits dredging and disposal activities. A marine licence can typically take between six and 12 months to come through, dependent upon the nature of the project, application requirements, which includes environmental assessments, and outcome of the public consultation.

Once a marine licence is in place, the planning of the annual dredge campaign usually starts in June, when hydrographic surveys are arranged for marinas subject to high levels of siltation. These surveys provide an invaluable picture of the existing depths across the marinas and allow us to review potential dredge requirements. The hydrographic surveys also enable us to identify short to medium-term patterns in siltation across each marina, allowing dredging in future years to be planned more accurately.

Dredging at MDL Marinas in the Solent region is carried out by a single contractor, whilst elsewhere it is tendered separately to a number of contractors. Dredging tends to start in early November, typically at Cobbs Quay in Poole, and then moves in an easterly direction towards Northney and Sparkes marinas in Chichester, to be completed by March. Once the dredging is finished a second hydrographic survey is undertaken to ensure the target depths have been reached.

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DID YOU KNOW?

Last year MDL dredged in excess of 50,000m3 (65,000 tonnes) of material across its marinas, enough to fill more than 20 Olympic-sized swimming pools.