Dredging: Maintaining all tides access at MDL’s marinas

We’ve just finished dredging at our Sparkes and Northney marinas in Chichester, the culmination of our £400K 2020/21 winter dredging programme.

All Tides Access

This year we dredged enough silt and mud from our marinas to fill ten Olympic sized swimming pools, weighing 33,000 tonnes and the equivalent of 8,000 elephants or the Spinnaker Tower in Portsmouth. This amount of dredging and much more in some years, is essential to providing year-round all tides access to our marinas for berth holders and visitors.

As with many marinas globally, our marinas are subject to ongoing siltation and require regular dredging to maintain water depths to ensure boats don’t run aground on the approach and are able to berth safely and stay afloat once inside the marina.

“The results of the investment in our comprehensive dredging programme are probably the least visible of all our investments.” says Alex Beere, our Director of Technical Services. “But it would soon be very noticeable if we didn’t complete this essential work annually, and would heavily impact on a customer’s experience of our marinas.”

Key to providing a second to none marina experience is the provision of excellent infrastructure, and 24hr access to our marinas. Dredging is the core activity that allows MDL to maintain all tides access.

Variable Dredging Requirements

Siltation is the natural process of sediment deposition, with a proportion of the sediment (or mud particles) suspended in the water settling on the sea or riverbed when water is static or slow moving. The issue is exacerbated by erosion of riverbanks further upstream or heavy rainfall depositing mud into the river, along with material washing in with the tide.

Each of our marinas is situated in its own unique, picturesque location meaning it’s subject to its own individual range of tidal and physical factors. Therefore, the pattern and scale of siltation will differ between the marinas in our network, with no two marinas dredging requirements ever being the same.

Minimal Disruption

Excavating accumulated sediment requires a large dredge to be operated with extreme care within the marina, ensuring no boats, marina pontoons or piles are damaged. To minimise disruption to marina activities, we always schedule the dredging during the winter months, typically November to March when many boats are out of the water and visitor levels are reduced.

We’re able to further manage the disruption by rotating which marinas are dredged each season and mostly dredging specific sections only, rather than the entire marina. The selected sections are dredged on a cycle, with the whole marina being dredged over a three to five-year period. Cobb's Quay, Saxon Wharf, Hamble Point, Northney and Sparkes, including part of the navigation channel, featured in our 2020/21 dredging schedule.

Planning for Success

Due to strict legislative requirements, our dredging programme takes careful planning with extended lead times. First off, we need to apply for a marine licence from the Marine Management Organisation, which permits dredging and disposal activities. This licence can take up to twelve months to come through, depending on the scope of the project, and includes an environmental assessment, ensuring that our dredging proposal will not adversely affect the local marine environment. We also obtain consent from the Harbour Authorities within whose areas we operate. They ensure, in addition to environmental protection within their boundaries, that safe navigation is not compromised during the dredging activity.

Once we have the marine licence, we start planning the annual dredge campaign. This planning usually starts in June with hydrographic surveys of the marinas subject to high levels of siltation. These surveys provide a picture of the existing depths across the marinas and allow us to review potential dredging requirements. The hydrographic surveys also help us to identify short to medium-term patterns in siltation across each marina, enabling us to plan dredging in future years.

Dredging at our marinas based in the Solent area is carried out by local contractor, Jenkins Marine, while elsewhere it is tendered separately across different contractors depending on location. Now the dredging for this season is complete, a second hydrographic survey will be undertaken to ensure target depths have been reached and berth holders and visitors can enjoy unfettered access to their berths.

We spent £400K on dredging this year to maintain the necessary depth in the marinas for all tides access, but some years the annual dredging requirements can be as high as £1m.

To find out more about our marinas and the many benefits of berthing with MDL, please click here.