Particle tracking study in the Mersey Estuary

Abstract

A nested modeling system has been applied to Liverpool Bay to accurately simulate the circulation during the three month period January to March 2008. The model has been applied for a particle tracking study investigating the movement of 2 sediment mixes (70:30 silt to medium sand and 50:50 fine sand to medium sand) for 3 different deposit tonnages (10, 500 and 1500 Tonnes). The disposal sites are those already used for maintenance dredging within the Mersey Estuary: IS120 (close to the mouth), IS128 and IS110 (with increasing distance into the inner estuary). Following the release of sediment at the start of all the scenario simulations the majority (83% or more) of the deposited sediment remains within the Mersey Estuary after the 3 month simulation. A small percentage of fine particles (<12%) are able to drift north along the English coast with a few leaving the Liverpool Bay coastal cell (11 a and b). When deposited in larger quantities a small percentage of coarse sediment is also found to leave the estuary (<17%) in a westward pathway towards the Welsh coast. Initially the sediment follows a pathway out of the estuary via the main navigation channel, where it is then influenced by the residual flow within the bay. This typically moves sediment back onshore before diverging at Formby Point. The simulated fine particles follow the path of the northerly drift interacting with the Ribble Estuary before continuing north. The coarse fraction continues to move west from the end of the main channel. The particles remaining within the estuary accumulate close to the shorelines in the inner region, potentially increasing the required maintenance dredging within the docks. The deposits close to the Mersey Narrows have greater tendency to become transported offshore than those from the disposal sites situated with greater distance into the inner estuary.

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