Sediment bypassing and re-entry in an estuarine salt wedge regime

Abstract:

Sediment trapping at a salt wedge front differs from trapping in a partially mixed estuary, because the lengthscale of a salt wedge frontal zone is an order of magnitude shorter than that of a partially mixed estuary. As a result, fine sediment is much more likely to bypass the frontal zones of a salt wedge, leading to a coarser estuarine sediment bed than would be found in a partially mixed estuary with the same riverine sediment input. A field and numerical study of frontal dynamics and sediment trapping in the Connecticut River provides clear evidence of the bypassing of fine sediment and the dominance of sandy estuarine sediment even in the presence of a large riverine input of suspended mud. Almost all of the newly delivered mud is exported during the spring freshet, but some mud does return during low flow conditions, remobilized and transported by the advancing salt wedge. The mud that returns is a small fraction of the total riverine delivery, and it tends to be sequestered in low-energy zones along the flanks of the estuary and adjacent embayments. This study suggests that the sedimentary environment of an estuary depends more on the characteristics of its frontal regime than on the characteristics of the sediment input.