Are inlets responsible for the morphological degradation of Venice Lagoon?

Through the centuries, Venice Lagoon has undergone morphological changes that can be attributed to both natural events and human actions. The lagoon has progressively deepened, and it is claimed to lose roughly one million cubic meters of sediments each year. In the ongoing debate concerning the possible means to counteract this morphodynamic degradation, inlet geometry is considered a major factor controlling the exchange of sediments. Our aim is to explore the causes of this loss. We focus first on sand, as this is the type of sediment present on the bottom of the near-inlet regions. We employ a simple model of the inlet hydrodynamics to estimate the net exchange of sand associated with the sequence of tidal events recorded for several years. Results suggest that in the absence of an excess supply from the sea, the yearly loss of sand through Venice inlets is an order of magnitude smaller than the total sediment loss usually claimed. We then show that this estimate is only slightly affected by the sand supply from wave resuspension in the far field whose effect is simply to store sediments in the near-inlet region. We finally argue that most of the sediment loss is wash load carried by the ebb currents overloaded by very fine sediments resuspended by wind in the inner lagoon and unable to settle within the channel network.