On the Hydrodynamic Geometry of Flow-Through versus Restricted Lagoons
The classification of a lagoon as a restricted lagoon is shown to depend not solely on its geometry but also on the tidal hydraulics. By numerically simulating the tidal exchange of two lagoons of similar geometrical dimensions, the Nidova lagoon and the Papas lagoon, in Western Greece, subject to very similar tidal forcing, applied to the two tidal inlets in the first case and three in the second, very different residence times are found, namely 2.5 days for the Nidova and 25-30 days for the
... 5-30 days for the Papas lagoon. This large difference is attributed to the fact that whereas the Papas lagoon functions as a typical restricted lagoon, in which the water renewal is achieved by mixing in the lagoon of the tidal prism water exchanged within a tidal cycle, the Nidova lagoon functions as a flow-through system because of the differential arrival of the tide at its two tidal inlets. It is suggested that this way of enhancing the flushing rate of a lagoon be considered, whenever possible, when creating a new tidal inlet to the lagoon. management actions, aiming at the improvement of environmental conditions and fishery exploitation. An innovative method of data analysis has been presented in recent studies, focusing on similar coastal basins in Southern Italy [8, 9] . The aim of these studies was to interpret hydrodynamic processes associated with these water bodies by identification and evaluation of hydrodynamic and meteorological data. Among the physical parameters that affect the water quality of coastal lagoons, renewal of water has long been identified as a key parameter and has been made the basis of the early classification by Kjerfve  in choked, restricted, and leaky systems according to the degree of water exchanged with the adjacent coastal ocean. More recently, the water renewal time has been proposed as the basis for the classification of lagoons  . As a result of the importance of water renewal in lagoons, the creation of new artificial tidal inlets has been proposed and applied as a solution in several instances (see e.g.,  ). The question then arises: is the amount of water exchanged in each tidal cycle the basic parameter-with all other parameters kept constant-determining the flushing time? Or, to rephrase the question, are two lagoons of similar dimensions and under the same tidal forcing expected to have similar flushing rates, or is it possible to have widely varying differences in flushing rates? The answer is provided by a real-life example of two lagoons situated in Western Greece, of similar dimensions and under very similar tidal forcing, namely the Nidova lagoon, part of the complex of the wider Messolonghi lagoon, and the Papas lagoon, both communicating with the Gulf of Patras. The former has a flushing rate on the order of a few days and the latter of over a month. In what follows, we analyze why the former, having only two tidal inlets, does not behave as a typical, restricted lagoon, as one might expect from mere inspection of its geometry, because the hydrodynamics circumvent, so to speak, the topography and render the lagoon a flow-through basin, whereas the latter, having three tidal inlets, behaves like a typical restricted lagoon and, possibly as a result, suffers recurring dystrophic crises.