AUV monitoring mission

One of the major problems related to the plume tracking studies is the sampling plan (Ramos, 2005). Usually, by observation of typical conditions in the vicinity of the outfall, namely density stratification and primary current directions that are most likely, investigators have some expectations of the effluent plume direction when it leaves the diffuser, its height of rise, thickness and even dilution.

However, oceanographic conditions seldom correspond exactly to those expected and values of density stratification, current speed and direction, etc will almost certainly be different in some degree from those predicted. When there is no flexibility in the plume tracking work plan, these changes in the expected oceanographic conditions can cause some problems: for example; (1) higher current speeds may result in an effluent plume that is more dispersed and that may extent beyond the survey area; (2) field transects not perpendicular to along-current direction may cause a distortion of plume characteristics such as spreading width, length of hydrodynamic mixing zone and minimum dilution, not valid to be compared with e.g. RSB model (Roberts et al.,1989) predictions which are to be observed in a vertical plane through the wastefield, perpendicular to current direction, at the end of the initial mixing region; (3) transects shape and resolution of the transects do not permit the end of the near field to be distinguished (Roberts et al., 2002); (4) the prescribed survey area may provide sampling out of the wastefield.

A race track design or any other type of complete sequential loops around the diffuser does not miss the plume, but, besides gathering useless data, is certainly not optimal for minimizing spatial and temporal aliasing effects.

Conducting previous exploratory transects to find gradients in the field that help to specify plume structure and behaviour may become a difficult task to natural tracer tracking. Minimum dilutions reported in field studies may not always have been measured at the end of near field. In some cases, as admitted by the authors, measurements may have been obtained in a still rising plume, in which case dilution was incomplete. When the expected near field sampling actually took place beyond the hydrodynamic mixing zone, upper dilutions were certainly considered.

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