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Revista de la Asociación Argentina de Sedimentología

versión impresa ISSN 1853-6360


ISLA, Federico I  y  CASSANELLI, Aníbal. Simulación del transporte dispersivo con decaimiento: futuro emisario submarino de Mar del Plata. Rev. Asoc. Argent. Sedimentol. [online]. 1999, vol.6, n.1-2, pp.61-73. ISSN 1853-6360.

Knowing the tidal delay along the 35 km between Mar del Plata and Mar Chiquita, the velocity of tidal currents (mesured at 10 m depth, approximately 3 km distant from the coast) were modelled and the dispersion of a merging sewage outfall was simulated. The mean and maximum discharges of the sewage disposal were considered for the conditions of the year 2025. The total coliform concentration of the plume were assumed to be as high as today (109 total coliforms in 100 ml). To simulate the dispersive transport of a decaying pollutant, it is necessary to apply three approximations to the problem: 1. the effect of the initial dilution (in the near field) of the polluting jet into the environmental waters, 2. the effect of the dispersion in the far field, and 3. the decayment of the pollutant in relation to the pollution degree of the environment. In the simulation of a sewage disposal system, we need to know the behaviour of the plume in the water (flotation and/or the possibility of the stratification of the plume within the water body) In order to simulate the behaviour of the future sewage outfall of Mar del Plata city, two hydraulic models were used: RSB is recommended for stratified waters, and UM for merging plumes. These programs permit to analyze several construction parameters of the pipeline as length with diffusers, number of diffusers, spacing between diffusers, port diameters, port elevation over the pipe, and angle between the pipeline and the dominant currents. In eight of the ten scenarios considered, the bathing conditions were obtained 2 km away from the outlet of the diffusers; in the remaining two, the bathing conditions were arrived at 2.5 km from the outlet. In regard to these simulations, a submarine outfall of 3 km would fulfill bathing requirements. The deacyment coefficient and the environmental concentration of colliform bacteria in the water are the more important factors to deal with in these models. On the other hand, the effects induced in the far field by the tidal currents, winds (local winds and waves) and Coriolis force were modelled by a special computer program developed for the bidimensional simulation of tide-dominated shelves (AQUASEA). In regard to the bacteria-decayment rates chosen (T90, or time lag for the dye-off of 90% of the coliform bacteria in water) based on controlled experiments, a sewage outfall of 3 km length would be able to dilute the plume during the sunlight hours, without altering the bathing conditions at the neighbouring beaches. This would not happen during the night hours. During intermediate conditions (low sunlight), bathing conditions would remain with a T90 of 3 hours, but the beaches to the south of the sewage disposal would be at risk if the T90 lasts 5 hours. A high-bathing risk would remain during the setup hours of a cloudy morning. Anyway, the open-ocean wave-diluting effects were not possible to be modelled although waves were probed to be a high dispersion agent at the Mar del Plata inner continental shelf.

Palabras clave : Dispersive transport; Pollutant decayment; Simulation; Sewage outfall; Mar del Plata.

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