Page 33 - GENERACION Y TRANSPORTE DE SEDIMENTOS

Generación y Transporte de Sedimentos 33

as the main contributor of suspended sediments of of the middle and lower Paraná through various
the Bermejo River Basin. mechanisms, shaping and renewing the surface
Once the contributions of the Bermejo River layers of the latter. In this context, it has been re-
in the Upper Basin to the Lower Basin have been ported that, making a balance, the wash load is
described, we analyzed the characteristics of them partly retained in the vast alluvial plain that ex-
and their behavior in the Paraguay-Paraná-Delta tends on the right bank of the main channel. The
and Río de la Plata system. fraction retained would be directly related to the
The sediments delivered by the Upper Bermejo strong flow of the Bermejo River. From the lat-
River to Junta San Francisco consist of 75% of fine ter, as has been expressed throughout the book,
particles (clay and silt, especially the latter), and comes the main supply of fine sediment to the
25% of sand. In the Lower Basin, in El Colorado, the Paraná river system downstream. The situation
ratio of sand in suspension varies between 10 and described is of paramount importance when con-
20%, on average, of the total suspended load. The sidering sediment control practices in the Upper
fine fraction is mainly composed of silt. The con- Bermejo River Basin, which must be carefully
centration of suspended sediments of the Paraná studied to avoid altering the river system.
River undergoes a sharp increase downstream the As for the sand transport in the Paraná, both
confluence with the Paraguay. As this river does in suspension and by bottom trawling, it may be
not have a significant suspension transport before calculated from saturated transport capacity for-
receiving the Bermejo, it is verified that the latter mulas. The prediction given by the expression of
causes such increase. Engelund-Hansen (Vanoni, V., 1975) are extremely
The different types of material in suspension, reliable for this river, and its application for its av-
entering the main channel of the Paraná in its mid- erage conditions gives a value of 25 million tons of
dle section, are mixed by the river in such a way total sand transport annually.
that form multi-modal suspended sediment, poorly In the navigation channels of the Río de la
selected, except when transporting the concentra- Plata, high proportions of silt in the bed samples
tion peaks produced by the Bermejo. are found, showing that only in these areas, con-
As a first approach, one can say that the fine ditions of current velocity are such that allow the
sediments carried in suspension by the Paraná Riv- deposit of suspended solids. The average diam-
er only find general favorable deposition conditions eters of the samples taken from the channel bed
upon reaching the Río de la Plata and its navigation Mitre (19 μm), access channel (10 μm) and Punta
channels. Indio channel (10 μm) confirmed the statement.
However, it is known that the finest fractions In summary, it may be said that the monitor-
(silt and clay), transported in suspension, are sig- ing of sediment produced in the Upper Bermejo
nificant parts of the deposits that usually, during River Basin, though its Lower Basin first, then go-
periods of overflow, sediment on the floodplains ing by the Paraguay and Paraná, to reach the Río