CHAPTER 9 . Geomorphological and Physical Characteristics of the Bahia Blanca Estuary
205
Since sand is not input from the inner reaches of the channel and sand that may
come from the inner shelf cannot bypass the ebb delta or the marginal channels (i.e., El
Toro channel), they have to rely only on the material contained within the basin. As
these dunes are subject to strong tidal currents, they migrate at a very high velocity in
comparison with their size. Therefore, the bedforms use their own material, in a kind
of auto feedback, to help them progress developing steeps lee sides in the process to
conserve sand and maintain the dynamic equilibrium.
Except for bedforms found in flood-dominated channels, all dunes and shoals observed within the Bahia Blanca Estuary have an ebb dominance. This is a very important factor because when combined with the lack of sediment input from the rivers
and the shelf (stopped by the ebb deltas), the relatively large concentration of suspended sediment into the estuary must be maintained from the erosion of the tidal
flats and islands' coasts.
The analysis of erosion/accumulation along tidal channels and island borders made
by Esposito (1986), Ginsberg (1993) and Ginsberg and Perillo (1997) clearly shows that
most places in the estuary are in clear retreating conditions. For instance, Ginsberg
and Perillo (1990) have described the presence of erosion cusps along most of the
channels in the estuary. A specific study of some of these forms indicated that anyone
of them may provide up to 18 m3 ye l of sediment to circulation. Those erosion cusps
have densities of the order of 25 per km in some places. Such figure plus the erosion
observed at the cliffy coasts of the islands (up to 1 myel) may give a clear picture of
the enormous amounts of material that is being extracted from the shores and put
into transport.
Perillo and Sequeira (1989) have shown that the southern coast of the Principal Channel across Ingeniero White Harbour has retreated up to 50 m during the period
1980-1986.They have also demonstrated that in the same period about 1.5 million m3
where net exported from a stretch of about 8 km of the middle reach of the channel.
Because most of the sediments in the estuary are silt and clay results in the fact
that it is maintained and transported in suspension. The strong currents and the small
intervals in which slack water occurs, impede sediment deposition within the channels. On the tidal flats, wave erosion (as will be described later on) also acts to both
erode the old sediment and preclude any accumulation that may occur. The sum of all
these factors results in the erosional stage that is found in most of the estuary and the
fact that almost all the sediment observed corresponds to the deltaic deposition period.
9.4
Physical Processes
9.4.1
Tides
The larger energy input into the Bahia Blanca system is produced by a standing,
semidiurnal tidal wave. The mean, spring and neap amplitudes of the tide at five stations along the channel and one outside the system are plotted in Fig. 9.6. The amplitude and phase angle for the most important semidiurnal and diurnal components of
the tide at the main tidal stations are shown in Table 9.1. Although the tide at the Ocea-
205
Since sand is not input from the inner reaches of the channel and sand that may
come from the inner shelf cannot bypass the ebb delta or the marginal channels (i.e., El
Toro channel), they have to rely only on the material contained within the basin. As
these dunes are subject to strong tidal currents, they migrate at a very high velocity in
comparison with their size. Therefore, the bedforms use their own material, in a kind
of auto feedback, to help them progress developing steeps lee sides in the process to
conserve sand and maintain the dynamic equilibrium.
Except for bedforms found in flood-dominated channels, all dunes and shoals observed within the Bahia Blanca Estuary have an ebb dominance. This is a very important factor because when combined with the lack of sediment input from the rivers
and the shelf (stopped by the ebb deltas), the relatively large concentration of suspended sediment into the estuary must be maintained from the erosion of the tidal
flats and islands' coasts.
The analysis of erosion/accumulation along tidal channels and island borders made
by Esposito (1986), Ginsberg (1993) and Ginsberg and Perillo (1997) clearly shows that
most places in the estuary are in clear retreating conditions. For instance, Ginsberg
and Perillo (1990) have described the presence of erosion cusps along most of the
channels in the estuary. A specific study of some of these forms indicated that anyone
of them may provide up to 18 m3 ye l of sediment to circulation. Those erosion cusps
have densities of the order of 25 per km in some places. Such figure plus the erosion
observed at the cliffy coasts of the islands (up to 1 myel) may give a clear picture of
the enormous amounts of material that is being extracted from the shores and put
into transport.
Perillo and Sequeira (1989) have shown that the southern coast of the Principal Channel across Ingeniero White Harbour has retreated up to 50 m during the period
1980-1986.They have also demonstrated that in the same period about 1.5 million m3
where net exported from a stretch of about 8 km of the middle reach of the channel.
Because most of the sediments in the estuary are silt and clay results in the fact
that it is maintained and transported in suspension. The strong currents and the small
intervals in which slack water occurs, impede sediment deposition within the channels. On the tidal flats, wave erosion (as will be described later on) also acts to both
erode the old sediment and preclude any accumulation that may occur. The sum of all
these factors results in the erosional stage that is found in most of the estuary and the
fact that almost all the sediment observed corresponds to the deltaic deposition period.
9.4
Physical Processes
9.4.1
Tides
The larger energy input into the Bahia Blanca system is produced by a standing,
semidiurnal tidal wave. The mean, spring and neap amplitudes of the tide at five stations along the channel and one outside the system are plotted in Fig. 9.6. The amplitude and phase angle for the most important semidiurnal and diurnal components of
the tide at the main tidal stations are shown in Table 9.1. Although the tide at the Ocea-