An integrated study in the estuary of Bahía Blanca (ARGENTINA)

Author: Guillermo Raúl Angeles
Departamento de Geografía (Universidad Nacional del Sur), 12 de octubre y San Juan, 8000, Bahía Blanca, Argentina (gangeles@criba.edu.ar)

The Bahía Blanca estuary is located at the southwest of Buenos Aires Province (Argentina) comprising an area of 2,300 km2. It is considered a mesotidal estuary formed by a complex system of tidal channels of variable size, islands and extended tidal flats. The last ones delimit the Main Channel of the estuary. It has a general NW-SE trend and constitute the access to one of the most important harbour complexes of Argentina.

In this thesis the study of the tidal channels and the environments around it was made considering that the knowledge in detail of the tidal channels geomorphology and dynamics constitute the basis to develop an adequate zoning and coastal planning in the area of study. To achieve this goal, several methodologies were used, such as oceanographic surveys; classical morphometric analisys; fractal geometry; remotely sensed images processing and geographical information system (GIS).

In a first stage, the results obtained by the oceanographic surveys were described. These surveys were made in 1996 and 1999. The analysis of both the physical and geomorphological characteristics reveals that:
1) The general circulation is dominated by a stationary and semidiurnal tidal wave. It is the principal agent responsible for both the dynamics of the sediment and the geomorphology in the estuary;
2) The scant freshwater discharge into the estuarine system, the higher evaporation on the tidal flats and the superficial washing on the coastal plains are the three processes that influence in the high values of salinity measured (between 34.4 and 38.5) along a complete tidal cycle (13 h). The action of these processes in the estuary provokes a mixture in the water column that defines it as a partially homogeneous estuary;
3) The temperature values measured showed an increase in the outward layer water temperature along the tidal cycle, due to the radiative and turbulent warming process between the outward layer water and the air. However, the temperature values presented a vertically homogeneous distribution in the water column;
4) The current vector was decomposed into a longitudinal component (U) with reference to the main axis of the channel (positive in the ebb direction), and transverse component (V) positive to the right looking toward the mouth of the estuary. The analysis of tidal currents during the tidal cycle showed that the estuary is dominated by ebb currents. The maximum speed values measured corresponding to the longitudinal component (ebb current 102 cm s-1 and flood current 79 cm s-1);
5) The bathymetric surveys make it possible to know the tidal channels morphology. In a sector of the Main Channel it was observed how the dredging affected its morphology. Also, a small slope of the channel bottom with a S-N and E-W trend was detected. The maximum depth measured was 17 m in the centre of the channel whereas in La Lista channel the bathymetric survey showed that its meanders are dominated by the ebb currents with the grater depth measured along the erosional bank (maximum depth 7 m).

In a second stage, a new methodology derived from the relationship between the Normalized Difference Water Index (NDWI) and Horton stream ordering was applied to classify the tidal channels network. On the NDWI image obtained was traced the tidal channel network. Later, according to Horton’s stream orders a hierarchical classification of the channels was made. Five orders of tidal channels and five different zones according to its drainage density and drainage frequency were identified. The rersults obtained reveal the existence of two areas perfectly delimited. The first one, called North, has the greater drainage density (8.4) and drainage frequency (10.6) values. The second one, called South, has both drainage density and drainage frequency values remarkably smaller than the other sector (5.5 and 4.6, respectively).

In a third stage, a fractal dimension was estimated for nine tidal channels depicted in TM Landsat-5 imagery to derive information about the degree of geological control on a tidal channel network. Two methods Box counting (Pfeifer and Obert, 1989) and Contiguity (Richardson, 1961) were used to estimate a dimension fractal (D) in each tidal channel considered. The results showed that all channels have D values near to 1. That means that the channels are selfaffine fractal features. However, these fractal dimentions would not be showing the meandering pattern complexity characteristic of these tidal channels. On the other hand, the fractal condition of the tidal channels reveals that the evolution of them is not chaotic and maybe would be being controled by both the morphology of the tidal flats and the action of the tidal currents.

In a fourth stage, Landsat TM data in the form of false colour composites (FCC) generated from bands RGB = 4-7-2 and RGB = 457 were used to elaborate two maps representing both the geomorphology and the vegetation characteristic of the Bahía Blanca estuary. These maps have been prepared based on images segmentation techniques of FCC to identify, classify and delineate various geomorphological units and vegetation types. Both maps were checked during the field works. In the geomorphological map five units were identified: 1 Salt marshes and Tidal flats; 2 Islands, sand banks and islets; 3 Coastal zone related to marine paleolevel conformed by frank-clayey sediments; 4 Coastal zone related to pelodeltaic area of the Colorado river basin; and 5 Tidal channels. In the vegetation map six classes were defined: 1 Halophytic shrub steppe; 2 Psamophylous herbaceous steppe; 3 Halophytic scubs;
4_ Xerophilous scrubs; 5 Land crops; and 6 Lands without vegetation.

The relationship among the two maps (geomorphologic and vegetation) and the oceanographic surveys results showed that 51% of the area is occupied by tidal flats and salt marshes. In the latter evolve both halophytic shrub steppe and halophytic scrubs (covering 20% and 3% of the area studied, respectively). The first one is covering the banks of the channels whereas the second one occupies the high lands of the salt marshes. Furthermore, it was observed that the spatial disposition of these halophytic species is in parallel to the more important channels (4th and 5th order according to Horton’s classification).

The importance of this phenomenon is related to the contribution of sediments into the estuary. It is very little and for this reason, this spatial pattern of the vegetation could be acting as a fine sediment trap during the ebb cycle. As a consequence, it could be contributing to delay the erosion process in the estuary.

Finally, an integrated analysis of the all results obtained in the different stages of this study was developed to establish a zoning proposal for the coastal planning in the Bahía Blanca estuary. This zoning was based on a new methodology never applied in the estuary which was used to identify seven landscape units: 1 Tidal channels associated with halophytic species developed over salt marshes; 2 Tidal channels associated with tidal flats without vegetation; 3 Sector dominated by the mouth of the estuary with presence of islands, sand banks and islets; 4 Sector dominated by the Sauce Chico river oulet and the head of the estuary; 5 The Main Channel or the harbour complexes channel access; 6 Coastal zone with extensively cultivated lands; and 7 Coastal line with development of harbour complexes and petrochemical complexes.

In each one of these landscape units was estimated an environmental preservation value (EPV) according to landscape, geomorphological, scientific, ecological and landuse type criteria. The highest EPV corresponding to the landscape units 1,2 and 3. Whereas, the landscape units related to activities pertaining to port and industry have the lowest EPV.

A brief description of the coastal planning proposals to be implemented in the Bahía Blanca estuary is given below:
a) Develop preservation activities on the landscape units defined with high EPV in order to prevent the evolution of higly specialized flora and fauna;
b) Elaborate a control plan for some tourist activities (p.e. water sports and fhising) in order to restrict their practice only in some specific areas of the estuary;
c) The environment impact assessment (EIA) should be considered a priority in those coastal areas with development of harbour and petrochemical complexes;
d) Restrict fishing activities practiced with inadequate ecological fishing techniques (e.g. trawler fishing) in those landscape units that involve secondary tidal channels, which constitute spawning and breeding sites for numerous ichtyic species.