Effect of the flowrate variations of Sauce Chico and Napostá Grande rivers over the inner part of Bahia Blanca estuary – Application of the mohid water modeling system

M. E. CARBONE, F. LIMBOZZI, E. ALBERDI AND P. ABALO

Perspectives on managing our estuarine heritage: examples from Latin

Lugar: Lisboa Portugal; Año: 2007;

In this chapter the hydrological behavior of Sauce Chico and Napostá Grande rivers at the lower part of their watersheds, in effect at their mouth neighborhoods, is analyzed. In those sectors, both their incidence on and their interaction with the hydrodynamics of the marine environment at the inner area of Bahía Blanca estuary are evaluated.The aim of this study is to investigate the effect that produces a significant flowrate increase over the inner section of the estuary, as a result of one storm event of a single exceptional rainfall. With this purpose a preliminary hydrodynamic model using MOHID Water Modeling System has been developed. The Bahía Blanca estuary is a mesotidal system conformed by an important channels network with diverse dimensions oriented in NW-SE direction. These channels have sinusoidal courses separated by islands and extensive tidal flats composed, in general, by silty clay sediments. The tidal regime is semidiurnal, being the average amplitude at the ports of Ingeniero White and Galván (inner zone) of 3.8 and 2.7 meters in spring tide and neap tide respectively (Perillo and Piccolo, 1999). Sauce Chico and Napostá Grande rivers   forman parte de las cuencas de pendiente atlántica and are the only permanent contributions to the Bahía Blanca estuary (García and García, 1964), which form a hierarchized drainage network of third order (Strahler, 1952). The Sauce Chico watershed has an area of 1,600 Km2 (Carrica, 1998) and the river length is 110 km This course drains from the middle sector of the pedemont of the Sierra de la Ventana ranges crossing the plain with a semicircular form in East-West direction. Afterwards, seven kilometers before arriving at the estuary, in the head of the Principal Channel, the river turns to the east and is branched off demarcating its alluvial fan. According to its regional morphology, this tributary is the main collector of all watercourses which had drained and nowadays drain to the estuary (González, 1997). Its hydrographic parameters were measured by the Dirección General de Agua y Energía having determined a module of 1.807 m3.s-1 (1952-1978), being the maximum and minimum flow rates 570 and 0.310 m3.s-1 respectively (Figure 1). To get a comprehensive view of a heavy rainfall event, a dense network of precipitation and stream gages is necessary.  However rainfall data in this study area is scarce. Data of two rainfall gage station are available for this study. They lies  respectively  at the middle and lower part of  Naposta Grande watershed. The first one, a Davis GroWeather Station is  located  in Tres Picos town (38º 17´ 26.2” S and 62º 10´ 16.6” W) and the another one en la Aerometeorológic Comandante Espora Station (38º 43´ 47.8” S and 62º 09´ 35.9”). The analysis of the precipitations registered since November of 2005 until September of 2007 at the middle and lower basin of Sauce Chico river and at the lower part of Napostá Grande watershed allowed to establish the relation between the rainfall and the rivers flow rates obtained for that period. Although the data series does not surpass the three years, it allows making a preliminary characterization in the studied region. Also the study of hydrograms arisen from precipitation events of a single heavy rain and their relation with the flow rate increase in the mouths of both courses was made. In order to analyze the interaction between fluvial and marine environments at the inner part of Bahía Blanca estuary, a preliminary hydrodynamic model using MOHID Water Modeling System was developed. This application allowed the simulation of the effects of exceptional flow rates upon the estuary according to the rainfall events above referred. According to this preliminary study, the Sauce Chico and Napostá Grande rivers shows  dissimilar hydrological behaviors and, although both belong to Atlantic slope river basins,  they have own features, flowing by lands with different slopes and characteristics. The land use and the land cover of the region have influence over the flowrates at the lower watersheds, where the flowrate changes reflect the pressures due to urban and agriculture activities.