Evaluation of the Santa Helena dam impacts on the Jacuípe river estuary, north coast of Bahia - Brazil

Abstract

River dams are normally associated to hydraulic and hydrologic impacts in estuaries. Hydraulic consequences, reported in literature, are the increase of the mean and the reduction of maximum discharges, with the attenuation of the flood waves. This study aimed to characterize the hydrodynamics and mixing patterns of the Jacuipe river estuary (northern State of Bahia), and to evaluate the influence of Santa Helena dam, erected in 2001, upon the estuarine flow. It was observed that the dam altered the natural discharge regime, impacting all ranges of fluvial discharge. The changes were, however, opposite to those related in the literature, as post-dam mean discharge value was reduced and peak discharge values were increased. Water release rates topped the extreme of the natural discharge curve, whereas null discharges were sustained for a much longer period. The estuary was partially mixed during neap and well mixed during spring tides, with gravitational circulation predominating with low and null dam discharge values in both tidal conditions. Larger discharge values (70 m³ s^-1) expelled the salt water from the estuary and caused ebbing flow throughout the tidal cycle. The implementation of water transposition from Santa Helena dam to the neighboring Joanes river will reduce peak flows and probably cause shoaling of the estuary channel and inlet. This process is likely to cause degrading environmental conditions in the estuary.