Mathematical modelling for reservoir water-quality management through hydraulic structures: a case study

Abstract

The U.S. Army Corps of Engineers' one-dimensional model (CE-QUAL-RI) was used to analyse the usefulness and feasibility of hydraulic structures in allowing management procedures for reservoir water-quality control. The model calculates the vertical hydrothermal and water-quality structure as a function of inflowing waters, meteorology and reservoir operation. It considers selective withdrawal as a result of the operation of spillways, intake for turbines and bottom outlets. The example herein is a study of the feasibility of a bottom outlet in altering water-quality tendencies of an Amazon hydroelectric power project — Cachoeira Porteira Reservoir — which filling forecast to first half of 1995. In order to obtain more realism in respect of water-quality dynamics of Amazon reservoirs, some modifications were introduced at the original model version, which was also adapted to 16-bit microcomputer hardware. A large hydrological series of averaged monthly flow rates was studied, and two sequential years were chosen for obtaining a critical reservoir water-quality situation. Field water-quality data from distinct hydrological periods were used, and daily local meteorological information was considered. The simulations involved four hypotheses, with consideration of different flow rates through a bottom outlet. The alterations to reservoir water-quality tendencies were analysed, emphasising the intensity of stratification, eutrophication, and anoxic conditions caused by the flooding of natural vegetation. The results show the low influence of bottom outlet utilization in the specific case of Cachoeira Porteira Reservoir water quality. Some considerations involving the features of the dam outlet structures and the simulations results are discussed.