On characteristic hydraulic times through hydrodynamic modelling: discussion and application in Patos Lagoon (RS)

Abstract

Although the analyses of characteristic hydraulic times are important diagnostic tools for studying water mass exchanges and identifying areas prone to stagnation that are potentially subjected to eutrophication effects, their concepts and uses are often misinterpreted. This paper compares similarities and differences between widely used characteristic hydraulic times, CHT, known as Residence Time, Times of Water Renewal Rates and Water Age. A proper definition for each of these characteristic hydraulic times is stated to avoid the existing confusion with multiple concepts in the literature. Methodologies to compute these CHT through hydrodynamic modelling systems are presented and, in order to enhance understanding, applied to three idealized cases in steady flow channels: (1) Channel with uniform flow; (2) Channel with a lateral inflow; and (3) Channel with a lateral embayment. Finally, a practical example is discussed by applying the methodologies to the Patos Lagoon (RS). The results for the idealized channel cases are non-intuitive and this theoretical discussion clarifies the interpretation and uses of different timescales and outlines the Water Age as the more versatile and multifunctional timescale if compared to the others addressed here. The results for the Patos Lagoon exemplify the valuable information that CHT can offer for environmental management in natural water bodies.