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.
Highlights
The analysis of characteristic hydraulic times (CHT) has been used as an important diagnostic tool
We adopted in this work the following definitions: a) Residence Time (RT): given a domain of interest in a water body with particles positioned at various locations in an initial time, the RT for each particle is the time it takes to leave the specified domain as it is passively transported by local currents; b) Times of Renewal Rates (TRR%) represents the time it takes to attain a water renewal of X% in different regions of the water body; and, c) the Water Age (WA) indicates, for a given position of the domain, the average time the water parcels that are in that position remain in the domain of interest as the flow circulates through the domain
The results presented in this paper showed that, even in idealized situations with simple domains and controlled flux alterations, non-intuitive results for CHT arise
Summary
The analysis of characteristic hydraulic times (CHT) has been used as an important diagnostic tool. These analyses are helpful to track the water mass exchange (Monsen et al, 2002), giving us an important knowledge of the hydrodynamics processes that helps water bodies to recover from local pollution. One can find numerous examples in the literature of CHT studies on water bodies pressured by the surrounding population or by the presence of industrial activity, that usually present environmental risks. Du and Shen (2016) studied the Residence Time in the Chesapeake Bay, the largest estuary in the United States and very important for local economy and ecology. Port zones have been investigated using CHT, assisting in harbor environmental management as stated by Grifoll et al (2014), where the authors analyzed the water renewal and mixing of the Barcelona harbor zone in the northwest Mediterranean Sea
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