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Some Like It Hot: Stratification, Circulation and Turbulence in a Shallow, Tropical Reservoir

Author(s): Stephen G. Monismith, Edmond Lo, Zikun Xing, Peipei Yang, James L. Hench, Derek A. Fong

Linked Author(s): Stephen Monismith

Keywords: Lakes, reservoirs, mixing, circulation, turbulence

Abstract: In this paper we give an overview of field work and computations that we have carried out over the last decade in Kranji Reservoir (Singapore) studying the fundamental hydrodynamics of the reservoir. This includes studies of the basic circulation dynamics of the reservoir including motions generated by wind stresses, inflows, and by surface heat fluxes. A key feature of the reservoir is that there is nearly as much variation in temperature in one week as there is in one year. As a consequence, there are no discernable seasonal variations in stratification, and thus diurnal stratification variations play a central role in the hydrodynamics of the lake. For example, wind stresses force complex baroclinic motions that are not seiching because internal seiche periods are generally longer than the time over which stratification develops due to heating and then decays due to surface cooling. Inflow events, which are short-lived, usually lasting a few hours, and intense, bring relatively large volumes of colder water into the reservoir, creating the most stable and long-lived stratification. Because winds are generally relatively weak on the reservoir, flows driven by cooling of shallow regions may be an important mechanism for transporting materials from the shallow areas away from the dam, towards the deeper waters near the dam. Because the practical motivation for our work was concern over nutrient-driven eutrophication, particularly involving the cyanobacteria Microcystis Aeruginosa, much of our effort has focused on quantifying rates of vertical turbulent mixing using temperature microstructure profiling. While the near-surface structure of turbulent mixing (turbulent kinetic energy, dissipation rates, etc. ) is well described by the existing concepts of surface mixed layer dynamics, more complicated and variable rates of mixing are observed below the mixed layer depending on the nature of the flows generated by cooling and by inflows. Nonetheless, consistent with observed stratification variations, vertical mixing is sufficiently rapid to ensure nearly complete vertical mixing throughout the water column every day, suggesting that given a very shallow photic zone, positively buoyant phytoplankton like Microcystis may be able to effectively compete with negatively buoyant ones like most diatoms. Overall, flows and the temperature structure in the reservoir appear to be quite three dimensional, suggesting that modeling transport processes in the lake require a 3d model. In this paper we give an overview of field work and computations that we have carried out over the last decade in Kranji Reservoir (Singapore) studying the fundamental hydrodynamics of the reservoir. This includes studies of the basic circulation dynamics of the reservoir including motions generated by wind stresses, inflows, and by surface heat fluxes. A key feature of the reservoir is that there is nearly as much variation in temperature in one week as there is in one year. As a consequence, there are no discernable seasonal variations in stratification, and thus diurnal stratification variations play a central role in the hydrodynamics of the lake. For example, wind stresses force complex baroclinic motions that are not seiching because internal seiche periods are generally longer than the time over which stratification develops due to heating and then decays due to surface cooling. Inflow events, which are short-lived, usually lasting a few hours, and intense, bring relatively large volumes of colder water into the reservoir, creating the most stable and long-lived stratification. Because winds are generally relatively weak on the reservoir, flows driven by cooling of shallow regions may be an important mechanism for transporting materials from the shallow areas away from the dam, towards the deeper waters near the dam. Because the practical motivation for our work was concern over nutrient-driven eutrophication, particularly involving the cyanobacteria Microcystis Aeruginosa, much of our effort has focused on quantifying rates of vertical turbulent mixing using temperature microstructure profiling. While the near-surface structure of turbulent mixing (turbulent kinetic energy, dissipation rates, etc. ) is well described by the existing concepts of surface mixed layer dynamics, more complicated and variable rates of mixing are observed below the mixed layer depending on the nature of the flows generated by cooling and by inflows. Nonetheless, consistent with observed stratification variations, vertical mixing is sufficiently rapid to ensure nearly complete vertical mixing throughout the water column every day, suggesting that given a very shallow photic zone, positively buoyant phytoplankton like Microcystis may be able to effectively compete with negatively buoyant ones like most diatoms. Overall, flows and the temperature structure in the reservoir appear to be quite three dimensional, suggesting that modeling transport processes in the lake require a 3d model

DOI:

Year: 2017

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