Understanding stratification and turnover dynamics of a tropical lake using extensive field observations and 3D hydrodynamic simulations.

The stratification and turnover dynamics of a tropical lake were evaluated using field observations and 3D hydrodynamic simulations. Located in the Philippines, Sampaloc Lake is a 104-ha and 27-m deep volcanic crater lake with enclosed watershed, which is at risk of the impacts of intensive aquaculture, rapid urbanization and climate change. Temperature, dissolved oxygen (DO) and chlorophyll-a (Chl-a) were measured at seven sampling stations using a multiprobe. Kruskal-Wallis test revealed that the three parameters are not significantly different among stations, indicating that one sampling station can represent the water quality of the whole lake. Schmidt's Stability Index (SSI) and thermocline strength, together with DO and Chl-a gradients decreased from October 2022 (stratified) to January 2023 (turnover). After successfully verifying the 3D numerical model, sensitivity analyses of water temperature to varying weather, together with particle tracking simulations, were implemented to determine the timing of isothermal state, upwelling, partial mixing, and full turnover. Compared to air temperature, variations in wind speed have more pronounced effects on the delay or progression of isothermal conditions in the lake based on SSI, Lake Number and Wedderburn Number. Isothermal conditions do not necessarily coincide with the timing of full turnover, with the latter being delayed by two days than the former, on average. Results revealed that full turnover can occur several weeks earlier with the decrease in AT and increase in WS. This study can advance the understanding of thermal and turnover dynamics of stratified tropical lakes, leading to better management of the water quality of these water bodies.

Factors contributing to the minimum water column stability and timing of the winter turnover in the Ogouchi reservoir.

Turnover in lakes and reservoirs causes circulation in the water column from the bottom to the surface when the water column stability becomes low. Previous studies commonly mentioned that turnover occurs when stratification indices become small, but the threshold is rarely discussed. While turnover phenomena have been extensively studied by evaluating changes in bottom dissolved oxygen (DO), the relationship between the disappearance of hypoxia and water temperature indices has not been determined. This study focused on the factors influencing the minimum thermal gradient (TG) and Schmidt Stability Index (SSI), and the timing of turnover events using DO as an indicator of mixing in the Ogouchi reservoir from 1992 to 2001. The results showed that the occurrence of minimum TG and SSI is mainly driven by inflow retention time and average maximum wind speed. Moreover, minimum air temperature and outflow retention time have few contributions to minimum SSI. It was found that 7 out of 10 years exhibited full winter turnover, while the remaining years showed incomplete mixing with persistent hypoxia at the reservoir bottom. This study identifies four cases based on onset thresholds of 0.0035 °C m-1 for TG and 30 J m-2 for SSI to explain turnover event: Case 1: an ideal state with stratification indices below the threshold, resulting in the disappearance of hypoxia; Case 2: indices above the threshold sustain hypoxia; Case 3: an irregular state where the indices exceed the threshold, yet hypoxia disappears; and Case 4: an unexpected persistence of hypoxia despite being below the threshold. The majority of the years (70 percent) were explained by thresholds. The multiple regression analysis indicated the importance of wind speed on the turnover event. Therefore, the effect of wind shear was analyzed for 30 percent of the years that cannot be explained by thresholds (cases 3 and 4). Case 3 shows turnover occurrence due to strong accumulated wind shear, despite exceeding thresholds. Conversely, Case 4 reveals weak wind shear preventing bottom water upwelling, even below thresholds. In conclusion, the precise TG and SSI thresholds for the onset of turnover event were determined using DO data. The thresholds explained the occurrence and non-occurrence of turnover event in most of the years and wind speed clarified unexplained cases by thresholds. The presented method successfully evaluated the timing of turnover and can be applicable elsewhere.

Thermal stratification responses of a monomictic reservoir under different seasons and operation schemes.

This study investigates the thermal stratification responses of a monomictic reservoir operated under different facilities. The analysis of 60-year long data showed that the reservoir's thermal regime varies with season and withdrawal scheme and is affected by upstream reach control through the vertical curtain. Isothermal conditions exist during winter (December-March) while stratification onsets in spring (starting April), intensifies in summer (August) and weakens during fall (October-November). Considering summer stratification, deep hypolimnetic withdrawals through the penstock intake promoted thicker epilimnion, with low values of thermal stability (Schmidt Stability Index, SSI) and thermocline strength index (TSI). Meanwhile, shallow withdrawals using selective outflow system resulted in narrower epilimnion, with larger TSI for no curtain scenario and larger SSI for with curtain scenario. Strongest thermoclines do not necessarily translate to largest magnitudes of thermal stability. Longer duration of stratification is associated with shallow withdrawals. Depending on the outflow depth and the occurrence of prolonged hot or cold atmospheric conditions, the onset of stratification could be likely shifted early or late. The 3D numerical simulation determined the individual effects of each operation, which strongly supported the results of the long term analysis. Since thermal stratification directly influences the reservoir's water quality regime, this study can be a helpful reference in optimizing the water quality management of the reservoir.

Direct measurement of secondary circulation in a meandering macrotidal estuary.

Secondary circulation, which induces the typical helical flow motion along a curved channel, is responsible for the complex morphodynamic processes in fluvial streams and estuaries with meandering nature. Classical secondary circulation has been extensively documented through flume experiments and numerical simulations but field observations about this phenomenon are scarce and only limited to few channel cross-sections. In this study, intensive measurements of flow velocities were performed using a vessel-mounted ADCP in order to illustrate the spatial distribution of classical secondary circulation in a meandering macrotidal estuary for both flood and ebb phases. Negligible salinity is noted during field measurements and velocity profiles manifesting logarithmic patterns confirm open channel flow conditions. As the flow enters the bend, primary and secondary velocities are shifted towards the outer bank. At the bend apex, circulation is accelerated with near-surface primary and secondary velocities skewed to the outer bank while bottom currents are skewed to the inner bank. As the flow exits the bend, bottom secondary currents gain dominance over surface currents while the bottom velocity vectors are directed towards the inner bank. Both the primary and secondary currents cause the propagation of the three-dimensional helical flow alongside with the asymmetry in the bend cross-section and the formation of point bar. This study successfully pioneers in demonstrating the three-dimensional structure of classical secondary circulation in actual field conditions through intensive ADCP surveys.