3.3. Model analysis

Hydrodynamic simulation in Bukit Merah Lake indicated that the major driving force of the hydrodynamic pattern in the reservoir is wind-driven motion. The hydrodynamic patterns at surface level as shown in Figure 5 were averaged from air-water interface to 0.5 m depth. The circulation pattern analyzed and modeled showed mixing and water movement in the lake, which is closely related to wind velocity and direction. For example, a northeast wind, with magnitude approximately greater than 3 m/s, can cause a substantial transport and circulation of water mass in Bukit Merah Lake. A much higher wind speed exceeding 7 m/s was recorded

Figure 4. Calibrated results of (a) current velocity and (b) temperature in Bukit Merah Lake.

in an earlier observation in 2014, and most wind direction was observed to be from north-east [17]. Wind-induced storm has been observed to have transported floating vegetation island from north to south of the lake [17]. In our simulation, water movement responded along the wind direction. As shown in Figure 5a, north-east wind moved water toward southeast direction. High wind exceeding 7.5 m/s induced mean surface current about 7.2 cm/s. Higher current of 14.4 cm/s occurs near constricted channel. Warmer temperature was observed in the west, while lowest temperature was observed in the southeast. High wind effects homogenized the surface temperature and induced turbulence that could well mix the water column (Figure 5a and b).

Figure 5. Pattern of (a) temperature and (b) current velocity pattern under high wind events and (c) temperature and (d)

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current velocity pattern under low wind events in Bukit Merah Lake.

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Figure 5. Pattern of (a) temperature and (b) current velocity pattern under high wind events and (c) temperature and (d) current velocity pattern under low wind events in Bukit Merah Lake.

in an earlier observation in 2014, and most wind direction was observed to be from north-east [17]. Wind-induced storm has been observed to have transported floating vegetation island from north to south of the lake [17]. In our simulation, water movement responded along the wind direction. As shown in Figure 5a, north-east wind moved water toward southeast direction. High wind exceeding 7.5 m/s induced mean surface current about 7.2 cm/s. Higher current of 14.4 cm/s occurs near constricted channel. Warmer temperature was observed in the west, while lowest temperature was observed in the southeast. High wind effects homogenized the surface temperature and induced turbulence that could well mix the water column

Figure 4. Calibrated results of (a) current velocity and (b) temperature in Bukit Merah Lake.

78 Applications in Water Systems Management and Modeling

(Figure 5a and b).

In addition to winds, inflows from tributaries are also important for Bukit Merah Lake. Water movement from Kurau River from simulated result responded directly with the inflow of Kurau River. The water mass moved from the river mouth in the east to the deeper area in the west and south. Wind effects conveyed further the water mass and spread in to larger areas depending on the speed and direction. Under low wind events or calm conditions, temperature gradient between shallow and deep areas promoted convective motion. Convective motions can be driven by differential heating and cooling of water mass of differed depth and volume. Since the lake depth was less than 5 m deep, both wind-induced and nocturnal mixing in Bukit Merah Lake extended to the lake bottom consistent with mixing pattern in other shallow lakes [18]. Well-mixed lakes were like to be more supersaturated with oxygen and thus more productive due to better light environment and nutrient supply such as from sediment. This hydrodynamic feature may support the findings on phytoplankton characterization between the three lakes, which indicated higher productivity in Bukit Merah Lake compared to the two lakes [19].

Unequal heating of surface layer between shallow and deep regions resulted in higher rate of change in temperature in the shallow areas due to small volume of water contained. This differential heating can induce movement of warm waters over the cooler waters [20]. Similarly, differential cooling or unequal cooling between shallow and deep regions resulting from larger heat loss in the shallow regions or weaker winds due to wind shelters resulted in downslope movement of cold waters to deeper regions [20]. Most of the areas at the south and west of Bukit Merah are shallower compared to the east side of Bukit Merah which induced movement of water toward the deeper areas in the east or center due to differential heating in the shallow areas during calm conditions (Figure 5c and d). In contrast to current velocity during wind events, current velocity during calm weather were small, in the range of nil to 2.9 cm/s. Differential cooling was observed in other large lake, such as the Lake Tanganyika in Africa, which induced large-scale convective motion throughout the lake [21]. Drought associated with long dry season may have strong impact on the Bukit Merah Lake due to its large size. Continuous abstraction of water for irrigation and higher evaporation rates can lead to significant drop of water level in the lake. Low water level reduces the lake volume and affects the hydrodynamic patterns with stronger mixing and temperature gradient in agreement with atmospheric forcing including wind, temperature, and solar radiation pattern.

The turbid water from the Pahang River may also induce vertical density gradient as surface temperature increases at the surface water due to reduction in penetration of solar radiation to a smaller depth as observed in the nearby floodplain Chini Lake [19]. Convective motion driven by water temperature gradient was also important in Bera Lake. In this lake, under still wind conditions and low flow, movement of current or convective motion was detected by the simulation which was likely driven by unequal temperature gradient between the macrophytes (shallow) and open water (deeper). During the night, warmer temperature at the deeper area in the east moved to shallower area in the west at the surface, while cooler water in the shallow areas move to the deeper areas of the open water at the lake bottom (Figure 6c). Density-driven flow was observed between reed beds and open water in a shallow lake in southern Sweden [22] and in laboratory experiments [23]. Current velocities induced by differential cooling and heating between floating-leaved bed and open water reported in the literature were between 1.6 and 1.9 cm/s [19]. Higher horizontal density flow was reported between open water and reed beds (1.5–2.8 cm/s) [22] and between the wetland and lake (3–5 cm/s) [24]. In this study, simulated current movements under no winds as illustrated in the simulation

Figure 6. Temperature and current velocity pattern in Bera Lake during (a) high winds, (b) strong inflow from Bera River,

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Hydrodynamic simulation in Durian Tunggal Lake showed the importance of wind effects in driving the hydrodynamic pattern in the reservoir. Wind records showed high wind conditions exceeding 6 m/s with wind direction mostly from the northeast. Durian Tunggal Lake was deeper compared to the other two lakes and may experience more apparent stratification. Lake isotherm under different wind conditions is shown in Figure 7. High wind speeds exceeding 6 m/s led to strong mixed layer throughout the water column within the upper (Figure 7a). In mid-afternoon and high solar radiation, increased surface heating and weak winds induce differential heating near surface layer (Figure 7b). The lake may also experience differential heating due to variation in heat absorption, given the high turbidity level in the

were in the range of 1.0–3.0 cm/s.

and (c) calm wind and low flow.

Hydrodynamic simulation in Bera Lake indicated that the major driving force of the hydrodynamic pattern in the reservoir is river inflow. Bera Lake is mostly surrounded by riparian forest and subsequently is sheltered from wind motions. Wind records showed low wind conditions with average winds of about 1 m/s. Wind-driven motion has low effects on this natural lake. However, higher wind speed (>3 m/s) occasionally alters water movement in the lake (Figure 6a). Similar to the nearby Chini Lake, this floodplain lake is very much influenced by inflows of the main river, namely the Pahang River that backflows into the lake through Bera River during monsoon and rainy seasons. Simulation showed the water movement responded directly with inflow from the Pahang River (Figure 6b). The inflow of cold and turbid water from the Pahang River is heavier and enters into the lake as underflows. Our qualitative observation revealed that inflows into the lake from all tributaries were negligible during dry period and drought, and together with no rain input led to severe drop of lake's water level.

In addition to winds, inflows from tributaries are also important for Bukit Merah Lake. Water movement from Kurau River from simulated result responded directly with the inflow of Kurau River. The water mass moved from the river mouth in the east to the deeper area in the west and south. Wind effects conveyed further the water mass and spread in to larger areas depending on the speed and direction. Under low wind events or calm conditions, temperature gradient between shallow and deep areas promoted convective motion. Convective motions can be driven by differential heating and cooling of water mass of differed depth and volume. Since the lake depth was less than 5 m deep, both wind-induced and nocturnal mixing in Bukit Merah Lake extended to the lake bottom consistent with mixing pattern in other shallow lakes [18]. Well-mixed lakes were like to be more supersaturated with oxygen and thus more productive due to better light environment and nutrient supply such as from sediment. This hydrodynamic feature may support the findings on phytoplankton characterization between the three lakes, which indicated higher productivity in Bukit Merah Lake

Unequal heating of surface layer between shallow and deep regions resulted in higher rate of change in temperature in the shallow areas due to small volume of water contained. This differential heating can induce movement of warm waters over the cooler waters [20]. Similarly, differential cooling or unequal cooling between shallow and deep regions resulting from larger heat loss in the shallow regions or weaker winds due to wind shelters resulted in downslope movement of cold waters to deeper regions [20]. Most of the areas at the south and west of Bukit Merah are shallower compared to the east side of Bukit Merah which induced movement of water toward the deeper areas in the east or center due to differential heating in the shallow areas during calm conditions (Figure 5c and d). In contrast to current velocity during wind events, current velocity during calm weather were small, in the range of nil to 2.9 cm/s. Differential cooling was observed in other large lake, such as the Lake Tanganyika in Africa, which induced large-scale convective motion throughout the lake [21]. Drought associated with long dry season may have strong impact on the Bukit Merah Lake due to its large size. Continuous abstraction of water for irrigation and higher evaporation rates can lead to significant drop of water level in the lake. Low water level reduces the lake volume and affects the hydrodynamic patterns with stronger mixing and temperature gradient in agreement with atmospheric forcing including wind, temperature, and solar radiation pattern.

Hydrodynamic simulation in Bera Lake indicated that the major driving force of the hydrodynamic pattern in the reservoir is river inflow. Bera Lake is mostly surrounded by riparian forest and subsequently is sheltered from wind motions. Wind records showed low wind conditions with average winds of about 1 m/s. Wind-driven motion has low effects on this natural lake. However, higher wind speed (>3 m/s) occasionally alters water movement in the lake (Figure 6a). Similar to the nearby Chini Lake, this floodplain lake is very much influenced by inflows of the main river, namely the Pahang River that backflows into the lake through Bera River during monsoon and rainy seasons. Simulation showed the water movement responded directly with inflow from the Pahang River (Figure 6b). The inflow of cold and turbid water from the Pahang River is heavier and enters into the lake as underflows. Our qualitative observation revealed that inflows into the lake from all tributaries were negligible during dry period and drought, and together with no rain input led to severe drop of lake's water level.

compared to the two lakes [19].

80 Applications in Water Systems Management and Modeling

Figure 6. Temperature and current velocity pattern in Bera Lake during (a) high winds, (b) strong inflow from Bera River, and (c) calm wind and low flow.

The turbid water from the Pahang River may also induce vertical density gradient as surface temperature increases at the surface water due to reduction in penetration of solar radiation to a smaller depth as observed in the nearby floodplain Chini Lake [19]. Convective motion driven by water temperature gradient was also important in Bera Lake. In this lake, under still wind conditions and low flow, movement of current or convective motion was detected by the simulation which was likely driven by unequal temperature gradient between the macrophytes (shallow) and open water (deeper). During the night, warmer temperature at the deeper area in the east moved to shallower area in the west at the surface, while cooler water in the shallow areas move to the deeper areas of the open water at the lake bottom (Figure 6c). Density-driven flow was observed between reed beds and open water in a shallow lake in southern Sweden [22] and in laboratory experiments [23]. Current velocities induced by differential cooling and heating between floating-leaved bed and open water reported in the literature were between 1.6 and 1.9 cm/s [19]. Higher horizontal density flow was reported between open water and reed beds (1.5–2.8 cm/s) [22] and between the wetland and lake (3–5 cm/s) [24]. In this study, simulated current movements under no winds as illustrated in the simulation were in the range of 1.0–3.0 cm/s.

Hydrodynamic simulation in Durian Tunggal Lake showed the importance of wind effects in driving the hydrodynamic pattern in the reservoir. Wind records showed high wind conditions exceeding 6 m/s with wind direction mostly from the northeast. Durian Tunggal Lake was deeper compared to the other two lakes and may experience more apparent stratification. Lake isotherm under different wind conditions is shown in Figure 7. High wind speeds exceeding 6 m/s led to strong mixed layer throughout the water column within the upper (Figure 7a). In mid-afternoon and high solar radiation, increased surface heating and weak winds induce differential heating near surface layer (Figure 7b). The lake may also experience differential heating due to variation in heat absorption, given the high turbidity level in the

water from interbasin water transfer from Muar River into the lake. Similar to Bukit Merah, northeast winds are common for Durian Tunggal Lake and such wind direction moved water toward southeast direction (Figure 8a). Mid-wind speeds and heavy rain exceeding 50 mm induced mean surface current more than 9.0 cm/s in certain western part of the lake (Figure 8b), while low wind induced wind surface current less than 2.5 cm/s (Figure 8c). Additionally, higher salinity of the water from interbasin water transfer was reported occasionally which may affect the hydrodynamic features [10]. However, this dynamic is not studied here due to limited observation data. Water level in this man-made lake is very much

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The hydrodynamic studies of the three lakes in Malaysia here indicate distinct variation of the circulation. The mixing regime is shaped by the bathymetry of the lakes in relation to changes in winds, temperature-solar radiation, and inflows. This study found variation in the major driving force of the hydrodynamic pattern between lakes. Hydrodynamic simulations showed that the Bukit Merah and Durian Tunggal reservoirs are more sensitive to wind-driven motion. Bera Lake is more sensitive to flood inflow by the main river during the monsoon season. Convective motion driven by water temperature gradient was important for Bukit Merah and Bera Lake. The horizontal gradient in the former was driven by variation in depth between shallow and deeper regions, while in the latter it was influenced by variation between sheltered areas and open areas. The hydrodynamic models were capable of evaluating the changes in the stratification pattern and physical process created by the effect of the wind on the lake. This will aid in identifying upwelling and downwelling areas and spatial water quality varia-

The model provides tool for understanding the hydrodynamic characteristics of lakes. The findings from the hydrodynamic analysis, such as water column and circulation pattern generated by the model, will become useful as the basis for the study of the water quality and

This research was supported by Ministry of Science and Technology of Malaysia, e-Science Fund grant (No. 04-03-09-SF0003). The authors are thankful to Kerian District, Perhilitan RAMSAR Tasek Bera and Syarikat Air Melaka Berhad for allowing us to conduct the research in field. Special thanks to Dr. Saim Suratman and Ir. Ahmad Jamalluddin Shaaban for their helpful support in this project and Kamasuahadi Yasin, Nizam, and Shukor for their technical

influenced by the water abstraction.

tion and plankton community structure.

environmental variations in water bodies.

Acknowledgements

support in the field.

4. Conclusion

Figure 7. Isotherm in the Durian Tunggal Lake during (a) high wind, well-mixed condition and (b) low wind, differential heating condition. Numbers represent profiling locations.

Figure 8. Temperature and current velocity pattern in Durian Tunggal Lake during (a) high wind, (b) mid-wind speed, heavy rain, and (c) low wind.

water from interbasin water transfer from Muar River into the lake. Similar to Bukit Merah, northeast winds are common for Durian Tunggal Lake and such wind direction moved water toward southeast direction (Figure 8a). Mid-wind speeds and heavy rain exceeding 50 mm induced mean surface current more than 9.0 cm/s in certain western part of the lake (Figure 8b), while low wind induced wind surface current less than 2.5 cm/s (Figure 8c). Additionally, higher salinity of the water from interbasin water transfer was reported occasionally which may affect the hydrodynamic features [10]. However, this dynamic is not studied here due to limited observation data. Water level in this man-made lake is very much influenced by the water abstraction.
