Effects of PRB Dams to Manila Bay Circulation

A lot of reservoirs and dams can be found within the Pampanga River Basin (PRB). You can learn more about them through one of our previous posts: http://bit.ly/PRB5_Dams. The two biggest reservoirs in PRB, Angat and Pantabangan, have volume capacities of 850 million cubic meters (MCM) and 2,996 MCM, respectively. These volumes are enough to cater the water demand in Metro Manila for approximately 7 months and 2 years, respectively. But how exactly do they affect the flows downstream of the current dam locations? What were the flow regimes like before the reservoirs existed? To investigate the effect of the presence (or absence) of these two dams to the downstream flows, a long-term continuous simulation was done to compare the with- and without-reservoir scenarios. Could their absence also somehow affect Manila Bay, specifically its current circulation patterns?

Before the Angat and Pantabangan Reservoirs existed, the simulations showed the following flows would have been observed. At the downstream of Angat Dam’s location, the annual average flows would have been higher by 8.5% (2.4 cubic meters per second or cms) than when the dam was already existing. This is already equivalent to 8% of the amount of water needed to irrigate the farmlands in Bulacan. The dry season mean flow would have been lower by 21.4% (3.1 cms) while the wet season mean flow would have been higher by 18.7% (7.7 cms).

For the case of Pantabangan Dam, the annual average flows downstream would have been higher by 36.3% (7.2 cms) than when the dam was already existing. This can already meet 24% of the irrigation requirement of Bulacan farmlands which is 30 cms. The dry season mean flow would have been lower by 38.3% (6.2 cms) while the wet season mean flow would have been higher by 88.0% (20.5 cms). In both cases, we can see how the reservoirs and dams are able to regulate the flows, since their absence causes the flows to change towards the extremes (i.e. wet season flows are higher and dry season flows are lower in the scenario without reservoirs). The changes are higher at the downstream of Pantabangan than Angat due to the former’s much larger capacity or storage to retain water before discharging.

While the Angat and Pantabangan Reservoirs are both located in PRB, they actually discharge at different outlets. Angat Reservoir discharges to Manila Bay through the Angat River while Pantabangan Reservoir discharges to the Bay through the Pampanga River. The effect of their absence is much decreased at the outlets since a lot of rivers or tributaries have already merged with the main channel downstream of these reservoirs before reaching the Bay. The absence of Angat Dam increases the annual average flow at the Angat River outlet by 5.8% (2.4 cms), while the absence of Pantabangan Dam increases the annual average flow at the Pampanga River outlet by only 2.8% increase (7.2 cms). Changes in watershed discharges significantly affect the prevailing seasonal circulation patterns in Manila Bay. This can be crucial to the material transport characteristics of the bay. Hydrodynamic simulations show that river discharges influence both the magnitude and direction of currents in Manila Bay especially in the near-surface waters, as illustrated in the images. Note that the “Without Discharge” simulations include flows from Pasig River, which is hydraulically connected to Laguna de Bay. It can be observed that larger discharges produce stronger influence in the current patterns, also as shown in the difference in the wet season (September) scenarios with and without river discharges. For the case of Pampanga River, wherein the simulations showed a 7.4 cms average flow for April 2018, the near-surface currents are expected to change as seen in the images. This flow is very much near the 7.2 cms increase in flow in the scenario without the Pantabangan Reservoir. It means that the absence of such a large reservoir could affect Manila Bay’s current patterns.

Here are some pertinent information about the hydrologic models done. The long-term continuous hydrologic model simulation was run from 1995 to 2019 for both the with- and without-reservoir scenarios These long-term simulations have been the basis of the flow averages state in previous slides. PAGASA weather stations were utilized for the weather data input. Since most of the gauges had gaps in them, a weather generator was first created from the same dataset. Fortunately, the collected secondary data covered a sufficient period enough to form the statistics to create the weather generator.

For the reservoirs, the elevation-area-capacity curves were used as guide to determine the volumes, surface areas, and other information needed for the reservoir inputs to the SWAT model. Daily outflows or dam discharges were also one of the main inputs that were collected. Other parameters were assumed based on the land-cover and soil maps used in the model.

The models were also calibrated using the Pampanga River Camba Station, which is located near Mt. Arayat. Model calibration and validation were based on a 2-year dataset. The calibration resulted to a Nash-Sutcliffe Efficiency value of 0.36. To cap it off, reservoirs and dams affect downstream flows. If these changes would be significant and if it would be applicable, the hydrodynamics of the receiving body of water at the river’s outlet could also be affected.