Pasig River’s Unique Hydrodynamics

We have learned some interesting facts about Pasig River in our previous post here: http://bit.ly/PMLDB1_PasigRiver. More than collecting discharge from its basin, Pasig River enables the exchange of water quantity and quality between the two large and important water bodies that it connects: Manila Bay, a natural harbor where the largest port of the country is located, and Laguna Lake, which is the country’s largest freshwater lake with multi-purpose functions. It serves as the sole drainage outlet of Laguna Lake. The river is also navigable by small crafts. As it passes through five major cities of Metro Manila (Taguig, Pasig, Makati, Mandaluyong, and Manila), and one municipality (Taytay), and receives inflows from four major tributaries (Marikina, Pateros-Taguig, Napindan and San Juan) and 43 minor tributaries from Laguna Lake basin, Pasig River is a major source of pollution for both Laguna Lake and Manila Bay.

Pasig River naturally flows to Manila Bay as it is sloping downward to the sea. The volume of water exchanged through Pasig River heavily depends on the water level dynamics between Manila Bay and Laguna Lake. It is estimated that the river discharges up to around 300 cubic meters per second* (cms) during wet season when the Lake has received much influx from its tributaries and the lake levels are highest. For the duration when Pasig River continuously discharges water to Manila Bay (around 200 days per year), the average amount of water drained by the river from Laguna Lake is 190 cms. This is almost 4 times the estimated raw water demand in Metro Manila.

As Pasig River drains Laguna Lake, it receives mostly freshwater especially during the wet season. During the dry months, however, lower lake water levels paired with high tides at Manila Bay causes Pasig River to backflow and receive saline water from the Bay. A confluence of brackish water is then formed along the river. At times, this confluence reaches Laguna Lake and forms a saltwater plume at the Lake that can extend until the South and Central Bays of the Lake. This saltwater intrusion in the Lake is not a yearly phenomenon but is rather dependent on the Lake’s annual hydrology.

During incidences of saltwater intrusion in Laguna Lake, a saltwater plume is formed at the northwest bay of the Lake. Saltwater reaches the fish cage and fishpen-loaded South and Central Bays through hydrodynamic action in the Lake, creating brackish water environment that is suitable for milkfish aquaculture. The freshwater runoff from Laguna Lake’s catchment area and the saltwater backflows from Pasig River contribute to the salt balance and water quality of the Lake. This is important in maintaining good conditions for fishery, which is the dominant use of the Lake.

The waters received by Pasig River from its direct catchment area and from the 24 river basins that flow into Laguna Lake drain to Manila Bay during the river’s normal flow conditions in the wet season. The freshwater plume formed by the river at its outlet at Manila Bay is dispersed by the hydrodynamic forces at the Bay.

As a tidal estuary whose flow depends on the water level difference between Manila Bay and Laguna de Bay, Pasig River affects both these major water bodies with pollution and garbage it brings. Solid wastes and dissolved pollutants from upstream sources that enter Pasig River end up in Manila Bay during the wet seasons and affect Laguna Lake during its backflows.

The hydraulic residence time is a measure of the average time a particular mass of water is stored in a given storage space before it gets flushed out. The lake’s residence time can have implications to the water quality, biochemical processes taking place, and the manner in which materials and pollutants are transported into, within, and out of the lake. Based on salinity concentrations and flow directions resulting from simulations, the average hydraulic residence time in Laguna Lake ranges from 6 to 8 months. The variation in lake residence time depends on the water level balance between the Bay and the Lake, effectively the annual hydrology of the Lake.