With the forecast of La Nina until February 2021, we wish to provide context to the issues particularly salient in Typhoon Cyclone Ulysses (TCU) flooding in Marikina, Rizal, and Bulacan.

  1. As part of the Integrated Warning Systems for the Angat and Ipo Dams, a Flood Forecasting and Warning System (FFWS) was established in 2007 WITH PAGASA, National Power Corporation (NPC), Office of the Civil Defense (OCD), and MWSS as members. To prevent the Angat dam from breaking when the water level breaches it spill level of 210 meters, as what happened in TCU, the Angat Technical Working Group composed of NWRB, PAGASA, MWSS, NIA, and NPC decides on the spill discharge proposed by the NPC.

Most of the excess water during the Angat Dam spilling operation goes to Ipo Dam, managed by MWSS.  To prevent Ipo Dam from overtopping, the MWSS opens the radial gates to release water slowly.

La Mesa Reservoir is the final diversion point of raw water coming from the Umiray-Angat-Ipo river systems. La Mesa Dam does not have gates when its elevation reaches the spilling level.

The responsibilities of the member agencies to the FFWS are delineated as follows:


Office Responsibility
PAGASA Issue Bulletins and Warnings
NPC Recommend water discharge volume to the Angat Dam TWG. Open with spillway gates based on agreed-upon discharge volume.
OCD Disseminate warning information to Disaster Coordinating Councils
MWSS Inform NPC and PAGASA on the gate openings of Ipo Dam and spilling of La Mesa Dam


  1. The Google map below traces the geopolitical boundaries where spills from Angat
    and Ipo Dams flow,  and the overflow path from the La Mesa reservoir.

Discharged excess water from the Angat Dam and Ipo Dam flows through the Angat River and drains into Manila Bay, though about 10% of its excess water affects the municipalities of Bulacan Province. In contrast, when water from La Mesa Reservoir overtops, the water flows through the Tullahan River that traverses Malabon and Valenzuela municipalities before it drains into the Manila Bay.

  1. Balancing the inflow and outflow of excess water at the Ipo and La Mesa Dams during intense rainfall eases the pressure on cities and flood plains surrounding its reaches. In the case of the Ipo dam, when the spill level is breached, surplus water is released at a maximum albeit safe rate to prevent catastrophic releases.  This helps reduce the flood threat to some extent and helps regulate the flow of water further downstream.

The elevation of Ipo and La Mesa a month before TCU was low enough to absorb the expected 112-200 millimeters of rainfall.  MWSS adhered to the protocol in the management of both Angat and Ipo dams prior to spilling.  Prior notice was given to concerned LGUs and other agencies involved on the spilling operations to ensure not only the safety of the dam but also residents that may be affected by the spill.

  1. La Mesa Dam Reservoir serves as a strategic reserve in case of an emergency, like in case of a water shortage or the “Big One” (7.2 magnitude earthquake).  During heavy rainfall, and since it does not have gates to control excess water, a

trigger system is followed to reduce the effect of flooding on the residents along the Tullahan River.

  1. To prevent excessive flood water to Metro Manila, MWSS promptly acted by initiating the closure of the Umiray tunnel to decrease the inflows to Angat Dam. Ipo Dam radial gate openings, including the aqueduct gates were adjusted to decrease inflows to La Mesa Dam.

A holistic view on the confluence of factors such as tropical cyclones from October-November, the forest moisture in the watershed, soil erosion, land conversion, congestion of waterways and drainage systems present the need for an integrated sustainable framework in the management of our dams.   For its part, MWSS has been undertaking medium and long-term solutions to address flooding from the perspective of an integrated watershed management approach. The MWSS’s annual program, which started in 2017, is the planting of one million trees to reforest Umiray, Angat, Ipo, and La Mesa watershed.  Through partnerships with like-minded organizations, the one million target has been exceeded annually, with other watershed areas expanded to include Upper Marikina, Laguna de Bay, Kaliwa, and Manila Bay.