The Metropolitan Waterworks and Sewerage System (MWSS) said local government units were informed before spilling operations of Angat and Ipo Dams commenced during Typhoon Ulysses.
In a statement, MWSS explained that it the inflow and outflow of excess water at the Ipo and La Mesa Dams needed to be balanced during intense rainfall to ease 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,” MWSS said.
“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,” it added.
MWSS said the dams’ radial gates had to be opened slowly for water to be released to prevent what it called “overtopping.”
“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,” it said.