Magat Dam, one of the largest dams in the Philippines, is a major source of renewable hydro-electric power for Metro Manila and Luzon, as well as irrigation water that helped transformed Cagayan Valley, mainly Isabela and Cagayan, as one of the major breadbaskets of the Philippines, especially for the Metro Manila metropolis -- Metro Manila, CALABARZON and Central Luzon.
Originally, Magat Dam was expected to last 50 years. However, contributing and interacting factors -- increased siltation and sedimentation of the reservoir (exacerbated by rapid deforestation from slash and burn agriculture and illegal logging), fish caging in the reservoir, the 1990 Luzon earthquake -- significantly reduced its lifespan to about 30-35 years.
Dams contribute to flash flood. Magat Dam on 17 Oct 2017 with three gates opened as a pre-emptive measure for rapid release of "excess water" held above the dam -- to make room for the massive rainwater that would come with the anticipated arrival of Typhoon Lando.]
The rapid urbanization of CALABARZON and Central Luzon the past few decades has essentially merged these adjacent regions to become integral part of the Metro Manila metropolis where more than a third of the Philippine population now reside. There will come a time in the future therefore when Cagayan Valley will supplant Central Luzon as the rice granary of the Philippines. In fact, Isabela is already the largest corn producer in the Philippines.
As shown in the video, eventually as many as four gates were opened before noon on the day indicated above to allow for more rapid release of the "stored dam water". This rapid release of water -- more than 120,000 m3/sec -- contributed to downstream flash floods in Isabela and subsequently in Cagayan.
The aforementioned precautionary measure is conducted on all dams throughout the Philippines -- if it was ascertained taken.
The path, intensity, speed of the typhoon could change -- all affecting the total amount of rainwater that would fall in the watershed of the dam. A miscalculation of the total volume of water released with respect to the eventual total amount of rainwater from the typhoon could impact the future hydro-electric power generation capacity of the dam as well as its ability to allocate the needed water for irrigation.