The Manggahan Floodway is an artificially constructed waterway in Metro Manila, the Philippines.
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On September 26, 2009, at about 6:00 pm PST, the 50-mph "Tropical Storm Ketsana" (called "Ondoy" in the Philippines) hit Metro Manila and dumped one month's rainfall in less than 24 hours, causing the Marikina River system, including the Manggahan Floodway, to burst its banks very rapidly.
Near the boundary of Marikina and Pasig cities, the river meets the gates of the Manggahan Floodway, a controlled waterway used to prevent flooding in Manila during heavy rains by diverting most of the water of the Marikina towards Laguna de bay instead of Pasig River.