The Manggahan Floodway is a 0.50 km. x 10.10 km flood control complex, with a total of 2.50 square kilometer embankment reserve area for silt deposit. The Floodway was primarily constructed to save Metro Manila from floodwater coming from San Mateo & Montalban, Rizal and Marikina, Metro Manila, flowing down through Pasig – Marikina River, diverting its flow via 250m wide x 8.50m deep Manggahan Channel towards Laguna Lake and regulated by Napindan exit gate in case of overflow.
When the rain subsides, and when the weather condition normalizes, the floodwater is released vice-versa through the same channel, controlled by a flood gate via PasigRiver towards ManilaBay.
Through the years, the silt deposit area or the embankment at both sides of the channel has been converted into a squatter’s area that continuously grew until 80% of the area had been occupied. The berm slope at both sides had not been spared as it is also 80% occupied.
As these happened, no periodic dredging was ever conducted. Silt deposits increased up to 3.75 meters deep per the investigation conducted in the year 2001 and it continues to increase. Now the volume capacity of the channel decreases for non-dredging and the width also decreases due to left and right backfilling of the berm slope, uncontrolled by the government agencies concerned, much more tolerated with the proclamation of Presidential Proclamation 458.
Laguna Lake is now highly contaminated with toxic organisms. The natural resources and marine life was unfortunately devastated. It is very famous and has in fact referred to, as the biggest septic tank in the world by an international newspaper.