Metro Manila suffered from severe flooding, and Manila City itself was particularly vulnerable. A few hours of moderately intense rainfall can inundate significant portions of the densely populated city. The worst and most destructive flood occured when the Pasig River overflows its banks during intense local rainfall.

The Mangahan Floodway Project was conceived as a practical solution to the over bank problem. The Project limits the discharge of the Pasig River in bank full stages by diverting the excess water through a man made channel to Laguna de Bay. The floodway can provide protection to the greater Manila areas from peak flows with up to 100 year recurrence interval.

However the low-lying areas of Metro Manila, were still threatened by flooding, because of the insufficient capacity of the Pasig and other river channels and drainage facilities. To cope with this problem, the study on flood control and drainage covering all of Metro Manila was completed in March 1990, and the detailed design of North Laguna Lake Flood Control and Drainage Project was completed in May 1993.