Manggahan Floodway

Also known as: The Floodway (common local shorthand) · Marikina River diversion channel (descriptive term)

Definition

The Manggahan Floodway is a flood control channel in Metro Manila, Philippines, built in 1986 at a cost of about 1.1 billion pesos. Roughly 10 kilometers long, it runs from Pasig to Taytay, Rizal, and diverts peak flood flows of the Marikina River away from the Pasig River and into Laguna de Bay, which serves as a temporary reservoir during the rainy season. The channel is headed by a fully gated diversion dam that regulates how much Marikina River water enters the floodway (Wikipedia, DBpedia).

The floodway was designed for a discharge of 2,400 cubic meters per second within a channel width of 260 meters, and it works in tandem with the Napindan Hydraulic Control System (completed 1983), which regulates flow between the lake and the Pasig River. It is a key structural component of the Pasig–Marikina river system described in the separate entries on the Pasig River and Laguna de Bay: without it, Marikina River floodwater would flow through the Pasig River corridor and inundate low-lying districts of Manila, Makati, Mandaluyong, and Pasig. The Department of Public Works and Highways has pursued the Pasig–Marikina River Channel Improvement Project — including a control gate structure 600 meters downstream of the diversion point — to restore the floodway toward its original design capacity (Wikipedia, DPWH).

Identities

Source Type	Identity
Wikipedia	Manggahan Floodway
Wikidata	Manggahan Floodway (Q16894592)
DBpedia	Manggahan_Floodway
ProductOntology	N/A
Wiktionary	N/A
Library of Congress Subject Headings (LCSH)	N/A
MeSH	N/A
NCBI Taxonomy	N/A
AGROVOC	N/A
Google Scholar	Manggahan Floodway Marikina River flood control Metro Manila
ConceptNet	N/A
OpenCyc	N/A

Also Known As

The Floodway (common local shorthand)
Marikina River diversion channel (descriptive term)

Examples and Analogies

Urban relief valve: Like a pressure-release valve on a boiler, the gated floodway bleeds off surging Marikina River flows before they overwhelm the narrower Pasig River downstream.
Overflow parking for floodwater: Laguna de Bay functions as a vast holding area, “parking” diverted storm water until it can drain through the Napindan Channel and Pasig River to Manila Bay.
Bypass road for water: Just as a diversion road carries traffic around a congested city center, the channel carries excess river flow around the densely built Pasig River corridor.

Usage Scenarios

1. Typhoon-Season Flood Diversion

During southwest monsoon rains and typhoons, operators open the gates of the diversion dam at the floodway’s head so that rising Marikina River water is split between the natural channel toward the Pasig River and the floodway toward Laguna de Bay. This reduces flood peaks along the Pasig River and protects central Metro Manila from deeper inundation (Wikipedia).

2. Lake-Level and Reverse-Flow Management

When Laguna de Bay’s water level is higher than the river’s, flow in the floodway can reverse, and the structure then becomes part of the lake’s drainage system toward the Pasig River. Coordinated operation with the Napindan Hydraulic Control System determines how much water moves between lake, river, and bay (Wikipedia).

3. Post-Disaster Channel Clearing and Relocation

After catastrophic events such as Typhoon Ondoy (Ketsana) in 2009, government agencies use the floodway’s banks as the focus of debris clearing, dredging, and informal-settler relocation programs intended to restore the channel’s design width and conveyance capacity (Wikipedia).

Strategies

Gated diversion control: Operate the headworks gates to match diversion volume to downstream river capacity, keeping Pasig River stages within safe limits.
Use the lake as a buffer: Treat Laguna de Bay as a temporary flood reservoir, releasing stored water gradually once storm peaks pass.
Protect the design section: Recover encroached bank areas and prevent new structures within the 260-meter design corridor so the channel retains its 2,400 m³/s capacity.
Integrate with complementary works: Coordinate with the Napindan Hydraulic Control System, and revive planning for a lake outlet to Manila Bay, the function of the cancelled Parañaque Spillway (Wikipedia).

Security and Safety Measures

Gated headworks: A fully gated diversion dam at the channel’s inlet provides positive control over inflow and can isolate the floodway when needed (Wikipedia).
Conveyance-width enforcement: Relocation and prohibition of settlements and structures within the channel aim to preserve the design width reduced by encroachment from 260 meters to an effective 220 meters (Wikipedia).
Maintenance of the waterway: Dredging and waste removal are required to counter siltation and debris that slow flood flows; uncollected garbage and waste from bank settlements were identified as aggravating factors during the 2009 floods (Wikipedia).

Historical Context

The Manggahan Floodway was constructed as part of the Metro Manila flood-control program of the late Marcos era and completed in 1986 at a cost of about 1.1 billion pesos; Japan’s assistance agency has dated the works to 1988 and subsequently conducted the Study on Flood Control and Drainage Project in Metro Manila (JICA). It was planned together with the Napindan Hydraulic Control System (1983) and a proposed Parañaque Spillway that would have drained Laguna de Bay to Manila Bay — the spillway was never built, leaving the floodway to divert water into a lake with limited outgoing capacity (Wikipedia).

The structure’s defining test came on September 26, 2009, when Typhoon Ondoy (Ketsana) dropped roughly a month’s rain in under 24 hours. The Marikina River’s flow peaked near 3,000 m³/s, and the river system, including the floodway, rapidly overtopped its banks; blocked drains, poor sewer maintenance, and uncollected waste worsened the flooding. The head of the University of the Philippines National Hydraulic Research Center assessed that the floodway could have conveyed the peak without overflowing had its banks not been occupied by settlers. In February 2010, President Gloria Macapagal Arroyo revoked Proclamation 160, which had reserved 20 parcels of land along the waterway for 6,700 poor families, and ordered the forcible relocation of settlers blocking the channel (Wikipedia).

Challenges and Controversies

Informal Settlements and Encroachment

More than 40,000 households line the floodway’s banks, and their structures narrowed the effective channel from 260 to about 220 meters, directly reducing flood conveyance. The February 2010 revocation of Proclamation 160 and the forcible relocation it authorized set off a running debate between flood-safety imperatives and the housing rights of poor families, many of whom lived on the banks precisely because of the area’s low-cost access to the metropolis (Wikipedia).

Capacity Limits and the Cancelled Parañaque Spillway

Ondoy’s near-3,000 m³/s peak exceeded the floodway’s 2,400 m³/s design capacity, exposing the ceiling of the 1980s scheme. Engineers and researchers repeatedly point to the cancelled Parañaque Spillway — intended to move excess lake water onward to Manila Bay — as the missing half of the original plan; without it, water diverted into Laguna de Bay can only exit slowly through the Pasig River, limiting how much the floodway can safely divert in successive storms (Wikipedia).

Pollution and Siltation

Waste from bank settlements flows directly into the floodway, adding pollution and accelerating siltation, while portions of the channel are intensively cultivated with kangkong (water spinach). These uses complicate maintenance, degrade water quality in both the floodway and Laguna de Bay, and illustrate how a piece of flood infrastructure has become entangled with everyday livelihoods (Wikipedia).