Issues Confronting Metro Manila Flood Management

Is there a solution to Metro Manila's perennial flood? Despite the multi-billion investments poured to Metro-Manila, flood is not totally eradicated or confined. And flood can not be ultimately confined.

Flood results from the topographical, hydrological social, political, and the available infrastructure conditions in the basin. The magnitude and severity, measured in depth, duration and extent, varies accordingly.

Hydrology

PAGASA has four types of climate according to Corona classification . Based on this, Metro Manila belongs to I, two pronounced seasons, dry from November to April, and wet during the rest of the year. Maximum rain period is from June to September. Maximum annual rainfall is approximately 2000 mm in the flat region and 3000 in the mountainous Sierra Madre mountains.

River Systems

There are four river basins traversing the metropolis: Tullahan River Basin in the northern part, San Juan River Basin and the Pasig-Marikina River Basin in the central part, and the Laguna Lake Basin influencing the eastern part.

The Pasig-Marikina River emanates from the Siera Madre mountains. It traverses Montalban, San Mateo, Marikina, Pasig, Mandaluyong, Manila down to the mouth. Montalban and San Mateo has gentler river slope, while Marikina has mild to gentle slope where meandering is observed. Central Metro Manila lies in an alluvial flat. It is relatively low, with waterways influenced by tidal fluctuations.

Western and Northern Metro-Manila is flat and influenced by tidal fluctuations. Flat and tidal influenced terrain usually has low energy head or velocity necessitating pumps for the drainage system to flow smoothly to the rivers under natural condition.

Tullahan River catches the discharge from La Mesa Dam. It finally flows to Valenzuela, and Malabon. Navotas and Malabon are lying in the delta which are influenced by tidal fluctuations. Combined with river inflows, the area is more prone to floods.   

Laguna Lake Basin catches inflows from 24 rivers from Laguna, Cavite, Batangas, Rizal, and Metro-Manila. It serves as a temporary retarding pond from the floodwaters from Marikina River via Mangahan Floodway. Slow drain of the lake is attributed to one outlet, the Napindan Channel.

Infrastructure Deterioration

Pumps to diffuse the flood waters to rivers are aging and surpassing its efficient years. On the other hand, drainage systems were designed decades back with short term foresight with two to ten year probability period. As a result, the drainage systems are outdated and insufficient to accommodate the present dense environment and erratic climatic highs.

Social Activities Aggravating Flood Problems

As population grows, built-up sprawls like a wave from the river mouth to the fringes of hills and mountains. Banks, bridges, foot of the mountains sprout with colony of informal settlers, land development, commercial establishments, public and private buildings, etc. Natural water detentions, streams, and natural outlets are diminishing and the land covers transform to paved or impervious materials. Run-off becomes fast and flash floods devastate residential, commercial, agricultural and other areas.

Indiscriminate dumping to the waterways plus the uncollected garbage which goes along with the runoff further disrupt the flow. Exacerbating the situations are the encroachment of buildings and structures erected illegally and made legally. Upstream development triggers soil erosion. Water flows with floating and suspended sediment and garbage load inevitably clog the waterways.

On the other hand, land subsidence is reported by UP NIGS in the Central, Northwest, and Southwest Metro-Manila, which is attributed to water extraction and soil compaction pressed down slowly by built up of residential and high rise establishment over some decades. Navotas, Malabon, Valenzuela, and Kalookan (known as KAMANAVA) swampy areas have been residential, fishing and docking areas.

Upstream dwellers do not spare the forests. Slowly the trees fall down as the settlers  need firewoods, timber for their huts and some woods were sold illegally or burned as commercial charcoal. Well, they also need food,  so for convenience and expediency to shop in far downtown for food, they cut-off trees on the hill and till the soil and plant bananas, corn, and other root crops. Slash and burn is indiscriminately practice in the upstream of Wawa Dam.

As heavy rain pours down, soil particles are dislodged and are carried with the run-off. Later they are transported in the rivers and waterways and settled on the river bed. Eventually, accumulated sediment  forms sand bars which  change the flow pattern of the river and reduce the flow capacity, hence, resulting to overflow of the river.

It was observed in Ondoy that sediments and garbage contributed to high run off in the Marikina River.

Structural Proposals to Mitigate Floods

Spillway

After Ondoy, some said the old master plan of Metro-Manila, which was formulated during Marcos era, must be reviewed. One of the major proposals was to build the Parañaque Spillway to hasten the subsidence of high water stage in Laguna Lake via canal with a maximum capacity of 350 m3/sec from the lake to Manila Bay. High water in Laguna lake affects the lakeshore communities in Eastern Metro Manila, Rizal and Laguna; likewise, the discharge from rivers, especially the Marikina-Pasig River and tributaries. The energy of the flow is diminished greatly as the lake and river waters converge. Overflow is experienced in the unprotected and low lying banks.

The spillway was a hot item then. It was heralded by a leading and prominent urban planner over media, both print and broadcast. However, as time ticks it becomes less and less feasible and outdated. Environmental considerations include the built-up areas which would be traversed by the channel and the possible salt water intrusion. In hydraulic point of view, the elevation of the Laguna Lake and the Manila Bay has a slight difference (say 3 meters under high lake stage and 0.5 meter during dry season and in consideration of the high tides). Therefore, it is not economically and technically sound. Even if a superpump is installed to gain energy head, the cost of operating and maintaining one, plus the efficient years of use, can not add up to its feasibility.  (A BIG NO)

Underground Tunnel

Another structures hoped to be installed is an underground tunnel, ala smart tunnel in Kuala Lumpur. The shield machine to build the tunnel is not available and very expensive to acquire from Japan or Europe. Again, consider the flat terrain and differences in gradient in flow dynamics. It requires pump again to speed up the flow. Also, the garbage thrown and deposited on the waterways will clog the tunnel. Operation and maintenance become steeply affordable.  (With Reservation)

Laguna Lake RingDike

Ring dike around Laguna Lake is another possibility. Ring dike will also serve as road which will hasten transportation around Rizal and Laguna and neighboring cities and provinces. Existing dike completed under JICA funds was the North Lakeshore Dike (now a part of the proposed C-6 project). Taguig, Pateros, portion of Pasig City and Taytay are partially protected by the structures. It is equipped with four pumping stations.

However, completing the dike around the lake will require land acquisition incurring monumental cost on the part of the government before the actual equally expensive dike is built. Likewise,  the lake ecosystem will be affected by the cutting and filling for embankment.(On Selected Sites)

Retarding Basin

Retarding Basin is one of the feasible options at present to reduce the impact of flood to Marikina-Pasig River. Retarding basin is a temporary storage from the discharge from the main river course to reduce the steepness of the peak discharge via diverting portion of flow to the the pond. Storage capacity depends on the volume the retarding basin can accommodate flood waters; hence, there is limitation for a prolonged rainfall lasting more than the time the basin can accommodate.

Proposed spots for retarding basins are in San Mateo and upstream of Tumana Bridge in Marikina City.

 In Japan, retarding ponds are used as multipurpose arena for recreation or sports, such as football fields during dry seasons. (Highly RECOMMENDED)

Dam Upstream of Wawa Dam

One alternative to reduce the peak discharge of Marikina River is to build a dam upstream of the existing Wawa Dam. Although, this will submerged and inundate a considerable portion of land, this  will surely reduce the impact of big discharge downstream. This can be dual purpose, flood control and water supply, which can help the Angat Dam, which is projected to be insufficient to supply the whole Metro Manila aside from possible destruction as it lies near the fault line.

One potential impact of Dam is displacement of informal settlers (mostly coming from provinces - i.e., Aklan). Environmental consideration includes biomass decay - methane is released into the atmosphere which contributes to global warming, preventing fish migration and spawning, and changeable pattern of dam release which eventually destroys ecosystem.

However, dam can compensate the negative impacts. It can generate funds through tourism, water supply and flood control or possibly small scale hydropower generation. (RECOMMENDED WITH ENVIRONMENTAL AND SOCIAL IMPACT MITIGATION)

Rainwater Harvesting

Rainwater harvesting is a technology that captures precipitation for domestic or other purposes. Small scale storage that will last for a week or so is the drums or other small containers directly attached to downspout or conduits from the rooftops. Collectively, the household rainwater harvesting may retard the flood peak relatively low. (Good Also, but little effect)

Non-Structural Measures

NOAH PROJECT

Noah Project - Is it reliable? I tried to access their site but what came up was this " 502 – Bad Gateway -
The page you requested is currently unavailable. Please try again later."

Maybe it is good only during fair weather when I don't need their information.

The move to have  forecasts by NOAH or ASTI  is confusing. Until when will the NOAH be managed by UP Prof Mahar Lagmay? Why not just improve  system of PAGASA? Just for clarification.


EFCOS

Planned to be expanded. But funding agencies are on wait and see attitude. MMDA has bad reputation for letting it decay slowly until Ondoy came. Whaaa!!! It was ineffective! Who says it is out-modeled?

EFCOS is being kept meticulously now at meager budget by its former ward. I hope the relay station will resurrect so that it can transmit valuable information to agencies and people concern.

RESETTLEMENT OF PEOPLE IN RISKY FLOOD PLAIN

MGB has the map of flood plain area in Marikina. Yet it seems the map was not referred to that's why  high risk areas are  thickly populated. Now, another flood risk maps are coming up, a by-product of World Banks's Master Plan for Metro Manila and Surrounding Areas and from the Greater Metro Manila Risk Assessment from NDRRMC through CSCAND  . But who will use it? The developers? LGUs? or the residents who have no place to go because they don't have the means.

Note: Pictures from WB Master Plan for Flood Management in Metro Manila and Surrounding Areas.

Policies

The water code stipulates that the Secretary of Department of Public Works and Highways can declare a flood control zones. On the other hand, the Local Government Code allows the Local Government Units to make land use plans and build infrastructures within their jurisdictions. LGUs issue permits even in flood plain areas encroaching the river systems. The DPWH on the other hand is too slow to look into the matter until the encroachment seems irreversible.

Also noted is the ignorance of  the Offices concern to give  the river banks to builders in the upstream areas. Just look around (esp. Montalban ) just downstream of the bridge going to Payatas. It is a head scratching activity.