NEW APPROACH TO FLOOD CONTROL
Jose Antonio Cangco
Floods are getting worse and annual typhoons and heavy seasonal rains are always pummelling us. The Philippines is more fortunate compared to Bangladesh where monsoon rains and melting snow from the Himalayas flowing through Nepal and India, cause enormous inundation on the flat lands which form about 80% of the country. Like a Bangladeshi, we can accept and learn to live with floods or do something before things get really bad.
The three-pronged approach to flood control concerns only Metro Manila because other parts of the country require area-specific ways and means to address the problem. The three-pronged approach is not the final solution but only supplementary, although the benefits can be lasting.
The Pasig River traverses Metro Manila, a coastal metropolis lying on a floodplain above sea level. The Pasig River has the Marikina and San Juan rivers as major tributaries and serves as the only outlet of Laguna Lake to Manila Bay. Meanwhile, Laguna Lake is the repository of the San Mateo, Tunasan, Pagsanjan and Sta. Cruz rivers, and a dozen plus other tributaries. Moreover, depending on the level of the lake and tidal conditions in the bay, the Pasig River can reverse its flow so floodgates were built in Napindan to contain the direction from and to the lake. Numerous canals, creeks, and esteros also drain into these different rivers.
Floods occur, among other things, when a catchment area receives more water faster than it can drain away. Before the onset of rapid urbanization when concrete became the ruler of the land, rain easily passed through the earth; now, it has to follow the drainage system designed by man to carry it to the canals and creeks. Metro Manila produces some 8,500 tons of garbage daily or about one-fourth of the country’s total, hence, we are no strangers that plastic bags, Styrofoam, and other non-biodegradable and solid trash litter the streets and clog the sewerage and drainage system. Other contributing causes are deforestation in the foothills of the Sierra Madre, land subsidence caused by over extraction of groundwater, soil erosion, siltation, and climate change. (A bigger danger of over extraction is, I think, it can result in sinkholes and ground collapse, bringing down buildings, cars, and people with it.)
Obstruction by vegetation and structures along creeks, esteros, and rivers slows down the torrent of floodwater, and along with siltation in Laguna Lake the volume these can carry or store during heavy rains is greatly reduced. Therefore, the three-pronged approach to flood control will focus on these factors.
The first approach is to prevent trash and garbage from collecting on the drainage systems on streets and then from massing-up and obstructing conduits and small canals. At first, depressions and shallow pools are just a minor inconvenience to commuters and pedestrians but as streets become impassable, destruction of property starts to pile up. Prevention during this early stage is still possible at the barangay level through stricter implementation of garbage disposal and efficient collection methods in each of the neighbourhood streets, blocks, and villages. The barangays can be empowered to police their jurisdictions and impose fines on polluters. As an incentive, prizes can be awarded to the cleanest barangays that have no thrash and garbage strewn on the streets and water corridors.
The second approach to flood control is more extensive. It involves regular dredging and clearing of canals and esteros irrespective of their location and size. Those that overflow are more than just a source of inconvenience; property damage as a result can run into millions of pesos and the dangers posed to people living in close quarters, as well as to transients and stranded travellers, are immeasurable.
The whole length of esteros, creeks, and rivers should be inspected, cleaned of surface trash and cleared from obstacles to ensure faster flow and exit. More so, this is to prevent solid articles from accumulating and slowing down the current especially in bottlenecks and choke points along the watercourses.
Clearing, rip rapping, and dredging of creeks and rivers must be part of their maintenance, done year-round. Siltation, which results in shallow waterways, is a natural process when heavy materials and particles accumulate on the river and estero bottom. Top soil washed away due to erosion contributes to the siltation of rivers and then with shallower depths the volume of water carried is less so any excess runway engulfs both banks. Moreover, structures built on these courses narrow their width and results in backflow build-up. Dredging makes them deeper to carry more volume while cleaning and clearing allow for faster passage and prevent further inundation in those areas.
Several suitable methods, machines and equipment for dredging and desilting are the long-reach excavators to dredge esteros and other shallow corridors while a grab digger or a dragline can operate in deeper parts. There are many ways of removing silt from the bottom and the methods can be customized by using the proper types of equipments because a deeper and free-flowing corridor can carry more volume than any shallow one.
The first and second approaches to mitigate the problem in Metro Manila have dealt with the prevention of blockages then in increasing the volume of surface water that can flow freely. Obstacles and clogged conditions cause backflow and flooding, damaging property with loss of life and limb not very distant possibilities.
The third approach to flood control in Metro Manila is directed to providing a reservoir for excess rainwater to prevent inundation of low-lying areas especially those traversed by the rivers and creeks. Being aware of this fact, the government commissioned studies to divert floodwaters to Laguna de Bay, and then to prevent the lake’s inundating coastal towns when the lake level is high, proposals were forwarded such as the construction of a spillway in Sucat, Paranaque to Manila Bay (Freeman, Fox and Associates), or toward the Sierra Madre mountains through Famy or Siniloan, Laguna, or between Pangil and Paete to Lamon Bay.
Therefore to relieve this recurring problem in some parts of the metropolis, the Manggahan Floodway was built in 1986, serving as a shortcut for waters of the Marikina River before joining the Pasig River, to spill out to the lake. Per its design, it can handle large volumes per second such as that brought by Ondoy but vegetation and structures constricting the channel greatly reduced its capacity.
What concerns us at this point is if some of the overflowing waters in the metropolis are directed toward Laguna de Bay (the rest presumably exiting in Manila Bay), how will the lake be prepared to contain such additional deluge? A lake that over-extends its banks will be disastrous to the residents of the lakeshore towns, while it seems it’s back to the drawing board for the Paranaque spillway, the Famy/Siniloan, and Lamon Bay channels.
There might be some truth that poor planning and implementation had been the norm during years of urbanization of Metro Manila. The Parañaque spillway, to help drain off excess water from overflowing Laguna de Bay during the rainy season and designed to be in tandem with the Manggahan Floodway, was not pushed through when the prices of real estate properties were still low. Another possibility that was not implemented when property values were still quite affordable was to build a direct passage between the opposite curves of the Pasig River because a straight course allows for faster exit to the sea. Looking at a map of Manila, this is applicable in Santa Ana and San Miguel where the river makes a triple horse-shoe bend, appearing as an inverted W.
Disregarding costs, a new option is to build great tunnels under the city where excess rainwater can drain out in reservoirs up north in parts of Bulacan or Rizal. This has been done in several cities such as in Tokyo, Las Vegas, and Kuala Lumpur where the tunnel doubles as a roadway for cars during the dry months. To ease similar problems in the city, Bangkok has started implementing a five-year plan to build four giant underground drainage tunnels that when completed, will have added to total about 50 kms. of tunnels.
The third approach, forwarded as not the final solution and to be taken with the first two approaches, takes a different view. The cost for constructing a spillway in Paranaque cutting through the South Super Highway will be tremendous, yet others say going eastward to Laguna will bring new opportunities to the region. While this debate is going on, proponents of the third approach are for dredging the lake to make it deeper and widen its boundaries in sections where it is possible to avoid inundating thickly populated lakeshore towns. With increased depth and a bigger surface area, the lake will offer livelihood for the people, doubling at the same time as a reservoir for our ever-burgeoning population. Hopefully, this will also change the lake’s moniker as a “large septic tank.” Building a dike to prevent flooding in some areas may work but it will not stop siltation.
Floods, recognizing no political boundaries, make their control a metro-wide coordinated effort. Since the metropolis is a flood basin, strong laws and ordinances should be passed against indiscriminate garbage disposal and pollution; heavy penalties imposed on subversion of infrastructure improvements, on theft and vandalism of PAGASA and Phivolcs apparatus and instruments. We should invest in the best machines and equipment possible and the budgets of the concerned departments, agencies, and local governments be increased to enable year-round maintenance to keep our watercourses streaming and flowing.
We must decide now and implement a comprehensive and long-range plan of flood control in Metro Manila. Because when we shall have done so, there will still remain the job of always cleaning, clearing, rip rapping, dredging, and desilting our numerous creeks, canals, rivers, and lake. Doing these will bring back life and nourish our waterways, and may be the answer to making the Pasig River beautiful again.
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