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HOW TO MINIMIZE FLOODING IN CENTRAL LUZON AND THE MARIKINA VALLEY AND METRO MANILA

August 21st, 2013 Posted in Dam water release

HOW TO MINIMIZE FLOODING IN CENTRAL LUZON AND THE MARIKINA VALLEY AND METRO MANILA

The San Roque Dam was designed for a 50-year return flood frequency. Since Typhoon Ondoy and Peping were of the order to a 75-100 year flood, it is thus imperative to lower the dam rule curve by 4-5 meters which is the equivalent of 1 major storm. Please note that it took 6 typhoons to fill up the San Roque Dam to its operating level to provide both power and irrigation during summer.

By operating at a lower level, the San Roque Dam is thus transformed from a 50-year return flood structure to a robust 75-100 year return flood structure. The Agno River System should likewise be re-designed and its riprap banks and dikes raised to contain 25-50 year return flood flows from the current 10-year return flood being observed by DPWH as per JAPAN government aid investment practice.

Responsible authorities should revisit the major recommendations of the Congressional Committee on Dam Safety that studied what could have been done to alleviate the impact of spillway releases by the San Roque Dam during the height of Typhone Ondoy and Peping. This includes modifying the dam rule curve to allow for sufficient ullage to contain an incoming storm and to raise the design specs of the downstream Agno River Flood Control System from the design flow of a 10-year flood to at least a 50-year flood.

Maintaining the old design means that future storms will cause a flooding every 10 years at a cost of P40 billion each time as in 2008 when the entire Pangasinan and Tarlac areas were flooded when San Roque dam released over 5,000-6,000 cubic meters per second (CMS) of spillway water. Please note that a dam is constructed mainly for flood control – to store excess water from the storm and release them in the future when it is safe to do so without flooding downstream areas – and that power generation and irrigation water for summer months is just a secondary function.

My simulation of the dam showed that had a pre-emptive discharge of 500-1,000 CMS at least 3-5 days before arrival of typhone Ondoy and Peping would have avoided the emergency release of 5,000-6,000 CMS to protect the integrity of the San Roque Dam from possible collapse. This time, the authorities should take seriously the recommendation of this Committee chaired by the Pangasinan Congressman Mark Cojuangco.

Paging also authorities in charge of monitoring the safe operations of the dams in Luzon to ensure that dam operators should ensure that dams have sufficient ullage (void space) to contain at least the volume of rainfall from 1 storm. They should include as part of their operating procedure pre-emptive discharge of at least 3-5 days before arrival of an incoming storm tracked by PAGASA using their newly installed doppler radar which measures velocity, time of arrival and expected rainfall intensity (mm per hour per square meter of catchment area). Typhoon simulation runs being conducted by PAGASA-DOST that provides information on typhoon location, speed, arrival and rainfall intensity and duration should also be used as a guide in making what-if runs with the dam simulation model.

There is also a need for DPWH to develop a hydraulic and hydrologic model of the entire Central Luzon and Marikina watersheds so that given the state of our river systems, esteros, drainage and flood control dikes, we can predict the impact and extent of flooding (time, elevation and area of water inundation after an elapsed time since start of massive rainfall). In this way, the flood control structures can be gauged if it can contain various scenarios of extreme rainfall as that of Ondoy and Peping, and now Maring. We must put science in our short term and long-term planning activities.

As long as the stilts of the SM Marikina occupies the flood plain of the Marikina River, flooding upstream from the mall will cause untold suffering and misery to the residents of Provident Village, Tumana, San Mateo and Montalban (Rodriguez). Somebody brave enough should tell this to the Fernandos and Marikina City officials that this added friction of the stilts and the roadway under the Marikina Bridge is causing backflow effect and raising of water elevation, thus causing floodwaters to overtop the riprap and protective walls of exclusive villages (e.g. Provident Village) along the banks of the river.

Paging the TV news services and newspapers to feature and take live pictures simultaneously of upstream and downstream conditions at the SM Marikina mall, and you will see the danger posed by SM Marikina that occupies the flood plain of the Marikina River. Paging brave souls willing to tell this to the country.

President PNoy recently announced that two major companies that constructed structures that impede the flow of the Marikina River, San Juan River and Pasig River that drains into Manila Bay will be investigated by the Department of Justice (DOJ) for obstruction of waterways.

What do you think?

Please share hour thoughts and comments and suggestions.

 

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