San Fernando, LA UNION (August 16, 2003)—Pangasinan of late has been battered by two strong typhoons, Chedeng on May 27 and Harurot on July 22. As in the past, floods would have been the normal episode to towns along the course of the Agno River.

But things looked different this time around. According to mayors of municipalities traversed by this river, unlike in the past, the Agno River was very calm and quiet at the height of these typhoons. Keen observers point to that one big difference—the San Roque Multipurpose Dam (SRMP) in Barangay San Roque, San Miguel, Pangasinan.

The flood control capability of SRMP was first proven during typhoon Chedeng, when at the height of the typhoon on May 27, 2003 PAGASA recorded a six-hour rainfall of 358.0 mm and a 24-hour rainfall of 722 mm.

According to the Mines and Geosciences Bureau Region 1, which deployed its Special Task Action Group (STAG) to monitor the situation that time, despite the heavy and prolonged precipitation that took place for about thirteen (13) days due to typhoon Chedeng there was no reported flooding adjacent to the Agno River.

On the contrary, the STAG report revealed, the cities of Dagupan and Urdaneta, and towns of Mangaldan, Mapandan, Sta. Barbara and Calasiao—areas that are not along the course of the Agno River but are traversed by Sinocalan, Pantal and Cayanga rivers—have suffered from 4 to 6 feet deep floods after several rivers have swollen due to the record-high precipitation.

“This only indicates that the SRMP has tangibly helped in flood-control in areas traversing the Agno River,” MGB Region 1 Director Samuel T. Paragas said.

The SRMP, a rock-filled clay-cored embankment dam, stands at 200 meters above the existing river channel and contains nearly 40 million cubic meters of fill materials including river aggregates and clay materials. The dam is 1,300 meters long at its crest elevation of 295 meters above sea level (masl). This imposing manmade mining-spawned structure has two principal functions, power generation and irrigation. However, there are two less recognized but equally critical roles of the dam, flood control and water quality.

According to San Roque Power Corp. report, the SRMP produces a marked attenuation or reduction in the perennial flooding of the Agno River affecting at least 16 Pangasinan and Tarlac towns. For floods up to a 50-year event, or such floods so large as to recur only once in 50 years, peak outflows from the dam are at least one-third (1/3) less than peak inflows to the reservoir.

The report explains that the reservoir does not contain floods, but rather attenuates them; that is, it reduces the outflows released downstream by storing a portion of the inflows received from upstream. In fact, the SRMP attenuates all floods up to the 200-year event—a flood so great as to recur only once every 200 years. The flood attenuation gradually reduces from about 1/3 for the 50-year flood to zero for the 200-year event. The projected flow pattern before and after the dam is completed is illustrated below.

During typhoons, the reservoir level is allowed to rise above the normal maximum operating elevation of 280 masl and may reach the maximum exceptional elevation of 290 masl, consistent with SRMP design criteria.

During extreme events; namely, floods beyond the 200-year event, all inflows received from the upstream Cordillera watershed must be released downstream. Then, flood conditions will be same as those prevailing had the SRMP not been constructed.

In other words, the spillway is operated so that the peak outflow during a flood event does not exceed the peak that would have occurred without the dam and the rate of rise of the water in the downstream channel does not exceed the natural rate without the dam. (By Roel A. Collado, Supervising Science Research Specialist, MGB-REGION 1)