Dam experts conducting safety study on the structural integrity of Angat dam has recommended the immediate rehabilitation of the 44-year old dam and its dike in Norzagaray town after it was found that it was sitting on an earthquake fault line.
The recommendation was made during a powerpoint presentation delivered by officials of the Metropolitan Waterworks and Sewerage System (MWSS), headed by Administrator Gerardo A.I. Esquivel, in a recent meeting with provincial officials led by Gov. Wilhelmino M. Sy-Alvarado at the provincial capitol here.
Dam officials maintain that the main dam and dike are generally in good order from visual inspection.
However, based on the draft final report submitted by John Grimston, Tonkin and Taylor International, the group commissioned by the government to conduct study on the structural integrity of Angat dam in coordination with concerned government line agencies, it was learned that during construction (of Angat dam), a series of fault traces were identified in the foundation of the main dike.
The dam experts said that these faults were understood to have been traced for a distance of approximately 15-kilometer upstream of the dike.
Earlier, the Philippine Institute of Volcanology and Seismology (Phivolcs) commenced a site-specific seismic hazard assessment for Angat Dam in 2002, but this work was not completed. The report of NGI (2002) summarizes preliminary conclusions presented verbally by the Phivolcs team.
Their opinion was that the Western Marikina Valley Fault (WMVF) should be considered active, and that rupture of the main fault could trigger movement on the splay beneath the dike of approximately one to two meters horizontal and 0.5 meter vertical.
According to the experts, an earthquake on the Western Marikina Valley Fault could, therefore, trigger movement on the splay of fault trace mapping, undertaken by Phivolcs, suggests that the WMVF runs several kilometers to the east of Angat Dam, and that the fault is present beneath the dike.
On the dam break study, the Grimston, Tonkin and Taylor team submitted main findings and key conclusions, which are summarized as follows: ’the modelling indicates that the dam break flood wave will not only affect the areas near the Angat River, but spread and occupy vast areas well beyond the Angat River channel, and extend both upstream and downstream into the floodplain of the Pampanga River.’
The group also reported that about 20 cities/towns in Bulacan, 7 cities/towns in Pampanga and three cities/towns in Metro Manila would likely be affected by a dam/dike failure scenario.
The map shows that inundation levels can reach as high as 10-30 meters in some areas of Norzagaray, Bustos and Baliuag, Bulacan due to the initial flood wave resulting from breakage. It may reach as high as 5-10 meters in some areas of Pulilan, Plaridel all the way to Calumpit and Malolos City.
It is also estimated that three-five meter flooding will reach areas of Pampanga (due to the Bustos Dam failure) and the flood plain all the way to Metro Manila.
Governor Alvarado said that he would endorse the Phase 2 of the project (detailed design and remediation works)to be implemented at the earliest opportunity after the MWSS administrator informed him that the endorsement of the governor was earnestly needed because it was a requirement prior to the NEDA-ICC approval.
Esquivel also told Alvarado that “considering that the safety of the Angat dam and dike is indeed a national security concern, which needs to be addressed immediately, there is therefore the need to undertake remediation works at the earliest possible time.”
It can be recalled that Alvarado has called the attention of the national government to conduct an immediate study on the structural integrity of Angat dam shortly after he assumed post as governor on 2010.