On September 26, 2009 intense and widespread flooding occurred in the provinces of Rizal and Laguna, as well as the City of Antipolo and the whole of Metro Manila after the relentless and unprecedented downpour brought about by typhoon “Ondoy.”

Militant environmental-advocacy groups, as reported by Manila newspapers such as “People’s Journal”, hastily and recklessly blamed quarrying operations for the disastrous deluge and prevalent landslides that occurred in the aftermath of Typhoon “Ondoy”. In response, Secretary Jose L. Atienza of the Department of Environment & Natural Resources (DENR) ordered the Mines and Geosciences Bureau to activate a Technical Team to conduct the proper ocular inspections and scientific review of quarry operations in relation to the alleged contribution of the said mining activities to the unfortunate catastrophe.

In order to further enhance their in-depth geo-hazard analysis that focuses on flooding and landslide susceptibility, the team further looked back into past studies and research by geoscientists from the MGB-IV-A (Panfilo O. Montero, et al dated October 1997).  Based on the said research, three (3) major river systems have been identified and delineated together with other minor drainage systems that drain most of Rodriguez, Antipolo, Angono and immediate vicinity/ties. These are:

1. The westerly flowing Wawa River found on the northern part of the area in Rodriguez municipality, the lower downstream extension of the bank of which is within the upper portion of the Marikina River Basin;
2. The west to northwesterly flowing Nangka River in Antipolo City and vicinity; and
3. The southerly flowing Morong River found in the mid-western part covering portions of Antipolo City, and Teresa, etc.

Based on detailed analysis of solid scientific evidences, the aforementioned scientists came up with the following findings and conclusions; to wit:

• The low lying areas along the major and most of the minor river basins investigated in Rodriguez and Antipolo City and vicinity are apparently susceptible to floods, erosions, and siltations, particularly during and in the aftermath of an intense and prolong rainfall spell;
• Apparently there is no significant impact of the mining activities found within the aforesaid various river basins in the area on the flooding characteristics of the rivers as well as on the rivers’ erosions and siltations. With or without mining operations in the various river basins, floods, erosions and siltations may still occur along the delineated flood hazard prone areas of the rivers due to the physiographic and hydrogeologic conditions that were extensively studied therein.  The foregoing is based on the following carefully considered factors:

1. The Marikina River Basin has an area of 53,500 hectares (total drainage area according to UP National Hydraulics Research Center data) which is large enough to catch large amount of rain/floodwater;
2. 2.    The river system upstream of the subject basin has relatively steep gradient, which is geologically controlled by several structures e.g, (Eastern and Western) Valley Fault System and folds among others, such that the retention time of surface run-off is consequently relatively short;
3. The basin is largely underlain by the impermeable rock formation, and this again results to cause more surface run-off because rain water flows down the stream as soon as it drops into land surface and drainage system.  As observed, excessive run-off during typhoon “Ondoy” intensely saturated the soil cover and the underlying fractured and weathered rock unit (sheared zones of the Valley Fault System), which in turn gave way to produce landslides and intense soil erosion even if some areas upstream and downstream of Marikina River System is covered by dense vegetation.  It has to be noted that flooding, erosion and subsequent siltation was observed in the whole part of the Marikina Watershed Areas and River Basin/s, as evidenced by murky/turbid water flowing along the river and floodplains during the typhoon, which could even be noticed in areas where no mining operations are to be found;
4. The basin in general is sparsely vegetated resulting to less evapo-transpiration to cause more surface run-off; and
5. High column of rainfall occurred in the area, which is the main factor in the occurrence of major flooding in Marikina Basin and numerous landslides uphill.  According to PAGASA, the amount of prolonged/continuous (six to eight hours, at about 400-450 mm) rainfall at the height of “Typhoon Ondoy” had exceeded the average monthly rainfall that occur for the month of September.

In a separate geohazard survey/study conducted by MGB scientists (Karlo Queano, PhD, et al) in both municipalities of San Mateo and Rodriguez, Rizal, the team had identified and confirmed several areas/barangays to be highly susceptible to floodings and landslides. Consequently, information and education campaigns were conducted in both towns attended by no less than the chief executives, councilors, key officials of the municipal LGU’s and barangay chairmen for their awareness. Memorandum of Agreements were likewise forged for the installation of landslide hazard warning signages in areas identified to be highly susceptible to geo-hazards, among or with the participation of the same municipal mayors, the Mines and Geosciences Bureau, Office of Civil Defense and the Regional Disaster Coordinating Council.

Geomorphologically, intense flooding and/or flash flooding that affected the country in recent years prove that deltas (ex. November 2004 Real-Infanta-Nakar (REINA) flood), valleys (September 2009 Marikina-Pasig-Cainta-Laguna flood, October 2009 Pangasinan flood) and plains (particularly alongside river channels) are highly prone to flooding.

The inspection and assessment carried out by the Technical Team created for the purpose of reviewing the impact of quarrying/crushing operations in and around the Marikina Watershed areas revealed the fact that the quarry operations have also been affected by the recent severe flooding and that the facilities are relatively intact.  Control facilities to regulate the siltation and pollution of the waterways are still functional and are considered sufficient to address the siltation problem.

The Team finally concluded that, considering that the total area (active and inactive/exposed; inclusive of Tanay and Taytay operations) of the quarry operations cover only 246 hectares as compared to the 53,500 hectares total drainage area (UP NHRC data) of the Marikina Watershed/River Basin, the contribution of the mining/quarrying operations to the flooding and siltation of the Marikina River and the Laguna Lake is very minimal or minuscule in terms of volume.  The foregoing figures translate to roughly equivalent to just 0.46 percent (%) in terms of active/exposed quarry area coverage against the total watershed area.  This conclusion is likewise bolstered by the fact that the regularly monitored quarry operations are equipped with the necessary siltation and pollution control facilities to effectively minimize the amount of silt and suspended material that may contaminate the waterways and consequently produce minimal negative impacts to water quality.

In their report to DENR Secretary Atienza, the Technical Team emphasized that mining or quarrying in general has no significant impact to the flooding susceptibility of the Marikina Watershed/River Basin. With or without mining operations in the various river basins, floods, erosions and siltation may still occur along the delineated flood hazard prone areas most especially in the aftermath of intense and prolonged downpour similar to the severe rainfall spawned by Typhoon “Ondoy”.