Peter Willing, Ph.D., Water Resources Consulting LLC, Bellingham, Washington
This review is based on first reactions to a reading of the 1984 Environmental Impact Assessment, 1994 Update to the Feasibility Study, 1997 Update to the 1984 EIA, and a small amount of checking and research.
Flood control function of the reservoir
The reservoir will supposedly contain something like the 5-year recurrence interval flood. This is a fairly small, and often recurring, event. It will not contain larger floods, which will have to be passed through the reservoir. The 1984 EIA is frank about this possibility, stating that
gThe reservoir is vulnerable to mismanagement with respect to flood routing. Irresponsible releases of flood waters (under flood condition downstream of the dam) will result in a severe flooding of the whole Pangasinan and most of Tarlac plains. Inattention by operators on the rising reservoir water level could also result in the breaching of the dam, its eventual loss, and the consequent catastrophic flooding downstream.h
In other words, trying to operate the reservoir and spillways in an emergency mode could make floods worse than they might be naturally, and losing the whole dam is conceivable. Northern Luzon is typhoon country; the dam site can expect one or two typhoons per year, on average. They can generate 480 mm of rainfall in a day.
Building a dam that will contain the five-year flood is asking those below the dam to develop a false sense of security, which will result in increasingly damage-prone uses of the floodplain, and far more devastating floods when the five-year magnitude of flood does arrive.
The EIA says that a flood warning system should be developed. There is about a 3 hour travel time for a flood to move from Binga to the populated areas downstream; it will be less for San Roque. There is no mention of how residents downstream are supposed to respond to a flood warning. There is no sophisticated appreciation of flood damages, stage-duration-damage curves, etc. A bad judgment on the part of an operator could stack a reservoir discharge on top of a natural flood and make it worse than it would be normally.
There are apparently nine tailings dams of various sizes, ages, and conditions in the tributary area to the proposed San Roque dam. Several of these are on tributaries that run directly into the reservoir. Several of these tailings dams have failed in the past, and all are presumably questionable in their design and ability to withstand a flood. If a large tailings dam breaks, it sends a slug of water and sediment downstream in a wave that can have fairly devastating effects. It could cause an over-topping of the San Roque dam, if the reservoir was near full supply level. This possibility is not considered in any of the materials reviewed.
Fish habitat is claimed as an offsetting benefit to construction of the dam; if it is restocked with the grighth species, a valuable fishery will become established. The acknowledged water chemistry problems, and an unstable reservoir, will make this very difficult. Fish kills because of mining pollution have happened before. @Even if the fish were to accumulate in the reservoir, they may not be fit for human consumption due to their likely high metal concentrations. The fish from Lingayen Gulf are too hot with mercury and cadmium.