Republika ng Pilipinas
Department of Agriculture
Pambansang Pangasiwaan ng Patubig
(National Irrigation Administration)
Regional Office No. 1
October 11, 2011
NIA’S POSITION ON THE EFFECT OF QUARRYING ALONG THE BUED RIVER DOWNSTREAM OF SAN FABIAN DAM.
Strongly supported by NIA’S more than forty years of invaluable experience in the operation of the San Fabian Irrigation System and considering the historical facts of the Bued River and backed by our technical assessment of the various phenomena that took place in the area and its effect on the San Fabian diversion dam, the Regional Manager and Staff of the National Irrigation Administration firmly believe and state that:
a. The lowering of the riverbed is the main reason for the damage and collapse of the old diversion dam. This is perceptible by examining the old and latest design of the dam whereby the downstream apron is typically set at riverbed elevation or lower. In the original design of the dam, the downstream elevation was then at 51.00 but needs to be lowered to 47.5 or 3.50 Meter below for stability and hydraulic considerations in the 2009 dam design.
b. The lowering of the riverbed is greatly affecting the delivery and application of diverted water for irrigation because of deep percolation due to a much lower water table. This is manifested by the diminishing irrigated area from 2,765 has. In the 1970’ s to a mere 1,144 has. even during the wet crop season since 1994 to present.
c. The more than 4.5 meters difference of elevation from the downstream apron to about One (1) kilometer of the Bued river near the quarry site is causing the slope of the river to become steeper thus increasing the velocity of flood water. Stronger current carries more sediments and scouring the embankment and nearby farmlands as they are made of finer and erodible materials.
d. The existence of sandbar in the middle of the river is causing the shifting of fiercer water current towards the western embankment thus the erosion of farmlands and houses at the same time scouring the riverbed.
e. To stop further the lowering of riverbed no quarrying is allowed downstream of the dam. This is to allow the river to negate the effect of retrogression and extensive quarrying, i.e., to replenish the extracted materials downstream of the dam up to the mouth of the river until such time that more gentle and more stable slope shall have been attained.
f. Quarrying if approved by all concerned agencies shall have limitations as to depth, width area and location of quarry site as it will ultimately determine the shift and direction of flow at various stages of river discharges which at present should preferably be at the upstream of the dam.
Gerardo C. Oculto, Principal Engineer C
Frida L. Nidoy, Operation Chief, SFDRIS
Helsy S. Bermudez, Manager, Pangasinan IMO
Dolores C. Nicer, Chief, Planning and Design Section
John N. Celeste, Regional Manager