UP Experts say dredging Cagayan River Will Not Solve Floods

Feb 21, 2009

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Dredging the Cagayan River will not prevent further floods, a team of experts from the University of the Philippines said Thursday.

 

The team from the National Institute of Geological Sciences (Nigs) of U.P. Diliman was headed by Prof. Fernando Siringan, Prof. Alfredo Mahar Lagmay and Prof. Emeritus Kelvin Rodolfo, also of the University of Illinois.

 

UP Prof F.P. Siringan presents his findings to the Rotary Club of Cagayan de Oro (Mother Club)

“Dredging the Cagayan River is not necessary,” Siringan said in his luncheon presentation made to members and guests of the Rotary Club of Cagayan de Oro (Mother Club) held at a local hotel Thursday, Feb. 19, 2009.  “It is costly, very short term and a recurring cost. It has very little effect in low areas and deepening an estuary only brings sea water inland, poisoning ground water and soils.”

 

Because the central and business district of Cagayan de Oro is built on a delta or flood plain built by the gradual accretion of sediments, floods are part of its natural cycle.

Large portions of the delta plain have elevations within two meters or lower of sea level and key infrastructure such as the Kagay-an Bridge rotunda are actually built on previous wetlands or recently abandoned river channels.

 

Photo showing constricted portions of Cagayan River channel by F.P.Siringan

Thus, the city’s central business district is influenced by tides, storm surges and tsunamis and with urbanization, floods have become a threat, Siringan said.

 

Siringan had previously conducted studies of the Cagayan River in 1989-90, 1999 and 2001 for the Cagayan de Oro River Development Authority (CORDA) previously headed by Mayor Jaraula when he was still a city councilor.

 

Siringan said the Cagayan River channel needs only to be widened in constricted portions and even out the protrusion of the flanks. This will bring the river mouth closer to the break of slope and make the water flow faster to flush out excess water and sediment straight to the sea instead of hindering its flow and causing floods inland.

 

River dredger by Dante Sudaria

To save on costs, the U.P. team recommended that the city government ask sand and gravel quarries to relocate their extraction activities where water flow needs redirecting.

Instead of costly dredging, the U.P. team recommends a comprehensive package of measures to prevent a repeat of the two weekend floods last January which devastated Cagayan de Oro.

 

“The first step is to avoid areas that are low lying and are thus prone to flooding,” Siringan said. Climate change, rising sea levels, intense storms and the migration of the Inter-Tropical Convergence Zone (ITCZ) southward all increase the possibility of increased precipitation and rising rivers and seas.                     

 

Should there be constructions in these areas, the lands over which constructions are to take place should first be elevated or the houses can be built on stilts and drainage systems put in place, Siringan said.

 

The team said the poor forest cover of the Cagayan River was instrumental in making the floods worse, contributing to poor water retention and high sediment loads of rivers. These factors were exacerbated by constrictions along the water way and shallow river mouth.

 

The Iponan River showing extensive landslide slumps or slips along its riverbank slopes

“Streams stop flowing when they reach sea level, and drop the sediments they are carrying,” Siringan explained. “Channels rapidly silt up.”

The team also validated Mayor Constantino G. Jaraula’s previously announced plan to conduct extensive reforestation to make up for the 2,000 hectares of forest cover the Cagayan de Oro Watershed lost to conversions to agriculture and other uses over the years. The mayor was present during the presentation.

 

“Reforestation is a must,” Siringan stressed. He also urged that new lands reclaimed by accretion in the Cagayan River delta be developed as mangrove reserves and existing wetlands be preserved and even expanded.

 

“These mangroves act as a natural filter to clear sediments from flood waters and are also beneficial to local fishermen by serving as spawning grounds for fish,” Siringan added.

Not the least, the team recommended that hydraulic mining should be totally banned. This illegal and destructive method of mining is particularly extensive along the Iponan River as evidenced by the brown color of its waters.

 

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