Ideal watershed and river system – the cure to flooding

By Engr. Edgar Mana-ay

 

Last January 28 the Sangguniang Panlalawigan held a Joint Committee hearing on the ongoing construction of the PHP12-billion Jalaur River Multipurpose II Dam (JRMP II) which is now about 40% completed.

The Committee on Disaster Risk Reduction and Management headed by Hon. Rolly Distura and the Committee on Infrastructure and Environment and Natural Resources headed by Hon. Matt Palabrica jointly presided.

The main purpose of the hearing is primarily to address the concerns of the Provincial Government on flooding as what happened to Cagayan Province in Northern Luzon last November 11 to 14 making Cagayan” looks like an ocean” according to its Governor Manuel Mamba.

As usual the bigwigs of the different offices of the Province were invited to hear the report of National Irrigation Administration (NIA) the government supervising agency in the construction of the JRMP II dam and its Contractor Daewoo and Consultant Dassan, both Korean firms. The trend of the discussion/presentation was focused more on the safety aspects of the Dam and how it will “prevent” destructive flooding as what happened in Cagayan where ignorant people blamed the opening of sluice gates of Magat Dam as the main cause.

To set our perspective in the right direction, it is the WATERSHED AND ITS RIVER SYSTEM THAT IS THE SIAMESE TWIN OF FLOODING. 150 years ago, 15% of rainfall penetrates the ground, becomes part of underground water resource, feeding the streams and rivers during summer. A hydro-geology study by Sweco in 1997 for the Tigum-Aganan Watershed (TAW) showed rain infiltration at only 8.56% and today it may even go down to as low as 8%. Less infiltration means more over land flow, more erosion of the mountain sides and more flooding in the plains as rainfall drainage moves towards the seas thru the systems of creeks and rivers.

Unlike the animals and the fishes, man has never learned to walk lightly on this earth. We have devastated the forests, increase impermeable surfaces in the form housings, basketball courts, boy scout jamboree sites, reared animals in the mountain sides that compacted the ground thus negating the cushioning effects of the leaves of trees for rainfall to fall slowly to the ground for more infiltration and less over land flow. Instead of focusing in returning and healing the devastated forests and its rivers system to its original condition, mankind invented the Dam which is just a make shift, feeble and bordering impotency measure to mitigate the destructions it has made to the environment that causes flooding. A classic example is China’s Three Gorges Dam designed to tame China’s longest and largest river, the Yangtze. Three Gorges Dam is so big that it is visible to the naked eye from space and spans 2.4 kilometers across the Yangtze, but it FAILED in its primary purpose to prevent/minimize the Yangtze from overflowing during a 60-years flood last June. The disastrous flood in the Yangtze basin flooded an area bigger than Mindanao causing 158 death, affected 54.8 million people with estimated economic loss of US$20.5 billion equal to the cost of the dam itself.

 

The right direction of preventing destructive floods is therefore NOT the construction of a dam (although it will help a little) but to concentrate on the healing and rehab of the watershed and its river system (the Siamese twin of flooding) to bring it nearer to its original condition. A healthy watershed is very vital because it provides the load to the creeks and river system for delivery to the sea. Adequate forest cover reduces sediment load and unloads drainage water to the river system in a gradual manner just enough for the river system to absorb and carry thus preventing flooding. The term rain forest condition is already Greek to the younger generation. As an oldie, this writer had a rare interaction with rain forests in Mawab, Davao where my father was awarded a “homestead” of a pure virgin rain forest. Rain infiltration can go as high as 20 percent because of the thick forest cover plus vines and shrubs undergrowth and centuries of decayed leaves covering the ground. This certainly reduces load to rivers and increase groundwater resource.

Then come the streams and rivers which carry the load (both sediments and water) that the watershed imposes on it to deliver towards the sea. Just like a human being, rivers react to the load given by the watershed. Within reasonable limits, it can adjust its depth, width and meander if its load is increased. If imposed with a load beyond its inherent equilibrium capacity and its physical well-being is ERRONOUSLY revised by man in the name of progress (the Jica STRAIGHT floodway!), then the river fights back by destroying its banks and overflows to the plains. This is what happened upstream in the unconcreted river banks of Tigum and Aganan rivers in Pavia and Sta. Barbara; because of the siphon effect of the straight floodway which this writer predicted in 1998 during the public hearing in Pavia for the Jica flood control project.

A river at grade or a stable river is one that seldom overflows its banks (given a watershed in good condition) and if in extreme cases it does overflow, it does not cause massive erosion. This is because the river hydraulic and geometric variables are at equilibrium and technically becomes a river at grade. The hydraulic variables are sediment load of the water, volume and timing of water delivered, all dependent on the condition of the watershed, while geometric variables are: width, depth, bank profile, sinuosity and grade or slope of the river bed. Like a growing child, there is a need to “baby” the river. Controlling, restoring and maintaining rivers at grade requires a soft approach without dependence on steel and concrete which is the hard approach of the Japanese, blitzkrieg style, reminiscent of the “shock and awe” U.S. invasion of Iraq. We have to enhance the natural tendencies of rivers to adjust itself both at low flow and in flood in order to return to a river at grade or at equilibrium.

What then must the provincial government do to address the watershed and river problem? The existing watershed and river management efforts are lousy to say the least. I was a member of the Tigum Aganan Watershed Management Council (TAW) during the time of Mayor Mike and there was no concerted effort to improve TAW. There is a need of a separate body whose job is purely the management of our watersheds and rivers in the province.

 

Note: The writer is a professional member of the American Water Works Association (AWWA) and Chairman of the Municipal Water Board and Hydro-geology Consultant during the term of then Pavia Mayor now 2nd district Rep. Mike Gorriceta.